Ancient sickness stays in the swim: Bernard Dixon finds a potent tummy bug down at the pool

SWIMMING pools, with their pungent smell of the powerful disinfectant, chlorine, hardly seem likely to expose users to infection. Yet they can pose dangers, not least as sources of Giardia lamblia, a microbe capable of causing gastrointestinal upsets known as giardiasis.

Evidence to support this comes from a study in Avon and Somerset, the first of its kind conducted in Britain.

Until recently most microbiologists thought of G lamblia as at most an infrequent cause of ill-health, though giardiasis is recognised as a truly ancient disease: the microbe has been found in dessicated human faeces deposited 1,800 to 2,200 years ago in caves in Israel and Tennessee.

Like those responsible for malaria and sleeping sickness, G lamblia belongs to a group of microbes known as protozoa, and changes its form during its life cycle. The infectious stage of G lamblia - as seen in the ancient faeces - takes the form of a spherical cyst. If the cysts are ingested and reach the duodenum, they burst open to release the feeding forms known as trophozoites. These attach themselves to the duodenal wall, move into the large intestine, then pass out in the faeces.

The Dutch microbiologist, Antonie van Leeuwenhoek, first described the cysts in 1681, when he observed his own faeces with a primitive microscope.

The typical effects of giardiasis are diarrhoea with pale greasy stools, nausea, flatulence, cramps and bloating. The infection can be acute or chronic, lasting from one or two weeks to months or even years, but some carriers remain unaffected.

The variable behaviour of G lamblia is the main reason why its significance -clarified in the Eighties by US microbiologists, who deliberately infected themselves - was for long uncertain. In Britain, sporadic cases occur throughout the year, although they peak in summer and autumn. The disease is more prevalent in young children than adults, and is commoner in tropical regions than more temperate climates, and among poor people. In Africa, Asia and South America, some 200 million people a year are infected, causing about 500,000 actual cases of disease and 10,000 deaths.

In the UK, however, the disease appears to be increasing; indeed, G lamblia is not difficult to find in our environment.

A survey last year at the University of Strathclyde and Stobhill General Hospital, Glasgow, showed that 11 per cent of canine faeces samples, collected at random from seven public parks in the west of Scotland, contained the cysts; in one park, 40 per cent of the specimens carried the parasite.

The Avon and Somerset study by Selena Gray and her colleagues at the South Western Regional Health Authority, Bristol, and the University of Bristol, asked patients, confirmed as infected by G lamblia during an 11-month period, a series of questions about how they might have become infected.

A variety of experiences, during the four weeks before submitting a sample of faeces found to contain G lamblia, were recorded. These included overseas travel, consumption of potentially contaminated water, attendance at nursery or day-care facilities, and indulgence in water sports such as canoeing, sailing, windsurfing, scuba diving or fishing.

The results, published in Epidemiology and Infection, revealed two statistically significant risk factors for giardiasis: swimming and travel abroad, especially when camping, caravanning and/or staying in holiday chalets. Of the 35 patients who had been swimming, 27 had been in pools and the others in rivers or the sea.

This is the first time swimming has been shown to increase the risk of giardiasis other than when frankly contaminated water has triggered an outbreak.

Although G lamblia can be destroyed by chlorine, it is more resistant to it than many other microbes, so infection is possible in poorly maintained pools where faecal contamination overwhelms the chlorination and filtration system. And the danger is acute in pools used heavily by toddlers, who are prone to 'accidents'.

Giardiasis is seldom life-threatening, and can be treated with various drugs. Good personal hygiene and avoidance of suspect water supplies should minimise the risks when camping and caravanning.