When Mosquitoes Go Bad part 1.......(West Nile Virus)


West Nile Virus

When you hear about West Nile Virus, it brings to mind images of dead crows and mosquito laden swamps.  In the past few years, West Nile Virus (WNV) has been diagnosed in multiple horses in Western New York.  Last year (2013) The New York Department of Agriculture and Markets reported 13 WNV positive horses throughout the state!


While the immune system of most horses infected by WNV can prevent the virus from crossing the blood-brain barrier, clinical signs are seen in some horses when the virus breaches the blood-brain barrier and causes encephalomyelitis, damaging the brain and spinal cord.  The clinical signs of WNV encephalomyelitis vary in range and severity, with the most frequently observed signs include incoordination or ataxia (especially of the hind limbs); twitching of the muzzle and lower lip, and twitching of the muscles in the neck, shoulders or pectoral (chest) region. Signs may be the same on the left and the right of the horse or may be one sided. Also reported are behavioral abnormalities such as depression or heightened sensitivity to external stimuli, stumbling, toe dragging, leaning to one side and in severe cases, paralysis of the hindquarters, recumbency, coma and death. Other clinical signs that may be noted include fever, generalized weakness, impaired vision, inability to swallow, aimless wandering, and convulsions. The nature and severity of clinical signs depend largely on the area(s) of the central nervous system affected by the virus and the extent of damage. There are reports of WNV occurring more commonly in older horses; however, last years cases in New York ranged from yearlings to aged horses.   

First isolated in Uganda in 1937, WNV is transmitted principally by mosquitoes and can cause inflammation of the brain and spinal cord (encephalomyelitis). Clinical disease caused by this virus is seen primarily in birds, equines and humans and very infrequently in goats, sheep, dogs, llamas, various reptiles and bears, among other species. Prior to its discovery in the northeastern U.S. in 1999, WNV was widely distributed in Africa, the Middle East, southwest Asia, and parts of Europe. 

WNV was first recognized in the western hemisphere in September 1999, when it was isolated from the tissues of sick flamingoes and pheasants at the Bronx Zoo and from dead crows in the New York City area. By 2002 over 15,000 horses were diagnosed with WNV in 41 states.

WNV circulates in nature between birds and mosquitoes. Various species of birds serve as hosts of the virus, allowing it to replicate within them.  Mosquitoes act as vectors of WNV by biting the infected birds and then transmitting it to a wide range of animals, including humans. The strains of WNV present in North America also cause disease in crows and blue jays, in which the infection is usually fatal. Humans, horses, and a wide variety of other species can also be infected with WNV.  Because there is only a very small amount of the virus in the blood of infected horses, mosquitoes are unable to transmit the virus from horse to horse or from horse to human. The virus is transmitted when a mosquito takes a blood meal from an infected bird and then feeds on a horse. During the process of taking a blood meal from the horse, the virus is transmitted by the infected mosquito.

As is typical of numerous other viral infections, many horses experience no clinical illness following exposure to the virus for the first time. 

WNV infection diagnosis is usually based on the nature of the clinical signs displayed by an affected horse, together with the detection of antibodies to the virus in the blood by laboratory examination. It is important to remember that many of the clinical signs of WNV encephalomyelitis closely resemble those of many other equine neurological diseases (e.g., Eastern equine encephalitis, rabies, equine protozoal myeloencephalitis, equine herpesvirus-1 and botulism).  Testing and progression of clinical signs will help to differentiate between these diseases. However, cases of WNV encephalomyelitis tend to occur during late summer or early fall when viremia (the level of infective virus in the blood) in the bird population is higher and mosquito populations are numerous and active.


Currently, there is no specific anti-viral treatment for WNV encephalomyelitis. Treatment focuses on controlling pain and inflammation. Anti-inflammatory drugs will be provided to control inflammatory changes in the central nervous system. Other supportive measures such as intravenous fluids, sedatives and nutritional support can be important components of therapy. It is important to consult your veterinarian immediately if you suspect your horse is showing neurological signs so that the appropriate treatment measures can be implemented without delay.


A number of measures can be taken to help protect your horse against WNV. These are comprised of vaccinating against the disease coupled with management strategies to reduce exposure to mosquitoes and. Horses vaccinated against Eastern, Western or Venezuelan equine encephalomyelitis are not protected against WNV.   A separate WNV vaccine is currently available as well as combination products that combine Eastern, Western, Tetanus as well as West Nile Virus.

The vaccine should be administered as a series of two doses given three to six weeks apart. Foals should receive three immunizations starting at 6 months of age if the mare was immunized against WNV 30 days prior to foaling. The duration of immunity from vaccination is not known. It is recommended to vaccinate every four to six months in regions where the virus is active. Contact your veterinarian for the appropriate vaccination schedule for your location.  In New York we typically recommend bi-annual vaccination against WNV in the spring and a booster during the early fall months.

Aside from vaccination against WNV, other measures should be taken to reduce the risk of your horse being bitten by a virus-infected mosquito. Eliminate or reduce potential mosquito breeding sites by disposing of old receptacles, such as tires and containers and eliminating areas of standing water in areas where horses congregate. 

Clean clogged roof gutters and turn over plastic wading pools or wheelbarrows when not in use. Thoroughly clean livestock watering troughs at least monthly. Screen stalls (if possible) or at least install fans over the horses to help deter mosquitoes. Avoid turning on lights inside the stable during the evening or overnight. Because mosquitoes are attracted to light, placing incandescent bulbs around the perimeter of the stable will attract mosquitoes away from the horses. Lights can also be used to draw mosquitoes to electric bug zappers. 

The use of insect repellant that contains pyrethrin on horses can also reduce the chance of being bitten by mosquitoes. Remove any birds (including chickens) located in or close to a stable. Some veterinarians have success by hanging cattle ear tags on horse halters. These ear tags have been impregnated with insecticide and often reduce the effects of not only mosquito biting, but also midges and the effects of “fly-strike” dermatitis around the ears.  Our Mosquito and Fly Control Measures blog post has additional information on vector control!

Because WNV can affect humans as well as horses, don’t forget to take actions to protect yourself as well. When outdoors in the evening, wear clothing that covers your skin and apply plenty of mosquito repellent.

Prevention is key to the control of this infection.