The South Dakota Department of Health announced the state�s first confirmed case of West Nile virus (WNV) on Friday, July 8. The case was reported in Brown County.
The state has reported 1,757 cases of WNV and 26 deaths since its first human case in 2002. The following year saw South Dakota�s highest occurrence of WNV, with 1,039 cases reported.
Statistics like those have receded drastically thanks to the efforts of state and local governments, which follow regulated plans to kill mosquitoes and their larva.
�The majority of mosquitoes you�re going to find aren�t going to be carrying West Nile � they�re more of a nuisance mosquito,� said Vermillion Parks Superintendent Aaron Baedke. �But we can get out and try to get them all at the same time.�
This year�s prevention efforts have been complicated somewhat by the ongoing floods and heavy rainfall.
�There�s definitely going to be more mosquitoes,� Baedke said. �The real issue is, Vermillion can only do what we can do. We�re trying to protect people within city limits, but there�s 1,000 times more standing water in the county than there is in town. It�s impossible to kill them all. We�re just trying to make a dent and maybe (kill) the one or two mosquitoes that might infect somebody with West Nile.�
State Epidemiologist Dr. Lon Kightlinger informed residents of the increase WNV risks last week via a press release.
�We do expect more mosquitoes this summer with so many areas affected by flooding and we expect other people to be bitten and infected with the West Nile virus,� he said. �West Nile has already been detected in mosquitoes in neighboring states (Iowa, Nebraska, Wyoming) � and the peak transmission period for the virus is approaching so now is the time to get in the habit of using insect repellent.�
The state assisted communities with their mosquito extermination programs during the first few years of the WNV problem through grant funding, which went toward the purchase of larvicide to put in ditches and kill mosquito larva.
�They�ve kind of graduated us from the grant system, but this year with all the flooding, the state came out and gave a whole bunch of chemicals away,� said David Nelson, director of Parks and Recreation. �They gave all the cities in this area, like Wakonda, Irene, Yankton, Vermillion free chemicals.�
Since 2002 the city of Vermillion has attempted to decrease the mosquito population through a variety of methods, including nighttime fogging on city streets and the continued use of larvicide. Standing water also is treated.
�We also have five traps in town, located in the different zones, so if the larva does turn into adults, then they�re caught in the traps and monitored every day,� Nelson said. �That way we can estimate how many adults are in the different zones.�
The traps � which are located on both city and private property � burn propane and emit a chemical called octanol that mimics mammalian breath.
The mosquitoes are attracted to the trap by this chemical and a vacuum sucks them into a bag, which is emptied each day, at which point the mosquitoes are counted and analyzed in terms of species.
Based on the number of mosquitoes there are, the city will perform another fogging of the streets. One such fogging already has taken place this year, Baedke said.
�The state sets a threshold of how many mosquitoes you want to have, compared to subjecting the whole town to fogging and insecticides,� Nelson said. �At that point, they figure it�s worth the attempt to try and stop it. There�s always going to be bugs, but it�s just how many can you tolerate.�
�(The traps are) by no means going to control the population,� Baedke added. �They�re just out there to give us an idea of what is there.�
Nelson said that even though only one WNV case has been announced this year, the public probably can count on learning of more. September tends to be the worst month in terms of cases, he said.
However, Baedke added that the incubation period for WNV is 2-15 days, and that 80 percent of the people who are infected with it do not get sick.
Less than 1 percent of that number actually get the West Nile disease, he said.
�The chances of knowing you have it are very slim,� he said.
Regardless of this, Nelson said people must do what they can to prevent themselves from being bitten in the first place by limiting how much skin they expose and to use mosquito repellents that include DEET.
Most importantly, they must get rid of standing water near their homes, he said.
Baedke agreed, saying, �We had standing water in Barstow Park after a heavy rain, and it took me two days to pump the water out of there. But in that two days, I was able to do surveillance and find mosquito larva that was just in the grass, just waiting to get wet. Once the water was standing, they hatched.�
Common nuisance mosquito eggs can lie around for years before hatching, Baedke said.
For more information on the prevention of WNV, visit http://doh.sd.gov/westnile/, or visit http://sdces.sdstate.edu/westnile.