SIOUX FALLS, S.D. (AP) - Officials have found that controlling West Nile virus is as much about educating the public as it is killing mosquitoes.
South Dakota, with the nation's highest rate of serious infections, is entering its seventh summer of the sometimes fatal virus that is transmitted by mosquitoes from infected birds to humans.
Not every mosquito species transmits the virus. Early in spring it's a nuisance mosquito called Ades vexans that's bothering people in the eastern part of the state but seldom carries the virus. It's first to show up because eggs laid in fall hatch in spring.
"Just add water and poof, you've got mosquitoes," said Mike Hildreth, a professor and researcher at South Dakota State University.
But the real threat of West Nile virus comes later in summer from the Culex tarsalis mosquito, which lays eggs in the spring and needs to repopulate and find infected birds before it turns its attention to humans.
That's why the number of human West Nile cases generally peaks in the second week of August and why public policy and personal precautions often are driven by the less-dangerous nuisance mosquito that shows up earlier.
"Last year, with the dry summer, what we were hearing from people is, `There's no mosquito problem, why are you spraying?', and yet the tarsalis, which handles the drier conditions much, much better, was in plentiful numbers," Hildreth said.
South Dakota had 208 human infections and six deaths last year. Twenty-six people have died in South Dakota from West Nile since it first appeared in 2002. More than 300 have developed the serious neuroinvasive complications of encephalitis and meningitis.
The virus already is circulating in some states. Human cases have been reported in Arizona, Mississippi and Tennessee, said Lon Kightlinger, epidemiologist in the state Health Department.
South Dakota's per-capita rate of serious complications from West Nile is the highest in the nation. Wyoming ranks second, Nebraska third and North Dakota fourth, Kightlinger said.
"For some reason the upper Great Plains is a good home for that Culex tarsalis mosquito," Kightlinger said.
"We're cooped up all winter here ... and people enjoy their summers (outdoors) right at the apex of the West Nile season."
Hildreth and other researchers are trying to piece together how precipitation and temperature can influence populations of the virus-carrying mosquito so a town can get the most out its mosquito control efforts by focusing on the virus carriers.
"They have a tough job in that they have to make the (mosquito control) decisions that will affect their community and they oftentimes have to go against the people's perceptions," Hildreth said.
Two years ago, the Culex tarsalis population was down but the percentage infected with West Nile virus increased.
"So in that particular year we should have seen very few human cases, and yet we saw a surprisingly large number considering how dry things were," Hildreth said. "And I think it's because people are just not taking any precautions when it's dry and they're saying, `There's no problem."'
The state Health Department has $500,000 available this year for local mosquito control programs but has changed the distribution formula. Rather than a 50-50 matching grant, towns will receive funding that takes into account their history of West Nile cases and population.