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.