Cryptosporidiosis is caused by protozoan parasites of the genus Cryptosporidium (family Cryptosporidiidae, order Eucoccidiorida, subclass Coccidiasina, class Sporozoasida, subphylum Cryptosporidiosis is caused by protozoan parasites of the genus Cryptosporidium (family Cryptosporidiidae, order Eucoccidiorida, subclass Coccidiasina, class Sporozoasida, subphylum Apicomplexa). Cryptosporidium are sm
all intracellular parasites, which occur throughout the animal kingdom and have been reported in many species of mammals, birds, reptiles, amphibians and fish. A more complete description of the life cycle was published (Tyzzer, 1910) and subsequently a second species, C. parvum, was also described from laboratory mice (Tyzzer, 1912). parvum differed from the type species, not only by infecting the small intestine rather than the stomach, but also because the oocysts were smaller. Following the initial discovery of Cryptosporidium, over 50 years passed during which the parasite was commonly confused with other Apicomplexa parasites, especially members of the coccidian genus Sarcocystis. The reason for this was that many Sarcocystis spp. have oocysts with thin walls that often rupture, releasing free sporocysts, and because each sporocyst contains four sporozoites like Cryptosporidium oocysts, a variety of named and unnamed species were erroneously assigned to the genus. Subsequent ultra structural studies, however, supported earlier light microscopy studies and showed that Cryptosporidium species possessed a unique attachment organelle (Hampton and Rosario, 1966), which is the key feature that currently defines the genus and family (Levine, 1985). More than 20 'species' of Cryptosporidium parasite have been described on the basis of the animal hosts from which they were isolated, however, the lack of host specificity with many species has brought into question the validity of many species classified in this way. For a while, limited transmission studies were used as evidence for the mono-specific nature of the genus Cryptosporidium, resulting in the widespread use of the name C. parvum for Cryptosporidium parasites from all kinds of mammals, including humans. Other Cryptosporidium parasites, such as C. meleagridis in turkeys (Slavin 1955), C. wrairi in guineapigs (Vetterling et al., 1971), C. baileyi in birds (Current et al., 1986) and C. saurophilum in lizards (Koudela and Modry, 1998), were however, considered to be separate species based on demonstrated biological differences from the established species C parvum and C. muris. In recent years, molecular characterizations of Cryptosporidium have helped to clarify the confusion in Cryptosporidium taxonomy and validate the existence of multiple species in each vertebrate class. As a consequence, the complete taxonomy of the genus has undergone major revision based on a number of parameters that included not only morphology but also developmental biology, host specificity, histopathology, and sequence-based differences, within the ribosomal RNA (rRNA) gene repeat unit between individual isolates within a previously ‘valid’ species. Species definition and identification of this genus is therefore constantly changing, with the addition of new species based primarily on molecular criteria. Currently there are 13 valid species namely: C. hominis found primarily in humans (previously known as C. parvum Type 1), C. parvum, found in humans and other mammals (previously known as C. parvum Type 2), C. andersoni in cattle, C. canis in dogs, C. muris in mice, C. felis in cats, C. wrairi in guineapigs, C. meleagridis in turkeys and humans, C. baileyi in chickens, C. galli in finches and chickens, C. saurophilum in lizards, C. serpentis in snakes and lizards, and C. molnari in fish (Xiao et al., 2004). Cryptosporidium species infect the microvillus border of the gastrointestinal epithelium of a wide range of vertebrate hosts, including humans (see Table 1). Infected individuals show a wide spectrum of clinical presentations, but the pathogenicity of Cryptosporidium varies with the species of parasites involved and the type, age, and immune status of the host. In many animals, Cryptosporidium infections are not associated with clinical signs or are associated with only acute, self-limiting illness. In some animals, such as reptiles infected with Cryptosporidium serpentis or individuals who are immunosuppressed, the infection is frequently chronic and can eventually be lethal.
