West Nile Virus (WNV) was first recognized in the Western Hemisphere in 1999. It is a vector-borne disease, meaning that it requires an intermediary creature -- in this case, a mosquito -- to be transmitted from one person or animal to another.
Although humans can become infected with WNV, it's actually most prevalent among birds. As with humans, mosquitoes circulate the disease among wild bird populations.
In addition to birds and humans, other vertebrate animals, such as horses, also vulnerable to WNV. Horses are particularly susceptible, in fact, and one-third of horses that become ill with WNV either die of the disease or must be put down.
In humans, West Nile Virus is most dangerous to the very old, the very young, and those who already have weakened immune systems. Fewer than one percent of otherwise-healthy humans who are bitten by an infected mosquito will become seriously ill.
As the map above shows, all U.S. states except one (New Hampshire) reported human cases of West Nile Virus in 2018, despite almost two decades of aggressive attempts in many parts of the country to control the spread of the disease through pre-emergence mosquito treatments and wide-area spraying.
Because mosquitoes are the only known vector of West Nile Virus in the United States, controlling mosquito populations is essential to reducing the incidence of the disease. Mosquito control may consist of any or all of the following:
Mosquitoes breed in water, so reducing sources of standing water is a very important part of any successful mosquito-control plan. Many mosquito species can breed in very small bodies of water such as a birdbath, an old tire, or a child's toy pail.
Homeowners should walk carefully through their property and eliminate man-made objects that may serve as breeding pools whenever possible. In addition, artificial fish ponds should be kept aerated, permanent swimming pools kept properly chlorinated, kiddy pools emptied and turned upside-down after use, and birdbaths emptied, cleaned, and refilled on a daily basis.
There are several pesticides labeled for use in bodies of water to kill larval mosquitoes before they pupate into adults.
One of the oldest mosquito control products is Bacillus thuringiensis (commonly called BTi), which is a bacterium that is deadly to larval mosquitoes but essentially harmless to other creatures. BTi briquettes, or "mosquito dunks," are simply tossed onto the surface of the water in the correct quantity for the surface area being treated, during the several days before or after mosquito eggs start hatching. Your state's Cooperative Extension service should be able to provide you with an estimate of when that will be based on counting degree days.
BTi is one of several over-the-counter mosquito larvicides that can be purchased without a license in most jurisdictions in the U.S. In fact, I'm not aware of any state that forbids the use of BTi by unlicensed individuals. It's about as easy and as low-hazard an application as pesticide applications get. Nonetheless, if you have any doubts about the legality in your jurisdiction, then I recommend that you consult with your state's Cooperative Extension department prior to purchasing or using BTi or any other mosquito larvicides.
There also are a number of chemical insecticides that are labeled for use in water, but they generally must be applied by specially trained individuals who hold advanced certifications in both Aquatic and Public Health Pest Control. Many jurisdictions also require special permits for every application of an aquatic pesticide other than BTi.
Several years ago, fish of the species Gambusia affinis ssp. began to be used for mosquito control. This practice has fallen out of favor due to the finding that so-called "mosquito fish" consumed many times more fry (young fish) and beneficial insects than they did mosquitoes; so nowadays, their use is pretty much limited to artificial ponds, drainage pools, ditches, and so forth.
Dragonflies are perhaps a more promising predator. Although many people are afraid of them, dragonflies are completely harmless (and in my opinion, very beautiful) insects who can consume several hundred adult mosquitoes each on an average night. Reintroducing dragonflies into areas where their populations have been decimated (ironically, in many cases, by the very insecticides intended to control mosquitoes) is a possibility worth considering.
There are a wide variety of mechanical devices designed to capture or kill mosquitoes, but the only one that catches more mosquitoes than harmless or beneficial insects is the Mosquito Magnet. I'm not a big fan of "mosquito zappers" because they attract and kill many more harmless and beneficial insects than they do mosquitoes. The mosquito magnet, on the other hand, catches very few non-target insects.
When large numbers of adult mosquitoes are found to be carrying West Nile Virus, it may be necessary to use pesticide sprays or fogs to quickly reduce their numbers. Wide-area applications can only be made only by properly trained and licensed individuals working closely with their local regulatory and public health agencies, but most states allow homeowners to fog their own land using something like a mosquito fogger or mist blower.
The problem, once again, is that these kind of treatments wind up killing a great many more harmless and beneficial insects than mosquitoes. Their use is justified only when balanced against a known finding of WNV or other mosquito-borne diseases in local mosquito test pools. It's also tolerable to treat an area that's going to be used for an outdoor gathering the next day. As a general rule, however, routine exterior application of mosquito fogs or mists for preventative purposes do more harm than good, except during times of known disease endemicity.
Non-chemical control methods, such as caulking gaps around doors and windows and installing or repairing window screens, are the most permanent way to keep mosquitoes and other insects out of your home. If you can, installing one or more bat houses on your property can also dramatically reduce bat populations. Bats eat hundreds of insects each every night, and a great many of those insects are mosquitoes.
If non-chemical methods aren't quite enough, there are many over-the counter liquid insecticides like Harris Home Insect Killer that may be sprayed on the exterior of buildings around the doors and windows to help prevent mosquitoes from getting into your home. You'll find it goes a lot more quickly if you invest in a pump-type insecticide sprayer rather than using the trigger pump included with most of these products. Spraying a roughly one-foot (about 0.3 meter) band around every door and window will dramatically reduce the number of mosquitoes who get into your home.