Jonathan F. Day, Professor of Medical Entomology, University of Florida, Florida Medical Entomology Laboratory, Vero Beach
What is your real risk of contracting a mosquito transmitted disease like West Nile virus? The answer depends on three factors: where you live, your daily mosquito exposure rate and, most importantly, local mosquito infection rates. Where you live is important because of the geographic distribution of WNV and other mosquito-transmitted diseases. There are regions of the United States where the virus is common and other regions where WNV is rare. If you live in California, Texas, Nebraska or Colorado where the virus is seasonally common, your risk of infection is higher than if you live in Maine or New Hampshire where the virus is rare.
Likewise, mosquito exposure rate is an important risk factor. People who live in homes without window screens or air conditioning are at a much higher risk of infection than people who live in mosquito-proof homes. Individuals such as night watchmen and emergency responders who work outdoors at night are at higher risk of infection than individuals who work in air conditioned buildings.
Finally, the local mosquito infection rate is the most important risk factor. During most summers, mosquito infection rates are extremely low. For example, a typical WNV mosquito infection rate in Florida is one in 10,000 (that is, one WNV-positive female mosquito is found in every 10,000 females that are sampled). The likelihood of a human WN infection under these conditions is very low. Contrast this with the mosquito infection rate of 1 in 100 observed in Colorado during the 2003 WN epidemic. If one out of every 100 female mosquitoes can transmit WNV, the risk of human disease, especially in humans with high mosquito exposure rates, increases tremendously.
Risk of WNV exposure can be reduced by avoiding mosquitoes and preventing them from biting, especially when mosquito infection rates are known to be high. The best ways to avoid mosquito bites are to avoid infested areas, wear protective clothing and wear insect repellent.
UPDATE: Due to current environmental conditions, FMEL has released the sixth FMEL Arboviral Epidemic Risk Assessment for Florida of 2010. Details can be found in the latest update (PDF) at my lab's website at http://fmel.ifas.ufl.edu. Go to the Mosquito Information Web Site and look in "MWTD Risk Analysis."
