Wanted, dead or alive: Mosquitoes that could carry West Nile virus

Minnesota has 50 species, and all are suspects

Associated Press

ST. PAUL -- Fifty species of mosquitoes call Minnesota home, and scientists are trying to crack the mystery of which ones are killers.

As the mosquito-borne West Nile virus spreads across the state, researchers are working to determine which types are passing it along to birds and mammals, leaving dead crows and infected horses in their wake.

Research is being done several places, including the Minnesota Department of Health, the University of Minnesota and the Metropolitan Mosquito Control District, which has been battling mosquitoes in the seven-county metro area for 44 years. That's left it positioned to help narrow the list of suspects to the handful of species believed to be transmitting the disease.


That research is important because it will allow scientists to target control efforts to specific mosquito species, time periods and breeding spots, lessening the ecological damage that could be caused by widespread spraying.

"We'd like to make the West Nile prevention effort as efficient as possible," said Kirk Johnson, vector ecologist for the district.

To find out which species have the virus, district employees have been placing traps, essentially vacuum cleaner-like devices, in key areas frequented by mosquitoes.

Workers bring trapped mosquitoes back to the office and freeze them. Technicians using a microscope and tweezers sort them by species and send them to the state Health Department for testing.

That agency, which is collecting its own samples outside the metro area, hasn't found any with the virus yet.

"We've barely started to scratch the surface," said Dr. Kirk Smith, state public health veterinarian. Laboratories have focused so far on bird, horse and human samples. With a growing backlog of mosquito samples, the agency will be busy testing those for many more months, he said.

Even without a positive test result in hand, Johnson, Smith and others already have begun forming opinions on the most likely agents.

This year, about 200 horses have been infected in Minnesota, and authorities believe the number could easily pass 300.


That leads scientists such as district entomologist Sandy Brogren to believe one or more culprits will have common characteristics, such as being plentiful throughout the state and being attracted to large mammals.

That seemingly lets a lot of mosquitoes off the hook. Many, for example, peak early in the year or hang out in isolated spots.

But Johnson said transmission is a much more complex issue. Mosquitoes that bite only birds, for example, can keep the disease alive in the wild while others that bite birds and mammals can spread it to horses and people.

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