Why so few West Nile Virus cases this year?
SIOUX FALLS, S.D. — At this point last year, hundreds of residents in the Upper Midwest had been sickened by West Nile Virus. Several stricken by the illness had died.
Then came 2019. If there's a bright spot to the frigid winter and this year's cool, rainy spring and summer, it's this: The mosquito that infects people with West Nile Virus hates those conditions.
The species, Culex tarsalis, is bad at surviving tough winters and, in the warmer parts of the year, loves stagnant water and heat. Minnesota, North Dakota and South Dakota weren't the most friendly places for the West Nile Virus mosquito in 2019.
"I think we really did luck out this year," said Josh Clayton, South Dakota state epidemiologist. "We'll have to see what next year brings, because no year is the same. "
The virus-carrying mosquito loved conditions in 2018, so the year-over-year comparison to 2019 is misleadingly dramatic. But this year, with mosquito season coming to a close, nobody in South Dakota, North Dakota or Minnesota has died from a West Nile Virus infection.
As of Tuesday, the federal Centers for Disease Control and Prevention reported only 23 cases across the three states, with the bulk split between North and South Dakota. Last year, the states combined for 436 cases, 160 hospitalizations and eight deaths.
All three states' departments of health release regularly updated numbers on West Nile Virus cases in each state, and the number of cases in each state tends to vary a lot each year. One big factor, said Jenna Bjork, vector-borne disease epidemiologist with the Minnesota Department of Health, is weather.
"Climate does have a big impact on our numbers from one year to the next," she said. "Typically, the worst years we have, when we have higher case numbers, tend to be hot and dry years."
There are many factors for why West Nile Virus might hit humans hard any particular year, cautioned Michelle Dethloff, program manager with the Division of Disease Control at the North Dakota Department of Health. She hesitated to credit any one factor for the lower number of cases this year.
Mosquitoes carry the disease from infected birds — the reservoir for the virus — to humans. Climate conditions that affect birds and mosquitoes will affect how many mosquitoes could potentially pass on the disease.
Then there are human factors. When humans properly protect themselves from mosquito bites — using bug sprays, wearing protective clothing and eliminating mosquito breeding grounds — infection rates will fall.
Dethloff cautioned that every year, North Dakota, like many other states, sees cases of West Nile Virus among its residents. A quiet year shouldn't be an excuse to grow complacent about the risk.
"It's very important for people to be aware of West Nile and always take those precautions," Dethloff said. "Even when there are less cases, you can be the one who gets it, so it's not something to take lightly."
West Nile Virus regional statistics
- Minnesota: 63 cases, 2 fatalities
- South Dakota: 169 cases, 4 fatalities
- North Dakota: 204 cases, 2 fatalities
2019 (through Oct. 15)
- Minnesota: 3 cases, no fatalities
- South Dakota: 10 cases, no fatalities
- North Dakota: 8 cases, no fatalities
Sources: South Dakota Department of Health, Minnesota Department of Health, North Dakota Department of Health, U.S. Centers for Disease Control