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West Nile Virus Vaccination Guidelines

Developed by the American Association of Equine Practitioners

This information is intended as a supplement to the AAEP's Vaccination Guidelines (January 2008). Practitioners are directed to consult this publication as the following discussion is to be read in conjunction with those general guidelines for vaccination.

West Nile virus (WNV) infection was first diagnosed in horses in the United States in 1999 and is now an important consideration in the differential diagnosis of horses presenting with signs of neurologic disease in all areas of North America. West Nile virus, a flavivirus, was first identified as a cause of infection and fatal encephalomyelitis (inflammation of the spinal cord and brain) in horses and people in Egypt, Uganda and France in the early 1960s. Further epizootics of disease in horses have occurred in Morocco in 1996, Italy in 1998, France in 2000, and the United States from 1999 to the present. Since 1999, thousands of cases of equine West Nile Virus encephalomyelitis have been reported in the US, with an estimated 30 to 40 percent fatality rate. West Nile virus is now considered to be endemic in all areas of North America.

The flaviviruses, like the other encephalomyelitis viruses, are transmitted by mosquitoes, and infrequently by other bloodsucking insects, to horses, human beings, and a number of other mammals from avian hosts, which serve as natural reservoirs for these viruses in nature. Horses and humans are considered to be dead-end hosts of the West Nile virus and, therefore, do not contribute to the transmission cycle. The virus is not directly contagious from horse to horse or horse to human. Similarly, indirect transmission via mosquitoes from infected horses is highly unlikely because horses do not experience a significant viremia (i.e. they have negligible amounts of virus circulating in their blood).

The incubation period for West Nile virus in horses appears to be 3 to15 days. Clinical signs of WNV infection in horses may include fever, ataxia (stumbling or incoordination), depression or apprehension, stupor, behavioral changes, weakness of limbs, partial paralysis, droopy lip, teeth grinding, muscle twitching, muscle fasciculation, muzzle tremors, difficulty rising, recumbency (inability to rise), convulsions, blindness, or death. Data has supported that 40% of horses that survive the acute illness caused by WNV exhibit residual effects, such as gait and behavioral abnormalities that were attributed to the illness by owners, 6 months following diagnosis.

The variable clinical signs associated with WNV infection necessitate inclusion of many neurological disorders in the differential diagnoses. These include: rabies; equine protozoal myeloencephalitis (EPM); neurological equine herpesvirus-1; botulism; eastern, western and Venezuelan encephalomyelitis (EEE,WEE,VEE); heat stress; trauma; bacterial meningitis; cervical vertebral myelopathy (wobbler syndrome); myeloencephalopathy; and equine degenerative myelopathy.

Serologic tests (blood tests) used to diagnose WNV include plaque reduction neutralization (PRNT), virus neutralization, hemagglutination inhibition, complement fixation, ELISA and antigen (IgM and IgG) capture ELISA. The virus can also be identified in central nervous system tissue using techniques such as virus isolation, PCR and immunohistochemistry. The IgM-capture ELISA is currently the most reliable test for confirmation of recent exposure to West Nile Virus in a horse exhibiting clinical signs. Horses exposed to WNV typically develop a sharp rise in West Nile virus-specific IgM antibody that persists for 4-6 weeks after infection. Little IgM is demonstrated in horses that are recently vaccinated. The antibody measured by the PRNT is stimulated both by vaccination and recent exposure, making this test difficult to interpret in the suspect horse.

Risk of exposure and geographic distribution of West Nile virus vary from year to year with changes in distribution of insect vectors and reservoirs of the virus. Because of the unpredictable nature of those factors and the effects of the disease, it is recommended that all horses in North America be immunized against West Nile virus.

Preventive management practices may minimize the risk of the spread and transmission of West Nile virus from infected mosquitoes. Reduction of mosquito numbers and exposure can be achieved by reducing or eliminating stagnant or standing water in your area, removing old tires, keeping horses in the barns from dusk to dawn (prime mosquito feeding times), setting out mosquito traps, keeping air moving with fans, use of mosquito fish in water troughs and removing organic debris promptly. Chemical controls include the use of topical anti-mosquito repellent agents approved for the horse and use of mosquito dunks in areas of standing water.

Vaccination is the primary method of reducing the risk of infection from West Nile Virus to the horse, although clinical disease is not completely prevented. There are several licensed vaccines currently available. One is an inactivated (killed) vaccine, one is a recombinant canarypox vector vaccine, and the third is an inactivated flavivirus chimera vaccine. Vaccination with a two-dose series of any of these vaccines has been proven in an intrathecal challenge model to be effective in the prevention of viremia in experimentally infected horses 28 days after vaccination.  In contrast, non-vaccinated control horses in that study developed both viremia and neurological signs. Earlier studies using mosquito and needle challenges also demonstrated protection against viremia in vaccinated horses. The results of the studies to date indicate high efficacy of vaccination. Given the widespread nature of the disease and efficacy of vaccination, WNV vaccination is recommended as a core vaccine for all horses in North America. Consult your veterinarian to discuss selection of the most appropriate vaccine and vaccination schedule for your horses.

Vaccination Schedules: (from AAEP’s Core Vaccine Guidelines)

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 as well as those horses that are immunocompromised. Therefore, more frequent vaccination is recommended to meet the vaccination needs of these horses.

Booster vaccinations are warranted according to local disease or exposure risk. Protection against disease for 12 months is likely with all WNV vaccines. 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 adult horses with a 2nd dose given 3-4 weeks after 1st dose. The label recommended revaccination interval is 12 months or prior to the onset of the next vector season.

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 killed vaccine. It is unknown if this is true for the other products. Vaccination of naïve mares while open is a preferred strategy. 

Foals of unvaccinated mares 

First dose: given at 3 to 4 months of age. Second dose: 4 weeks after 1st dose. Third dose: 60 day interval after 2nd dose. 

Foals of vaccinated mares

Administer a primary 3-dose series beginning at 4 to 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 this product; however protection from clinical disease has not been provocatively tested in foals.

Animals may be vaccinated more frequently with this product if risk assessment warrants.

Recombinant canary pox vector vaccine: Administer a primary 3-dose series beginning at 4 to 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.

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

Inactivated flavivirus chimera vaccine: Administer a primary 3-dose series beginning at 4 to 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.

Animals may be vaccinated more frequently with the 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 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.

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. 

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.

Horses having been naturally infected and recovered

Recovered horses likely develop life-long immunity. Consider revaccination only if the immune status of the animal changes the risk for susceptibility to infection. Examples of these conditions would include the long-term use of corticosteroids and pituitary adenoma.

Foals/Weanlings

Yearlings

Performance Horses

Pleasure Horses

Broodmares

Comments

Born to unvaccinated mare: First dose: 3 to 4 months.

Second dose: 30 days after 1st dose.

 

Third dose: 60 days after 2nd dose.

 

Born to vaccinated mare: First dose: at 4-6 months of age. 2nd dose: 4-6 weeks after 1st dose. Third dose: 10-12 months of age.

Annual booster, prior to expected risk. Vaccinate semi-annually or more frequently (every 4 months), depending on risk.

 

Annual booster, prior to expected risk. Vaccinate semi-annually or more frequently (every 4 months), depending on risk.

Annual booster, prior to expected risk. Vaccinate semi-annually or more frequently (every 4 months), depending on risk.

Annual,

4 to 6 weeks prepartum (see full text inguidelines).

Annual booster is after primary series. In endemic areas, booster as required or warranted due to local conditions condusive to disease risk. Vaccinate semi-annually or more frequently (every 4 months), depending on risk. 

Note: As with the administration of all medications, the label and product insert should be read before the administration of all vaccines.