West Nile Virus Outbreak Worst in Years

This year’s outbreak of West Nile virus has been particularly severe in Texas, which has been host to almost half of all cases.

More cases of West Nile virus have been reported so far in 2012 than at this point in any year since the virus was first detected in the United States in 1999, according to the Centers for Disease Control and Prevention (CDC). At this rate, the total number of infections for the year will most likely meet or exceed those of 2002 and 2003, in each of which more than 260 patients died of the disease.

As of August 28, 2012, there have been 1590 cases of the virus reported, of which 889 have been classified as neuroinvasive cases (including meningitis, encephalitis, and acute paralysis), and there have been 66 deaths. So far, 70% of the cases have been reported in 6 states—Texas, South Dakota, Mississippi, Oklahoma, Louisiana, and Michigan—and 45% have been reported in Texas alone. However, West Nile virus infections in people, birds, or mosquitoes have been reported in every state except Alaska and Hawaii, and infections have been reported in people in 43 states.

Infections with West Nile virus tend to peak in mid-August, but there is generally a lag in reporting of cases, which have to make their way from local clinics to state health officials to the CDC. In addition, the agency estimates that just 2% to 3% of all non-neuroinvasive West Nile virus (also referred to as West Nile fever) are diagnosed and reported to the CDC as there is no specific treatment for the condition.

Rates of West Nile virus infection had trended downwards since 2003 until this year. In a conference call with reporters, Lyle Petersen, MD, MPH, the director of the CDC’s division of vector-borne infectious diseases, said that the reason for this year’s outbreak is unknown, although the summer heat wave in some parts of the country may have played a role. (The outbreaks in 2002 and 2003 coincided with heat waves, but other heat waves have not accompanied outbreaks.) In addition, Dr. Petersen said that Hurricane Isaac is unlikely to have a significant impact on rates of West Nile virus infection.

On the conference call, Dr. Petersen noted that West Nile virus is endemic throughout most of the United States, so everyone should take precautions to avoid mosquito bites and eliminate standing water where mosquitoes breed. The virus is transmitted from birds to people via mosquitoes. For more information on prevention, symptoms, and spread of West Nile virus, click here for a fact sheet from the CDC.