Among the currently recognized species of Cryptosporidium, C. hominis and C. parvum are the most common isolates from people, although human infections with several other species have been reported: C. canis, C. felis, C. meleagridis (birds) and C. ubiquitum (many hosts); and, less commonly, C. andersoni (cattle), C. cuniculus, C. muris and C. suis.  All the species infecting people can also infect mammals or birds.  Oocysts of Cryptosporidium are very common in untreated surface water, and the parasite’s zoonotic potential leads to concerns about the possible contamination of water supplies for people with faeces from infected domestic animals and wildlife.  Examination of 55 Cryptosporidium isolates from infected dairy calves and from people with sporadic cases of cryptosporidiosis in Ontario revealed an array of genotypes, three of which were found in both calves and people.  A similar recent study in England and Wales identified genetically identical (gp60 locus) C. parvum in cattle and in people with diarrhea on seven of eight farms examined. A study of 8,000 Cryptosporidium isolates from people in England and Wales during the period 2000-2003 revealed 45.9% of infections were by C. parvum alone, 49.2% by C. hominis alone, 0.4% by C. parvum and C. hominis, 0.9% by other genotypes, and 3.5% were untypable.  These data have recently been updated for 2004-2006 (n=4509) , and show an increase in the proportion of C. hominis, a decrease in C. parvum, and the first detection of C. ubiquitum (two cases), a newly described species that infects primarily ruminants and was previously referred to as the cervine genotype. More recently,
of 14,469 Cryptosporidium isolates from people with gastro-intestinal symptoms in the UK between 2000 and 2008, 38 were C. felis, and one C. canis (plus 109 C. meleagridis and 11 C. ubiquitum, and four with other, non-speciated genotypes). Immunosuppression and contact with cats were significantly linked to infection with C. felis.

The oocysts of Cryptosporidium in the faeces of hosts are immediately infective and are relatively resistant to adverse environmental conditions. Also, very few oocysts (<5) are required to establish an infection, even in an immunocompetent host.  People can acquire the parasite by contact with (the faeces of) infected people, animals or birds, and from the ingestion of contaminated foods, drinking water, other drinks (e.g. fresh-pressed apple cider), or recreational water, especially swimming pools, for example a pool in a hotel accommodating groups of Ukranian dancers in Manitoba in 2001.  Also, there is evidence that sometimes heavy rains will wash the oocysts into streams, rivers and lakes used as sources of drinking water.  In Canada there have been several outbreaks associated with municipal water supplies - perhaps most notably the 2001 outbreak in Battleford and North Battleford, Saskatchewan, with an estimated 5,800 to 7,100 cases.  The first sign of this outbreak was abnormally high sales of anti-diarrheal medications, and was shown to be the result of defective functioning of a water treatment system, coupled with the water intake for the community being downstream on the river from the sewage outlet.  The largest North American outbreak of Cryptosporidium occurred in 1991 in Milwaukeee, Wisconsin, where an estimated 400,000 people showed clinical disease as a result of a breakdown in the water treatment system.  There were a number of secondary cases and there were deaths among people immunosuppressed by HIV - people for whom Cryptosporidium is a real danger.  This outbreak was thought to be caused by a human genotype of the parasite, but very few samples were examined..

In Canada, both Cryptosporidium and Giardia in people are federally notifiable diseases and so there are data from (primarily passive) surveillance for the occurrence of the parasite in people in this country.  National data from the Public Health Agency of Canada (PHAC) indicate 726 human cases of cryptosporidiosis in 2006 (the most recent year for which data are available) - approximately 2.3 cases per 100,000 population, of which an unstated number were likely travel-related.    No data are available on the species/genotypes involved in these cases. The incidence rate varies with season, location and age of the affected people.  Recent data from the US indicate that cases are more common in children 1 to 9 years old and in adults 25-39 years old, and that most cases occur during June through October. 

When interpreting passive surveillance data of this type it is important to realize that each case reported likely represents many more cases of illness that do not have contact with a physician, or are not accurately diagnosed, and, therefore, are not captured by the surveillance system.  A recent published estimate from the US Centers for Disease Control and Prevention (CDC) indicate that the raw surveillance data for Cryptosporidium should be multiplied by approximately 100 and those for Giardia by approximately 45 to better reflect the actual occurrence of the diseases.  These calculations suggest that in Canada in 2006 there were 72,000 cases of human cryptosporidiosis, about one tenth of the number estimated in the US. 

More information about Cryptosporidium is available under Dogs .