Hantavirus pulmonary syndrome (HPS) - a lung infection which causes eye pain, haemorrhaging in the lungs and other side effects - first appeared in the USA in 1993 after the 1991/2 El Nino.

The disease was first identified in people living in Utah, New Mexico, Colorado and Arizona - known as the Four Corners region because they intersect each other, but has since spread around the US.

Around five cases of the pulmonary disease were noted then, but since 1995 there have only been around four cases a year in the Four Corners region.

However, scientists from the University of New Mexico report in the week's Lancet, that they have seen 14 cases between January and August 1998 which they think are linked to the El Nino of 1997/98.

They say the new cases have all been recognised as HPS by the US Public Health Service.

Eight are men and six are women. The average age of patients is 35. Five people have died.

Climate change

The scientists say their studies show the impact of climate changes on the disease.

The milder winters and increased rainfall brought by El Nino, a weather phenomenon which occurs every two to seven years, caused the deer mouse population to increase in the US and made them more likely to nest in buildings used by humans.

The hantavirus is carried by mice

Deer mice carry the hantavirus which causes HPS. It is transmitted by breathing in dust from mice faeces and may also be passed on through mouse bites.

People living in poor housing or in agricultural areas are thought to be more at risk.

The scientists say: "Preliminary analysis would suggest that even in the face of considerable public education, the incidence of considerable public education, the incidence of HPS might still be linked to ecological events that are not subject to control by human activities."

Kidney pain

HPS, which has a mortality rate of up to 40% in the USA, is linked to haemorraghic fever with renal syndrome (HFRS).

This has a sudden onset. Symptoms include headaches, fever, backache and chills. It can lead to haemorrhaging, kidney pain, vomiting and death in up to 10% of cases, depending on the severity of symptoms.

It was first noted in the West in 1951 when US troops serving in Korea became infected.

There are up to 200,000 cases a year, most of them in the Far East. Half are in China.

However, there have been cases in France, Hungary, Greece, the former Yugoslavia, Japan and Sweden.