[Mosquito Control] Tracking West Nile Virus

Features - Mosquitoes

Scientists are learning more every day.

April 16, 2014

Mosquitoes are known vectors of West Nile virus, which sickened 2,374 people and killed 114 in the U.S. last year, reported the Centers for Disease Control and Prevention.

West Nile virus primarily is a bird disease; mosquitoes bite infected birds and then bite humans, transmitting the virus, explained Mike McLean, an entomologist at the Metropolitan Mosquito Control District in St. Paul, Minn.

Scientists soon may be able to predict when and where West Nile virus outbreaks will occur. A team led by Dr. Michael Wimberly of South Dakota State University is refining a model that uses ecological forecasting to understand how temperature, precipitation and vegetation greenness affect the severity of outbreaks in a given region or season.

Recently, the team added data that lets scientists look at the overwintering conditions before a West Nile virus year, said Wimberly.

In the Northern Great Plains, Midwest and South Central regions – areas with high risk of West Nile virus – there is a “very strong and consistent relationship with winter temperatures,” said Wimberly.

“Basically, the colder it is, the harsher the winter, the less chance that you will have a big West Nile virus outbreak in the subsequent year.”

What’s his prediction for these regions given this past winter’s low temps? “I’d be willing to go out on a limb and say I expect that is going to help reduce the burden of West Nile virus transmission in the upcoming year,” said Wimberly.

He will be updating the models as spring approaches and making more formalized predictions, as well as testing their accuracy.

The incidence of West Nile virus “really does depend on the weather,” said McLean. A wet, early spring followed by a hot, dry summer “tends to amplify the virus,” he said. Given the snowpack in some parts of the country, it’s likely some areas will see lots of standing water come spring, which could increase mosquito populations in general, he said.

People are most likely to contract West Nile virus in South Dakota, followed closely by North Dakota and Nebraska, said Wimberly. He said a number of factors “make a brew of increased risk.” These include fewer numbers of residents, a highly efficient vector, large numbers of farmers who work outdoors during peak mosquito times, and a fair amount of rain that collects easily on agricultural land.

California had the highest number of West Nile virus cases in 2013, but it also has higher population density so the risk of getting the disease is lower, Wimberly noted.

For several years, the number of West Nile virus cases in Minnesota dropped to the single digits. Scientists thought maybe people had developed “herd immunity” to the disease, said McLean. That theory was proved wrong when cases increased over the past two years.


Primary West Nile Virus Vectors

More than 43 mosquito species can transmit West Nile virus, according to the National Institutes of Health. Experts consider these the top three:

Culex tarsalis is the primary vector in the West, Great Plains and Upper Midwest, said Mike McLean, entomologist at the Metropolitan Mosquito Control District in St. Paul, Minn. C. tarsalis lays eggs in irrigation channels and ditches, and low lying areas of fields and farms. In drier seasons it takes advantage of what little water is still standing, he said. A “really a competent vector,” it spends the first half of the summer feeding on birds; in mid-summer it switches behavior and starts biting mammals, McLean explained.

Culex pipiens transmits West Nile virus in the East and Great Lakes Basin. This species breeds in the storm water catch basins and cisterns often found under city streets, said McLean. These man-made habitats are mostly found in heavily populated urban areas.

Culex quinqefasciatus or the Southern House mosquito is the virus vector in the far South. It breeds in containers, storm sewer catch basins and canals, ground pools, ditches, drains, and septic tanks. It ranges from Virginia across the southern plains to southern California, and from southern Iowa to Texas and Florida.


Dengue, Chikungunya Making Inroads

West Nile virus isn’t the only mosquito-borne disease grabbing headlines.

According to the Centers for Disease Control and Prevention, 2013 was a banner year for dengue fever in the United States.

Outbreaks of locally acquired disease (not returned travelers who were bitten elsewhere in the world) cropped up in southern Texas and Florida; a case also was detected in Suffolk County, New York. Puerto Rico has experienced a sizeable dengue epidemic since late 2012.

The CDC reported the two mosquitoes capable of transmitting dengue, Aedes aegypti and Aedes albopictus, are present in the southern United States and have been found as far north as Chicago and New York.

Cases of locally acquired chikungunya virus, transmitted by the same mosquito species, were reported on the island of St. Martin by the World Health Organization last December.

The symptoms of both diseases include fever, headache, muscle and joint pains, and skin rash. In some cases, dengue can be life threatening.