West Nile virus (WNV) is a viral disease that can cause encephalitis or meningitis, infection of the brain and the spinal cord or their protective covering. Prior to 1999, the disease was found only in Africa, Asia and southern Europe. Since 1999, WNV has spread throughout North America and has continued to cause disease in the United States. The yearly number of cases and fatalities has fluctuated depending on the weather conditions throughout the nation.
WNV is spread to people by the bite of an infected mosquito. The principal transmitter of WNV is the Northern House Mosquito (Culex pipiens). Mosquitoes first become exposed to the virus when they feed on birds that are infected with WNV. Once the mosquito is infected, it may transmit the virus to people or other animals when it bites them. Many birds can be infected with WNV, but crows and blue jays are most likely to die from the infection. Horses, too, are prone to severe WNV infection. People cannot get WNV from another person or from a horse that has the disease.
There is no vaccine currently available for WNV for people or most animals. However, there are vaccines available for horses, which represent 96.9 percent of all reported nonhuman mammalian cases.
WNV has become endemic in the United States and remains a serious disease threat; however, the number of cases and deaths vary significantly as climatic conditions vary by region from year to year. Sporadic seasonal epidemics will continue to occur in various regions of the country when weather and environmental conditions are most suitable. State, federal and local agencies continue to work together to address the health risks of WNV to Ohio families and their animals. Mosquito control efforts are often increased in urban areas during the summer to protect people from potential exposure to the disease.
Q. Do all mosquitoes carry WNV?
A. No, mosquitoes are generally considered a nuisance pest, but they occasionally can transmit diseases such as WNV. Sixty-four different species of mosquitoes are known to occur in Ohio. While most cannot transmit WNV, several mosquito species common to Ohio are known to be carriers of WNV. Only female mosquitoes bite. They do this to get a blood meal for developing their eggs.
Q. Where do mosquitoes live and breed?
A. The mosquito that carries WNV typically lays its eggs in stagnant water and water-holding containers. Weeds, tall grass and shrubbery provide an outdoor harborage for adult mosquitoes.
Q. When are mosquitoes most active?
A. Many mosquitoes are most active two to three hours before and after dusk and again at dawn when the air is calm. This is when the females are most likely to bite. However, some species will feed at any time of the day.
Q. When is the greatest risk of being exposed to an infected mosquito?
A. Most people have become infected in summer or early fall when mosquitoes are most numerous.
Q. Can the virus survive the winter?
A. Yes, during winter 2000, health workers in New York City found overwintering mosquitoes that contained evidence of WNV.
Q. Are crows and blue jays the only birds that can be infected?
A. No, the states reporting WNV activity have found many different bird species infected with WNV. However, crows and blue jays appear to be the most susceptible, or the most likely to be found dead because of WNV. This observation is not completely understood.
Q. Can other animals also be infected?
A. Yes, other animals have also been found to be infected and have died from WNV. During the year 2000 when surveillance was heightened, reports from the eastern states found WNV infecting 58 horses, two bats, a domestic rabbit, a skunk, a cat, a gray squirrel and a chipmunk.
Q. Is it possible to get infected from an infected person or animal?
A. No, the virus is not spread by person-to-person contact, and there is no evidence that people can get the disease by handling infected animals.
Q. What happens if a mosquito carrying WNV bites someone?
A. Most people who are bitten by an infected mosquito will demonstrate no signs or symptoms. However, some will experience a mild infection with a slight fever, headache, body aches and sometimes a skin rash or swollen lymph glands. Symptoms usually occur five to 15 days after being bitten by an infected mosquito. A very small number of people will suffer from a severe infection that is marked by a rapid onset of a high fever, a severe headache, neck stiffness, nausea or vomiting, confusion, muscle weakness or paralysis, seizures, coma and, rarely, death.
Q. Are some people more susceptible to the infection?
A. While everyone exposed to a mosquito that carries WNV is susceptible, people at greatest risk are those older than 50. Those who are immune-compromised may also be at greater risk. During the outbreak in New York City in 1999, everyone who died from WNV infection was 75 years of age or older. However, in 2001, two people in their 40s died from WNV infection.
Q. How is WNV diagnosed?
A. A doctor will need to test either blood or cerebrospinal fluid from a spinal tap for antibodies to the virus. A second blood test is required two to three weeks later to confirm the diagnosis.
Q. Is there a treatment for WNV encephalitis?
A. No, there is no specific treatment for WNV infection. While many people will not know that they have been exposed, nearly all of those with symptoms will fully recover. However, in some severe cases, hospitalization may be needed. There is no human vaccine for WNV. There are no antibiotics or antiviral medications that can be used in the treatment of WNV. All care is supportive.
Q. Do mosquitoes in Ohio carry other diseases?
A. Yes, there are several other viruses circulating among mosquitoes in Ohio that can cause encephalitis. St. Louis encephalitis, which is closely related to WNV, caused a major epidemic in 1975, resulting in 416 human cases and 29 fatalities. Every year, about 14 Ohioans, primarily children, are affected with La Crosse encephalitis. In 1991, an outbreak of eastern equine encephalitis affected horses in the Killbuck Marsh area in Holmes and Wayne counties. Of the 19 laboratory-confirmed cases, 17 horses died. The Ohio Department of Health, in collaboration with local health departments, has an ongoing program to monitor for these diseases. Although each of these viruses is somewhat different, prevention is basically the same: Reduce the mosquito population and protect yourself from mosquito bites, especially during the summer and early fall.
Q. How can I control mosquitoes around my home and neighborhood?
A. You can reduce the number of mosquitoes around your home and neighborhood by eliminating places where they lay their eggs. Young mosquitoes are aquatic, and they must have standing water to develop from egg to adult. Here are some simple steps you can take:
- Dispose of unwanted tin cans, plastic containers, flower pots or similar water-holding containers that have accumulated on your property. Do not overlook containers that have become overgrown by vegetation. Properly dispose of discarded tires. Water in tires is an excellent breeding site for disease-carrying mosquitoes.
- Empty bird baths and fill with fresh water weekly.
- Check and clean clogged roof gutters at least twice annually so they will drain properly. Roof gutters are easily overlooked but can produce millions of mosquitoes each season.
- Turn over plastic wading pools when not in use.
- Turn over wheelbarrows.
- Aerate ornamental pools or stock them with fish. Water gardens are fashionable, but they become mosquito producers if they are allowed to stagnate. Clean and chlorinate swimming pools that are not being used. A swimming pool that is left untended for a month can produce enough mosquitoes to infest an entire neighborhood. Mosquitoes may even breed in the water that collects on swimming pool and hot tub covers.
- Use landscaping to eliminate standing water that collects on your property. Mosquitoes will develop in any puddle that lasts for more than four days.
- Children's toys as well as tarps covering cars, boats and other equipment can also hold water and breed disease-carrying mosquitoes.
Q. How can I protect myself from WNV?
A. The best way is to avoid being bitten by mosquitoes. Use personal protection while outdoors when mosquitoes are present. These following actions will also reduce your chances of being bitten by mosquitoes:
- Wear light-colored clothing, long-sleeved shirts or jackets, and long slacks.
- Use mosquito netting when sleeping outdoors or in an unscreened structure. Protect small children when outdoors.
- Avoid mosquito-infested areas or stay indoors when mosquitoes are most active.
- Avoid physical exertion, and use colognes and perfumes sparingly, as these may attract mosquitoes.
- Consider the use of mosquito repellant, according to directions, when it is necessary to be outdoors. Chemical repellents are available in aerosol sprays, sticks, lotions, towelettes and pills. Pills have questionable effectiveness. Sprays and lotions are the commonly available formulations. The most common active ingredients are: N,N diethyl-meta-toluamide (DEET); ethyl haxanediol; dimethyl phthalate; and dimethyl carbate. Some common brands are Off!; Rutgers 6-12; Cutter's; Repel; Deep Woods Off!; Muscol; and Ben's 100.
- It is generally recommended that persons should use products containing 30 percent or less DEET. Use repellents sparingly and in the weakest concentration that does the job, especially on children. Products that contain 15 percent or less are considered "child safe." Read and follow label directions in using DEET. The amount of active ingredients is important! The higher the percent of active ingredients in a mosquito repellant, the more that is absorbed into the body. Health problems have been reported with the use of 75 to 100 percent DEET. Read the ingredients list on the container.
- In outdoor areas, aerosol bombs, smoke pots and citronella candles all have limited use. Mosquito "wands" such as "Skeeter Beater" usually contain moth balls. Electrocution devices attract mosquitoes into the yard. If used, place at the farthest distance from your area of person use. Sonic repellers do not work.
- In indoor areas, citronella candles and mosquito coils (which are actually a pesticide) can be used if well-ventilated.
- Put 16-mesh screens on all doors and windows, and keep them in good repair.
The Status of WNV in Ohio
WNV has been confirmed in Ohio every year since 2001. Infected mosquitoes, birds, horses and humans have been found in all Ohio counties. Therefore, the virus can be present throughout the state.
For the current status of WNV in Ohio and for more information, you can visit the following websites or contact your local health department.