West Nile Disease in Horses

Robin Paterson, D.V.M.
Cerbat Cliffs Animal Hospital

West Nile Virus (WNV) is a mosquito-borne virus that was first recognized in U.S. horses in 1999.  Thirteen years later, the disease is found throughout North America, parts of Central and South America, Europe, Asia and Africa, and continues to cause serious, sometimes fatal, neurologic conditions of affected equids as well as other mammalian and avian species.

Mosquitoes, or occasionally other blood-sucking insects, can transmit the virus from an infected avian reservoir host to the horse, human or other mammal. A reservoir indicates that the virus replicates inside the animal but unless the viral level inside the animal is extremely high, the host usually does not die, but mosquitoes can transmit the virus from this host to another animal.  Horse and humans are the dead-end hosts. This means that the virus at this stage does not replicate in very large quantities (unlike in a reservoir host), so the virus is unable to spread from one host to another.  Rather, the goal of the virus is to take over the “machinery” of the host cells and by doing so, kills the cell. If enough cells in enough systems are destroyed, the host dies. This only happens in about one third of equine cases.

The incubation of WNV in horses is 3-15 days. Clinical signs are variable and include the following: fever, depression, apprehension/behavioral changes, ataxia (stumbling, incoordination), partial paralysis, weakness, light sensitivity, droopy lip, twitching, muscle fasciculations/tremors, convulsions, difficulty rising, lameness, colic, blindness and death. The most recent data from the American Association of Equine Practitioners (AAEP) states that the mortality rate of horses with clinical signs of WNV is 33%. Of the horses that do recover from WNV, many retain some degree of neurologic dysfunction, gait or behavioral changes for months afterward.

The other diseases that must be differentiated from WNV include rabies virus, the encephalomyelitis viruses (Eastern, Western and Venezuelan), equine protozoal myeloencephalitis (EPM), botulism, trauma, heat stress, wobbler’s syndrome, and a few other relatively uncommon neurologic disorders. There are several definitive tests for WNV, and the testing is important to differentiate from the conditions mentioned above. Currently, the IgM-capture ELISA test is the most reliable for identifying recent exposure to WNV in clinical horses.

Vaccination is the most effective way to reduce a horse’s risk of developing disease from WNV. Currently, there are two types or strains of vaccination in four different USDA-approved vaccines. Three of these vaccines are a form of an inactivated strain, and the fourth is a live strain enveloped in a canary-pox vector. Any vaccine should be administered at the appropriate dosing schedule and by a licensed veterinarian. Your veterinarian can advise you on the appropriate vaccination type and schedule for your horse. Vaccination is recommended for all horses living in endemic regions of the United States, which, at this time, includes the entire continental U.S. The equine WNV vaccines have been shown to be effective in llamas and alpacas, although this is off-label use and you should discuss this with your veterinarian if you have questions.

Other important factors in lowering the risk of exposure to WNV include understanding the life cycle of the mosquito, and eliminating the breeding grounds whenever possible. Mosquitoes lay their eggs in stagnant waters, preferably near the adult mosquito food supply (plant sources and blood meals for females, plant nectar for male mosquitoes). The larvae are born and develop in these waters, mature to the pupa stage and then pupate (transform) to the adult mosquito.  Mosquito populations vary yearly and are influenced by temperature, moisture, organic debris, stagnant water and air availability, and availability of food sources. Reducing or eliminating sources of standing water can greatly diminish mosquito larvae. Remove items such as old tires, cans, buckets, bird baths, empty garden pots, unused troughs, stall muck, and limit  exposure to ponds and marshy areas, clean gutters, consider adding fans to barns/stalls, consider keeping horses in the barns during prime mosquito feeding (dusk to dawn),  filling in holes or ditches, and  use horse-approved mosquito repellent products, if possible.

Vaccination protocols frequently change as new technology is developed. Be sure to speak with your veterinarian for the most updated information on both West Nile Virus and what vaccinations and other preventative measures are available to best protect your horse.

American Association of Equine Practitioners recommendations for equine West Nile Virus vaccination schedules, as of August 2012.  (www.aaep.org)

Vaccination Schedules:

Adult horses previously vaccinated:

Vaccinate annually in the spring, prior to the onset of the insect vector season.

For animals at high risk or with limited immunity, more frequent vaccination or appropriately timed revaccination is recommended in order to induce protective immunity during periods of likely exposure. For instance, juvenile horses (<5 years of age) appear to be more susceptible than adult horses that have likely been vaccinated and/or had subclinical exposure. Geriatric horses (>15 years of age) have been demonstrated to have enhanced susceptibility to WNV disease. Therefore, more frequent vaccination may be recommended to meet the vaccination needs of these horses.

Booster vaccinations are warranted according to local disease or exposure risk.  However, more frequent vaccination may be indicated with any of these products depending on risk assessment.

Adult horses previously unvaccinated or having unknown vaccinal history:

Inactivated whole virus vaccine: A primary series of 2 doses is administered to naïve horses. A 4 to 6 week interval between doses is recommended. The label recommended revaccination interval is 12 months.

Recombinant canary pox vector vaccine: A primary series of 2 doses is administered to naïve horses with a 4 to 6 week interval between doses. The label recommended revaccination interval is 12 months.

Inactivated flavivirus chimera vaccine: A primary series of 2 doses is administered to naïve horses.  A 3 to 4 week interval between doses is recommended. The label recommended revaccination interval is 12 months.

Pregnant mares:

Limited studies have been performed that examine vaccinal protection against WNV disease in pregnant mares. While none of the licensed vaccines are specifically labeled for administration to pregnant mares at this time, practitioners have vaccinated pregnant mares due to the risk of natural infection. It is an accepted practice by many veterinarians to administer WNV vaccines to pregnant mares as the risk of adverse consequences of WNV infection outweighs any reported adverse effects of use of vaccine.

Pregnant mare previously vaccinated: Vaccinate at 4 to 6 weeks before foaling.

Pregnant mares previously unvaccinated: Initiate a primary vaccination series (see adult horses previously unvaccinated) immediately. Limited antibody response was demonstrated in pregnant mares vaccinated for the first time with the originally licensed inactivated, whole virus vaccine. It is unknown if this is true for the other products. Vaccination of naïve mares while open is a preferred strategy.

Foals:

Limited studies have been performed examining maternal antibody inference and inhibition of protection against WNV disease. The only data currently available is for the originally licensed, inactivated whole virus product in which foals were demonstrated to produce antibody in response to vaccination despite the presence of maternal antibody. No studies have been performed evaluating protection from disease in foals vaccinated in the face of maternal immunity.

Foals of vaccinated mares

Inactivated whole virus vaccines: Administer a primary 3-dose series beginning at 4-6 months of age. A 4 to 6 week interval between the first and second doses is recommended. The third dose should be administered at 10 to 12 months of age prior to the onset of the next mosquito season.

Data indicates that maternal antibodies do not interfere with the originally licensed, inactivated whole virus vaccine; however protection from clinical disease has not been provocatively tested in foals less than 6 months of age. Animals may be vaccinated more frequently with these products if risk assessment warrants.

Recombinant canary pox vector vaccine: Administration of a 3-dose primary vaccination series beginning at 4 to 6 months of age. There should be a 4 week interval between the first and second doses. The third dose should be administered at 10 to 12 months of age prior to the onset of the next mosquito season.

There are no data for the recombinant canary pox vector vaccine regarding maternal antibody interference. Protection from clinical disease has not been provocatively tested in foals less than 6 months of age. Animals may be vaccinated more frequently with this product if risk assessment warrants.

Inactivated flavivirus chimera vaccine: Administration of a 3-dose primary vaccination series beginning at 4-6 months of age. There should be a 4 week interval between the first and second doses. The third dose should be administered at 10 to 12 months of age prior to the onset of the next mosquito season.

There are no data for the inactivated flavivirus chimera vaccine regarding maternal antibody interference. Protection from clinical disease has not been provocatively tested in foals less than 6 months of age.  Animals may be vaccinated more frequently with this product if risk assessment warrants.

Foals of unvaccinated mares

The primary series of vaccinations should be initiated at 3 to 4 months of age and, where possible, be completed prior to the onset of the high-risk insect vector season.

Inactivated whole virus vaccines: Administer a primary series of 3 doses with a 30-day interval between the first and second doses and a 60-day interval between the second and third doses. If the primary series is initiated during the mosquito vector season, an interval of 3 to 4 weeks between the second and third doses is preferable to the above described interval of 8 weeks.

Data indicates that maternal antibodies do not interfere with the originally licensed product; however protection from clinical disease has not been provocatively tested in foals less than 6 months of age. Animals may be vaccinated more frequently with these products if risk assessment warrants.

Recombinant canary pox vaccine:  Administer a primary series of 3-doses with a 30-day interval between the first and second doses and a 60-day interval between the second and third doses. If the primary series is initiated during the mosquito vector season, an interval of 3 to 4 weeks between the second and third doses is preferable to the above described interval of 8 weeks.

There are no data for this product regarding maternal antibody interference. Protection from clinical disease has not been provocatively tested in foals less than 6 months of age. Animals may be vaccinated more frequently with this product if risk assessment warrants.

Inactivated flavivirus chimera vaccine: Administer a primary series of 3 doses with a 30-day interval between the first and second doses and a 60-day interval between the second and third doses. If the primary series is initiated during the mosquito vector season, an interval of 3 to 4 weeks between the second and third doses is preferable to the above described interval of 8 weeks.

There are no data for this product regarding maternal antibody interference. Protection from clinical disease has not been provocatively tested in foals less than 6 months of age.  Animals may be vaccinated more frequently with this product if risk assessment warrants.

Horses having been naturally infected and recovered:

Recovered horses likely develop life-long immunity, but this has not been confirmed. Consider revaccination if the immune status of the animal changes the risk for susceptibility to infection or at the recommendation of the attending veterinarian.  Examples of these conditions would include the long term use of corticosteroids and pituitary adenoma.

Share Button