LIFE CYCLE STRATEGIES AMONG PROTOZOA

The life cycles of some parasitic protozoa are direct (monoxenous), involving transmission from one definitive host to the next. For most monoxenous parasites, a protective cyst stage is used to enhance parasite survival in the environ­ment between active infections; the intestinal flagellate Trichomonas gallinae is an exception, having no cyst stage.

Many parasitic protozoa, however, have an indirect (heteroxenous) life cycle, involving a definitive host and at least one intermediate host. Most heteroxenous protozoa use hema­tophagous arthropods. Examples include the malaria parasites, piroplasms, and the trypanosomes. In other cases, the protozoa require an intermediate vertebrate host, or may use an optional (paratenic) host. In a few cases, protozoa rely on transmission through helminth life cycles, where the protozoa are incorporated with the helminth ova and transmitted to the susceptible host with the helminth infection. In humans, the flagellate Dientamoeba fragilis is transmitted by ova of the pinworm Enterobius Vermicularis (Ockert and Schmidt 1976, BonDurant and Wakenell 1994) and possibly Ascaris lumbricoides ova (Sukanahaketu 1977). In gallinaceous birds, the intestinal flagellate Histomonas meleagridis is transmitted by ova of the cecal nematode Heterakis gallinarum (Cole and Friend 1999).

In addition, there are a few free-living soil and water protozoa, primarily amebae, that are opportunistic parasites; these agents generally are not transmitted directly between susceptible hosts, but involve common-source infections derived from contaminated envi­ronments (Visvesvara and Stehr-Green 1990). Examples include species of Naegleria, Acan- thamoeba, Balamuthia, and Hartmannella (Ma et al. 1990, Martinez and Visvesvara 1997, John 2001).

Control strategies typically are directed at perceived weak points in the parasites’ life cycles. For arthropod-borne or helminth-borne parasites, this may involve habitat manipulation or the use of chemicals. For monoxenous para­sites, or those using vertebrates for intermediate hosts, breaking the chain of infection by treat­ing infected individuals or preventing infec­tion by habitat manipulation also is common. Examples are addressed with specific parasites.