Mosquito borne Diseases


Some mosquitoes in our area may carry viral diseases that can affect humans as well as horses, alpacas, llamas, emus, and other animals. EEE and WNV are in a large class of viruses known as “arboviruses,” or arthropod-borne-viruses.  Arboviruses generally are rare and can unpredictably appear and disappear in an area, but they can cause very serious or even fatal illnesses. The best defense against these diseases is information and education, and that is what we are providing here.

What is EEE?

Eastern Equine Encephalitis (EEE) is an arbovirus that is maintained in a cycle between mosquitoes (a species called Culiseta melanura) and many species of birds, usually in or near hardwood swamps. The virus is maintained in nature when an infected mosquito feeds on a bird for a blood meal and transmits virus to that bird, or when a mosquito picks up virus from an infected bird and then feeds on another bird.  EEE may also be transmitted by mosquitoes from birds to several mammal species, including humans. This most likely occurs through several other species of mosquitoes, called “bridge vectors,” that feed on infected birds, pick up the virus, and then bite humans. The EEE virus is transmitted only through the bite of an infected mosquito. Disease transmission does not occur directly from person to person.

EEE tends to build up within mosquito and bird populations in our area as the summer progresses and is most likely to be transmitted to humans in late summer or autumn (i.e., August or September), though it is possible for transmission to occur earlier or later. EEE is related to Western Equine Encephalitis and Venezuelan Equine Encephalitis, and occurs naturally throughout the eastern United States, up into Canada, and down into the Caribbean and South America. On average, this viral disease is rare in humans, with about 5 – 10 reported cases per year in the U.S.

The Disease and its Symptoms:

The incubation period (the time from infected mosquito bite to onset of illness) for eastern equine encephalitis virus (EEEV) disease ranges from 4 to 10 days. EEEV human infection can result in one of two types of illness: systemic or encephalitic. Systemic infection has an abrupt onset and is characterized by chills, fever, malaise, and joint and muscle pain. The illness lasts 1 to 2 weeks, and recovery is complete if there is no central nervous system involvement. If the virus gets into the central nervous system (brain and spinal column) it is called “encephalitic” and is a much more serious disease. In infants, the encephalitic form is characterized by abrupt onset; in older children and adults, encephalitis is manifested after a few days of systemic illness. Signs and symptoms in encephalitic patients are fever, headache, irritability, restlessness, drowsiness, anorexia, vomiting, diarrhea, a blue skin tone, convulsions, and coma.  Approximately a third of all people with the encephalitic form will die from the disease, usually 2 to 10 days after onset of symptoms. Of those who recover, many are left with disabling and progressive mental and physical problems, which can range from minimal brain dysfunction to severe intellectual impairment, personality disorders, seizures, paralysis, and cranial nerve dysfunction.

It is not known how often people are bitten by infected mosquitoes and do not get ill. One study from the 1960s suggests that as many as 96% of persons exposed to the EEE virus through a mosquito bite are asymptomatic (i.e., never get seriously sick).

Some facts:

  • EEE has been reported in many parts of Vermont.
  • People of all ages are at risk, though individuals under 15 and over 50 are at greater risk of severe disease (encephalitis).
  • Severe illness can occur in previously healthy individuals.
  • Obviously, the greater your exposure to mosquitoes, the greater the risk of contracting an arbovirus.
  • There is an EEE vaccine for horses, which has also been used successfully in other susceptible species; there is no vaccine for humans.
  • Approximately 85% of unvaccinated horses that get EEE die or are euthanized. Make sure your horses, alpacas, and llamas are vaccinated!

What Can be Done to Avoid the Disease?

BLSG will do all it can to reduce the mosquito population by killing larvae and adults. However, the mosquito species responsible for maintaining the EEE cycle in birds is very difficult to control due to its habitat preferences and unusual breeding habits (e.g., we cannot kill their larvae because they are inaccessible to us). Even if we could kill 99% of all mosquitoes, there are some that inevitably will escape treatment and  contact humans. Thus, personal protection is your best chance of avoiding nuisance mosquitoes and arboviral diseases. Here is how to do that:

1) Protect Yourself!

We cannot emphasize enough the importance of everyone protecting themselves from mosquito bites. The following methods will dramatically reduce your chances of being bitten and contracting an arboviral disease:

  • When outside in summer use insect repellents that are labeled as being effective against mosquitoes. Effective ingredients include DEET, Picaridin, and oil of lemon eucalyptus. For more information about choosing a repellent see:
  • Limit the amount of time spent outdoors when mosquitoes are most active – usually at dawn and dusk.
  • Consider curtailing evening activities if there are lots of mosquitoes around athletic fields where children are playing sports, and other areas where people gather.
  • Wear long-sleeved shirts and long pants outside when mosquitoes are active.
  • Cover baby carriages or outdoor play spaces with mosquito netting.
  • Fix any holes in the screens in your house and make sure they are tightly attached to the doors and windows.

2) Reduce mosquitoes near your home:

  • Remove standing water around your house.
  • Dispose of, or regularly empty any water-holding containers (including trash cans) on your property.
  • Drill holes in the bottom of recycling containers that are left outdoors, so water can drain out.
  • Remove old tires from your property or drill holes in them to drain water.
  • Clean clogged roof gutters of leaves and debris that prevent drainage of rainwater.
  • Turn over plastic wading pools and wheelbarrows when not in use.
  • Do not allow water to stagnate in birdbaths. Change it every three or four days.
  • Keep swimming pools clean and properly chlorinated. Remove standing water from pool covers.
  • Use landscaping to keep standing water from collecting on your property.

For more information on EEE, go to the Center for Disease Control website at


Here are steps you can take to prevent mosquito bites:

  • Wear long-sleeved shirts and long pants when outdoors
  • Limit the amount of time you spend outdoors at dawn and dusk
  • Use EPA-registered insect repellent labeled as effective against mosquitoes
  • Cover baby carriages or outdoor playpens with mosquito netting
  • Fix holes in your screens. Make sure they are tightly attached to doors and windows
  • Mosquitoes breed in standing water. Drain areas where water can pool, such as rain gutters, wading pools and old tires

What is WNV?

West Nile virus (WNV) is another arbovirus that can affect humans. Similar to other mosquito-borne encephalitic viruses, it is generally cycled between birds and mosquitoes, and is transmitted to mammals, including horses and man, by infected mosquitoes.  WNV infections have been confirmed in at least 326 species of birds. WNV has historically been found in Africa, Europe, Asia, and the Middle East, and was first detected in the United States in 1999 in New York City, where it likely arrived in an airline passenger.

WNV infections in humans are much more common than EEE infections. In 2012, nationwide, there were 5387 confirmed cases of WNV infection, with 243 deaths.

How do people and animals get West Nile virus?

As with EEE, West Nile virus is transmitted from bites by a mosquito infected with the virus. A mosquito becomes infected by feeding on a wild bird that has WNV in its blood. The mosquito then transmits the WNV in its saliva when it bites a person or animal. 62 of 176 species of mosquitoes tested have been found infected with WNV; however, only very few of these species are actually involved with disease transmission.

As with EEE, WNV is maintained in bird populations. Crows, jays, hawks, ravens, gulls, pigeons, ducks, owls, sparrows, and finches seem especially sensitive to WNV. Dead birds can be the first indication of virus transmission in an area. If you find any dead birds not obviously killed by other means, please call and report your sighting to the local Department of Health District Office (Middlebury: 802-253-8804 or 802-388-4644; Rutland: 888-253-8802  or 802-786-5811) or call the state Health Department Office at 1-800-913-1139 (8:00 a.m. – 4:30 p.m. Monday through Friday).

Symptoms of West Nile Virus

Only a small percentage of infected humans show severe symptoms of disease and death (on the order of 1 out of 150 infections).  About 20% will have milder, flu-like symptoms. Thus, WNV is far less serious than EEE, though it can still be debilitating and even deadly. Symptoms of West Nile fever, resulting from infection with West Nile virus, include fever, headache, rash, and body aches. In severe cases, encephalitis and meningitis may occur.   Severe symptoms may include high fever, disorientation, tremors, coma, paralysis, and death. Fatalities are more likely in people over the age of 50 and those with weakened immune systems. The time between the mosquito bite and the onset of illness—the incubation period—ranges from 5-15 days in humans. If there is any suspicion of EEE or WNV infection, contact your personal physician immediately. There is no current vaccine or cure for West Nile fever, but supportive care can be very important in recovery.

Protecting Yourself from WNV

Please see the section above on EEE for personal protection.

For more information on WNV, go to:

And for more information on either disease, go to: