Cryptosporidium spp. (PCR)

A parasite that also causes diarrhoea in healthy people; with a weakened immune system, the infection can be particularly severe and prolonged. It survives chlorine disinfection – which is why outbreaks via drinking water and bathing water keep occurring.

How Cryptosporidium lives and multiplies

Cryptosporidia are single-celled organisms from the same group as the toxoplasmosis pathogen. They are taken up as thick-walled oocysts through contaminated water, food or contact with people and animals, such as calves; just a few oocysts can be enough for an infection. In the small intestine, motile sporozoites hatch, attach to the gut cells and become enveloped by their membrane – so they sit on the cell surface, but protected in a sheath. There they multiply both asexually and sexually. Thin-walled oocysts can infect new cells within the same gut; thick-walled ones are excreted and are infectious immediately.

How Cryptosporidium affects the gut lining and the immune system

Infected gut cells die or are shed, the intestinal villi shorten, and the surface area through which nutrients and water are absorbed becomes smaller. At the same time, inflammatory messengers stimulate the lining to release chloride, and with it water, into the gut – this is how the watery diarrhoea arises. The infection is ended mainly by cellular defence: T helper cells and the interferon-γ they produce, together with interferons from the gut cells themselves. With a healthy immune system the illness therefore usually settles on its own after a few days to a few weeks, even though oocysts continue to be excreted for a while.

What happens when Cryptosporidium multiplies heavily?

If T helper cells are lacking – for example with advanced HIV infection, after organ transplantation or during chemotherapy – the cycle of self-reinfection in the gut continues unchecked. This leads to long-lasting, very watery diarrhoea with large losses of fluid and salts, impaired nutrient absorption and weight loss. The pathogens can then also colonise the bile ducts and trigger inflammation there. In young children in regions with poor water supply, repeated infections have been linked in studies with delayed growth.

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