For more than 10 years, scientists and public health officials have been trying to predict where West Nile virus will hit next. This has proven a difficult task because the hot spots change from year to year. Transmitted by mosquitoes, West Nile virus typically causes flulike symptoms, but in rare cases can lead to brain inflammation or death. In 2002, Illinois and Michigan reported over 500 cases each (with 67 deaths in Illinois), but the next year the biggest outbreaks were in Colorado and Texas.
So what’s going on? The current line of thinking goes like this: Ever-shrinking bird habitat plus unusually warm, dry spring weather forces birds and mosquitoes to share the same watering places. The mosquitoes pick up the virus from the birds, and when it finally rains, thousands of infected mosquitoes reproduce and start biting people, horses and other birds. By late summer, an outbreak of West Nile virus is in full swing.
The good news is that overall human/animal cases of West Nile virus have declined since 2006, which many researchers think is due to widespread vaccination of horses, the mammals most commonly infected with the virus. Vaccines for horses have been available since 2002, but clinical trials of human vaccines are still three years away. However, many people, birds and horses bitten by infected mosquitoes develop immunity without getting sick, which over time may further suppress outbreaks.
Besides covering up when outdoors and using a good repellant, the best way to protect yourself from possible infection is to eliminate the sources of standing water that mosquitoes need to breed, or to make such sources uninhabitable to mosquitoes. You can drill drainage holes in old tires, but for rain barrels, water troughs or small ponds, the best solution is to use a product containing Bacillus thuringiensis israelensis (Bti). The active ingredient in Bti products is a naturally-occurring bacterium often found in damp leaf litter. It damages the guts of mosquito larvae and kills them. Under most conditions, Bti kills the larvae and remains effective for about 30 days. Used correctly, Bti is nontoxic to mammals, birds, fish and most other insects. Commonly used Bti products are Summit’s Mosquito Dunks (see photo in the Image Gallery), and Vectobac, which is used by cities such as New York and Fort Collins, Colo.
Click the link to view maps of the geographical spread of West Nile (created by the U.S. Centers for Disease Control and Prevention).
Contributing editor Barbara Pleasant gardens in southwest Virginia, where she grows vegetables, herbs, fruits, flowers and a few lucky chickens. Contact Barbara by visiting her website or finding her on Google+.