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West Nile virus outbreak may be worst in US history

Robin Wulffson MD's picture
West Nile virus, mosquitos, outbreak, encephalitis, meningitis, death

On August 22, the Centers for Disease Control and Prevention (CDC) reported that this year, an outbreak of West Nile virus has claimed 41 deaths. To date for 2012, 47 states have reported West Nile virus infections in people, birds, or mosquitoes. A total of 1,118 cases of West Nile virus disease, including the 41 deaths, have been reported to CDC. Of these, 629 (56%) were classified as neuroinvasive disease (i.e., meningitis or encephalitis) and 489 (44%) were classified as non-neuroinvasive disease. The CDC notes that this year’s outbreak might become the worst in US history.

The number of reported cases soared in just a week’s time from 693, as of August 14, to 1,118, as of August 21. The 1,118 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 US in 1999. Approximately 75% of the cases have been reported from five states: Texas, Mississippi, Louisiana, South Dakota, and Oklahoma. Furthermore, almost half of all cases have been reported from Texas: 537 reported cases and 19 deaths. Last week the mayor of Dallas, which has been impacted by the disease, declared a state of emergency as a prelude to aerial spraying of insecticide to kill off mosquitos. To date, the only states not to report West Nile virus infections so far are Alaska, Hawaii, and Vermont.

“We’re in the midst of one of the largest outbreaks of West Nile virus ever seen,” noted Lyle Petersen, MD, MPH, director of the CDC’s Division of Vector-Borne Infectious Diseases, in a press briefing. He explained that West Nile virus infections typically peak in mid-August; however, the CDC expects the case count to continue rising through the end of September because of delays in the cases getting reported. He noted that it usually takes a couple of weeks for an infected individual to get sick, go to the doctor, and get reported.

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Dr. Petersen explained that is not clear why 2012 is such a bad year for the virus; however, he notes that the heat wave that has scorched much of the nation is a likely suspect. He noted that hot weather seems to promote West Nile virus outbreaks and lab experiments have found that higher temperatures increase the transmissibility of the virus through mosquitos.

The neuroinvasive form of the disease is the more serious because it results in encephalitis and meningitis. The death rate for infected individuals with neuroinvasive diseases is 10%; furthermore, those that survive often suffer from neurological or cognitive problems for years. Dr. Peterson noted that the proportion of reported cases with neuroinvasive diseases is misleading because the fever cases are extremely underreported. Only one in 150 individuals infected with the virus develop a neuroinvasive disease; however, almost all of them require hospitalization. In contrast, up to 20% of infected individuals experience fever and other symptoms considered milder than neuroinvasive disease; however, many of them never bother to consult a physician. When they do, the doctor may not diagnose a West Nile virus infection. Furthermore, the CDC does not recommend routine testing for infections presenting with fever.

To date, public health authorities can battle the outbreak only by spraying mosquitos with insecticide and reminding people to take precautions such as using insect repellent and emptying standing water from outdoor buckets and similar places where mosquitos might breed. There is neither a vaccine nor a treatment for the virus. Infected individuals are cared for with supportive measures such as hospitalization, intravenous fluids, respiratory support, and prevention of secondary infections.

Reference: CDC