West Nile Virus

Each year hundreds of Americans are reported with mosquito and tick-borne diseases such as Lyme disease, Eastern and Western equine, Lacrosse and St. Louis encephalitis. These diseases are endemic in the United States, which means they have a low-level of occurrence but consistent presence in human and animal populations. West Nile Virus (WNV) has emerged in recent years in North America and presents a threat to public and animal health. The most serious manifestation of WNV infection is fatal encephalitis (inflammation of the brain) in humans and horses, as well as mortality in certain domestic and wild birds, especially crows.

WNV was not detected in the Western Hemisphere until an outbreak occurred in New York City during the summer and fall of 1999. The virus survived in mosquito populations through the winter in the New York City area and subsequently spread down the east coast and then west across the rest of the country. The WNV is now considered to be endemic throughout the United States.


The only vectors found to be associated with WNV outbreaks are mosquitoes. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for several days. This is known as the mosquito/bird/transmission cycle. Occasionally the virus will spill over into the human and animal population when infected mosquitoes bite them. The virus is located in the mosquito's salivary glands. During blood feeding, the virus may be injected into the animal where it can multiply and cause illness. The virus primarily causes disease in birds but may infect horses and other mammals, including humans. In rare cases humans may be infected via blood transfusions, transplants, during pregnancy and breast feeding.

Experimental studies conducted by USDA/APHIS Veterinary Services have concluded that horses are not involved in the transmission cycle of WNV. Horses are considered dead-end or incidental hosts because they do not maintain a sufficient level of the virus in their system to infect mosquitoes or other mammals. This means infected horses are not considered a threat to other horses and people will not contract the virus by caring for infected horses.1 However, universal precautions should be taken when handling blood, spinal fluid, brain or spinal tissue from suspected animals since these may contain the virus.


Horses infected with WNV can have a variety of clinical signs. They may have a mild flu-like syndrome with fever, depression and listlessness which can progress to more serious neurologic or encephalitis signs such as muzzle twitching, in-coordination, stumbling, circling, aimless wandering, head pressing, hyper-excitability, coma and death.

Even when encephalitis signs occur, over 75% of affected horses recover. Recovery is usually complete and the animal shows no permanent neurological impairment after 3-4 weeks.


There is no definitive treatment for WNV infection. Like all other viruses, there are no specific medications that will destroy the virus. Supportive treatment, by a veterinarian, can assist is recovery and with early intervention and aggressive treatment most horses will recover fully.


There are currently two commercially produced vaccines available for protection against WNV. The suggested WNV vaccination guidelines calls for each horse to be given two vaccinations initially, 3-4 weeks apart, followed by a semi-annual booster. The timing in giving the vaccine is important. Research indicates that immunity may not develop sufficiently to protect horses until 3-4 weeks after the second dose is given. Therefore, it is essential that horses be vaccinated well before the summer months (mosquito season) begin.

Horse owners should vaccinate their horses for eastern equine encephalomyelitis (EEE), which has a similar suggested protocol as the WNV vaccine, with 2 initial vaccinations followed by semiannual boosters. It is important to know that the EEE vaccine will not protect horses against WNV. Because every situation is unique ask your veterinarian to recommend a vaccination protocol specific to your horses.

The use of vaccines to protect horses against these diseases is only a tool and should be used in conjunction with good mosquito reduction/avoidance measures implemented to protect yourself and your horse. The key is mosquito control to stop the bird-mosquito infection cycle. The most effective method is to destroy the mosquito larval habitat by removing all potential sources of stagnant water in which mosquitoes might breed. Mosquitoes can breed in any puddle that lasts more than four days. Water buckets, water troughs, wading pools, bird baths, wheelbarrows, clogged roof gutters, discarded tires, plastic containers or any water-holding container should be cleaned or emptied on a weekly basis. Drill holes in the bottom of containers that are left out-of-doors, turn over wheelbarrows, aerate ornamental pools and stock them with fish or chlorinate them. In addition to reducing mosquito populations, preventing animals from being exposed to adult mosquitoes is important. Horses should be stabled inside during peak mosquito feeding times which are dawn and dusk. Use of mosquito resistant structures such as well maintained insect screening and fans may reduce potential access of mosquitoes to equine and other livestock hosts. Insect repellants approved for use on horses may be of some value in decreasing exposure, however, there are limitations to the coverage area that may be achieved on any given horse due to limited duration of effectiveness of some formulations under certain conditions (e.g. rain, perspiration). Remember to always follow label instructions. Horse owners are encouraged to contact their veterinarian immediately should they notice any signs or symptoms of WNV infection in their horses, especially those exhibiting neurological signs.

Florida West Nile Virus Surveillance and Response Plan

On July 6, 2001, the presence of WNV in Florida was confirmed when a dead crow was found to be positive for the virus in Jefferson County. Since then, an extensive surveillance program has been implemented in Florida. This program involves the monitoring of mosquito pools, sentinel chickens and equine cases. Based on experience from previous years, one of the first indications that WNV might be present in an area is the unusually high death rate of birds, particularly crows or blue jays. The appearance of dead birds in an area might be an early warning that the virus is present. Dead birds should be reported by contacting the local county Department of health office or by entering the information on the Florida Fish and Wildlife Conservation Commission's dead bird reporting web site at: http://legacy.myfwc.com/bird/default.asp

The Florida Department of Health (DOH), in conjunction with the FDACS, the Florida Fish and Wildlife Conservation Commission (FWCC), local health and mosquito control agencies have developed a WNV Plan which provides a tiered prevention, control and response program for WNV depending on the location and type of evidence indicating an outbreak has occurred. For complete updates and information on the WNV Surveillance and Response Plan you may access the Florida Department of Health web page at: http://www.doh.state.fl.us/Environment/medicine/arboviral/index.html, or you may contact: The Florida Department of Agriculture and Consumer Services, Division of Animal Industry, 407 S. Calhoun St., Mayo Building, Tallahassee, FL, 32399-0800. The division personnel can be contacted at (850) 410-0900.

1Jonathan Plamer, DVM, DACVIM, University of Pennsylvania, School of Veterinary Medicine