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Many Cryptosporidium species have been described that collectively infect a range of animals, including humans. Cryptosporidium parvum and C. hominis (formerly known as C. parvum anthroponotic genotype or genotype 1) are the most prevalent species that cause disease in humans, but human infections by C. felis, C. meleagridis, C. canis, and C. muris have also been been reported.
Since the first reports of human infection by Cryptosporidium in 1976, cases have been reported worldwide. Outbreaks have been reported from several countries, the most remarkable being a waterborne outbreak in Milwaukee, Wisconsin (U.S.A.) in 1993 which affected more than 400,000 people.
Like that of many parasites, the life cycle of C. parvum and C. hominis is complex. Sporulated oocysts, containing 4 sporozoites, are excreted by the infected host in feces and possibly by other routes such as respiratory secretions. Transmission of C. parvum and C. hominis occurs mainly through contact with contaminated water (e.g., drinking or recreational water). Occasionally, food, such as chicken salad, may serve as a vehicle for transmission. Many outbreaks in the United States have occurred in water parks, community swimming pools, and daycare centers. Zoonotic and anthroponotic transmission (i.e., transmission by non-human animals and animals, respectively) of C. parvum and anthroponotic transmission of C. hominis occur through exposure to infected animals or exposure to water contaminated by feces of infected animals. Following ingestion (and possibly inhalation) by a suitable host, excystation occurs. The sporozoites are released and parasitize epithelial cells of the gastrointestinal tract or tissues elsewhere, such as in the respiratory tract. In these cells, the parasites undergo asexual multiplication (schizogony or merogony) and then sexual multiplication (gametogony) producing microgamonts (male) and macrogamonts (female). Upon fertilization of the macrogamonts by the microgametes, oocysts develop that sporulate in the infected host. Two different types of oocysts are produced: thick-walled oocysts, which are commonly excreted from the host, and thin-walled oocysts, which are involved primarily in autoinfection. Oocysts are infective upon excretion, thus permitting direct and immediate fecal-oral transmission. Oocysts of Cyclospora cayetanensis, another important coccidian parasite, are unsporulated at the time of excretion and do not become infective until sporulation is completed (see Cyclospora cayetanensis).