The facts about...

Cryptosporidium

The Organism

Cryptosporidium is a microscopic single celled parasite somewhat smaller than a red blood cell, which, if swallowed, can cause gastroenteritis. It is not a bacterium or a virus, but belongs to a group of micro-organisms known as protozoa. It infects humans, animals such as cattle and sheep, and sometimes, dogs, cats, rodents, birds etc.. Most of the infections in man and livestock are with just one species, Cryptosporidium parvum.

Cryptosporidium can grow only in a living host and does not multiply in the environment. The parasite develops mainly in the cells lining the gut where it goes through a complex life cycle. The last stage of this cycle is the production of oocysts, the infective stage. These are passed out of the body in the stools and can survive (but not multiply) in the environment, especially in cool moist conditions. Oocysts contain four motile (free moving) banana-shaped bodies known as sporozoites. When oocysts are swallowed, the sporozoites are released and attach to the cells lining the gut and start the life cycle over again.

The Disease

In the United Kingdom the infection is most common in children aged 1-5 years; younger adults are the next most commonly affected group. Infection is less common in infants under 6 months of age or adults over 45 years.

The incubation period may be as short as two days but is more usually about a week. Symptoms may start with the loss of appetite, nausea and abdominal pain. This is usually followed by profuse, foul smelling, watery diarrhoea, vomiting (especially in children), and there may be mild fever and noticeable weight loss. In otherwise healthy people the symptoms usually persist for 1 to 3 weeks but some symptoms can recur for longer periods (usually not more than a month).

There is no effective specific treatment (eg antibiotics) available. As with any form of gastroenteritis, it is important to take plenty of suitable fluids. Patients may sometimes be admitted to hospital, usually because they have become dehydrated.

In patients with seriously deficient immunity, including those with AIDS, the infection can be much more severe and the symptoms may persist for much longer, occasionally resulting in death.

Laboratory Diagnosis

This depends upon the detection of oocysts in the stools, usually by microscopic examination of a stained preparation of the patient's specimen submitted to the laboratory by their doctor. The patient may then be contacted, for example by an Environmental Health Officer to help identify the most likely source of the infection and to offer advice to help limit further spread.

Transmission

The size of the infective dose (ie the number of oocysts which need to be swallowed to start an infection) is not known, but is thought to be small, probably 10 to 100 oocysts but perhaps as few as one.

The infection may be passed either directly from an animal or from person to person. It may also be passed indirectly through the environment, especially in water; unpasteurised milk, offal and occasionally undercooked meat such as sausages. Further transmission may then result in secondary cases, for example in families or play groups. This is especially so between children who are not toilet trained or adults caring for them. The risk of this happening is lessened through careful attention to personal hygiene and care with food handling and preparation. Symptomatic infection is uncommon among rural adults who are regularly exposed to livestock, and therefore maintain high levels of immunity. Visitors (on holiday or to open farms), are less likely to have immunity and may become infected by contact with bottle fed lambs or calves, or by hands soiled from the farm environment.

Control of transmission

Oocysts are highly resistant to most disinfectants including bleach. They are killed by some strong chemical solutions such as hydrogen peroxide, ammonia, or caustic soda, and by desiccation, prolonged freezing and moderate heat (as in pasteurisation).

Despite the resistance of the oocysts to disinfectants, the risk of passing on the infection can be considerably reduced by good personal and domestic hygiene. This includes thorough washing of hands followed by complete drying (especially if you are preparing food which will not be cooked). Contaminated surfaces should be washed down with a soapy disinfectant solution followed by allowing the surface to dry thoroughly.

It is important for farm visitors to warn children not to eat sweets or suck their fingers. Visitors should wash their hands before eating food and again when they leave. Remember that footwear may re-contaminate hands when boots are removed back at the car or at home.

Some patients considered by their doctor to have very poor immunity may be advised that they should try to limit or avoid contact with livestock and children in nappies. They should not drink unboiled water from a tap or bottle.

Cryptosporidium and water

Drinking water: In recent years there has been increasing recognition of outbreaks of cryptosporidiosis arising from contamination of water supplies. Although resistant to many chemical disinfectants, including chlorine used to treat water, properly functioning water treatment systems are usually able to deal with the small numbers of oocysts which are present from time to time in water sources such as rivers. However, water treatment may occasionally fall below the required level of efficiency because of operational problems at the treatment works, especially following particularly heavy rain or agricultural contamination.

In this event, the public may be advised by the water company or health officials to boil their tap water. Prolonged boiling of water is not required - simply raising water to the boil in a kettle is sufficient. Boiled, cooled water should be used for food preparation, making ice cubes etc. The water directly from the tap should still be satisfactory, however, for other general purposes such as washing hands, dishes, clothes and for bathing or showering.

Swimming pools: Properly maintained pools with adequate water circulation, filtration and disinfection, do not usually represent a hazard to bathers. If the mains water supply becomes contaminated, the risk to pool users is minimal because the parasite does not multiply in water, the dilution of the water used to top up the pool and the effects of the water treatment system (continuous filtration, chlorination, etc.).

If an infected person does have 'an accident' and defaecates in the pool, other bathers sharing the pool at the same time may be infected. However the risk is minimised by the effects of dilution in the pool and pool water treatment. Bathers, especially young children in nappies, should not use the pool if they are unwell, particularly if they have signs of a 'tummy upset'.