BACTERIAL AND VIRAL TRANSMISSION PATTERNS

There is a rich variation in the strategies used by both bacteria and viruses for transmission to susceptible hosts (Smith 1982, Moore 1995, Graczyk 2002, Moore 2002, Swinton et al.

Horizontal forms of transmission are far more common and involve a wide variety of strategies. In an effort to simplify this complexity, we organize the various forms of horizontal transmission into five broad patterns: physical contact, ingestion, aerosol transmission, arthropod transmission, and helminth-mediated transmission.

Physical Contact

A variety of bacteria and viruses can be trans­mitted by direct physical contact including sexual activity, biting, and fomites. Examples of transmission by physical contact include papillomavirus infections in mammals (Sundberg et al.

Some transmission also may occur through direct contact with fomites. Fomites are inani­mate objects that can become contaminated by infectious material and serve as a source of these infective agents to susceptible hosts com­ing in contact with the contaminated surface. A bird feeder having perches contaminated with the bacterium Mycoplasma spp. from infected songbirds may serve as a source of these bac­teria to other songbirds (Friend 1999a). The bacterium Yersinia pseudotuberculosis also is transmitted via fomites (Wetzler 1971)

Ingestion

Ingestion of contaminated food or water is an important means of transmission for many bacteria and viruses (Smith 1982), especially intestinal bacteria and viruses transmitted by an fecal-oral route with contaminated food or water (Bopp et al. 1999, Farmer 1999). Preda­tion or scavenging of infected prey or carcasses also are means by which bacteria and viruses are ingested (Rosen and Morse 1959, Smith 1982), as is the casual licking of infected car­casses by curious ungulates. Although a rare occurrence, bacteria and viruses can be trans­mitted by cannibalism; epizootics of bubonic plague, caused by the bacterium Yersinia pestis, can be sustained in colonies of laboratory rats through cannibalism alone (Rust et al.

Canine parvoviruses commonly are transmit­ted by ingestion of fecally contaminated food or water, or by contact with contaminated fomites near latrine sites, marking sites, or other areas contaminated by virus shedders (Barker and Parrish 2001).

Aerosol Transmission

Some infective agents can be transmitted by means of contaminated aerosols through inhalation, nasal infection, or aerosol con­tact with the eyes. Among viruses, influenza virus (Slemon and Brugh 1994) and canine distemper virus (Williams 2001) commonly are transmitted in this fashion. Among bacte­ria, Pasteurella multocida can be transmitted to waterfowl through aerosol inhalation in the laboratory (Titche 1979); Mycoplasma conjuncti­vae also can be transmitted among susceptible ungulates through conjunctival contamination (Giacometti et al. 2002).

Arthropod Transmission

Many viruses and bacteria can be transmitted by arthropods such as fleas, mosquitoes, lice, biting flies, ticks, or mites (Smith 1982). Viruses transmitted by arthropods include a variety of arboviruses (arthropod-feorne viruses), includ­ing flaviviruses such as the West Nile virus, orthomyxoviruses such as eastern and western equine encephalitis viruses, as well as poxvi­ruses such as myxoma and avian poxviruses. Among the bacteria using arthropod transmis­sion are Borrelia burgdorferi, the cause of Lyme disease, Yersinia pestis, the cause of bubonic plague, Francisella tularensis, the cause of tula­remia (Smith 1982, Williams and Barker 2001), and most rickettsiae.

Most of these transmissions occur through the bite of hematophagous (“blood-feeding”) arthropods, but some also can result from surface feeding or excretions by flies or other infected arthropods (Smith 1982). For exam­ple, besides aerosol transmission, Mycoplasma conjunctivae also can be transmitted among susceptible ungulates by flies feeding on eye secretions (Giacometti et al.

Helminth-mediated Transmission

Only a few bacteria and viruses are trans­mitted through the life cycle of a helminth parasite. However, members of the bacterial genus Neorickettsia are consistently transmit­ted through the life cycles of several intestinal trematodes (Rikihisa et al. 2004). Another bacterium, Brucella spp., may be transmitted by infected lungworms (Parafilaroides spp.) to harbor seals (Phoca vitulina richardsi) (Garner et al. 1997, Dunn et al. 2001). Viruses that have used helminths for transmission include swine influenza virus and hog cholera virus, both of which are associated with transmission through the swine lungworm, Metastrongy- lus spp. (Shope 1941, 1943a, 1943b, 1958; Sen et al. 1961; Shope 1965). The lymphocytic choriomeningitis virus may be transmitted through the life cycle of Trichinella spiralis (Syverton et al. 1947). Transmission of micro­organisms to susceptible vertebrate hosts in this transmission pattern involves the same mechanisms as found for acquiring the respec­tive affected helminths, such as ingestion of, or penetration by, infective helminth stages.

Not surprisingly, some pathogens may use two or even several means of transmission. Francisella tularensis, the agent of tularemia, can be transmitted by infected ticks, ingestion of infected prey or contaminated water, bacte­rial physical contact with lacerated or even intact skin, through ocular mucous membranes, or by inhalation of infected aerosols (Jellison 1974, Morner and Addison 2001).

Hosts have developed a number of impor­tant strategies to reduce the success of various parasite transmission strategies. Immune sys­tems are an important component in fending off parasite attack. Physical barriers to parasite entry are also important. These were developed more fully in Chapter 2 (on immunity). Social barriers are an additional defense against successful transmission of infective agents (Loehle 1995). In contrast, habitat fragmenta­tion and other anthropogenic disturbances have been influential in changing patterns of transmission (Goldberg et al. 2008).