New West Nile virus strain may worsen epidemic

By Maggie Fox, Health and Science Editor

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."