Jim Gathany / Centers for Disesase Control via EPA
A Culex quinquefaciatus female mosquito feeds on human blood. This species is a known vector for West Nile Virus.
The number of U.S. cases of West Nile virus rose by 19 percent in the latest week, slowing slightly from its earlier pace but still putting this year on track to be the worst outbreak since 2003, government figures showed on Wednesday.
So far this year, 3,142 cases have been reported to federal health officials as of Sept. 18, up from 2,636 reported the week before, the Centers for Disease Control and Prevention said in its weekly update of outbreak data.
While still a sharp rise, the rate has slowed somewhat. Earlier this month, the number of cases jumped by more than 30 percent in one week. CDC officials have said they believe the peak for new infection was likely in August, and recent spike were the result of the lag time between infection and states reporting cases.
Almost 40 percent of all cases have been reported in Texas. Other states with large number of cases include Mississippi, Michigan, South Dakota, Louisiana, Oklahoma, and California.
A total of 134 people have now died from the disease, compared with 118 reported one week ago.
Of the cases reported to the CDC this year, just over half are the severe neuroinvasive form of the disease, which can lead to meningitis and encephalitis.
The milder form of the disease causes flu-like symptoms and is rarely lethal. The disease is thought to have originated in Africa and was first detected in New York City in 1999.
West Nile outbreaks tend to be unpredictable. Hot temperatures, rainfall amounts and ecological factors such as the bird and mosquito populations have to align just right to trigger an outbreak like the one this year.
The CDC said the number of cases this year is the highest reported to federal health officials through the third week in September since 2003, the year with the most cases.