Calcutta/Krishnagar, May 7: The mystery fever that has claimed three lives in Nadia is being blamed on a rare virus that is spread by pigs and bats and killed 49 people in Siliguri six years ago.

The Nipah virus, discovered in Malaysia in 1999, has since been known to strike only two other places, Bangladesh and Bengal, causing epidemics in both.

The Nadia infection, which now afflicts at least 30 people, was first thought to be a new strain of dengue. But blood samples of the three dead patients have tested positive for Nipah at the National Institute of Virology, Pune.

The wife of one of the three dead, Nityagopal Sarkar, confirmed that bats are common in her Krishnagar locality.

“They even enter our home and bite us, but we never bothered about it before,” Sephali Sarkar said. She has a temperature and is undergoing regular check-ups at the district hospital.

“Fruit bats are its natural hosts,” a virologist said. “The bats can infect pigs and both can infect humans. Transmission requires close contact with infected tissues or body fluids.”

“We have asked families to cut the leaves of fruit plants and stop eating pork,” said Mohan Basu, deputy chief medical officer of heath, Nadia.

All the 30 patients may not have the Nipah virus, though. The hitherto unexplained fever had afflicted 50 people since February but some 15 have recovered and two of the five dead actually died of encephalitis.

No medicine has been found to work against Nipah, which spreads rapidly and has a high mortality rate. It killed 105 of the 265 patients in Malaysia, 18 of 30 in Bangladesh in April 2004, and three-fourths of the 66 it infected in Siliguri in 2001.

In Siliguri, most patients were either hospital employees or visitors, suggesting human-to-human spread is a possibility. Virologists said domestic cats and dogs are suspected carriers, too.

Death can occur in 48-72 hours. The symptoms are high temperature, muscle pain, nausea and convulsions leading in some cases to a coma.