Back Other Health News Other Health News Topics Influenza West Nile Virus Outbreak May Be Largest Ever in U.S., Says CDC

West Nile Virus Outbreak May Be Largest Ever in U.S., Says CDC


Based on the number of cases so far this year, 2012 looks like it will have one of the worst outbreaks of mosquito-borne West Nile virus in the U.S. to date, Centers for Disease Control and Prevention (CDC) announced during a media telebriefing this week.

According to new figures posted on the CDC's West Nile virus information web site on August 23, 2012, there have been 1118 cases of West Nile virus infection in human reported in 38 states, 41 of which resulted in death.

Lyle Petersen, director of the CDC's Division of Vector-borne Infectious Diseases, said the number of cases has "risen dramatically in recent weeks," indicating that "we're in the midst of one of the largest West Nile virus outbreaks ever seen in the United States." 

West Nile virus was first identified in Uganda in the 1930s and made its appearance in the U.S. in 1999. The highest ever annual number of reported cases in this country was 9862 in 2003, with 264 deaths. Since then the number has steadily fallen, down to about 700 cases for the full season last year.

But infections are on the rise again, especially in the southern U.S. Nearly half the cases this year have occurred in Texas, with 270 cases and 11 deaths in Dallas County alone. Another 4 states (Louisiana, Mississippi, Oklahoma, and South Dakota) togetheraccount for another 25% of cases, but 47 states have reported detecting West Nile virus infection in either humans, birds, or mosquitoes-- all except Alaska, Hawaii, and Vermont.

The 1118 cases reported so far this year is the highest number ever reported to the CDC through the third week of August. Petersen said it is not clear why there has been more West Nile virus activity in 2012 than in recent years, but the unusually mild winter, early spring, and hot summer in many parts of the country may have played a role.

Late summer is the peak season for West Nile virus. While the CDC had received only 25 case reports in July, that number skyrocketed during August. Petersen explained that it takes a couple weeks before people develop symptoms and get diagnosed, so cases now being reported reflect infections from at least a week ago. Thus, he said, "we expect many more cases to occur and the risk of West Nile virus infection will probably continue through the end of September."

A majority of people infected with West Nile virus will not have symptoms. About 20% experience West Nile fever, characterized by fever, chills, swollen glands, and other flu-like symptoms lasting about a week, though fatigue may last considerably longer. Less than 1% of people develop neuroinvasive disease including meningitis or encephalitis, which can result in cognitive impairment, paralysis, coma, and death.

Petersen said that 629 (56%) of this year's reported cases were classified as neuroinvasive, reflecting that fact that most asymptomatic infections and many cases of West Nile fever are never diagnosed and reported.

There is no known treatment for West Nile virus infection and no available vaccine for humans (though there is one for horses). Once infected, people have lifelong immunity.

Because West Nile virus is a blood-borne infection, all blood donors in the U.S. are screened for the virus. So far this year 242 potentially viremic blood donors have been reported from 26 states. The virus may also be transmitted from mother to child during pregnancy, delivery, or breast-feeding.

The surest way to prevent West Nile virus infection is protection against mosquito bites.

"We encourage the public to use insect repellents when they go outdoors, wear long sleeves and pants during dawn and dusk, install or repair screens on windows and doors, use air conditioning if you have it, empty standing water from items outside of your home such as gutters, flower pots, buckets, kiddie pools, and birdbaths. And support your local mosquito control program," Petersen urged.

The 1118 cases reported thus far in 2012 is the highest number of West Nile virus disease cases reported to CDC through the third week in August since West Nile virus was first detected in the United States in 1999. Approximately 75 percent of the cases have been reported from 5 states (Texas, Mississippi, Louisiana, South Dakota, and Oklahoma) and almost half of all cases have been reported from Texas.

For more information:



Centers for Disease Control and Prevention. CDC Telebriefing on West Nile Virus Update. Transcript. August 22, 2012.

Centers for Disease Control and Prevention. West Nile Virus: What You Need To Know. CDC Fact Sheet. Updated August 7, 2012.