Cryptosporidial infection

What is Cryptosporidium?

Cryptosporidium parvum is an infectious parasite found in humans and animals. You may hear it referred to as Cryptosporidium, C. parvum or “Crypto”. It can cause a disease called cryptosporidiosis. Cryptosporidium multiplies in the gut of infected humans and animals and forms little eggs called oocysts that are excreted in faeces. These oocysts are very hardy and survive many months in the environment in water and soil.

If ingested the oocysts can transmit the infection and the whole cycle starts again. Although uncommon in children and adults with a normal immune system, Cryptosporidium can cause a temporary diarrhoeal illness lasting about two weeks usually followed by a full recovery. However, for a child or an adult with an impaired immune system, severe diarrhoea, which is sometimes life threatening, may occur. In some immune compromised patients, infection may get into the liver and cause a persistent infection leading to liver damage known as sclerosing cholangitis.

Who is most at risk?

  • Children or adults with some types of PID notably CD40 ligand deficiency and combined immune deficiency syndromes.
  • Children or adults with weakened ‘immune systems, i.e. those receiving immune suppressive treatment such as chemotherapy, and organ or bone marrow transplant patients.

How is it spread?

Cryptosporidium is spread by the faeco-oral route – in other words the organism has to be ingested or swallowed following direct contact with infected human or animal faeces.

Particular sources of infection are:

  • Adults or children with cryptosporidial infection
  • Infected animals
  • Drinking or contact with water contaminated with Cryptosporidium
  • Uncooked food or foods washed in contaminated water
  • Soil contaminated with human or animal faeces.

How safe is drinking water?

Very rarely, outbreaks of Cryptosporidium have occurred in mains supplied drinking water, when it is usual for warnings to be issued by suppliers to affected households.

The risk of acquiring Cryptosporidium from water supplies during non-outbreak periods is uncertain but thought to be low. However, as a precaution the Department of the Environment recommends that:

“All water, from whatever source, that might be used by an immuno-compromised person should be brought to the boil and allowed to cool before use.”

Households with private water supplies have a higher risk of being contaminated, and individuals should be aware that some water fountains, campsites or remote holiday accommodation may have private water supplies which can not be considered universally safe from Cryptosporidium.

Water from the tap or from bottles and ice used in restaurants, bars, theatres and other such places cannot be relied upon to be safe. Note that in the UK some bottled water has been noted to have higher levels of bacteria than tap water, this risk is increased when the source of the bottle water and processing is not known.

The use of water filters may reduce the risk of Cryptosporidium but no studies comparing boiling versus filtering water have been performed. If filters are used, individuals should be aware that appropriate filters must be professionally fitted into your water system, with a filter pore size no greater than 1 micron wide and that great care is needed to ensure that filters are maintained and changed regularly.

Is it safe to have pets?

Newborn and very young animals may pose a small risk of cryptosporidial infection. When contemplating acquiring a new pet, avoid:

  • bringing any animal with diarrhoea into the household
  • acquiring a puppy or kitten less than six months old
  • adopting stray animals.

For those at risk of Cryptosporidium it is not generally necessary to destroy or give away pets that have already been acquired but every effort should be made to keep them in good health with advice from a vet if necessary.

Minimising the chance of acquiring cryptosporidial infection

It is not possible to totally eliminate the risk of developing cryptosporidial infection and despite rigorous efforts to reduce exposure, some individuals may go on to develop the infection. The following is a practical guide that may help reduce but not eliminate the risk completely.

  • All tap or bottled water should be boiled before drinking.
  • Sparkling or carbonated water might be a safer option than still water as it is slightly more acidic when boiling is not possible (e.g. travelling).
  • Ensure dairy products and fruit juices are pasteurised.
  • Avoid paddling or toddler pools (older children and adults could swim in chlorinated swimming pools but avoid swallowing excessive quantities of water).
  • Avoid contact with human or animal waste (in lakes, rivers, beaches, sand and outdoor children’s sandpits, parks and garden soil).
  • Avoid exposure to farm animals (especially calves and lambs), newborn domestic animals less than 6 months old (such as kittens and puppies) and any stray, sick or very old domestic pets.
  • Careful hand-washing after accidental contact with faeces or animals.
  • Avoid excessive contact with adults or children with diarrhoeal illness.

See also our food and safety advice.

Monitoring for signs of Cryptosporidial infection

Anyone thought to be at particular long term risk from Cryptosporidium should be monitored closely for signs of infection. Any episode of diarrhoea should be promptly investigated by your doctor and stool samples should be analysed regularly. Occasionally your doctor may recommend assessment by a liver specialist who will look more closely for signs of infection in the liver. Sometimes it may be necessary to take a small sample of the liver by carrying out a liver biopsy.

Medicines effective against Cryptosporidium

Children and adults with a normal immune system rarely need treatment for cryptosporidial infection but those with impaired immune systems may benefit from treatment either as preventative therapy or for treating the infection once it occurs. At present not enough information is available to recommend any specific therapy for cryptosporidiosis but some drugs have been shown to offer some effect against Cryptosporidium parvum.

Paromomycin is an antibiotic that is thought to reduce the symptoms of cryptosporidial infection but does not usually clear or “cure” it completely. Your doctor may suggest taking it if the infection is present to lessen the effects of the illness.

Azithromycin is another medicine that may be given once cryptosporidium infection is present and it may lessen the severity of diarrhoea and in some cases may clear the oocysts from the stool.

Further information

If you want to know more about Cryptosporidium, you can contact the Family Resource Centre at Great Ormond Street Hospital (tel: 020 7813 8558) or consult your GP or hospital specialist.

Further information about water supplies can be obtained from:

England and Wales

Drinking Water Inspectorate
Nobel House
17 Smith Square

Contact details can be found at



If you live in Scotland you should call the Drinking Water Quality Regulator For Scotland (DWQR)

Telephone: 0131 244 0190

Northern Ireland

If you live in Northern Ireland you should call the Drinking Water Inspectorate – Northern Ireland (DWINI)

Telephone: 028 9056 9282

Private drinking water: all enquiries should be directed to the Environmental Health section of your local authority. You can find contact details for all local authorities in the UK here.

Reference: “Cryptosporidium in Water Supplies – Advice to the immuno-compromised individual” from The Department of the Environment, Transport and the Regions.

This information was reviewed by Dr Matthew Buckland, Medical Panel, August 2022.