Remember last year’s mild winter followed by a hot, mostly dry summer with a few heavy rains now and then? That weather pattern created conditions favorable for mosquitoes, increasing transmission of West Nile virus nationwide. Missouri is not known for a high incidence of West Nile, but reported cases in the state doubled from 10 in 2011 to 20 in 2012, according to the U.S. Centers for Disease Control and Prevention.
So far, 2013’s weather pattern has brought more consistent rain, and summer temperatures are not predicted to be as high as last year’s intense heat wave, but it’s too soon to tell exactly how this climate will affect the mosquito population and subsequent West Nile virus cases.
“We see a relationship between heat and incidence of West Nile that seems to be almost entirely related to mosquito populations. The more mosquitoes, the more likely people are to get bitten, and the more likely there’s transmission,” says Dr. Morey Gardner, director of the division on infectious disease and infection prevention at SSM St. Mary’s Health Center. When asked to make a prediction about this year’s West Nile virus incidence, Gardner says, “we just can’t be certain. We hadn’t anticipated what happened this past year.”
Regardless of whether there will be swarms or just a few token mosquitoes annoying us this summer, avoiding their bites decreases your risk of contracting West Nile virus. There are two primary strategies that everyone should use: environmental precautions and individual protection.
First, don’t put out a welcome mat for mosquitoes. Any standing water is a potential breeding ground—and mosquitoes breed fast. Birdbaths, fire pits, even a stray frisbee or extra flowerpot filled with rainwater should be emptied regularly. Window screens should be tightly in place and holes or tears should be repaired to keep mosquitoes out of your home.
To protect yourself, stay indoors or wear long pants and sleeves if you plan to be outside at or after dusk, when mosquitoes are most active. When you are enjoying the great outdoors, use a repellant to deter mosquitoes, ticks and other biting insects. The CDC recommends repellants containing DEET, picaridin, IR3535, and some oil of lemon eucalyptus and para-menthane-diol products, which provide longer-lasting protection. Follow instructions on the label for safe application and wash or bathe to remove the product after returning indoors.
“Of every 100 people who are infected by a mosquito, about 75 percent never even develop any symptoms,” says Dr. Michael Diamond, a specialist in infectious diseases and expert in West Nile Virus with Washington University Physicians. “In these people, the infection is so minor that they don’t even get a fever. It’s what we call an ‘inapparent infection.’ Of the last 25 in that group of 100 people infected, about one person will go on to get severe disease. The other ones would get relatively mild flu-like symptoms.”
In general, people who have a chronic illness, are immuno-compromised or elderly are at greatest risk for developing ‘neuroinvasive’ West Nile virus, the dangerous form of the disease that causes meningitis or encephalitis, both of which require hospitalization. “Of those who develop the severe neuro-invasive disease, about 10 to 20 percent will die,” Diamond says.
Individuals who develop neuro-invasive West Nile virus typically “become very, very ill. They develop profound muscle weakness and an inability to breathe normally,” Gardner says. They require intensive care, including ventilator support in most cases.
However, it is important to remember that the vast majority of West Nile virus infections go virtually unnoticed. Be cautious, protect yourself, and see your primary-care physician if you develop serious flu-like symptoms that last for more than a few days or have any difficulty breathing. But aside from being aware and taking reasonable precautions, “there’s no reason for hysteria,” Gardner concludes.