Cryptosporidium parvum - Intestinal Parasites

Cryptosporidium parvum

Cryptosporidium parvum is a ubiquitous coccidian parasite that causes cryptosporidiosis in humans; however, other species are known to cause infection in immuno-compromised patients and in total this genus comprises 19–20 distinct species. Recently there has been a renaming of the major species in this group to reflect the host specificity of the particular species or genotype. The life cycle of the parasite is complex but is completed in a single host. Infection follows the ingestion of oocysts associated with contaminated water or food. According to the World Health Organization (WHO) the health significance of C. parvum is high due to the persistence of the organism in the environment. Cattle represent the most important reservoir of C. parvum but other mammals, domestic and wild, can be infected and act as carriers of the disease, even if asymptomatic. It is now known that a number of genetically distinct subspecies exist that can be divided into two groups. Group I organisms infect humans only (soon to be reclassified as C. hominis); group organisms infect a wider range of hosts. There have been several large outbreaks of this infection in the UK and USA in which water was identified as the initial vehicle for transmission. Members of this genus are intracellular parasites infecting the intestinal mucosal epithelium. The two major life cycle forms are the oval oocyst and the sporozoite.

C. parvum infections are often asymptomatic, but symptoms such as profuse watery diarrhoea, stomach cramps, nausea, vomiting and fever are typical. The symptoms can last from several days to a few weeks in immun-ocompetent individuals, but in immuno-compromised patients infection can become chronic, lasting months or even years. The mean infective dose for immuno-competent people is dependent on the strain of C. parvum but it is considered to be approximately 100 cells, and infants are more vulnerable to infection. Diarrhoea is a major cause of childhood mortality and morbidity as well as malnutrition in developing countries. Cryptosporidium is the third most common cause of infective diarrhoea in children in such countries, and consequently it plays a role in the incidence of childhood malnutrition.

Cryptosporidium infection has a higher nutritional impact in boys than girls, becausee of the need for micronutrients in boys to build up larger muscle mass. However, breastfeeding does offer some protection against infection. In immuno-compromised individuals Cryptosporidium infection causes a severe gastroenteritis, and often the parasites infect other epithelial tissues causing pneumonia; the mortality rate due to C. parvum in AIDS patients is between 50% and 70%. Infection occurs when oocysts (Figure 6.2f) excyst following environmental stimuli (typical intestinal conditions) and parasitize the epithelial cells which line the intestine wall (Figure 6.8). After several further stages of the cycle, two forms of oocyst are produced; soft-walled oocysts reinitiate infection of neighbouring enterocytes while hard-walled cysts are expelled in the faeces.

Little is known about the mechanism by which these organisms cause disease. They are known to invade cells but this process is atypical in that the parasites form a vacuole just below the epithelial cell membrane. During infection a variety of changes are seen such as partial villous atrophy, crypt lengthening and inflammation; these responses are probably due in part to cell damage that occurs during the growth of the intracellular forms. It has also been proposed that parasite enzymes and/or immune-mediated mechanisms may also be involved. It should be remembered, however, that cryptosporidiosis is resolved by the immune system in healthy patients normally within 3 weeks.