All you ever wanted to know about West Nile virus


A mosquito feeds on the hand of Susan Paskewitz, a University of Wisconsin-Madison entomology professor.

A mosquito feeds on the hand of Susan Paskewitz, a University of Wisconsin-Madison entomology professor. //Credit: Jeff Miller, UW-Madison

On Tuesday, 23 June 2015, Public Health Madison Dane County (PHMDC) reported a bird in Dane County had tested positive for West Nile virus.

The virus, which can sicken people, is often transmitted by mosquitoes to birds, humans, horses and other animals, including domestic and wild dogs, chickens, llamas and even alligators.

Susan Paskewitz, a UW-Madison entomology professor, works with the Wisconsin Department of Natural Resources each year to test mosquitoes for the virus in Dane and Milwaukee Counties. Public health authorities place traps in the field, Paskewitz says, and collect mosquitoes that are then brought or mailed on ice to her lab. There, the blood-feeders are sorted, counted and identified. Culex pipiens mosquitoes – among the prime carriers of West Nile virus – are then tested for the virus.

“I monitor for Dane County beginning in June and for Milwaukee beginning in July,” Paskewitz says.  “We continue until the end of August for Dane and until sometime in September for Milwaukee, depending on the season.  If it stays hot, we go longer.  If it cools quickly, then we stop when classes get going because the workers are often students.”

This year, Paskewitz has help from undergraduate Tom Richards.

American crow//Credit: Walter Siegmund

American crow//Credit: Walter Siegmund

Birds in the corvid family, like wild crows and blue jays, are especially vulnerable to West Nile virus, says Paskewitz and Kristen Bernard, a professor in the UW-Madison School of Veterinary Medicine. When a dead bird is found and reported to the dead bird hotline (800-433-1610) in Wisconsin or to local public health authorities, it’s sent for testing, which is how West Nile virus and other pathogens are often detected in these animals.

Blue Jay

Blue Jay//Credit: U.S. Fish and Wildlife Service, Dave Menke

“West Nile virus is maintained in nature in a cycle between birds and mosquitoes,” says Bernard. “When an infected mosquito bites a bird, virus is transmitted in the mosquito saliva. The virus then replicates in the bird’s tissues, which can result in high levels of virus in the blood. New mosquitoes then become infected when they feed on a bird that has high levels of virus in the blood.”

The cycle repeats when an infected mosquito feeds on another bird.

Humans can become infected if they are bitten by a mosquito harboring the virus, through blood transfusions and organ transplants, or by handling sick or dead birds carrying West Nile. Most of the time, people don’t show signs of illness.

“Most people are asymptomatic after WNV infection (75%), and 25% of people experience febrile disease (West Nile fever),” says Bernard.

According to the Centers for Disease Control and Prevention (CDC), West Nile fever (also called West Nile disease) can be accompanied by fever, body aches, joint pain, vomiting, diarrhea, or rash. Most people completely recover, though fatigue and weakness can persist and Bernard says there is growing evidence the virus can stick around for months to years after initial infection.

In a small number of cases (about 1%, says the CDC) the virus invades the nervous system, causing severe inflammatory illness like encephalitis or meningitis. About 10% of people who develop neuroinvasive disease die.

“People over 65 have 5-fold greater risk – 1 out of 50 infections will result in neuroinvasive disease,” adds Bernard.

Photomicrograph of brain tissue from a West Nile encephalitis patient, showing antigen-positive neurons and neuronal processes

Photomicrograph of brain tissue from a West Nile encephalitis patient, showing antigen-positive neurons and neuronal processes (in red).//Credit: Centers for Disease Control and Prevention, W.-J. Shieh; S. Zaki

There is currently no antiviral treatment for West Nile virus and Paskewitz says it’s unlikely we will be able to completely control the virus.

“Unlike malaria or dengue, where hundreds of millions of people get sick every year and scientists are working on genetic strategies, there are too few human illnesses to warrant large-scale measures to eliminate WNV transmission,” Paskewitz says. “And there are many species of mosquito and birds that play roles in maintaining the virus, making it less likely that we will find a solution that would eradicate the virus.”

Bernard’s lab is interested in the role mosquito saliva plays in the disease process; their studies have shown it enhances the ability of the virus to make more copies of itself. The lab is trying to identify what in the saliva makes this possible.

“This is not only important for understanding the disease process of WNV, but it is also a model for other mosquito-borne viruses, such as dengue and chikungunya viruses,” Bernard says. “The long term goal would be to use the mosquito salivary factor as a vaccine to inhibit the enhanced virus replication, which would reduce disease and transmission to mosquitoes. This would most likely be delivered in conjunction with a more traditional vaccine.”

While a vaccine against West Nile virus would be impractical – humans are “dead-end hosts” for the virus and thus do not pass it back to mosquitoes, and vaccinating birds presents numerous challenges – the technology could apply to human vaccines for viruses like dengue and chikungunya, which can be passed from humans to the mosquitoes that feed on them.

However, a salivary factor-derived vaccine could potentially dampen West Nile disease in individuals who get sick, Bernard says.

Her lab is also studying how the body ultimately clears the virus and is collaborating with a human blood researcher to understand whether persisting virus can be transmitted after blood transfusion.

“In my work with mouse models, I have found that West Nile virus can persist in the central nervous system (CMS) for up to six months and I am working to understand the animal’s immune response to this persisting virus,” she says. “Our long term goal is to understand how the body finally clears virus from the CNS in order to identify therapies to promote viral clearance.”


Because there is no vaccine or antiviral treatment for West Nile virus, your best bet is prevention. Public Health Madison Dane County offers the following tips to avoid bites and reduce your chances of exposure:

  • Limit time spent outside at dawn and dusk, when mosquitoes are most active.
  • Apply insect repellant to clothing as well as exposed skin because mosquitoes are capable of biting through clothing.
  • Make sure window and door screens are in good repair to prevent mosquitoes from getting into your residence.
  •  Trim tall grass, weeds and vines because mosquitoes use these areas to rest during hot daylight hours.
  • ELIMINATE SOURCES OF STANDING WATER BY:
    • Getting rid of items that can hold water, such as tin cans, plastic containers, ceramic pots or discarded tires. Turn over wheelbarrows, wading pools, boats and canoes when not in use.  Water left in these items provides breeding grounds for mosquitoes.
    • Cleaning roof gutters and downspouts for proper drainage – again to avoid standing water.
    • Changing the water in birdbaths and pet dishes at least every three days.
    • Cleaning and chlorinating swimming pools, outdoor saunas, and hot tubs; draining water from pool covers.
    • Landscaping to prevent water from pooling in low-lying areas.