Epidemiology

Prevalence estimates vary greatly because of a lack of uniformity in access to health care, diagnosis, reporting, and surveillance. Cases are under-reported.[27][28]​​​ One pooled global estimate was 7.6% (95% CI 6.9% to 8.5%) that varied greatly by country and population group studied.[29]

In the US, there has been an increasing incidence; 13,979 confirmed cases were reported in 2019 (annual incidence 4.3 per 100,000 population).[30]​ However, <2% of the estimated 823,000 cases occurring annually are diagnosed and reported.[31]​ Between 2009 and 2017, there were 440 Cryptosporidium outbreaks in the US reported to the Centers for Disease Control and Prevention, with the average number of outbreaks increasing by around 13% per year.[32] The main causes of outbreaks included contaminated water in pools or water playgrounds, contact with infected cattle, and contact with infected persons in child care settings. Between 2015 and 2019, a total of 76 Cryptosporidium outbreaks were associated with treated recreational water.[33]​

In the EU in 2018, 20 European Union/European Economic Area countries reported 14,252 confirmed cases, 4.4 per 100,000 population, more than in 2017. Four countries (Germany, the Netherlands, Spain, and the UK) accounted for 76% of all confirmed cases, with the UK alone accounting for 41%, reflecting increased ascertainment.[34] Case numbers peaked in the autumn (September), and a smaller peak was also observed in spring (April). The highest notification rate was in children aged <5 years (15.8 cases per 100,000 population).​

In the UK in 2017, there were 4292 cases reported in England and Wales (annual incidence 7.3 per 100,000 population), most were children aged <5 years, and a higher number of boys than girls were affected. There was a second peak in women aged 25-34 years.[35]

The strong seasonality reported in Europe, with peaks occurring during late spring and late summer-early autumn, is thought to have some link with environmental contamination and animal contact in the spring and increased use of recreational water venues and foreign travel in the late summer/early autumn.[36] Global epidemiological trends differ between geographical regions and are influenced by climate and socio-economic status.

Cryptosporidiosis and young age are consistently linked. In low-middle income countries Cryptosporidium is a major cause of moderate-severe diarrhoea, particularly in children aged <2 years.[37] Among these young children, where malnourishment is prevalent, Cryptosporidium is significantly associated with death, stunted growth and cognitive deficit.​[3]

C. parvum (zoonotic) and C. hominis (anthroponotic) are the most common species, and many subtypes have caused disease; distribution depends on acquisition risk factors. Routine diagnosis does not identify infecting species.[38][39]

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