Cryptosporidium is a coccidian protozoan which is well-known as a ubiquitous human pathogen. Cryptosporidium begins its life cycle as sporulated oocysts, which are excreted to an external environment via the faeces of the infected host.

The pathogenic oocysts reside in food and water. Transmission of Cryptosporidium takes place mainly through contact with contaminated water. The protozoan can be present in recreational waters, such as pools and lakes. It can also be found in drinking water supplies, as their oocysts are resistant to chlorine treatment and can tolerate low temperatures. The resistance of oocyst to the external factors results in its prolonged survival in the environment. The other transmission routes include fecal-oral and person-to-person contact. Therefore, Cryptosporidium outbreaks commonly occur in day-care canters, where young children spend lots of time in groups in small areas, as well as they share toilets and play rooms.

Ingestion of oocytes initiate the infection. While in the intestine, oocysts become activated to release four infective sporozites. These motile sporozites invade the epithelial linings of the intestines where they reproduce. The sporozites first enter asexual intracellular multiplication (schizogony) stage and then undergo sexual multiplication (gametogony). Sexual reproduction results in differentiation of male microgametes and female macrogametes, which is followed up by fertilization. The final product of the fertilization is a zygote, which can develop into two different types of oocysts. The first type is the thicked-walled oocyst that will be excreted and second type is a thin-walled oocyst that will be mainly involved in autoinfection of the host. Both kinds of oocyst form infectious sporozites within what makes them infective upon excretion allowing for direct and instant faecal-oral transmission. The Cryptosporidium life cycle differs from the most of the parasites as the entire cycle is monoxenous - occurs within a single host. Multiplication or reproduction does not take place in an external environment. Other characteristics differentiating this pathogen from the others are an ability to infect variety of hosts (humans, wildlife, reptiles and fish) and ability to inhibit most climates due to production of robust oocyst.

Cryptosporidium infection can be symptom free, but in most cases it causes gastrointestinal illness. Cryptosporidiosis’s most common symptoms are acute, voluminous and watery diarrhoea accompanied by abdominal cramps, vomiting, fatigue, nausea, chills and sweats. In immunologically healthy individual symptoms can magnify, fade or remain constant over period of 2-26 days; whereas, cryptosporidiosis in immunosuppressed individuals can be chronic and even life threatening.

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