GLOSSARY

Italicized words also are defined in the glossary.

aberrant host A host in which a parasite is not normally found and where it often cannot complete its life cycle; in some cases aberrant hosts die from the effects of the parasite.

abomasal parasite count (apc) The total number of abomasal nematodes (Family Trichostrongylidae) in the abomasum of a deer or other cervid; the mean APC is used by some managers as a measure of the condition of the habitat as well as an indicator of the health and nutritional status of the cervid population.

abscess A collection of pus in a local area of tissue necrosis.

abundance Number of individuals of a particular parasite infecting a single host, regardless of whether the host is infected. The value would be 0 (zero) in an uninfected host, but uninfected hosts would be included in calculating mean abundance. Contrast to intensity.

acanthella Larval stage of an acanthocephalan, occurring between the acanthor and cystacanth stages.

acanthocephalan A phylum of nonsegmented intestinal helminths characterized by having an anterior attachment organ (proboscis) covered with spines; also called spiny-headed worms.

acanthor First larval stage of an acanthocephalan in the intermediate host. acetabulum A sucker. It can be the ventral sucker on a trematode or one of the suckers on the scolex of a cestode.

acoelomate Lacking a body cavity (coelom), as is characteristic of the Phylum Platyhelminthes.

acquired immunity The portion ofthe immune system acquired after exposure to foreign agents or chemicals. This may include active immunity or passive immunity. Contrast to innate immunity.

active immunity Protection acquired against specific foreign materials (antigens) through production of antibodies by B-lymphocytes or through cell-mediated immunity by the action of T-lymphocytes. Also called specific immunity.

acute Having a short and relatively severe course.

adenopathy A disease of the glands, commonly the lymph glands.

adiaspores Fungal spores characteristic of Emmonsia spp. which, after inhalation, produce very large spherules but no further spore formation or other type of reproduction.

aerobe Organism using free oxygen for

its respiration.

agglutination test A simple antigen-antibody test used for bacteria and larger antigens.

alopecia Hair loss.

AMAsτιcoτE A stage in the life cycle of trypanosomes that lacks a flagellum and develops in the tissues of the vertebrate host.

ameba A eukaryotic protozoan using pseudopodia as a means of locomotion.

amplifyinc host A host in which infectious agents can multiply rapidly to high levels, providing an important source of infection for vectors in vector-borne diseases.

anaerobe Organism living and metabolizing in the absence of free oxygen, with a final electron receptor other than oxygen; oxygen is toxic of some anaerobes. Facultative anaerobes can live in the presence or absence of free oxygen.

anamorph A mitotic, asexual form offungus that does not produce ascospores, basidiospores, or any other spores.

anaphylaxis A form of immediate (rapid) hypersensitivity to an antigen associated with IgE antibodies and cytokines produced by mast cells and basophils.

anapolysis In cestodes, the shedding of eggs from the strobila while the proglottids remain attached.

anemia A reduction below normal expectations of the amount of blood an animal carries, as measured by erythrocyte count, packed cell volume, or hemoglobin levels. Also a reduction in the quality of the blood as in a deficiency of hemoglobin.

anorexia Loss or lack of appetite for food.

anthelminthic (anthelmintic, antihelmintic) A drug used specifically against helminth infections.

Anthropocenic “Human origin”; caused by humans.

Anthropophilic “Human loving”; infecting humans, but with little or no transmission to other species.

anthropozoonosis Disease transmitted from humans to nonhumans; typically the nonhumans serve as dead-end hosts. Contrast to zooanthroponosis.

antibody A protein (immunoglobulin) produced in the blood of vertebrates in response to an antigen. The antibody produced is able to bind specifically to that antigen, and may or may not have a protective effect for the host by the inactivation or removal of that antigen.

anticen A chemical substance foreign to a host, capable of stimulating antibody formation. anticen-antibody test A test for determining the presence of a specific antigen, or a specific antibody, based on the known presence of one of these components.

anticen processinc/presentation Intracellular breakdown of complex antigens by specialized cells that result in antigenic fragments (epitopes) binding to major histocompatibility complex receptors and being expressed on the cell membrane.

antiserum Serum with antibodies.

apolysis In cestodes, the shedding of eggs after proglottids have detached and left the host.

arbovirus General term for an “arthropod-borne- virus.” More specifically, a taxonomic group of single-stranded RNA viruses in the Families Togaviridae and Flaviviridae that are transmitted by arthropods.

Arthroconidia See arthrospores.

arthropod “Jointed foot,” a phylum (Arthropoda) of bilaterally symmetrical invertebrates with chitinous exoskeletons and jointed legs, and a true hemocoel.

arthropod-vertebrate reservoir A reservoir for microorganisms comprising a vertebrate host and hematophagous arthropod. Occasionally, non-hematophagous arthropods that feed on conjunctival or other body fluids may be part of this reservoir type.

arthrospores (=Arthroconidia) Environmentally resistant spore-like structures among some ascomycete fungi, formed asexually by disarticulating the hypha.

ascites An accumulation of serous fluid in the abdominal cavity.

ascospores A sexual spore produced within an ascus, among fungi of the Phylum Ascomycota.

ascus A spore case consisting of a single terminal cell; characteristic of the Phylum Ascomycota among fungi,

asymptomatic With no signs of illness.

ataxia Lack of muscular coordination; irregularity of muscle action.

autoinfection Reinfection by a juvenile parasite without the adult leaving its host.

b-lymphocytes Lymphocytes that mature in the bone marrow of mammals or the bursa of Fabricius of birds and, in response to specific antigen, proliferate, becomes plasma cells, and produce antibodies specific to that antigen.

bacillus A general term for a rod-shaped bacterial cell; also refers specifically to the genus Bacillus among bacteria.

bacteremia The presence of bacteria in the blood stream, whether or not they are pathogenic. Compare to septicemia.

bacteria Prokaryotic organisms with cellular rather than organismal properties; generally have single-stranded DNA and divide by binary fission rather than mitosis or meiosis.

bacteriophage A virus that infects bacteria. One of few viruses carrying enzymes.

barrier host See zooprophylactic host.

basidiospores Among fungi, sexual spores that form on the outside of a basidium; characteristic of the Phylum Basidiomycota.

basidium Among fungi, a club-shaped to elongate structure on which basidiospores are formed; characteristic of the Phylum Basidiomycota.

basophil “Base loving”; a type of polymorphonuclear leukocyte of the innate immune system that releases histamine and other vasoactive substances active in moderating the process of inflammation. The internal cytoplasmic granules are stained readily with basic dyes.

benign Favorable for recovery; not malignant or recurrent.

binary fission The simple division of one cell into two daughter cells. Contrast to multiple fission.

biocoenosis The biological association of a pathogenic parasite, its wild vertebrate reservoir species, intermediate hosts or vectors, and any wild vertebrate amplifying hosts.

biological transmission Transmission after a parasite undergoes development or reproduction in the arthropod vector.

biological vector A vector in which an infectious agent lives or develops. Contrast with mechanical vector.

biotype A group of organisms possessing the same genotype; also, a variant strain of a bacterial species differing in identifiable physiologic characters.

bird louse See chewing louse.

biting louse See chewing louse.

black fly Hematophagous fly of the Family Simuliidae, associated with fast-moving rivers and other aquatic habitats, with a nearly worldwide distribution.

blow fly Insect of the Family Calliphoridae that deposits its eggs in decaying matter, on wounds, or in body openings.

bot fly Robust and hairy insect of the Family Oestridae that is an obligate ectoparasite on mammals.

bradyzoite Small merozoite-like stage developing in Toxoplasma gondii and similar protozoa.

brood capsule A capsular projection on the internal membrane of a hydatid cyst.

bubo Swollen lymph node.

bursa A flattened section of cuticle at the posterior end of some male nematodes (e.g., Order Strongylata) used to clasp the female nematode during copulation. A similar structure occurs in all male acanthocephala.

bursa of fabricius In birds, the invaginated tissue of the cloacae in which B-lymphocytes mature.

cancer A malignant neoplasm.

capitulum Anterior (“head”) region of ticks, also called gnathosoma.

capsid Protein coat surrounding the nucleic acid core of a virus.

capsomere Individual protein units of a

virus capsid.

capture myopathy Degenerative changes in striated muscle tissue of wild animals following handling or other severe stress.

capsule A sheath or continuous enclosure

around a parasite stage or other structure generally formed as a defensive measure by the host. Also, a protective polysaccharide or polypeptide structure around a bacterium, and a mucopolysaccharide sheath around the cell or spore of a fungus.

carcinogen A chemical or a process (e.g., radiation) that stimulates formation of cancer.

carcinoma A malignant neoplasm (cancer) of epithelial cells. Contrast to sarcoma.

carrier Infected animal that sheds potentially pathogenic agents, but often remains asymptomatic.

cecum Blind pouch or diverticulum of an intestine.

cell-mediated immunity A portion of the acquired immune system, moderated by T-lymphocytes, that provides defense against parasite-infected cells and cancer cells. Contrast to humoral immunity.

cercaria Juvenile form of a digenetic trematode produced in the snail intermediate host by asexual reproduction within a sporocyst or redia.

Cerciophacy Ingestion of trematode cercariae by some aquatic invertebrates.

cestode (tapeworm) A dorsoventrally flattened helminth of the Phylum Platyhelminthes, usually segmented, and lacking an intestinal tract.

chewinc louse Louse of the Suborder Mallophaga or Ishnocerca (depending on authors) that feed primarily on feathers, skin, or hair.

chlamydia Specialized form of bacteria that is a non-motile, gram-negative, obligate intracellular parasite with no apparent ability to generate its own ATP. Chlamydiae have a special reproductive cycle involving binary fission and budding that distinguishes them from all other microorganisms.

chronic Continued for an extended period; in opposition to acute.

chytrid A type of fungus (Order Chytridiales) associated with serious population declines and extinctions among amphibians.

cilium A small hair-like organelle of motility attached to a cell. Found among protozoa.

cladistics Method of classification based on evolutionary genealogies alone.

clinically active infection The presence of an infective agent that results in overt signs of disease. Among microorganisms, this can be a reservoir type in which the microbial parasites depend on an unbroken series of active infections among susceptible hosts to sustain the microbes among the hosts of a region.

coccus A general term for a spherical-shaped bacterial cell.

coenurus A cysticercus-like larval stage among cestodes of the Family Taeniidae, with numerous scoleces remaining attached to the bladder wall.

co-evolution Changes in the genotypes of two or more species that are a direct consequence of each species' interactions with one another. cohort A group of individuals sharing a common characteristic, such as all being born in one year.

complement A family of circulating proteins that work as a series of enzymes; various components can label parasite antigens, enhance inflammation, enhance antigen-antibody reactions, or contribute to lysing target cells.

complement fixation test An antigen-antibody test used to test for antibodies that bind (or fix) complement.

component population All individuals of a specified life history phase at a particular place and time.

conidiospore Among fungi, a spore produced at the tips or sides of a hypha.

conjunctiva The membrane covering the eyelids and exposed surface of the eyeball.

contacious Capable of being transmitted from an infected individual to a susceptible individual.

coprophacic Ingesting fecal material.

coxa The short, most proximal base jointed segment of the arthropod leg.

cutan eous Pertaining to the skin.

Cyclodevelopmental transmission

See cyclopropagative.

Cyclopropacative transmission Transmission process in which the parasite undergoes a life history stage change (metamorphosis) and multiplies asexually within the vector before transmission to the next vertebrate host.

cyst Any sac generally containing liquid or semi-solid material. Among protozoa, usually considered a quiescent vegetative stage that is environmentally resistant and often the stage important in transmission of the parasite.

cystacanth Encysted form of an acanthocephalan life cycle found in the intermediate host that is the infective stage for the definitive host.

Cysticercoid A larval cestode with a non- invaginated scolex, lacking a fluid-filled sac.

cysticercus A larval cestode with an invaginated scolex in a fluid-filled sac.

Cystocenous “Cyst-producing”; a term often used in relation to the cyst-producing glands of trematodes.

cytokine One of a series of intercellular chemicals produced by leukocytes that regulate immune and inflammatory responses; actions include enhancing inflammation, attracting other leukocytes, and destroying some target cells.

cytotoxic ò-lymphocytes T-lymphocytes that respond to foreign antigens presented on cell surfaces by killing the cells that produced the antigens.

dead-end host A species that can serve as a host for a parasite, but does not serve as a source of the parasite for another host.

decimating factor A factor that kills a host directly. Predation and many diseases are examples. Contrast to welfare factor.

definitive host Host in which the parasite reaches sexual maturity. Normally this term is limited to protozoa and macroparasites.

dendritic cell A nonphagocytic antigen­processing cell.

density The number of individuals of a particular parasite species taken from a host or habitat; commonly expressed in relation to area, volume, or weight.

density dependent A state where the impact or risk of infection by a causative agent varies consistently with changes in the density of the host population.

density independent A state where the impact or risk of infection by a causative agent does not vary consistently with changes in the density of the host population.

dermatitis Inflammation of the skin.

deutonymph A nonfeeding stage in the life cycle of some mesostigmatid mites that molts into the adult.

developmental transmission Transmission after the parasite develops and metamorphoses, but does not multiply within the vector.

diagnosis Identifying the cause of a clinical disease.

diapause A genetically programmed quiescent stage in arthropods when most physiological processes are reduced or suspended.

dilution effect Reduced risk of infection to arthropod vectors by a parasite because a high biodiversity of vertebrate hosts reduces the likelihood that the vector will encounter a host of the infected species in its habitat. dimorphism Having two distinct forms. Some fungi have the capacity to both produce mycelia, and to live as single-celled yeasts.

dioecious Having separate sexes; the two sexes are in separate individuals.

direct fluorescent antibody test Test for an antigen by use of known antibodies with a dye attached that fluoresces under ultraviolet light.

direct life cycle A life cycle in which the parasite is transmitted directly from host to another, without the need for an intermediate host or vector. Monoxenous.

disease A reduction in fitness, as assessed by an absence of some measure of health or a decreased adaptability to change. Disease can be monitored by pathologic changes in host tissues in response to causative agents, with changes in anatomic, chemical, or physiologic features.

disease spillover Process where pathogens continually invade native wildlife populations after an introduced host artificially raises overall host population density to allow transmission— even when the native host population has dropped to levels below which the pathogen normally is transmitted effectively.

dyspnea Difficult or labored breathing.

ecdysis Shedding of the exoskeleton of arthropods and other invertebrates during their life cycle.

ectoparasites Parasites living on the exterior of the host and having only a partial dependence on the host to regulate their relationship with the external environment when they are infesting the host.

elimination A condition in which a parasite or disease no longer is occurring in some areas, even though some transmission still is occurring in other areas. Contrast to eradication.

elisa Enzyme-linked immunosorbent assay.

edema An excess accumulation of fluid in cells, tissues, or cavities; often applied to the subcutaneous tissues.

embryonated egg A nematode or other helminth egg in which the developing larva is visible.

emerging disease Infections that either have newly appeared or have existed and are rapidly expanding their geographic range, with a corresponding increase in detection, prevalence, mortality, or morbidity. encephalitis Inflammation of the brain.

encephalomyelitis Inflammation of the brain and spinal cord.

encysted Enclosed in a cyst or sac.

endemic (=enzootic) Having a low incidence but regularly present in a host population; alternatively, characteristic of a geographic region.

endogenous Growth from within an organism. Compare to exogenous.

endoparasite Parasites living within a host's body and having a complete dependence on the host to regulate their relationship with the external environment during their presence in the host.

endothelial cells Cells lining the blood vessels, heart, lymphatic vessels, and serous cavities of the host body.

en dotoxi n A heat-stable toxin associated with the outer membranes of gram-negative bacteria, and released only following damage to the bacterial cell.

enteritis Inflammation of the intestine.

envelope An encompassing membrane. In virology, a coat surrounding the nucleocapsid, usually furnished in part by the host cell.

enzootic see endemic.

enzyme-linked immunosorbent assay (elisa) An antigen-antibody test using an enzyme reaction as an endpoint; the endpoint may be a color change, a change in fluorescence, or an electrochemical change.

eosinophil “Eosin loving”; a phagocytic polymorphonuclear leukocyte of the innate immune system that can attack helminths and other multicelled parasites, detoxify some compounds, and ingest antigen-antibody complexes. The cytoplasmic granules stain readily with acidic (eosin) dyes.

eosinophilia The formation and accumulation of a large number of eosinophils in the blood.

epidemic (epizootic) Disease affecting many hosts of an area at a given time and at a far greater rate than expected. Also defined as a sudden, rapid increase in the prevalence or intensity of a parasite or disease. The term epidemic often is restricted in usage to human populations, and epizootic to nonhuman populations.

epidemiology (=epizootiology) The study of diseases in populations of humans and other animals, including their causes and distributions. epimastigote A stage in the life cycle of the genus Trypanosoma (protozoan hemoflagellate) that occurs in the insect midgut.

epipharynx A structure which overlaps the mouth of certain insects, and is adapted to form a tube for sucking blood.

epitope The small region of an antigen that is specifically recognized by antigen-binding receptors of B- and T-lymphocytes.

epizootic An epidemic among a nonhuman population.

epizootiology See epidemic.

epornitic An epidemic in an avian population; not a commonly used term.

eradication A parasite or disease no longer is being transmitted anywhere in the world, and all control or prevention measures can cease. Contrast to elimination.

erythrocyte “Red cell”; a red blood cell.

erythropoiesis Formation of red blood

cells (erythrocytes).

etiology Study of the origins and causes of diseases and how they are introduced to their hosts.

eukaryote Organism with a true nucleus containing double-stranded DNA, an endoplasmic reticulum, mitochondria, Golgi apparatus, lysosomes, and other cellular organelles. Traditionally these include algae, protozoa, fungi, and all multicellular plant and animal cells.

excystation To break out or escape from a cyst, as for a parasite after its cyst has been ingested by the host.

Exoerythrocytic merogony A part ofthe process of merogony among the Apicomplexa (sporozoa) protozoa that occurs outside of erythrocytes and occurs within liver, spleen, or other tissue cells.

exogenous Growth by addition to the exterior of an organism or structure. Compare to endogenous.

Exopterygote An insect in which the wings develop externally and gradually. The insect undergoes no pupal stage, and its young are called nymphs.

exsanguination Loss of blood to the extent that life is threatened or lost.

exotoxin A bacterial toxin that is excreted

by the living bacterial cell into the surrounding environment.

facultative Optional, not obligatory.

facultative anaerobe Among bacteria, organisms that can grow both in the presence and absence of oxygen.

false negative rate The proportion of exposed (diseased) animals incorrectly assessed as being negative for a test.

false positive rate The proportion of healthy/unexposed animals incorrectly assessed as being positive for a test.

festoon Small section on the posterior of some hard ticks (Family Ixodidae) demarcated by suture lines.

fibroma A tumor composed of fibrous or connective tissue.

filarial nematodes Nematodes (Superfamily Filarioidea) of tissues and tissue spaces of many vertebrates, in which the females are ovoviviparous or viviparous.

filariform esophagus A nematode esophagus with parallel sides, with no constriction or terminal bulb.

fitness Survival and reproductive success on an organism; the ability to produce viable descendants into the next generation.

flagellum An organelle of motility among bacteria and protista. The eukaryotic flagellum is structurally quite different from that of the bacterial flagellum.

flea A hematophagous insect of the Order Siphonaptera that is laterally flattened and has hind legs adapted for jumping.

flesh fly Insect member of the Order Sarcophagidae, some of which are parasitic on wild vertebrates.

fly Member of the Order Diptera among insects.

fluke See trematode.

fomite An inanimate object that is a source of parasites to a susceptible host coming in contact with infectious material left on it (e.g., tree trunk or fence post and wire for scratching, bird feeder bars and platform on which birds can perch).

fungus A kingdom of eukaryotic organisms with cell walls of chitin or cellulose, but lacking chlorophyll, roots, leaves, stems, or other plant characteristics. Most are non-motile and reproduce by spores, and most form tube-like hyphae that can further develop into multicellular mycelia.

gametocyte “Gamete cell”; among sporozoan protozoa, the cell that follows the merozoites in the life cycle and will become either the microgametocyte or macrogametocyte in the process of gametogony.

gametogony “Gamete formation”; among sporozoan protozoa, the sexual reproduction leading from merozoites, through gametocytes and gametes, to the formation of the zygote. The term gamogony sometimes is used.

gamogony See gametogony.

gel electrophoresis A technique to separate protein molecules or nucleic acids in a gel matrix by means of an electric current.

genome The full set of genes of an organism.

germling Among chytrid fungi, a developmental stage between the zoospore and mature sporangium consisting of maturing fungal cells with root-like hyphae.

gnathosoma Anterior (“head”) region of ticks and mites, containing the hold-fast and feeding organelles.

gonopod A secondary genital appendage by which a male arthropod transfers the spermatophore directly to the genital opening of the female.

gonotrophic cycle The reproductive cycle of hematophagous insects of feeding on and digesting blood, egg maturation, and ovipositing.

gram-negative Bacteria with a relatively high level of lipopolysaccharides in their cell wall that are susceptible to decolorization by alcohol; thus they do not retain crystal violet stain, a blue stain, but are counterstained by safranin, a red dye.

gram-positive Bacteria with a thick layer of protein but a low level of lipopolysaccharides in their cell wall, that resist decolorization by alcohol and retain a blue-colored crystal violet stain.

granulocyte Polymorphonuclear leukocyte with distinct granules in its cytoplasm. Includes basophils, eosinophils, as well as neutrophils (in mammals) and heterophils (in birds).

granuloma A site of chronic inflammation initiated by a variety of infectious and noninfectious agents that result in tissue necrosis; the site often includes macrophages, lymphocytes, and other leukocytes.

cRAviD Condition of a uterus having eggs or developing embryos.

Cubernaculum An accessory structure in some male nematodes to guide the spicules out through the cloaca.

haller’s orcan A depression of the first tarsi of ticks that functions as an olfactory and humidity receptor.

halter A small knobbed structure found as a pair in some two-winged insects; they are flapped rapidly to maintain stability when flying.

hapten A molecule of low molecular weight that is antigenic only which attached to a carrier protein.

hard ticks Ticks of the Family Ixodidae characterized by having a hard shell (scutum) on their dorsal surface, and having their capitulum extending anterior to the main body (idiosoma). Contrast to soft ticks.

health A complex condition assessed by one or a combination of specific physical characteristics; also can be assessed by the adaptability of a host to change.

helminth A worm parasite and a member of one of four endoparasite groups trematodes or_ flukes, cestodes or tapeworms, nematodes, and acanthocephala. Pentastomes (tongue worms) sometimes are included.

HELMiNTH-VERTEBRATE RESERVOIR A reservoir for microorganisms involving both a vertebrate host and one or more helminth parasites infecting that vertebrate.

helper t-lymphocyte T-lymphocytes that initiate or increase the function of other lymphocytes and macrophages after having antigen presented in conjunction with major histocompatibility complex molecules.

Hemacclutination INHIBITION test An antigen­antibody test used on viruses that spontaneously agglutinate erythrocytes in vitro, in which the presence of antibody inhibits that capacity.

Hematophacous “ Blood eating”; characterized by ingesting the blood of other animals as a source of nutrition.

hemimetabolism Incomplete metamorphosis; a term used to describe the mode of development of certain insects that includes three distinct stages: the egg, nymph, and adult stage, or imago. These groups go through gradual changes; there is no pupal stage. The nymph often resembles the adult somewhat, as it has compound eyes, developed legs, and wing stubs visible on the outside. Compare to paurometabolous.

Hemocoel “Blood coelom”; the main body cavity of arthropods, containing a blood-like fluid (hemolymph).

Hemoflacellate “ Blood flagellate”; a flagellated protozoan, such as a member of the genus Trypanosoma or Leishmania, living in the blood of a vertebrate host.

hemolysin “Blood lysing”; a chemical substance that causes a breakdown (lysis) of red blood cells.

hemorrhace Loss of blood from the blood system.

hepatitis Inflammation of the liver.

hermaphrodite Single organism having both male (“Hermes”) and female (“Aphrodite”) sexual organelles; monoecious.

Heteroceneity Generally refers to the property of diversity. More specifically, a feature of acquired immunity reflecting that there are at least a million different types of B-lymphocytes, each capable of producing a specific antibody, and a comparable number of specific T-lymphocytes.

heterophil A type of leukocyte found only in birds, comparable to the neutrophil of mammals.

heteroxenous Having an indirect life cycle, requiring at least one intermediate host.

hexacanth See oncosphere.

Hippoboscid fly Member of the Family Hippoboscidae among insects parasitizing mammals and birds. Also called louse fly.

HiSTOLYTic “Tissue-lysing”; a chemical that can break down (lyse) tissues.

hookworms A family (Ancylostomatidae) of nematodes.

horizontal transmission Transmission of parasites to other members of the vertebrate population of the same generation or cohort, or between a vector and vertebrate host. Contrast to vertical transmission.

humoral immunity Portion of acquired immunity associated with B-lymphocytes and antibody production. Contrast to cell-mediated immunity.

HYDATi D A cysticercus-like larval stage with many scoleces produced in brood capsules that float in the fluid of the main cyst.

Hyperdisease Disease in immunologically naive populations that results in very rapid and extensive population declines.

hypersensitivity An allergic stage involving an exaggerated reaction to a particular antigen, following prior exposure to that antigen.

hypha The tubular filament offungi representing the vegetative unit.

hypostome Portion of the mouthparts of ticks on the capitulum, used to thrust into and hold firmly onto the tissues of the host.

i diosoma The single, main body of ticks and mites.

imago Adult or final instar in the development of an insect.

immunoblot test See western blot test.

immunoglobulin Serum proteins (gamma globulins) produced by specific B-lymphocytes of a host in response to a specific antigen. There are several molecular classes (isotypes).

immunohistochemistry Process of localizing proteins in cells of a tissue section by the use of labeled antibodies.

immunological memory The capacity of the acquired immune system to respond more strongly and more quickly to a foreign material on a repeat exposure to it.

immunosuppression Prevention of an immune response, or limitation of the normal immune response.

incidence The number of new hosts that become infected with a particular parasite during a specified time interval, divided by the number of uninfected hosts present at the start of the time interval. Contrast to prevalence.

incubation period The period oftime after a host is infected with a parasite, but during which the parasite cannot yet be detected. Called the prepatent period in the study of malarial protozoa.

indicator host Susceptible hosts that give evidence of the presence of a parasite in a host without symptoms (silent carrier).

indirect fluorescent antibody test (=indirect immunofluorescence test) An antigen­antibody test in which presence for an antibody is tested using an indicator dye that fluoresces under ultraviolet light.

indirect life cycle A life cycle that requires one or more intermediate hosts or vectors for completion. Heteroxenous.

in fection The persistent presence of a parasite in a host, whether or not it causes disease. Some scholars prefer to limit the term infection to endoparasites and use a related term, infestation, to describe the persistent presence of ectoparasites.

in Fectivity Ability of a parasite to initiate and maintain an infection in the host.

i n festation An infection with ectoparasites.

inflammation Process of returning an injured site to a condition of homeostasis through regeneration of some cells and through scar formation; inflamed tissue often is characterized by redness, heat, swelling, pain, and loss of function.

infrapopulation A parasite infrapopulation includes all individuals of a parasite species in an individual host, at a particular time.

innate immunity A protective portion ofthe immune system that is always in place and does not require previous exposure to infective agents, toxins, or foreign chemicals. Examples include the skin and mucous membranes, phagocytic cells, and a variety of antimicrobial peptides. Also called nonspecific or natural immunity. Contrast to acquired immunity.

insect A class (Insecta) of arthropods with jointed exoskeletons, clear segmentation that results in three distinct body regions, and three pairs of jointed legs.

instar Molt stage in the life of an arthropod.

intensity Number of individuals of a particular parasite species on an infected host—the number of individuals in an infrapopulation. In contrast to abundance, intensity can be determined only for those hosts infected by a particular parasite; uninfected hosts would not be included in calculating mean intensity.

interferon A type of cytokine in the innate immune system that is tied to antiviral and anticancer defenses.

interleukin A cytokine produced by a leukocyte that affects its own activities or those of other leukocytes.

intermediate host Required host for some helminths and protozoa, where the parasite undergoes developmental change and may multiply, but does not reach its sexually mature stage. Normally a different taxonomic group from the definitive host.

intensity The number of parasites per infected animal.

i Ntercellular “Between cells”; activity occurring between cells. Contrast to intracellular.

intracellular “Within cells”; activity within a cell. Contrast to intercellular.

intrauterine Within the uterus.

invasiveness Ability of a pathogen to progress further into the host from the initial site of infection.

isotype A molecular class of immunoglobulins (Ig) such as IgM, IgG, IgA, IgD, IgE, and IgN.

ked A hippoboscid fly.

kinete A motile zygote of piroplasms (Apicomplexa), similar to the ookinete of Plasmodium. After invading cells, kinetes form sporozoites.

laceration A torn, ragged wound.

landscape epidemiology The study of diseases from a holistic ecological perspective, including risk patterns and environmental risk factors. Based on the notion that most infective agents, their host, and vectors are limited in their geographic and ecologic distribution by environmental determinants.

larval migrans A disease caused by nematode larvae migrating through the host, but not developing to adulthood. Cutaneous larval migrans is migration under the skin. Visceral larval migrans involves migration through internal organs.

larviparous “Larva producing”; female flies producing early instar larvae that are inoculated directly to a susceptible vertebrate host. Characteristic of ovoviviparous flies.

latent infection Among microorganisms, the capacity to persist in the tissues of apparently healthy carrier hosts for extended periods, often for the life of the infected host. Infected hosts may serve as reservoirs for the pathogen and shed them to other susceptible hosts.

latent period Among microorganisms, the time from initial infection to when the host can transmit the infection to another host. leech Invertebrate member of a Class (Hirudinea) among the Phylum Annelida; members may be predatory or parasitic and affect all classes of vertebrates.

lesion An injury or wound, often with a circumscribed area of pathology.

leukemia A cancer of leukocytes or their precursors, resulting in a large increase of affected leukocytes and their precursors in the blood.

leukocyte “White cell”; a white blood cell that is part of the host defense system. Examples include lymphocytes, monocytes, macrophages, eosinophils, neutrophils (in mammals), and heterophils (in birds).

lice Plural form of louse.

louse Ectoparasitic insect of the Order Phthiraptera; all are dorso-ventrally flattened and wingless. Two main groups are sucking lice and chewing (or biting or bird) lice.

louse fly See hippoboscid fly.

lumen Space within any hollow organ.

lymphocyte A type of leukocyte important in acquired immunity. Two major groups are B-lymphocytes, which produce antibodies, and T-lymphocytes, which are part of cell-mediated immunity.

lymphoid tissue Lymphocyte-rich tissues that may be concentrated in lymph nodes or more diffuse in other tissues.

lymphokine A cytokine released by a lymphocyte.

lymphoma A cancer of lymphocytes, which reside in the lymphatic system and in blood-forming organs.

lysosome An intracellular vacuole containing lysozyme and other digestive enzymes.

lysozym e A digestive enzyme often playing an important role in host defenses by disrupting chemical bonds in bacterial cell walls.

macrogametocyte Cells produced during the process of gametogony among the protozoan Apicomplexa (sporozoa) that give rise to the larger, quiescent, “female” macrogametes.

macroparasite Commonly, a parasite visible to the naked eye; these are parasites that generally do not multiply in their adult stage within their definitive host, but instead produce transmission stages such as eggs and larvae that pass into the external environment or to vectors. Common examples are parasitic helminths and arthropods. macrophage A phagocyte (leukocyte) of the innate immune system that inhabits host tissues and can remove and destroy invading microorganisms and remove tumor cells.

major histocompatibility complex (mhc) A closely linked group of gene loci that encode cell surface proteins and are used by the immune system to recognize these cells as their own (self), or as foreign material (antigen) that subsequently will be attacked.

malignant Tending to worsen and to cause death. Term used in cancers based on the property of these cells to invade new tissue and to metastasize to other sites distant from the primary lesion.

mange Dermatitis caused by several species of parasitic mites.

mast cells A leukocyte of the innate immune system found in tissues that produce many chemicals (cytokines) to enhance the process of inflammation.

mechanical transmission Transmission through contaminated appendages, usually mouthparts. May also follow regurgitation of an infectious blood meal or after a parasite passes through the arthropod gut unchanged. Contrast to biological transmission.

mechanical vector A vector that transmits infective agents by mechanical means only. Contrast to biological vector.

memory cells Long-lived lymphocytes that survived an earlier response to their specific antigen and are able to respond quickly and strongly to the subsequent appearance of that same antigen.

meninges The membrane s surrounding the brain and spinal cord.

meningitis Inflammation of the meninges.

meningoencephalitis Inflammation of the brain (“encephalon”) and the meninges.

merogony “Merozoite production”; among sporozoan protozoa, an asexual stage of multiplication occurring by the process of multiple fission in the vertebrate host, leading from individual infective sporozoites to numerous merozoites. Also called schizogony in older literature.

meront An asexual stage among protozoa Apicomplexa (sporozoa) in the process of merogony; called a schizont in older literature.

merozoite “Animal part”l the final stage in the process of merogony, in the Apicomplexa

(sporozoa) among protozoa. This stage follows the meront and either reinvades new host cells to form more meronts or develops into a microgametocyte or macrogametocyte stage.

mesothorax The second of three segments in the thorax of an insect, and bears the second pair of legs.

metacercaria The infective stage to the definitive host for digenetic trematodes.

m etacestode A developmental larval stage of a cestode, following the oncosphere.

m etacyclic stage Stage in the life cycle of a parasite that is infective to its vertebrate host.

metamorphosis Developmental changes among insects through their succession of life history changes.

m etastasis Transfer of malignant cells or disease from one tissue to another not directly connected with it.

metathorax The most posterior of the three segments in the thorax of an insect, bearing the third pair of legs.

metazoan Any multicellular animal.

mhc Major histocompatibility complex.

Microaerophil A bacterium requiring free oxygen for growth, but at lower levels than is present in the atmosphere.

microfilaria A first stage young of any filarial nematode.

microgametocyte Life cycle stages produced during gametogony among protozoan Apicomplexa (sporozoa) that give rise to slender, active “male” microgametes.

microparasite Commonly, a parasite not visible without microscopic magnification; microparasites generally multiply within their definitive host. Examples typically include bacteria, including rickettsia and chlamydia, as well as fungi and protozoa. For convenience, viruses commonly also are included in this group, even though they are not generally considered living organisms.

midge A hematophagous fly

(Family Ceratopogonidae).

miracidium The larval stage of digenetic trematodes that develops in the eggs shed by the adult; some remain in the egg until ingested, while those of other species are ciliated and free swimming.

mite Small (oviparous.

pandemic A widespread or worldwide epidemic; generally this term is used in reference to human diseases.

panzootic A widespread or worldwide epizootic among nonhumans, often involving more than one host species. (Uncommon usage).

parasite An organism that lives in or on a host, from which it derives food and other biological necessities. The parasite benefits by using host resources, often reducing the host's survival or reproductive success.

parasitemia “Parasites in blood”; the occurrence of parasites in the blood stream.

paratenic (transport) host Host in which no development or reproduction of the parasite occurs; often considered an optional host.

parenchyma The functional elements of a tissue or organ, in contrast to the structural framework (stroma).

parous The state of a female arthropod that has experienced one or more gonotrophic cycles and has oviposited.

pericardium The serous membrane (serosa) surrounding the heart.

parthenogenesis Development of an egg without fertilization by a sperm; unisexual reproduction.

passive hemagglutination test An antigen­antibody test in which the antigen is attached to an erythrocyte to make a positive reaction more visible.

passive immunity Immunity to a fetus or neonate from antibodies transmitted by the female through the placenta, milk, or egg yolk. This type of immunity lasts for a short period only.

pastoral cycle Parasite life cycle involving domestic animals. Contrast to sylvatic cycle.

patent period The period of time characterized by detectable parasite activity in the host, including disease. Often used in the study of malarial protozoa.

pathogen pollution Human introduction of pathogens (with or without their host) to new geographic regions.

pat hogenic Capable of causing injury or death to tissues or a host.

pathognomonic Distinctive or characteristic signs on the basis of which a diagnosis of a disease or pathologic condition can be made.

Paurometabolous A simple, gradual, direct metamorphosis in which immature forms resemble the adult except in size and are referred to as nymphs. Compare to hemimetabolism.

pcr Polymerase chain reaction.

Pentastome A phylum of helminth-like parasites, likely evolved from arthropods, found in the respiratory systems of vertebrates.

peridomestic A term used to describe animals that are not domesticated, but which nevertheless live in close proximity to humans.

peritoneum The serous membrane (serosa) lining the abdominal cavity.

peyer's patches Lymphoid tissue on the wall of the small intestine.

phagocyte One of several types of leukocytes that can ingest foreign material in a host and that can protect the host from the effects of that material. Examples include monocytes, macrophages, and eosinophils, as well as neutrophils (in mammals), and heterophils (in birds). phoresy A process by which some lice are transported to a new host by temporarily attaching to other arthropods.

piroplasms A group of Apicomplexa (sporozoan) protozoa transmitted by ticks of the Family Ixodidae.

plasma The fluid portion of blood after red and white cells have been removed, but in which the clotting elements still remain. Contrast to serum.

plasma cells B-Iymphocytes that are actively producing antibodies.

pleomorphic “Many forms”; occurring in various shapes.

pleura The serous membrane (serosa) that covers both lungs and lines the thoracic cavity.

pneumonia Inflammation of the lung, typically following infection by pathogenic organisms.

polymerase chain reaction (pcr) test A technique for amplifying a specific sequence of DNA by repeated cycles of synthesis, driven by pairs of primers.

polymorphonuclear leukocyte “Many-shaped nucleus, white cell”; a leukocyte with irregular, lobulated nuclei containing several nuclear segments connected by strands of chromatin.

polyphyletic Organisms placed in one taxonomic group that evolved from two or more different taxonomic groups; a polyphyletic taxon does not contain the most recent common ancestor of all its members. Contrast to monophyletic.

precipitin test An antigen-antibody test used for viruses and other particles too small to be seen with a light microscope.

prelarva A nonfeeding, quiescent stage among mites that occurs between the egg and larva stage.

prepatent period The time interval between parasite infection in a host, and detection of eggs or larvae (for helminths), or blood stages (for hematozoa) in the host; similar to incubation period. Contrast to patent period.

presoma The holdfast organ (proboscis), neck, and attached muscles of an acanthocephalan.

prevalence The number of hosts infected with one or more individuals of a particular parasite species (or taxonomic group) divided by the number of hosts examined for that parasite species. It is commonly expressed as a percentage when used descriptively and as a proportion when incorporated into mathematical models. Contrast with incidence and intensity.

prion “Proteinaceous infectious particle”; an infectious protein, with no evidence of associated nucleic acids, but whose aberrant folding pattern may serve as a template for aberrant folding of the host-generated protein.

proboscis The retractile, spiny, hold-fast organ of an acanthocephalan.

procyclic stage A developmental stage among hemoflagellates occurring in the invertebrate host, often in the arthropod midgut.

proglotti d One segment of a cestode strobila, including a complete set of reproductive organelles.

prokaryote Organism lacking true nuclei and most cellular organelles. It reproduces by fission rather than mitosis or meiosis. Examples include the Archaebacteria and Eubacteria; some also include viruses in this category.

promastigote A stage in the life cycle of trypanosomes (protozoa) found in the midgut of infected insects, especially in the genus Leishmania.

propagative transmission Transmission to

the next vertebrate host only after a parasite multiplies asexually within the vector and after a suitable incubation period.

prophylaxis Treatment given to prevent a disease.

protist A single-celled eukaryotic organism, without cell differentiation into tissues. Includes many protozoa.

protonym ph Early hematophagous stage in the life cycle of mites; a quiescent, nonfeeding stage for some mites.

protoscolex Small scolex associated with the hydatids of the cestode Echinococcus (Family Taeniidae).

protozoa A traditional term for unicellular eukaryotes, comprising predominantly non- filamentous heterotrophic species. Most protista are considered protozoa.

Pseudocoelomate Having a fluid-filled cavity between the body wall and intestine formed from a persistent blastocoel, without a mesoderm lining; characteristic of nematodes and acanthocephala.

pseudocyst A cyst-like structure often containing numerous protozoan parasites that is formed by host cells rather than by the parasites. pseudopodium “False foot”; a temporary, retractile cytoplasmic protrusion characteristic of amebae, and used for motility or feeding.

pupiparous Insects that retain developing larvae within the female's body until they are ready to pupate.

pus A fluid or semisolid product of inflammation, including leukocytes, cell breakdown products, and tissue fluids.

redia An asexual stage among digenetic trematodes that occurs in the snail intermediate host.

reservoi r An ecologic system that supports the needs for a parasite population to survive indefinitely.

reservoir competence Probability that an uninfected vector feeding on an infected host will acquire an infection of a parasite.

restriction fragment length polymorphism

(rflp) A variation in the DNA sequence of a genome that can be detected by breaking the DNA into pieces with restriction enzymes and analyzing the size of the resulting fragments by gel electrophoresis.

rhabditiform esophagus An esophagus among some nematodes in which a constriction separates the main body of the esophagus from a bulb in the posterior portion.

ribosome A cell organelle composed of RNA and protein, having two subunits, that is the site of protein synthesis.

rickettsia Specialized form of bacterium that is a small, gram-negative obligate intracellular parasite with a specialized cell wall chemistry; almost all are transmitted by arthropods.

rostellum An anteriorly projecting knob on

the scolex of some cestodes, often with encircling hooks.

salivaria Trypanosomes that develop in the midgut or foregut of their arthropod hosts. These trypanosomes are transmitted by the bite of the arthropod.

sand fly Fly of the Family Psychodidae, in which the female is hematophagous.

saprophyte An organism that lives on dead or decaying organic matter, characteristic of many fungi and bacteria.

sarcocyst “Flesh cyst”; the last generation meront of the Apicomplexa (sporozoan) Sarcocystis spp., deposited in the muscle tissue of the intermediate host.

sarcoma A malignant neoplasm composed of embryonic connective tissue. Contrast to carcinoma.

scabies A form of mange caused by the mite Sarcoptes scabei.

schizogony See merogony.

schizont “Divided being”; an older, alternative name for meront in the life cycle of the protozoan Apicomplexa (sporozoa).

scutum A hardened plate on the dorsal surface of ticks in the Family Ixodidae (“hard ticks”).

scolex The anterior, hold-fast organelle of a cestode, often containing hooks or suckers.

sensitivity In antigen-antibody tests, the proportion of samples truly positive for an antibody for which positive results are detected with the test used. Contrast to specificity.

septicemia A systemic disease associated with infection by pathogenic bacteria in the blood stream. Compare to bacteremia.

septum A cross wall in a fungal hypha.

serology The study of specific antibodies present in a host by the use of antigen-antibody tests.

seroprevalence The prevalence of a certain antibody in a population of hosts.

serosa A serous membrane such as the peritoneum, pleura, or pericardium.

serotype A specific designation for a microorganism based on the kinds and combinations of antigens present.

serovar See serotype.

serous Having the nature of serum.

serum The fluid portion of blood remaining after all red and white blood cells and clotting elements have been removed; also called antiserum (serum with antibodies). Contrast to plasma.

sexual selection Selection of a mate by a member of one sex based on variations in characteristics within the opposite sex.

sign Any objective evidence of a disease that is perceptible to the attending veterinarian or manager.

silent carrier Apparently healthy carrier hosts for a pathogen that typically do not experience clinical illness, but that can serve as a source of the pathogen for other, susceptible hosts (indicator hosts).

soft ticks Ticks of the Family Argasidae, lacking a scutum on their dorsal surface and having their entire capitulum covered dorsally by the main body of the tick in nymphs and adults. Contrast to hard ticks.

soil and water organism reservoir Use of soil and water by some bacteria for their long-term survival, and as a source for infecting other susceptible animals.

specific immunity lymphocyte-mediated responses that are characterized by the specificity of their response, the heterogeneity of the chemicals to which lymphocytes respond, and the presence of an immunological memory. Also called acquired immunity. Contrast to innate immunity.

specificity In antigen-antibody tests, the proportion of truly negative animals for an antibody that have negative results with the test used. Contrast to sensitivity.

spherule A stage of the fungus Coccidioides spp., as well as Emmonsia spp. and others, in which endospores develop.

spicule A thin, needle-like structure adjacent to the bursa in some nematode males; used in the process of copulation.

spiracle Respiratory opening in many arthropods.

spirochete A spiral-shaped bacterium; often a general term representative of the Families Spirochaetaceae (including Borrelia) and Leptospiraceae (including Leptospira) among Eubacteria.

splenomegaly “Large spleen”; a clinical condition in which the spleen becomes enlarged.

sporangiospores Fungal spores produced within a sporangium.

sporangium Among fungi, a sac-like structure whose entire contents are converted through endogenous asexual cleavage into spores.

spore An environmentally resistant reproductive element offungi, and some bacteria and protozoa. May also refer to an oocyst of the Apicomplexa among protozoa.

sporocyst “Spore cyst”; an asexual stage found among some trematodes occurring in the snail intermediate host. Also, a stage of development of sporozoan protozoa that occurs inside the oocyst. sporogony “Spore formation”; an asexual form of multiplication among sporozoan protozoa in which individual zygotes develop further to produce oocysts, sporocysts, and sporozoites.

sporozoite “Spore animal”; the infective stage for the vertebrate host among sporozoan protozoa, typically formed within sporocysts.

stercoraria Trypanosomes that develop in the midgut or hindgut of their arthropod hosts. These trypanosomes are transmitted by the host rubbing in infected feces deposited by the arthropod host.

stigma A sclerotized plate associated with spiracles in mites.

stylet A relatively rigid elongated organ or appendage of an animal, such as a piercing mouthpart.

strobila The main body of a cestode behind the scolex. Among the Eucestodes, it encompasses the chain of proglottids.

Strobilocercus A cestode larval stage with a fully- developed scolex and neck attached to a bladder.

stroma The supporting matrix of a tissue or organ, as opposed to its functional element (parenchyma).

subacute Intermediate in duration; between acute and chronic.

subcutaneous Occurring below the skin (cutaneous) level.

sucking louse Lice of the Suborder Anoplura that feed on blood of mammals

suprapopulation All developmental phases of a species at a particular place and time.

susceptible Vulnerable to infection by a particular parasite.

sylvatic cycle Parasite life cycle involving wild animals. Contrast to pastoral cycle. Usually limited to helminths.

symbiosis “Living together”; a broad term denoting a mutual sharing of physiological mechanisms by different species, with no specification of the outcome. For example, the relationship may be mutually beneficial (mutualism), or one species may gain and another is disadvantaged (predation, parasitism). Used by some to denote only mutually beneficial relationships.

symptom Any subjective evidence of a disease, generally as perceived by the patient. This term generally is not applicable to animal diseases since those symptoms usually cannot clearly be communicated by an animal to the veterinarian or manager.

syngamy “Gamete fusion”; fusion of gametes to form diploid cells.

synlqphe Cuticular ridges on the cuticles of a nematode.

systemic Occurring throughout a host body, rather than being localized in one tissue or organ.

t-lymphocyte Lymphocyte that matures in the thymus and confers self/nonself recognition in conjunction with MHC proteins; these can be further divided into several subsets.

tachyzoite Small merozoite-like stages of the protozoan Toxoplasma gondii.

tapeworm See cestode.

target cell A cell that is selectively affected by a leukocyte or cytokine, as well as a drug or other chemical. These may include cancer cells or cells infected with parasites.

tarsus Most distal segment of the leg of an insect or tick or mite.

teleomorph Among fungi, a meiotic sexual form that produces ascospores or basidiospores.

test and slaughter A strategy to eliminate infected animals in a herd by capturing individual hosts, testing them for the presence of the infective agent, and killing those that have been infected. This approach has been used for controlling Brucella abortus in bison.

thallus The vegetative body of a fungus composed of many hyphae.

thorax The chest cavity encased by the ribs, including the lungs and heart.

threshold density In the context of density­dependent transmission, the host density below which a pathogen normally is unable to maintain transmission, and disappears.

thymus The organ in which T-lymphocytes mature in both mammals and birds.

tick An exclusively ectoparasitic order (Ixodida) of arthropods in the Class Arachnida, having a single body segment (idiosoma), four pairs of legs as nymphs and adults, and no antennae. Closely related to mites, but generally larger, have a hypostome and Haller's organs on the dorsal surface of the first tarsus in all stages. Two families generally are recognized, Ixodidae (hard ticks) and Argasidae (soft ticks).

titer The inverse of the highest dilution of a serum giving a clear positive test in an antigen­antibody test.

toll-like receptors A family of proteins of innate immunity that respond to molecules important to survival of many bacteria, viruses, and other parasites, such as bacterial lipopolysaccharides and nucleic acids of viruses.

Transovarial transmission Transmission of a parasite through the egg to the next generation of vertebrate host or arthropod vector.

Transstadial transmission Transmission of a parasite in arthropod vectors through one or more molts, from one life cycle stage to another.

trematode (fluke) Member of a subclass (Trematoda) of the Phylum Platyhelminthes that is characterized as being dorsoventrally flattened, bilaterally symmetrical, having an oral sucker and pharynx, but no coelom.

Trichomonad Protozoan intestinal flagellate, including members of the genera Trichomonas, Histomonas, and Dientamoeba.

tritonymph A final quiescent nymphal stage, occurring between the deutonymph and adult in some mites.

trophozoite “Feeding animal”; any protozoan in an asexual stage of development. Among sporozoan protozoa, it refers to a specific stage in the life cycle following the sporozoite.

trypanosome A general term for protozoan hemoflagellates, including the genus Trypanosoma.

Trypomastigote Stage of trypanosomes (protozoa) resembling the typical adult form of the genus Trypanosoma with a kinetoplast, basal body at the posterior end, and a flagellum running along an undulating membrane; it typically is found in the peripheral blood of the vertebrate host.

tsetse fly Member of a family (Glossinidae) of hematophagous insects important in the transmission of trypanosomes and other parasites of medical and veterinary importance.

ulcer An excavation or depression on the surface of the skin or an organ, produced by the sloughing of inflammatory and necrotic tissues.

Unembryonated egg A helminth egg in which the larval stage has not yet developed.

unilocular hydatid A hydatid cyst for the cestode Echinococcus granulosus (Family Taeniidae), in which up to several million protoscolices develop in brood capsules through the process of endogenous budding.

vaccination The process of stimulating host immunity to an infectious agent or toxin by exposing the host to reduced or deactivated amounts of the antigen and allowing an immune response to develop.

vasoactive Having an influence on the permeability of blood vessels.

vector In a broad sense, any host or object that transmits parasites, including intermediate, definitive, and paratenic hosts, as well as water or wind. In a more restricted sense and in common usage, vectors refer to arthropods that transmit parasites between vertebrate hosts. Such arthropods may serve only as a physical means of movement (mechanical transmission), as a host in which the parasite necessarily increases in number (multiplicative or propagative transmission), as a host in which some required development of the host occurs (developmental transmission), or a host in which both maturation and an increase in numbers occurs (cyclopropagative transmission).

vector capacity Measure of ability of an arthropod to serve as a vector between individual hosts in a community.

vector competence Measure of vector species to become infected, remain infectious, and then transmit to naive and susceptible hosts.

venereal Pertaining to sexual contact.

verminous Pertaining to helminth parasites, often nematodes.

vertical transmission Transmission of a parasite across generational lines, between parents and offspring. Contrast to horizontal transmission.

viremia The presence of viral particles in the blood system.

virion An intact viral particle, consisting of a nucleic acid core of DNA or RNA (but not both), surrounded by a protein coat.

virulence Collective effects of a parasite on host fitness, including mortality, morbidity, and fecundity. Three specific characteristics of virulence are infectivity, invasiveness, and pathogenicity.

virus Nonliving infectious entity with a genome, but generally lacking any independent metabolic activities. The complete particle is called a virion.

vitellaria Genital glands in trematodes and cestodes which secrete yolk and shell for the fertilized egg.

viviparous To produce living offspring within the female body. The young derive nutrition directly from the female. Contrast to ovoviviparous.

warble fly Robust and hairy insect of the Family Oestridae that is an obligate ectoparasite on mammals.

welfare factor A nonlethal factor such as a shortage of food, water, or cover that reduces reproductive success, or makes hosts more susceptible to direct mortality factors such as disease or predation. Contrast to decimating factor.

western blot test (immunoblot test) A test for specific protein based on initial gel electrophoresis followed by an enzyme assay such as ELISA.

xenodiacnosis Diagnosing an infective agent by infecting a test animal.

yeast Single-celled fungal cell that is unable to form hyphae or mycelia.

zooanthroponosis Disease transmitted from nonhumans to humans; typically, humans serve as a dead-end host. Contrast to anthropozoonosis.

zoonosis Disease transmitted between humans and nonhumans.

zoophilic Infecting primarily nonhuman mammals or birds.

zooprophylactic (barrier) host A host that kills the pathogens in infected vectors after being fed upon by those vectors.

Zoosporancium Among chytrid (Order Chytridiales) fungi, a reproductive, host­dependent structure in which zoospores are formed.

zoospore Among fungi, flagellated, asexually produced spore characteristic of the Class Chytridiomycetes.

Zycospores Among fungi of the Phylum Zygomycota, spores formed by the conjugation of morphologically identical gametes.