Cryptosporidium and legionella


Cryptosporidium is a microscopic parasite which can infect a wide range of animals. There are many species of cryptosporidium, some of which can cause illness in humans, called cryptosporidiosis, the symptoms of which include stomach upsets, fever and diarrhoea.

A resistant form, called an oocyst, can be found in the faeces of infected people and animals. Contact with an infected person or animal or exposure to contaminated food or recreational water, for example, at a swimming pool, are all common routes of infection. It is very rare that cryptosporidium can be caused by contamination of a drinking water source.

All sources of public drinking water supply are risk assessed for their vulnerability to contamination. If a source is found to be at risk from cryptosporidium steps are taken to reduce the risk to the source water.

Cryptosporidium is resistant to the chlorine used as a disinfectant in drinking water, so additional treatment measures are applied to either remove the oocysts or kill them before the water is distributed.

Drinking water sources considered to be at risk are routinely monitored for cryptosporidium and health authorities continuously monitor for cases of cryptosporidiosis in the community.

The symptoms described above are common to many illnesses. Anyone who is unwell should contact their doctor.


Although legionella bacteria are widely distributed in the aquatic environment and can be found in all types of water only certain strains cause legionellosis, a collective term for diseases caused by legionella bacteria including the most serious Legionnaires’ disease, a severe and potentially fatal form of pneumonia.

In drinking water, legionella bacteria are controlled by disinfection of supplies which is effective at killing them.

Legionella bacteria only become a risk to health where conditions, particularly warm temperatures, exist that allows them to grow and survive. Legionellae can be inhaled if released in large numbers into the air as a fine mist of water droplets. This can occur in poorly designed, installed or maintained artificial water systems such as cooling towers, hot and cold water systems and spa pool.

All organisations or self-employed individuals, including those who rent property or rooms in their home are responsible for the water system in their premises. They have a legal duty to ensure that any risk of exposure to legionellae, for those working and living in their premises, is assessed and controlled properly.

A competent person must perform a legionella risk assessment.

We do not provide advice or a risk management service for legionellae. For more information, please contact a suitably qualified independent expert.

Wessex Water Scientific Services does carry out legionella testing.

Our tests explained

Each year we analyse around 50,000 samples of water taken from treatment works, reservoirs and customers’ taps