Risk of West Nile grows for Utahns

The population of Culex tarsalis mosquitoes — the ones that carry West Nile virus in Utah — is starting to decrease slightly, but the chances of being bitten and possibly infected is actually greater now because more mosquitoes are infected.

"There couldn't be a worse time for people to be bitten by mosquitoes as right now," Sam Dickson, Salt Lake City Mosquito Abatement District manager, said Monday.

The Utah Department reports that the virus has been detected in nine Utah counties, including Box Elder, Cache, Davis, Duchesne, Emery, Grand, Salt Lake, Tooele and Washington. Last week, Utah County officials also said they had detected infected mosquitoes, but that's not yet reflected in the state's count.

That count, through July 27, includes two humans, one horse, five sentinel chickens and 28 mosquito pools (22 of them in Salt Lake County) infected with the virus.

The answer to the seeming contradiction between dwindling population and chance of being bitten by an infected mosquito lies in the habits and life cycle of the mosquito itself.

About mid-September each year, adult female C. tarsalis stop biting because they need a blood meal, and they start eating nectar to built up fat so they can overwinter. Then all they have to do is come out of hibernation the next spring, have a blood meal and start laying eggs. This year, that stage began in mid-April.

C. tarsalis lays its eggs on top of water, so the largest populations in early spring were on the wetlands northwest of the Great Salt Lake. Those eggs hatched within hours, but it took the hatchlings a couple of weeks to become adults, Dickson said. Treatments can kill the mosquitoes, but not their eggs, so the day after mosquitoes are killed, new ones may be hatching.

The hotter it gets, the shorter the time it takes to go from egg to adult. By now, it's down to a seven-day cycle, Dickson said.

The wetlands are also home in early spring to young birds that are easy pickings for the mosquitoes craving blood, since the birds are in nests, featherless and home-bound. They become the reservoir for the virus, perpetuating the cycle.

Around July 24, the C. Tarsalis mosquito population peaks. Around then, too, the marshes are drying up, and since the young birds now have feathers and new aviation skills, they fly off, depriving the mosquitoes of their handy food source.