Texas records worst outbreak of West Nile virus on record

West Nile virus infections in the United States climbed by about 25% in one week, says the latest report from the Centers for Disease Control and Prevention (CDC). The cumulative number of severe infections reached 1993 and deaths 87.

“That is the highest number of West Nile disease cases reported to CDC through the first week of September, since West Nile virus was first detected in the United States in 1999,” said Lyle Peterson during a telephone news conference with reporters on 5 September. He directs the CDC division of vector-borne infectious diseases.

“We expect this increase to continue for the next several weeks, probably until October,” he said. That is when the mosquito population will begin to die off in cooler weather and the level of new infections should decline.

The most current numbers were that 1069 (54%) of the infections involved neuroinvasive disease and 924 (46%) were non-invasive disease. The summary reflects information submitted to the CDC by 3 am on 4 September.

Peterson anticipates that the numbers will rise further over the next reporting period because Monday 3 September was a national holiday and many agencies reported cases only up until the previous Friday.

“Even if West Nile virus transmission were to stop today, we would continue to see reports of cases for several weeks,” said Peterson. That is because of the time lags between exposure to the mosquito-borne virus, the development of disease, its identification as West Nile infection, and reporting of those infections through state departments of health to the CDC.

Texas continues to be the epicentre of the outbreak, accounting for about 45% of the national caseload.1 But West Nile infections have been identified in all of the 48 continental states and deaths have been reported in more than half of those states.

David Lakey, commissioner for the Texas Department of State Health Services, said, “2012 is now officially our worst year in the State of Texas for West Nile disease. About a fourth of all the cases that have been reported in Texas, we reported last week.”

There have been 1013 cases of disease; 495 cases of neuroinvasive disease; and 40 deaths in Texas, he said, drawing upon more recent data than what were included in the CDC totals. The previous worst year was 2003 when the state reported 439 cases of neuroinvasive disease and 40 deaths.

Initial monitoring suggests that the epidemic may have peaked in north Texas, partly because of aerial spraying of insecticides to control the mosquito population. Areas that received two sequential nights of aerial sprayings saw a 93% decline in mosquito populations that carry West Nile virus. Lakey said, “[In] the areas that did not receive aerial spraying, the number of mosquitoes actually went up.”

The portion of mosquitoes carrying West Nile virus also has declined, at least in some parts of the state. About half of the pools of insects tested positive for the virus in early August. But in north Texas, where most of the subsequent spraying has occurred, that has declined to about 5-6%. However, about 28% of the mosquitoes tested positive for the virus in the Austin area, where little or no spraying has taken place.