The number of people who have become ill with West Nile virus disease continues to go up, and the CDC expects the numbers will be high at least through October. As of September 11, 2012, 48 states have reported West Nile virus infections in mosquitos, birds, and people. Only Alaska and Hawaii reported no West Nile virus activity. A total of 2,636 cases of West Nile virus disease in people, including 118 deaths, have been reported to the CDC, which is an overall increase of about 35 percent over the previous week’s numbers. Of the 2,636 cases, 1,405 (53 percent) were classified as neuroinvasive disease (such as meningitis or encephalitis) and 1,231 (47 percent) were classified as non-neuroinvasive disease. For comparison, the numbers reported as of September 4 were 1,993 cases total, with 1,069 cases of neuroinvasive disease, and 87 deaths.
The 1,405 cases of West Nile virus neuroinvasive disease is the highest number of such cases reported to the CDC through the second week of September since West Nile virus was introduced into the United States in 1999. The number of neuroinvasive disease cases is the best indicator of the size of a West Nile epidemic. The 2,636 year-to-date cases are the highest that have been reported through the second week of September for any year since 2003, but this is the most serious outbreak in terms of severe illness since West Nile was first detected in the U.S. in 1999.
West Nile outbreaks in the United States tend to peak in mid- to late August. The longer the weather stays warm, the more transmission occurs. In some areas of the country the outbreak may not have peaked yet. All of the major outbreaks in the United States to date have occurred when the temperature was abnormally warm. In 2002 and 2003, they occurred during periods when the temperature was above normal. When the virus was discovered in 1999 in New York City, it was one of the hottest summers on record.
In almost every area of the United States, the virus is endemic and circulates every year. However, this year two-thirds of all cases have been reported from six states: Texas, Louisiana, South Dakota, Mississippi, Michigan, and Oklahoma. Forty percent of reported cases have originated in Texas. Fifteen people have died from West Nile virus neuroinvasive disease in a 16-square mile area of two zip codes in North Dallas County. The number of deaths in that area reflects the high number of neuroinvasive disease cases that have occurred in Dallas and the surrounding counties. There have been a tremendous number of cases of severe disease in that area.
Why the outbreak has been so intense in the Dallas area is still a matter of speculation. West Nile virus outbreaks are highly focal. They depend on a number of ecological factors which are hard to measure, and which are influenced by weather. The CDC will continue to investigate what factors seemed to precipitate this outbreak, particularly in the Dallas area. The CDC will also investigate the effect of the control operations done in Dallas. Part of the area was subjected to aerial spraying with insecticides to reduce the number of vector mosquitos. In some sprayed areas the number of vector mosquitos went down by 93 percent.
One of the important aspects regarding outbreaks of West Nile is weather. Temperature and rainfall affect a lot of the factors causing the outbreaks such as the number of mosquitos, the number of birds, how these birds interact with mosquitos, the lifespan of the mosquito, the speed at which the virus replicates in the mosquito and becomes more transmissible from a mosquito. Mosquitos become infectious faster so there's a very complicated relationship of temperature and rainfall to all of these factors that are involved in West Nile virus transmission. As the CDC gathers more data, it may be able to get a better idea about what are the precipitating factors that actually cause outbreaks.
See the CDC teleconference transcript