West Nile virus cases jumped 35 percent in past week, but outbreak may have peaked, CDC says

By Kay Lazar, Globe Staff

The reported number of people infected with West Nile virus nationwide has jumped 35 percent in the past week, and the numbers are expected to climb until at least October in the midst of the deadliest outbreak of the mosquito-borne disease this country has witnessed, federal health officials said Wednesday.

All told, 2,636 cases of West Nile, including 118 deaths, have been recorded, said Dr. Lyle R. Petersen, director of the Division of Vector-Borne Infectious Diseases at the federal Centers for Disease Control and Prevention.

Thirteen of those cases -- and one of those deaths -- are from Massachusetts.

But Petersen said that, based on historical data, health officials are hopeful that the nation has “turned the corner” on the epidemic for this season.

“We are hopeful the worst of the outbreak is behind us,” Petersen said during a national teleconference with reporters.

He cautioned that if the weather continues to stay unusually warm in various areas of the country, then the virus will not have peaked in those regions. Mosquitoes tend to turn more sluggish as the weather gets chillier and die in the first hard frost. But Petersen noted that even Massachusetts has reported isolated cases of infections as late as November, and urged the public to continue to take precautions against being bitten, including use of insect repellant, covering up exposed skin, and avoiding outdoor activities between dusk and dawn when mosquitoes are most active.

He said that based on the 13 years the virus has been detected in the United States, the number of new infections typically peak in late August. But the number of cases reported to health officials will likely climb for several more weeks, he said, because there is a lag time between infection and when a person becomes sick, seeks medical care, and is diagnosed with the illness.

Slightly more than half of the infections nationwide, roughly 53 percent, developed into serious illness, such as meningitis or encephalitis, health officials said.

“We ... believe this year’s outbreak is the largest to date and certainly the most serious,” Petersen said.

Two-thirds of the cases nationwide have been reported from just six states: Texas, Louisiana, South Dakota, Mississippi, Michigan, and Oklahoma, and 40 percent of all cases have been reported from Texas.

Federal health officials are uncertain what fueled this year’s deadly outbreak of the virus and plan to study weather patterns, including temperature and rainfall amounts.