Troubled waters need filtration systems

Medical Matters:The availability of a good water supply and effective sewage disposal for public health was recently voted the most important medical milestone since 1840.

Writing in the British Medical Journal, Prof John Mucherback of Erasmus University in Rotterdam, said providing clean water and the safe removal of sewage "plays a vital role in improving public health now and in the future".

This is especially the case in the developing world. The global burden of disease study in 2001 found more than 1.5 million deaths from diarrhoeal disease, in low and middle income countries, were due to unsafe water and poor sanitation.

But no matter how pure water appears to be in developed countries, it can still be a source of infection, as last week's outbreak of gastric illness in Co Galway has shown.

A significant jump in the number of cases of cryptosporidiosis in the county since the beginning of the year meant that public health and council officials have advised some 90,000 people to boil water before use. Water contaminated by the bug cryptosporidium parvum is now known to be the source of the outbreak.

Cryptosporidium is a protozoa found in animal faeces which causes severe diarrhoea in both animals and humans. 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.

The diarrhoeal illness in humans caused by the cryptosporidium bug is an example of a zoonosis - a disease of animals that can be transmitted to humans. The protozoan lives in the intestines of sheep, cattle and domestic animals and is passed onto the ground via the stool of an infected animal.

So contact with animals, such as occurs at this time of the year during the lambing season, is one method of infection. Swallowing infected water and eating uncooked contaminated food are others. But given that the Galway outbreak includes many people living in the city, recent torrential rain and flooding may have washed the parasite into the water supply.

Much of Galway's water is extracted from Lough Corrib, as is that supplied to group water schemes in the county, making the lake a likely common source for the recent outbreak of cryptosporidiosis.

Infected animals shed large quantities of tiny cysts measuring 4-6 microns in diameter. Because of their small size and the difficulty separating them from other particles, they can be hard to detect in water. And the cysts are resilient, surviving for long periods in a cool, wet environment. They also resist chlorination.

The first reported outbreak of cryptosporidiosis in the Republic that was strongly associated with drinking water occurred in April 2002 in the midlands. Some 26 cases of the disease were linked to the presence of cryptosporidium in a lake serving a population of 25,000 people.

The investigation concluded that heavy rains facilitated the entry of animal excrement into the lake.

Studies carried out in Cherry Orchard Hospital in Dublin and the Regional Hospital, Galway in 1987 found that 4 per cent of admissions for gastroenteritis in children, aged 14 years or younger, were due to cryptosporidiosis. The peak months for infection with the bug were April and May; the majority of the patients were from rural backgrounds and were aged from one to five years.

Symptoms of cryptosporidiosis generally begin from two to 10 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 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 patient's faeces.

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 Aids or cancer - cryptosporidial infection is more serious and could be fatal.

All water used for drinking, preparing food, brushing teeth and bathing infants should be boiled during an outbreak.

But the best method to prevent infection with the protozoan is to remove it from the water supply. Slowly filtering the water through fine grains of sand is an effective way of catching the cysts. They can then be removed at the settlement stage of the filtration process.

While this is carried out in the larger water treatment plants in the State, fine sand filtering is not the norm in smaller water schemes. With this serious outbreak of cryptosporidiosis, we clearly need to upgrade all water treatment facilities in the Republic.

It is a reminder that, despite our best efforts, still waters can run into trouble.

Dr Muiris Houston is pleased to hear from readers at mhouston@irish-times.ie but regrets he cannot answer individual medical queries.