West Nile Virus

Author: Karen Marquardt

West Nile Virus has been in the news recently here in the Austin area with positive mosquito test pools reported in both Williamson and Travis counties. Many people are concerned about what West Nile Virus (WNV) might mean for themselves and their families. Here is some information about the virus, how it is monitored and how it is transmitted, to help you understand the state of WNV in Central Texas right now.

What is a "positive mosquito test pool?” This is an important starting point in understanding WNV. A ‘positive test pool” means that at least one mosquito in a given area has tested positive for WNV. It does not mean that a human has been diagnosed with the disease.

How are mosquitoes monitored? Many Texas counties, including Williamson and Travis, monitor mosquito populations for WNV. They set up dozens of mosquito traps across the county, then regularly collect the mosquitoes, sort them by species and gender, and send them to be tested for WNV. A "test pool" is up to 50 female mosquitoes of the same species caught in a single trap. Female mosquitoes are the only ones that bite people, which is why only female mosquitoes are included in the pool. They sort mosquitoes by species to determine which kind are carrying WNV and to limit testing to only known vector species.

CDC Gravid Trap

CDC Gravid Trap

What is West Nile Virus (WNV)? WNV is a zoonotic disease, which means it is carried by an animal (birds) and may be transferred by a vector (mosquito) to humans. WNV can cause serious health issues, but about 80% of those who catch the virus show no symptoms. Of those that do show symptoms, most get West Nile Fever, which looks like a lot of other viral illnesses, with symptoms like fever, headache, fatigue, muscle aches, and sometimes a rash. A very small percent, only about 1% of affected individuals, will develop a more serious illness called West Nile neuroinvasive disease.

Which mosquitoes carry WNV? In Central Texas, the most common vector of WNV is the Southern House Mosquito, or Culex quinquefasciatus. Luckily, these mosquitoes don't travel very far, frequently less than one mile from where they are hatched. This means that a positive test pool can show officials exactly where the risk of human exposure is highest. It also means that there is less much lower exposure risk outside of the immediate area where the mosquito was found.

What can be done to prevent humans from contracting WNV? By tracking WNV in the mosquito population, officials can take action before the virus reaches the human population. If a positive mosquito pool is identified in your area, you may receive a warning from the county. When this happens, you should limit your activities outdoors when mosquitoes are most active, which is at dusk and dawn. You should wear mosquito repellent when you must be outside, and you should be aware of any symptoms that you begin to exhibit. If you develop WNV symptoms, be sure and tell your doctor if you have been bitten by a mosquito.

Most importantly, every citizen should do their part by eliminating potential mosquito breeding sites around their homes by following the Five Ts of mosquito control. Tip over containers that fill up with water from rain and sprinklers, Toss out yard waste and lawn clippings, Turn over larger containers and store them so that water can’t collect there, stretch out Tarps so that they don’t collect water, and Treat for mosquitoes if they are still a problem around your home.

Mosquito Squad customers should see an 85% to 90% reduction in mosquitoes in their yards, which will reduce, but not eliminate, the risk of contracting WNV. If you receive notification of a positive test pool in your area, please take it seriously!

Your county may also arrange for a limited mosquito spraying program, like Williamson County did last weekend in the Sun City area.

Both Travis and Williamson counties have information on their websites including information about WNV monitoring, news and abatement. Texas Agriculture Extension has an excellent, informative factsheet on WNV. Hays County also has a nice website that includes information about trapping and monitoring.