Submitted to: American Society of Tropical Medicine and Hygiene
Publication Type: Abstract Only
Publication Acceptance Date: November 1, 2002
Publication Date: December 1, 2002
Citation: FAYER, R. ANIMAL RESERVOIRS, VECTORS, AND TRANSMISSION OF PATHOGENIC PROTOZOA. AMERICAN SOCIETY OF TROPICAL MEDICINE AND HYGIENE. 2002. Technical Abstract: Cryptosporidium Cryptosporidiosis in humans and ruminants presents primarily as a diarrheal disease. Although some highly virulent isolates have caused mortality in cattle, other less virulent isolates can cause morbidity and mortality in immunosuppressed humans and animals. Of 12 named species of Cryptosporidium only Cryptosporidium parvum is widespread in humans and an estimated 150 or more other species mammals but 4 other species have now been identified in human infections. Cryptosporidia develop intracellularly in the gastrointestinal tract and form oocysts that are excreted in the feces. Fecal-oral transmission of the oocyst stage results in infection from ingesting contaminated drinking water, food, and recreational water. Species and isolates are nearly identical and therefore difficult or impossible to identify by microscopy. Molecular methods identify genotypes of Cryptosporidium by RAPD, RFLP, and sequence analysis. Because most reports of cryptosporidiosis have not been characterized by methods other than microscopy, infections may actually result from several species. Most Cryptosporidium isolated from immunocompromised persons have been the human and cattle genotypes, but a few persons have been found infected with genotypes originally associated with cat, dog, and turkey hosts. Two healthy persons have passed oocysts resembling C. muris, originally a parasite of rodents. These findings indicate that a variety of animals might be sources of Cryptosporidium for humans. Filth flies, birds, and shellfish have been identified as mechanical vectors of oocysts. These findings indicate the need for molecular characterization of specimens so the epidemiology can be better understood (Fayer et al., 2001) Toxoplasma Toxoplasma gondii is an obligate intracellular parasite with a coccidian life cycle. It develops in the intestine of members of the cat family, forming oocysts that are excreted in the feces. To become infectious, oocysts require several days exposure to moisture and air. In vertebrates other than felids they parasites develop in extra-intestinal sites causing tissue damage, inflammation, and disease. In immunocompetent hosts that survive the acute infection, cysts develop around slowly multiplying bradyzoites. Oocysts can be transmitted by ingestion of contaminated food or water, or by direct exposure to cat feces. Tachyzoites can be transmitted via the placenta to a fetus, or by blood transfusion causing asymptomatic infection, clinical illness, or death. When meat or organs containing cysts are eaten by carnivores, including humans, they initiate parenteral infection. When persons that harbor cysts become immune compromised, tachyzoites develop, often in the central nervous system, causing lesions, illness, and possibly mortality. Transplantation of organs harboring cysts can result in the same sequence of events. All stages (oocysts containing sporozoites, tachyzoites, and bradyzoites) are infectious for felids. Flies and worms have been shown to mechanically transport oocysts. Over 350 vertebrates are reported to have been infected with T. gondii. Domesticated and wild felids are the only final hosts; i.e., only felids excrete oocysts. The prevalence of T. gondii in felids, based on seroepidemiological studies, is high. Antibodies to T. gondii in trapped lynx and bobcats from Quebec, Canada using the modified agglutination test (Labelle et al., 2001) was 44 and 40%, respectively. These findings suggest that T. gondii is widespread in the wild and that exposure to wild felids and game animals may represent a potential source of infection for humans. Seroprevalence data for T. gondii in 865 captive neotropical felids in Brazil, indicated antibodies to T. gondii in 54.6% of cats, 45.9% of jaguarundis 57.7% of ocelots, 51.9% of oncillas, 55.5% of margays, 12.5% of Pampas-cats, 75% of Geoffroys-cats, 63.2% of jaguars, and 48.2% of (Ramos Silva et al., 2001). Despite a wide host range and world wide distribution, T. gondii has low genetic diversity. Most isolates group into 2 or 3 lineages. Most Type I strains have been isolated from human clinical cases. Types II and III strains are considered avirulent. Recently, 17 of 25 T. gondii isolates from asymptomatic chickens from rural areas around Sao Paulo, Brazil were Type I (Dubey et al., 2002). This is the first report of Type I from a food animal. Cattle, pigs, sheep, goats, and deer are also food animals implicated as sources of human infection. Microsporidia Microsporidia is the non-taxonomic name applied to over 1000 species in the phylum Microspora. Thirteen species infect humans, mostly immunosuppressed patients. Diarrhea is the most common symptom. Enterocytozoon bieneusi, is the species found most frequently in humans. It also has been found in pigs, cattle, a cat, dogs, and monkeys. Encephalitozoon intestinalis, also found in humans, has been identified in feces from a donkey, dog, pig, cow, and goat. Encephalitozoon hellem, also found in humans, has been found in several psittacine birds, an ostrich, and hummingbirds. Encephalitozoon cuniculi, reported in AIDS patients, has been found in guinea pigs, wild rabbits, hamsters, shrews, muskrats, mice, rats, dogs, cats, arctic foxes, goats, pigs, sheep, horses, and nonhuman primates. Little is known of the prevalence of these species in human or animal hosts and the routes of transmission are unknown. However, microsporidia spores have been found in surface water, suggesting that drinking water or recreational water could be vehicles of transmission. Spores are excreted in feces or urine. Reports of microsporidia in cattle and pigs suggest that food animals or food could be sources of infection. Giardia Giardia is the most common flagellate of mammals and birds and thought to be the most common intestinal parasite of humans. Prevalence in children worldwide is high, especially the very young (0-9 years of age). Those in day-care centers, pre-school, and those living in unhygienic conditions are at greatest risk. Transmission is by the fecal-oral route. Primary routes are personal contact, and contaminated water and food. Recently, Giardia was found in dill, lettuce, mung bean sprouts, radish sprouts, and strawberries, and cysts were also detected in irrigation water used for bean sprouts (Robertson and Gjerde, 2001). Close association between people and domesticated animals, especially dogs and cats, has suggested zoonotic infection. Of 271 dogs examined in Brazil 12.2% were found positive for cysts (Oliveira- Sequeira et al., 2002). Prevalence in lambs and calves has been reported up to 100% (O'Handley et al, 2001).