Coccidian Parasites Cyclospora Cayetanensis, Isospora Belli, Sarcocystis Hominis/Suihominis Vitaliano Cama
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CHAPTER 3 Coccidian Parasites Cyclospora cayetanensis, Isospora belli, Sarcocystis hominis/suihominis Vitaliano Cama 3.1 PREFACE Cyclospora cayetanensis, Isospora belli, and the Sarcocystis spp. Sarcocystis homi- nis and Sarcocystis suihominis are parasites that infect the enteric tract of humans (Beck et al., 1955; Frenkel et al., 1979; Ortega et al., 1993). These parasites cause disease when infectious oocysts are ingested by humans. The routes of transmission can be direct human to human contact or through contaminated food (Connor and Shlim, 1995; Fayer et al., 1979) or water (Wright and Collins, 1997). Taxonomically, these parasites are very distinct: Cyclospora and Isospora belong to the Eimeriidaes, whereas Sarcocystis belongs to the Sarcocystidae. Nonetheless, some similarities are noteworthy: infectious stages of these parasites have morphological similarities, they have been reported to be food-borne, they cause infections of the intestinal tract of humans, and their clinical presentations have similarities (Mansfield and Gajadhar, 2004). Thus, relatedness and differences between several aspects of cy- closporiasis, isosporosis, and sarcocystosis will be covered in this chapter. 3.2 BACKGROUND/HISTORY Cyclospora is probably the most important foodborne pathogen of the three parasites. It is endemic in several regions of the world, primarily in developing countries (Markus and Frean, 1993; Ortega et al., 1993), whereas in the developed world it has been associated with important foodborne outbreaks (Charatan, 1996; Herwaldt, 2000; Herwaldt and Beach, 1999). It has also been reported in travelers returning from endemic areas (Gascon et al., 1995; Soave et al., 1998). Isospora belli is an infrequent parasite of humans, with most cases reported from tropical areas. In the immunocompetent population, Isospora infections are usually asymptomatic (Teschareon et al., 1983). Immunocompromised patients, however, can suffer I. belli infections associated with severe clinical disease (Soave, 1988). Human sarcocystiosis is even more infrequent, and infectious parasites are in- cidentally identified in feces (Bunyaratvej et al., 1982; Fayer, 2004). The clinical significance of the detection of oocysts or sporocysts in feces is unknown in most cases (Dubey, 1993) . Cyclospora cayetanensis was fully recognized as a parasite less than 15 years ago (Ortega et al., 1993), and gained significant recognition because of its association with several food-borne outbreaks in the United States. The name and description 33 34 V. Cama of human infectious Cyclospora dates back to 1992, however, previous studies described organisms similar to Cyclospora. One of the oldest, if not the first doc- umented report was published in 1979 (Ashford, 1979), describing a parasite with microscopic characteristics of immature (nonsporulated) C. cayetanensis oocysts. This report concluded that the described organism probably was a new species of Isospora infecting humans (Ashford, 1979). In the following years, other investigators reported the organisms autofluoresced when observed under UV light (Long et al., 1991). This finding led some inves- tigators to believe it was a blue-green algae or a cyanobacterium. Thus, the term cyanobacterium-like body or CLB was coined and used for some years. Other terms found in the literature that also referred to Cyclospora were coccidian-like body (also CLBs) (Hoge et al., 1993) or big Cryptosporidium, as reported by Naranjo et al. in the 1989 Annual Meeting of the American Society of Tropical Medicine and Hygiene. Findings in this report were on the basis of microscopy of acid-fast stained organisms and transmission electron micrographs that showed sections of organisms resembling encysted flagellates. Shortly thereafter, studies led by Ortega demonstrated that CLBs were indeed a coccidian parasite, with each oocyst containing two sporocysts, and each of them containing two sporozoites. These findings were first presented at the Annual Meet- ing of the American Society of Tropical Medicine and Hygiene in November 1992. The morphological characteristics of the parasite allowed its classification as a pro- tozoan parasite belonging to the genus Cyclospora (Ortega et al., 1993). Shortly thereafter, the parasite was named C. cayetanensis, in honor of Universidad Peruana Cayetano Heredia, a Peruvian University where preliminary studies were conducted (Ortega et al., 1994). Thus far, it is the most recently recognized coccidian parasite affecting humans. In contrast to Cyclospora, Sarcocystis, and Isospora have been known to in- fect humans for several decades. I. belli is an enteric parasite exclusively from humans (Ferreira et al., 1962; Panosian, 1988). Although it infects, replicates, and completes its life cycle in enteric epithelial cells, it does not cause any signifi- cant disease in the immune-competent population. People with impaired immu- nity, however, may develop extra-intestinal infections and isosporiasis may pose a risk to their lives (Arnaud-Battandier, 1985; Figueroa et al., 1985; Furio and Wordell, 1985; Henry et al., 1986; Jonas et al., 1984; Kobayashi et al., 1985; Ma et al., 1983; Modigliani et al., 1985; Ng et al., 1984; Ros et al., 1987; Shein and Gelb, 1984; Whiteside et al., 1984) (Table 3.1). Sarcocystis spp. were first reported by Miescher in 1843, who described white milky lesions in the muscles of house mice. Since the initial report, multiple authors described similar lesions and infections in the flesh of various animal species. The transmission cycle, however was not yet elucidated. In 1943, Scott demonstrated that there was no direct transmission from infected to healthy sheep. Despite the negative results, it gave an indication that there was no direct transmission between members of the same species. It took several decades until the life cycle of S. hominis and S. suihominis were finally described in the 1970s (Heydorn, 1977; Rommel and Heydorn, 1972), when it was demonstrated that both of these parasites required two Coccidian Parasites 35 Table 3.1. Similarities and differences between Cyclospora, Sarcocystis, and Isospora species that infect humans. Cyclospora Sarcocystis hominis, cayetanesis S. suihominis Isospora belli Life cycle Number of hosts 1 host 2 host 1 host Type of cycle Anthroponotic Prey-predator anthroponotic Transmission Fecal–oral Ingestion of Fecal oral contaminated meat Epidemiology Developed countries Epidemic (infrequent) Very rare. Opportunistic infection in immune impaired people Developing countries Endemic in certain Endemic in areas Opportunistic areas where undercooked infection in immune beef/pork is impaired people. consumed Infrequent in healthy people. Symptoms Immune competent Diarrhea Mild diarrhea Asymptomatic Immune impaired Diarrhea Mild diarrhea Chronic diarrhea, ocular infections Oocysts Shape Round Ellipsoidal Ellipsoidal Size (μm) 8–10 13 × 10 30 × 12 Infectious stage Sporulated oocysts Sporocyst Oocyst or sporocyst No. of sporocysts 2 2 2 Shape sporocysts Round 4 Ellipsoidal ellipsoidal Sporozoites per 2 2 sporocyst Schematic representation Sporulation Outside host Intestine of host In or outside host Stage found in stools Unsporulated oocysts Sporulated Unsporulated oocysts sporocysts hosts: a definitive host; also described by others as the predator species, and an intermediate host or prey. Infections in the definitive hosts usually infect the enteric tract, where asexual and sexual stages develop. In contrast, in intermediate hosts the parasites develop sexual stages only, forming white milky threads (sarco = flesh and kystis = bladder) in their muscles. Shortly thereafter in 1975, it was demonstrated that cattle and sheep could be the intermediate hosts for more than two species of Sarcocystis, and that simultaneous infections may occur (Heydorn et al., 1975). 36 V. Cama 3.3 BIOLOGY The oocysts of C. cayetanensis are spherical, with 8–10 μm in diameter, and although its life cycle has not been fully demonstrated, evidence so far supports the notion that this is a parasite that only infects humans (Ashford, 1979; Ortega et al., 1994; Ortega et al., 1993). Cyclospora has been reported in feces of some animal species, including ducks (Zerpa et al., 1995), chickens (Garcia-Lopez et al., 1996), dogs (Carollo et al., 2001; Yai et al., 1997), and monkeys (Chu et al., 2004); however, no evidence of tis- sue infections was described in these animals. Cyclospora spp. were observed in non- human primates and genetic analysis based on the small subunit RNA gene revealed significant host specificity. Cyclospora papionis was exclusively found in baboons, vervet monkeys were only infected with Cyclospora cercopitheci, and only colobus monkeys had infections with Cyclospora collobi (Eberhard et al., 2001). Animal infectivity studies using nine strains of mice, including adult and neonatal immuno- competent and immune-deficient inbred and outbred strains, rats, sandrats, chickens, ducks, rabbits, jirds, hamsters, ferrets, pigs, dogs, owl monkeys, rhesus monkeys, and cynomolgus monkeys, were unsuccessful. These findings suggest that C. cayeta- nensis is a species specific parasite that only infects humans (Eberhard et al., 2000). There are two species of Sarcocystis that can infect humans, S. hominis and S. suihominis. As their names suggest, these are two-host parasites and man becomes infected by ingesting viable cysts in the meat of bovids or swine, respectively. Oocysts of these parasites are indistinguishable by microscopy; elliptical in shape, measuring 13 by 10 μm. Two