Medical Matters: Recent health scares in northern England and Scotland are reminders that even that purest of resources, water, can be a source of human infection.
The most serious outbreak was in Cumbria where an elderly man has died and many others have been struck down with Legionnaires' disease, a topic dealt with in this column on April 1st this year. The bacterium thrives in stagnant water at temperatures between 20 and 45 Celsius, and if thrown into the air in water droplets - for example by an air-conditioning unit - and inhaled, it can cause a flu-like illness and pneumonia.
In the same week, health officials in Glasgow reported that water supplies to a quarter of the city had been contaminated by a parasite. Routine testing at a reservoir found high levels of Cryptosporidium, a protozoan found in animal faeces which can cause severe diarrhoea.
Protozoa are large microbes and are classified separately from bacteria, viruses and fungi. The humble amoeba, which readers may remember from their school science classes, is a protozoan, as is another organism which causes gastroenteritis, giardia lamblia.
Cryptosporidium parvum is an example of a zoonosis - a disease of animals that can be transmitted to humans. It lives in the intestines of sheep, cattle and domestic animals and is passed on to the ground via the stool of an infected animal. Recent torrential rains in Scotland are thought to have swept the microbes from nearby fields into the reservoir. People in rural areas whose water supply comes from wells are also at risk of infection if the protozoan gets washed into their water system.
While viruses are by far the commonest cause of gastroenteritis in young children, cryptosporidium is now thought to be a relatively common cause of watery diarrhoea. In one study of 62,241 patients in England and Wales with presumed infectious diarrhoea, 2 per cent were excreting cryptosporidia in their faeces. Positive tests for the protozoan were highest in children aged one to four years.
Symptoms generally begin two to ten days after infection. They include watery diarrhoea, stomach cramps, nausea, loss of appetite and weight loss. The attack usually subsides in one to two weeks, although I recall one seven-year-old girl who developed intermittent diarrhoea, swollen glands and who failed to put on weight over a period of months, before the diagnosis of cryptosporidiosis was made.
Diagnosis involves the microscopic examination of a faecal sample. Small cysts, measuring 4-5 micrometres in diameter, are seen in the stool. Cryptosporidia can also be identified by electron microscopy of biopsy specimens taken from the small bowel.
Unusually, there is still no effective medication for the infection and so treatment consists of supportive care using anti-diarrhoeal agents and replacing fluids and electrolytes. In the case of children, properly formulated oral hydration agents such as Dioralyte are best.
For patients with a suppressed immune system - for example those with advanced HIV disease - cryptosporidial infection is more serious and is difficult to eradicate. Infected patients are usually started on anti-viral therapy in an effort to stimulate the immune system to fight off the bug.
All water used for drinking, preparing food, brushing teeth and bathing infants should be boiled during an outbreak. This is the best method of preventing infection with the protozoan.
Another major cause of infective gastroenteritis caused by an organism similar to cryptosporidium is giardia lamblia. It too is commonly acquired from water contaminated with faeces. Once the bug has been ingested it attaches itself to the wall of the small bowel, causing symptoms one to three weeks later.
Usually, the diarrhoea is acute but some people develop chronic flatulence, bloating and malabsorption problems. The diagnosis is made from a faecal sample in which the cysts of the protozoan are identified. And unlike cryptosporidium, treatment with an antibiotic called metronidazole is effective against giardia. Cooking food adequately and boiling and filtering suspect water are the best means of preventing infection.
Both cryptosporidium parvum and giardia lamblia are reminders of the daily struggle endured by those in less developed countries, who are continuously exposed to serious and potentially fatal illnesses because they lack a reliable and clean water supply.
• E-mail Dr Muiris Houston, Medical Correspondent, at mhouston@irish-times.ie or leave a message at 01-6707711, ext 8511. He regrets he cannot reply to individual medical problems.