Cryptosporidium^[9] (see also  Chapter 79) is an Apicomplexan parasite of the group known as coccidia. It was thought to be restricted to non-human mammals, but is now recognized to be one of the commonest worldwide human enteric infections, especially in children and immunocompromised adults; symptomless infections are common. At least eight species have been described, but it appears that these may comprise a dozen or more distinct genotypes, each of which may or may not represent a distinct species. Humans are susceptible to at least two genotypes, the ‘human’ and ‘cattle’ genotypes, the former of which appears to be restricted to humans.

Cryptosporidiumis transmitted by a resistant oocyst in the faeces of infected persons or other animals, ingested with contaminated food or water. Four sporozoites emerge from the oocyst in the small intestine, enter cells of the microvillous border, and undergo merogony. The parasites remain intracellular, …