Outbreak of West Nile Virus May Be Worst Ever

August 22, 2012

August 22, 2012 — An outbreak of West Nile virus infections that has claimed more than 3 dozen lives this year could shape up to be worst ever, the US Centers for Disease Control and Prevention (CDC) announced today.

The number of reported cases jumped in just a week's time from 693, as of August 14, to 1118, as of August 21, according to the agency. During that same period, the number of reported deaths rose from 26 to 41.

"We're in the midst of one of the largest outbreaks of West Nile virus ever seen," said Lyle Petersen, MD, MPH, director of the CDC's Division of Vector-Borne Infectious Diseases, in a press briefing today. The number of disease cases reported in 2012 through August 21 is the highest for this time frame since the mosquito-borne virus was first detected in the United States in 1999.

West Nile virus infections typically peak in mid-August, but the CDC expects the case count to continue rising through the end of September because of delays in the cases getting reported, said Dr. Petersen. "It takes a couple of weeks for [an infected] person to get sick, go to the doctor, and get reported."

If the case count stays on its current trajectory through year's end, he said, "this will be among the biggest or the biggest outbreak."

Texas is the epicenter of the outbreak, with 537 reported cases and 19 deaths — nearly half of the nationwide totals. Last week the mayor of hard-hit Dallas declared a state of emergency as a prelude to aerial spraying of insecticide to kill off mosquitos. The only states not to report West Nile virus infections so far are Alaska, Hawaii, and Vermont.

Dr. Petersen said it is not clear why 2012 is a bad year for the West Nile virus, but the heat wave that has scorched much of the country is a likely suspect.

"One observation that has occurred in the United States and elsewhere is that hot weather seems to promote West Nile virus outbreaks," he said. Lab experiments show that higher temperatures increase the transmissibility of the virus through mosquitos.

Fever Cases "Very Underreported"

For the 1118 cases of West Nile virus infection reported through August 21, roughly 44% of patients had fever while the rest contracted neuroinvasive diseases — in particular, encephalitis and meningitis. The death rate for infected individuals with neuroinvasive diseases is 10%. Survivors often struggle with neurological or cognitive problems for years.

The proportion of reported cases with neuroinvasive diseases is misleading because the fever cases "are very underreported," Dr. Petersen said. Only 1 in 150 individuals infected with the virus develop a neuroinvasive disease, but almost all of them end up in the hospital. In contrast, up to 20% of infected individuals experience fever and other symptoms considered milder than neuroinvasive disease, but many of them never bother to consult a clinician. When they do, clinicians may not diagnose a West Nile virus infection. In addition, the CDC does not recommend routine testing for infections presenting with fever.

"The most important data we have is for neuroinvasive-disease cases," Dr. Petersen said. "We do believe they're reasonably reported."

The remaining 80% of individuals who acquire the virus experience no symptoms whatsoever.

So far, public health authorities can battle the outbreak only by spraying mosquitos with insecticide and reminding Americans to take precautions such as using insect repellent and emptying standing water from outdoor buckets and similar places where mosquitos might breed. There is no vaccine against the virus, nor any treatment, other than supportive measures such as hospitalization, intravenous fluids, respiratory support, and prevention of secondary infections for those with more severe forms of the viral infection, according to the CDC.

More information on the West Nile virus outbreak is available on the CDC Web site.


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