Mosquito sample tests positive for West Nile virus in Minnesota

The Metropolitan Mosquito Control District (MMCD) has found its first mosquitoes to test positive for West Nile virus this year in samples collected June 17-18 in Oak Grove in northern Anoka County.

“This is a bit earlier than normal,” said Alex Carlson, an MMCD spokesman, adding that the first positive samples are not usually found until the first week of July.

No cases of the mosquito-borne illness, which can cause fever, headache, body aches, vomiting or rash, have been reported in the state as of July 1, said Andrea Ahneman, of the Minnesota Department of Health

The Culex species of mosquitoes are the local carriers of West Nile virus. Three varieties are most common in Minnesota, the pipiens, restuans and tarsalis, Carlson said. While the pipiens and restuans are most abundant of the three, they don’t bite humans as often as the tarsalis, Carlson said.

The tarsalis was found in only 51 pools all of last year, when it was much drier than in 2024. Last week, MMCD found tarsalis in 22 pools during sampling, said MMCD vector ecologist Kirk Johnson.

“This is just the beginning of the season for tarsalis and with the amount of standing water we now have, expect their numbers to grow considerably over the next couple of generations,” he said.

West Nile virus risk tends to increase during July, August and September because the Culex species is more active in late summer and has had more time to pick up the disease from an infected bird and spread it to humans, Carlson said.