The cryptosporidiosis
Causing agent
Cryptosporidium organisms are microscopic protists belonging to the group of Apicomplexa (Fig. 1). More than 20 species of the genus Cryptosporidium can lead to cryptosporidiosis, a parasitic infection that can develop in human and many animals. Six Cryptosporidium species can infect human beings worldwide but most human infections have been attributed to C. parvum and C. hominis.
Figure 1. Oocysts (resistant stage) of Cryptosporidium stained with Ziehl-Neelsen (left) or with fluorescent monoclonal antibody (right). Photo: G. Certad, BDEEP laboratory, Pasteur Institute of Lille.
Life cycle
Animals or human beings contaminate themselves by ingesting the resistant and infectious form of Cryptosporidium : the oocyst that contains 4 infectious sporozoites. The oocysts could be present in recreational or drinking water as well as on vegetables or fruits. The transmission then occurs through the fecal-oral route.
Following ingestion, oocyst excystation occurs and the parasite invades epithelial intestinal cells. In these cells, the parasite undergoes asexual and sexual multiplication, thus giving rise to newly formed oocysts which are immediately infectious to a new host once in the environment or which can maintain an auto-infection cycle.
Prevalence
Cryptosporidium organisms are distributed in all regions of the world except Antarctica. They are most common in less developed countries.
Their transmission is zoonotic since human beings (or other animals) can acquire the infection by ingesting agricultural products or surface water contaminated with feces from humans, cattle or other livestock.
Cryptosporidium spp. are amongst the most common and virulent enteropathogens in humans because (i) the oocyst is immediately infectious upon excretion, (ii) a low number of oocysts is sufficient to ellicit the infection, (iii) the oocysts can resist for months in the environment and (iv) can survive in properly chlorinated recreational water venues for 10 days.
Two Cryptosporidium piscine species have been described in fish but none of them have been shown to infect mammalian species. Nevertheless, subtypes of Cryptosporidium that are infectious to humans have been found to infect fish (the Fish-Parasites network, unpublished data and Koinari et al., 2013). The risk of ingesting raw fish contaminated with Cryptosporidium oocysts remains rare but cannot be excluded.
Clinical signs
The disease severity can vary according to the age, the nutritional and immune status of the host and also in relation with the virulence of Cryptosporidium species or strains. Many infections are asymptomatic, self-limited and often not diagnosed. The main symptom of cryptosporidiosis is watery diarrhea. Other symptoms can be abdominal cramps, fatigue, nausea, and anorexia. Fever and vomiting may occur. Diarrhea due to Cryptosporidium tends to persist longer (median of 5-10 days) than that seen with gastrointestinal pathogens.
In immunocompetent adults, the disease resolves spontaneously. Among children (especially in developing countries), the diarrhea often lasts for 14 days or longer. Cryptosporidiosis is the second biggest cause of diarrhoeal disease and death in infants. In persons with HIV/AIDS (CD4 counts lower than 100 cells/ml), diarrhea can be severe and accompanied by malabsorption, weight loss, and high case fatality.
Prevention
Prevention against Cryptosporidium organisms is difficult given the organism’s high infectivity and resistance to disinfection. The hope to improve on the prevention relies on the development of an effective vaccine for the population at risk, mainly the children and immunocompromised persons. Meanwhile, one may recommend to these susceptible persons to consume mineral water, to avoid recreational waters (even properly chlorinated), to cook thoroughly food that may be contaminated by oocysts (fruits, vegetables, fish, etc.) and to wash hands with soap and water before preparing or eating food.
References
Chalmers RM & Katzer F. 2013. Looking for Cryptosporidium: the application of advances in detection and diagnosis. Trends Parasitol. 29(5):237–51. doi:10.1016/j.pt.2013.03.001
Checkley W, White AC Jr, Jaganath D, Arrowood MJ, Chalmers RM, Chen XM, Fayer R, Griffiths JK, Guerrant RL, Hedstrom L, Huston CD, Kotloff KL, Kang G, Mead JR, Miller M, Petri WA Jr, Priest JW, Roos DS, Striepen B, Thompson RC, Ward HD, Van Voorhis WA, Xiao L, Zhu G, Houpt ER. 2014. A review of the global burden, novel diagnostics, therapeutics, and vaccine targets for Cryptosporidium. Lancet Infect Dis. Sep 29. pii: S1473-3099(14)70772-8. doi: 10.1016/S1473-3099(14)70772-8.
Koinari M, Karl S, Ng-Hublin J, Lymbery AJ, Ryan UM. 2013. Identification of novel and zoonotic Cryptosporidium species in fish from Papua New Guinea. Vet Parasitol. 198(1-2):1-9.
Shirley DA, Moonah SN, Kotloff KL. Burden of disease from cryptosporidiosis. 2012. Curr Opin Infect Dis. 25(5):555-63.
Striepen B. 2013. Parasitic infections: Time to tackle cryptosporidiosis. Nature. 503(7475):189–91.
Useful links
French national network of cryptosporidiosis surveillance, Crypto-Anofel