Oklahoma County has the highest concentration of West Nile virus infections in the Oklahoma City metro, and Carter County has the highest concentration statewide. But the state epidemiologist says there are too many factors at play to know why some areas are worse than others.
Weather, vegetation, bird populations and human population density are among factors that influence where West Nile will be most widespread. The number of factors is high enough, and variance within those factors broad enough, that West Nile can spread unpredictably.
“It causes challenges in predicting from one year to the next if this is going to be a more severe season than we expected, and also where to target our surveillance and our mosquito control efforts,” said Kristy Bradley, the state Health Department epidemiologist.
The Health Department had reported 150 confirmed cases of human West Nile infection in Oklahoma as of Tuesday, which represents about four infections per 100,000 state residents. Eighty-seven people have been hospitalized, and eight have died.
Carter County has the highest infection rate in the state by far, no matter how large the sample size, according to the health department. Fifteen residents of Carter County, which features Ardmore as the largest city, have been infected by the virus, representing nearly 32 infections per 100,000 county residents.
Oklahoma County has 50 confirmed cases, which along with Pottawatomie County's five infections represents about seven per 100,000 residents. The rate drops to about five infections per 100,000 residents in Logan County and about two per 100,000 residents in Cleveland and Canadian counties.
Tulsa County's infection rate of about five people per 100,000 residents is only slightly higher than the state average. Most other counties with high rates of infections have a comparatively small sample size, with the exception of Pittsburg County's 10 infections and a rate of about 22 cases per 100,000 residents.
Southern areas are worst
Pittsburg and Carter counties are in southern Oklahoma, which along with North Texas has the highest West Nile infection rates in the region. Infection rates are generally more isolated this year in central and northern Oklahoma.
Illustrating the unpredictable nature of West Nile infections, previous outbreaks have had their highest impact in other parts of the state, Bradley said. There's no telling where the next one will occur.
But the presence of mosquitoes, or even the density of local mosquito populations, doesn't necessarily have an effect on whether the virus will be concentrated in a given area. A high rate of infection reflects a high rate of infected mosquitoes, not a particularly large localized mosquito population.
Once about 20 percent of an area's mosquitoes have been infected with the virus, a tipping point has been reached.
“That's what really tips the scales and translates into a high transmission rate to people and to horses and other animals that we would call incidental hosts in this cycle of transmission,” Bradley said.
The virus is transmitted to mosquitoes by birds, and to people and other animals by the mosquitoes. An infected person cannot spread the disease to other people, and more than 80 percent of people bitten by an infected mosquito don't contract the virus.
The cyclical virus peaks every few years. Oklahoma, which set a new state record of confirmed cases this year, had 107 infections and nine deaths when the virus spread in 2007.