West Nile Virus Risk Now Sooner Than Later
Though mosquitoes often become a nuisance in early summer, the breeds that are known to carry and spread West Nile virus (WNV) typically don't increase their activity until the warmer summer months, and this activity can continue well into the fall as long as temperatures remain warm. That time is near as Missouri's summer heat and humidity climb and mosquitoes that carry WNV enter the height of their breeding and feeding season. With that in mind, the Missouri Department of Health and Senior Services is reminding people that the threat of WNV is real, the disease can be serious, and prevention is simple but effective.
"We hope that, by now, most Missourians know WNV is here to stay and that it's a reality that must be dealt with every year," said Howard Pue, Missouri State Public Health Veterinarian "But we also know that people can let down their guard, even with potentially debilitating, deadly diseases like WNV. That's why we are compelled to remind people that WNV is here, it can be very serious, and that effective prevention is as simple as getting rid of sources of stagnant or standing water weekly and using the right repellent."
Pue said that while increased mosquito activity created by flooding this spring did not pose a higher WNV risk, stagnant water and debris left by receding floodwaters create the perfect breeding grounds now for mosquitoes that can carry the virus.
"We are not as concerned about the nuisance mosquitoes that develop in lowland areas known for flooding every few years," said Pue. "Floodwater mosquitoes often emerge in large broods and are annoying, aggressive biters, but they are not associated with increased risk for West Nile virus infection and are not a public health concern."
Pue warned, however, that the flooding has left water behind levees, carved out depressions in crop fields, and plugged culverts with debris, creating conditions where stagnant pools of water can later develop. It is in these types of stagnant pools that West Nile virus vector species prefer to breed and lay their eggs. Further, debris from flooded communities and farmsteads can create mosquito-breeding habitat where it did not exist before, such as trash piles with empty cans, buckets, barrels, or discarded tires transported by floodwaters or removed from flooded buildings during clean-up efforts.
"We don't want to cause undue alarm, but citizens and local governments should be aware that this could be another bad year for WNV in Missouri," Pue said. "The number of West Nile virus cases has gone up the past two years, with five deaths each year. We obviously don't want to see that trend continue. Where receding flood waters leave standing water and debris, special steps might need to be taken to prevent mosquitoes from breeding, such as increasing drainage or applying mosquito larvicides."
"Our message about West Nile virus has not changed - reduce mosquito breeding habitat and protect yourself from mosquito bites," said Pue. "Homeowners should properly dispose of trash and debris created by floodwaters as soon as possible. A ruined washing machine allowed to sit in a ditch and collect rainwater throughout the summer becomes the perfect breeding ground for mosquitoes. Taking precautions, including cleaning up trash and debris now, may help prevent an explosion in mosquito numbers during the summer and fall."
Most people infected with WNV either show no symptoms or develop a flu-like disease that they might attribute to another cause. The flu-like illness can last for a few days, though even healthy people have become sick for several weeks. However, some people develop a severe form of the disease involving the brain and other parts of the nervous system and never fully recover. Other people, particularly those over 50 years of age and those with underlying medical conditions, can die from WNV infection.
Fifteen of Missouri's counties will conduct surveillance for West Nile virus in mosquitoes during the 2008 season. The St. Louis County Health Department Vector Control Program collected mosquitoes in May and June that subsequently tested positive for West Nile virus. No other counties have had positive test results so far. As of July 22, 2008, DHSS had not received reports of any confirmed human cases of West Nile virus disease for the year.
For personal protection, people should wear protective clothing such as long-sleeved shirts and long pants. They should also use an insect repellent that contains an active ingredient shown to be effective in repelling mosquitoes. A wide variety of EPA-registered mosquito repellent alternatives are now available, including conventional repellents like DEET and picaridin, as well as repellents like oil of lemon eucalyptus and insect repellent 3535 (IR3535), which are derived from natural materials. As always, following label directions is critical in making sure you will have safe and effective protection from mosquito bites.