Cryptosporidial infection, with special reference to nosocomial transmission of Cryptosporidium parvum: a review

Cryptosporidium is a coccidian protozoan parasite found in a wide variety of host species including mammals, birds and lower vertebrates (poikilotherms). With the exception of one report of infection in a severely immunocompromised man with a Cryptosporidium which was thought to have characteristics suggestive of C. baileyi (Ditrich et al. 1991), human infections usually appears to be with C. parvum. This species seems to lack both the host and tissue specificity typical of most other coccidia. The developmental site of C. parvum is usually in the brush border of enterocytes, particularly of the small bowel, in an intracellular but extracytoplasmic location which appears to be unique. Cryptosporidial infection is usually asymptomatic in small mammals such as rodents but C. parvum is the cause of enteritis (scours) in young livestock animals such as calves and lambs. Older animals seem to be refractory to symptomatic infection although intermittent excretion has been demonstrated. Periparturient recrudescence may be important for the parasite in ensuring transmission to young animals which often become rapidly infected after birth despite antibodies in the colostrum. Once active, symptomatic infection starts in a group of young animals it spreads rapidly and diarrhea may be an important mechanism from the parasite perspective, in ensuring rapid transmission to new hosts. (DBO)