New West Nile virus strain may worsen epidemic

A mosquito is about to begin feeding in this 2005 file photo.

WASHINGTON (Reuters) - A new strain of West Nile virus is spreading better and earlier across the United States, and may thrive in hot American summers, researchers said on Thursday.

The virus infected an estimated 175,000 people last year, the U.S. Centers for Disease Control and Prevention reported in its weekly report on death and disease on Thursday.

The mosquito-borne virus caused an estimated 35,000 cases of fever, was reported to have killed 117 people and caused serious disease such as encephalitis and meningitis in 1,227 people in 2007, the CDC reported.

A second team of researchers said a new strain of the virus that has completely overtaken the original strain is particularly well suited to hotter weather -- which in turn means West Nile outbreaks may worsen in the north.

It also means that North America may suffer more from West Nile virus than other parts of the world, said Lyle Petersen, who helps lead West Nile surveillance at the CDC.

West Nile was introduced to the United States in 1999 -- during a particularly hot summer in New York City.

"In Europe, Africa and West Asia, where the virus was previously endemic, you'd see these big outbreaks and then they'd kind of disappear and then not come back for years on end," Petersen said in a telephone interview.

"What we have seen in the United States, we've had repeated outbreaks every single year since 2002 -- in fact, big outbreaks. This is an unusual pattern that not been seen before."

Hot American summers may be to blame, said Petersen and Marm Kilpatrick of the Consortium for Conservation Medicine and the University of California Santa Cruz.

FASTER AND WARMER

Writing in the Public Library of Science journal PLoS Pathogens earlier this week, Kilpatrick and colleagues said they showed the new strain, first seen in 2002, replicates faster in the bodies of mosquitoes when it is warm.

"The warmer the temperature, the faster it replicates in mosquitoes and the faster the mosquito can transmit the virus," Kilpatrick said in a telephone interview.

"It also indicates that increases in temperatures due to global climate change would have major effects on transmission of the virus."

West Nile virus infects birds, and it can spread to people via mosquitoes that bite both.

Petersen and Kilpatrick said it is known that mosquitoes transmit all sorts of diseases more efficiently when it is warm.

Kilpatrick and colleagues, working in a lab, showed the new, 2002 strain of West Nile, does particularly well in warmer temperatures.

The new strain appears to have evolved naturally, said Petersen. "We can no longer find the 1999 strain. It's pretty dramatic," he said.

The Kilpatrick findings fit in with what the CDC has seen, Petersen added.

"What we observed is, at least in temperate climates, these big West Nile Virus outbreaks tend to occur in heat waves," Petersen added.

Petersen said it is too soon to show any links between climate change and West Nile Virus, however.

If it gets too warm, mosquitoes die sooner, before they can spread the infection, Kilpatrick said. So in southern states the new strain may not have an advantage. But in the northern states and Canada, hot summers could make a big difference, he said.

"It is probably going to push the northern boundary farther north," Kilpatrick said.