Texas West Nile outbreak now worst in history

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The outbreak of West Nile disease in Texas this summer has become the most active and lethal in the state's history, the state health commissioner said Wednesday.

Dr. David Lakey, commissioner of the Texas Department of State Health Services, said the state's 1,013 reported cases more than double the previous high. Its 40 reported deaths tie the previous high, Lakey said in a telephone news briefing.

More cases and deaths have been recorded at the local level but not yet reported to the state.

"As of this week, 2012 is now officially our worst year in the state of Texas for West Nile," Lakey said during the briefing held by U.S. health officials.

More Information Safety tips To reduce exposure to the West Nile virus, public health officials urge: 1 Use repellent. 1 Wear long sleeves and pants at dawn and dusk when mosquitoes are most active. 1 Make sure all doors and windows have screens. 1 Regularly drain any standing water.

Neither Lakey nor a leading U.S. Centers for Disease Control and Prevention official could explain why Texas has been so hard hit. Dr. Kyle Petersen, a West Nile specialist for the CDC, said the agency would look carefully at that question at the end of the season.

Lakey said state health officials are seeing signs that the mosquito-borne outbreak may have peaked in the northern part of Texas, the area by far the hardest hit. But he added that the number of cases and deaths in the state will keep rising at least into October.

439 cases in 2003

The state's previous worst year was 2003, when 439 people were diagnosed with West Nile disease. This year, 495 people already have been diagnosed with neuroinvasive West Nile, the severe form of the disease that affects the brain and can cause paralysis, coma and death. Neuroinvasive West Nile only affects 1 in 150 people infected with the virus, and most non-neuroinvasive cases are not reported.

Harris County has reported 38 cases, 22 of them neuroinvasive and three resulting in deaths. Nineteen cases have come in the last 10 days, the Houston area's peak period historically.

Petersen said Wednesday that the 1,993 cases of disease is the highest number reported to the CDC through the first week in September since the virus was first detected in the U.S. in 1999.

But the number would appear to have to continue to be reported at record levels the rest of the year to rival the nation's all-time high of 9,862, reported in 2003. The season typically lasts into October, though reporting can continue thereafter. The CDC numbers lag behind the latest state and local numbers.

The year's 1,993 cases represent an increase of 25 percent from the previous week. Petersen said that number is lower than last week's 40 percent increase because the Labor Day holiday shortened states' reporting period.

Spraying has helped

Three North Texas counties - Dallas, Tarrant and Denton - account for 719 of the state's cases, according to those counties' websites. Lakey said those numbers aren't increasing as quickly, however, an improvement he attributed to aerial spraying of insecticides.

Lakey said the state's analysis found the infected mosquito population in areas of Dallas sprayed on consecutive nights decreased 93 percent; the infected mosquito population in areas not sprayed increased 10 to 20 percent.

In Harris County, the mosquito population in areas sprayed by air in mid-August decreased 82 percent.

The state's 1,013 cases include 89 found in donated blood by people without symptoms.

About 80 percent of people infected with the virus develop no or few symptoms. One in five develops mild symptoms such as headache, joint pain, fever, skin rash and swollen lymph glands.

No vaccines exist to prevent West Nile infections, and no medications have been developed to treat it.

Infected mosquitoes spread the virus from birds to people, most commonly between June and October.

todd.ackerman@chron.com