West Nile Infections Rise as Outbreak Spreads to Northern States

The worst-ever U.S. outbreak of West Nile virus is spreading across the U.S. with a 25 percent increase in the number of people infected by the mosquito-borne disease, health officials said.

There were 87 deaths among 1,993 cases of the disease reported this season through yesterday, the Centers for Disease Control and Prevention said today. The cases have increased the most in the northern U.S., with 44 states having at least one human infection, said Lyle Petersen, director of the division of vector-borne infectious disease at the CDC.

Health officials said the outbreak may have peaked in August though new reports of infections may occur through October. The outbreak could become the deadliest since the 2002- 2003 season when 260 people died. One factor contributing to the high number of cases may be record-high temperatures, which can make mosquitoes more infectious, said Peterson.

“The temperature may be influencing this year’s outbreak, though many areas that have had record-high temperatures haven’t had outbreaks,” Peterson said. “Temperature is certainly a factor in mosquito-borne outbreaks.”

Of the infections, 1,069 were infections of the nervous system, such as meningitis and encephalitis, that are particularly dangerous, the CDC said. Most cases of West Nile are mild with about one in 150 people infected developing a severe illness, according to the Atlanta-based agency. Only about 2 to 3 percent of people who come down with a fever from the virus are reported to the government, the CDC said last week. Symptoms can range from a high fever and stiff neck to coma and paralysis.

This year’s outbreak is the worst since the disease was first reported in the U.S. in 1999. Almost half of the reported illnesses are in Texas, with the outbreak concentrated in that state and South Dakota, Mississippi, Oklahoma, Louisiana and Michigan.

To contact the reporter on this story: Shannon Pettypiece in New York at spettypiece@bloomberg.net

To contact the editor responsible for this story: Reg Gale at rgale5@bloomberg.net