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West Nile virus (WNV) is the leading cause of arbovirus encephalitis in horses and humans in the United States. Since 1999, more than 25,000 cases of WNV encephalitis have been reported in U.S. horses. Horses represent 96.9% of all reported non-human mammalian cases of WNV disease.

This virus has been identified in all of the continental United States, most of Canada and Mexico. Several Central and South American countries have also identified WNV within their borders. The virus is transmitted from avian reservoir hosts by mosquitoes (and infrequently by other bloodsucking insects) to horses, humans and a number of other mammals. West Nile virus is transmitted by many different mosquito species and this varies geographically. The virus and mosquito host interactions result in regional change in virulence of the virus; therefore, no prediction can be made regarding future trends in local activity of the viruses. Horses and humans are considered to be dead-end hosts for WNV; the virus is not directly contagious from horse to horse or horse to human. Indirect transmission via mosquitoes from infected horses is highly unlikely because these horses do not circulate a significant amount of virus in their blood.

The case fatality rate for horses exhibiting clinical signs of WNV infection is approximately 33%. Data have supported that 40% of horses that survive the acute illness caused by WNV still exhibit residual effects, such as gait and behavioral abnormalities, 6 months post-diagnosis. Thus vaccination for West Nile virus is recommended as a core vaccine and is an essential standard of care for all horses in North America.

Three challenge models have been used to license currently available vaccines.  The mosquito and needle challenge were the two models used in early studies. These challenge models result in 90% of nonvaccinated control horses developing viremia, while only 10% of these horses demonstrated clinical disease.  More recently, the intrathecal infection challenge model (by injection in the atlanto-occipital space) has been employed. In this model, 70 to 90% of nonvaccinated control horses become viremic and 90 to 100% develop grave signs of encephalomyelitis.

West Nile virus vaccines are licensed either as 1) an aid in prevention of viremia, or 2) aid in reduction of viremia, encephalitis and clinical disease, or 3) aid in prevention of disease, viremia, and encephalitis or 4) aid in prevention of viremia and mortality, and an aid in reduction of severity of clinical disease.

Vaccines:

Four USDA licensed vaccines are currently available (two are inactivated whole WN virus vaccines, one is a non-replicating live canary pox recombinant vector vaccine and one is an inactivated flavivirus chimera vaccine):

Inactivated whole virus vaccines with an adjuvantLabel instructions call for a primary vaccination series of two intramuscular injections administered 3 to 6 weeks apart followed by a 12-month revaccination interval. These products are labeled as an aid in prevention of viremia or as an aid in prevention of viremia and mortality and an aid in reduction of severity of clinical disease.   

Recombinant canary pox vaccine with protective antigens expressed in a vaccine strain canary pox vector which does not replicate in the horse. The vaccine contains an adjuvant. Label instructions call for a primary vaccination series of two intramuscular injections administered 4 to 6 weeks apart followed by a 12-month revaccination interval. The product is labeled as an aid in prevention of disease, viremia, and encephalitis.

Inactivated flavivirus chimera vaccine with protective antigens expressed in a vaccine strain yellow fever virus vector and contains an adjuvant. Label instructions call for a primary vaccination series of two intramuscular injections administered 3 to 4 weeks apart followed by a 12-month revaccination interval. This product is labeled as an aid in reduction of disease, encephalitis and viremia.

All of the current WN vaccine products carry one-year duration of immunity, with challenge, consistent with their respective label claims.

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. Only one of the currently licensed WN vaccines carries a safe for use in pregnant mare label claim.  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 interference 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 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 the originally licensed, inactivated whole virus vaccine; however, protection from clinical disease has not been prospectively 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 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 inactivated flavivirus chimera vaccine regarding maternal antibody interference. Protection from clinical disease has not been prospecitvely 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 4-week interval between the first and second doses and an 8-week 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.

Recombinant canary pox vaccine:  Administer a primary series of 3-doses with a 4-week interval between the first and second doses and an 8-week 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.

Inactivated flavivirus chimera vaccine: Administer a primary series of 3 doses with a 4-week interval between the first and second doses and an 8-week 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.

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 equine pituitary pars intermedia dysfunction (PPID).