THE PROTOZOA (CAT)

GENERAL: Protozoan parasites of animals are typically single-celled organisms. Protozoa differ from bacteria in that the protozoan cell contains a discrete nucleus with a nuclear membrane. Also, undulipodia (flagella), when present on the protozoan cell, have a structure that is distinct from the flagellum of bacteria but not unlike that of the cilium of mammals and other animals. The protozoa differ from fungi in that fungal cells do not have undulipodia and typically are binucleate organisms. Protozoa differ from plants and animals by the fact that both plants and animals develop from an embryo, a developmental stage lacking from all protozoan forms.

Although the protozoal king dom contains somewhere around 35 phyla and myriad species of organisms (Margulis L, Corliss JO, Melkonian M, Chapman DJ (eds.). 1990. Handbook of Protoctista. Jones and Bartlett Publishers, Boston, MA, 914 pp), only 4 phyla are implicated as pathogens of the cat. Protozoan within these phyla differ markedly in their biology which is an indication of their widely divergent relationships. Certain aspects of the biology of protozoa, the ability to form resistant stages, utilization of vectors, and genetic ex change of material through sexual union, are important in a general understanding of these parasites because they are direct ly related to the transmission of these pathogens between feline hosts.

Protozoan parasites are typically transmitted between cats in one of four different ways (Table 1). First, direct contact is the means of transmission used by Trichomonas felistomae, a parasite of the mouth of the cat. In this form of transmission, the stage of the parasite is not resistant to environmental extremes and will die rapidly if deposited by the cat in drinking water or on skin by licking. Second, exposure to resistant stages in the environment, is the means by which cats are infected with Giardia felis, Cryptosporidium parvum, Isospora spp., and on some occasions by Toxoplasma gondii. These parasites all have a resistant stage that is protected by a thick protective wall, and once these stage enter a favorable environment, they are capable of persisting for months to years. A third means of transmission is via the ingestion of other hosts containing resistant stages; this type of transmission occurs with Sarcocystis spp., Hammondia heydorni, Toxoplasma gondii, and occasionally with species of Isospora. In this case, the host eaten by the cat has become infected by the ingestion of a resistant stage shed into the environment in the cat's feces; with the notable exception being that there can be vertical transmission in the non-feline host in the case of toxoplasmosis. The host protects the protozoan from environ mental extremes, and the parasite is capable of persisting within the host for months to years. Fourth, there is trans mission by a blood-feeding arthropod vector, which is the means by which cats typically become infected with Leishmania spp., species of Trypanosoma, and the apicomplexan genera Cytauxzoan and Babesia. For the most part, the arthropod is required to increase the quantity of infectious agents ingested when the arthropod bites to a quantity sufficient for the infection of the next host. The arthropod also serves to protect the parasite from environmental extremes as it moves from host to host. Other less frequent forms of transmission of protozoa between cats does occur, but in general, these four modes of transmission are the most typical.

Some protozoan parasites of the cat are rare or only occasionally seen while others are very common (Table 2). Factors that affect the prevalence of these parasites include such things as the geographic range of the parasite or its vector, local conditions of housing or environment, and age of the cats being surveyed. For instance, Trypanosoma cruzi is restricted to the Americas, mainly south of Mexico. The reason for the restriction of this parasite is in part due to vectors, triatomid bugs, that are adapted to human dwellings in that part of the world, typically these bugs are found mainly in the wild. Local conditions can have a great effect on the prevalence of parasites in a cat population. If the cats are hunters and spend a good deal of time outdoors, they will develop parasite infections different from those of the indoor cat, e.g., the indoor cat would be free from tick bites under most circumstances. If one looks at kittens, the prevalence of protozoan parasites in this population is much different than in adult cats. Kittens are more likely to be shedding large number of oocysts than adult cats. Of course, the truly rare parasites are probably rare because they are not typically parasites of the domestic cat.

The stages involved with the transmission of protozoan parasites are often, but not always, the same as those involved with diagnosis (Table 1). In those cases where the resistant stage is shed in the cat's feces, it can often be recovered using routine methodologies of flotation. However, in cases with severe diarrhea and rapid intestinal motility, the resistant stages may not form making diagnosis more difficult. In the case of Giardia felis, it becomes necessary to examine the feces in a manner that will allow the diagnosis of trophozoite stages. This would also be the case in a cat with amebic dysentery and perhaps in one with severe coccidiosis. The protozoan parasites of the cat that produce stages that are found in blood or tissue samples are typically those transmitted by biting arthropods. However, it would not be uncommon for the number of circulating organisms to be at such low levels that they may not be diagnosed by a routine blood smear or biopsy specimen. Thus, other forms of diagnosis might be required.