September 5, 2012 — In what is shaping up as the worst nationwide outbreak of West Nile virus, mosquitos might want to heed a familiar warning: Don't mess with Texas.
Two consecutive nights of aerial pesticide spraying last week in some stretches of Dallas and its suburbs reduced the number of mosquitos carrying the virus by 93%, David Lakey, MD, commissioner of the Texas Department of State Health Services, said today in a press briefing held by the Centers for Disease Control and Prevention (CDC).
Successful eradication of the disease-bearing insect was good news in an otherwise dismal update of the outbreak provided by the CDC and Texas, the state with the most infections and deaths.
The number of reported cases of infection nationwide from August 28 to September 4 rose from 1590 to 1993, while the number of deaths increased from 66 to 87, said Lyle Petersen, MD, MPH, director of the CDC's Division of Vector-Borne Infectious Diseases. The case count through September 4 is the highest for any year since West Nile virus was first detected in the United States in 1999. The record for the most cases during an entire calendar year is 9862 in 2003, while the highest death count was 284 in 2002.
The tallies for infections and deaths as of September 4 might have been higher except for the Labor Day holiday on Monday, which shut down state health departments and their reporting. "We may see a bigger jump [in cases] next week from cases that didn't make it into the numbers this week," said Dr. Petersen.
August is historically the peak month for West Nile virus infections. Mosquitos will continue to transmit the virus through October, Dr. Petersen said. However, even if virus transmission stopped today, the number of cases would continue to snowball because of the lag time between getting bitten by an infected mosquito and seeking medical attention once symptoms emerge, according to Dr. Petersen.
So far, Texas accounts for about 45% of the reported infections and 40% of the deaths. The only states not reporting any cases are Alaska, Hawaii, Maine, Rhode Island, Vermont, and Washington.
The total number of West Nile virus infections goes grossly underreported because 80% of infected individuals do not experience the usual symptoms, which include fever, headache, and nausea, according to the CDC. Symptomatic individuals often do not consult a physician, or their condition is misdiagnosed or not tested. Only 1 in 150 infected individuals typically develop a neuroinvasive disease such as meningitis or encephalitis, and their mortality rate is roughly 10%. As of September 4, nearly 54% of the reported cases fell into the neuroinvasive category.
Dr. Petersen said there is no evidence to suggest that a higher percentage of infected patients are succumbing to the more deadly neuroinvasive version of the disease in this year's outbreak.
Far Fewer Mosquito Pools in Northern Texas Harbor the Virus
With roughly 4 months still left in the year, 2012 already has set the record for the worst outbreak of West Nile disease in Texas, according to Dr. Lakey. Citing state numbers more up-to-date than those in the CDC counts, he said that the 495 cases of West Nile neuroinvasive disease — a bellwether statistic — as of September 4 exceeds the 439 cases in all of 2003, which had been the record year. In addition, the 40 deaths so far match the death count in 2003.
Although the number of reported infections and deaths will continue to increase, transmission of the virus could be peaking in some parts of the state, especially in northern Texas, which includes the hard-hit city of Dallas, Dr. Lakey said.
Texas is not just letting the outbreak take its course. Public health authorities there have responded with mosquito pesticide spraying, both on the ground and from the air. The air war has produced the most success. Last week, in areas of Dallas County and neighboring Denton County, where the weather permitted 2 consecutive nights of aerial spraying, the number of virus-carrying mosquitos decreased by 93%, according to Dr. Lakey.
On another positive note, the percentage of mosquito-breeding pools of water in northern Texas that tested positive for the virus has fallen from roughly 50% in late July and early August to roughly 5% to 6% in September. However, 28% of mosquito pools in the Austin area continue to harbor the virus.
More information on the national outbreak of the West Nile virus is available on the CDC Web site.
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