Only two kingdoms of living creatures have members that are true parasites of domesticated animals. These parasites belong to the kingdom Animalia and the kingdom Protista. Most of the parasites of domesticated animals belong to the Animal kingdom (flukes, tapeworms, roundworms, thorny-headed worms, arthropods, and leeches). The rest of animal parasites belong to the kingdom Protista. This kingdom contains the unicellular, or one-cell, organisms (better known as the protozoans).
Most protozoans are free-living organisms; however, those protozoans that are parasitic may produce significant pathology in domesticated animals and humans. Within the kingdom Protista are several phyla, which contain (1) flagellated protozoans, (2) ameboid protozoans, (3) apicomplexans, and (4) ciliated protozoans. These phyla contain the primary protozoans that may cause significant pathology in domesticated animals and humans.
Characteristics of the Protozoans
Protozoans are unicellular organisms; that is, they are one-cell organisms. Protozoans vary greatly in size, form, and structure; most are microscopic, and a very few are macroscopic, that is, visible to the naked eye. The kingdom Protista is divided into several phyla. These phyla differ in the manner in which the protozoans move within their tiny microenvironments. In veterinary parasitology, the most important phyla are Sarcomastigophora (containing the flagellates and amoebae), Ciliophora (containing the ciliates), and Apicomplexa (containing the coccidia, malarial organisms, and piroplasms).
The flagellates are those protozoans that possess at least one flagellum (a long, whiplike or lashlike appendage) in their trophozoite, or moving, form. This flagellum allows the protozoan to move about in a fluid medium. As a result of this activity, parasitic flagellates live in the liquid world of the host’s blood, lymphatic fluid, or cerebrospinal fluid. Flagellates are often pear-shaped or bullet-shaped and are able to swim in their host’s body fluids, meeting very little resistance. These flagellates vary greatly in pathogenicity (disease-causing potential). Some flagellates are highly pathogenic, whereas others appear to cause little or no harm to the host. Some important genera of parasitic flagellates of domestic animals are Leishmania, Trypanosoma, Trichomonas, Histomonas, and Giardia species. Figure 10-1 is a diagram of a representative flagellate, Tritrichomonas foetus, a flagellated protozoan of the reproductive tract of cattle.