American Association of Equine Practitioners

Core Vaccination Guidelines HEADING_TITLE

The AVMA defines core vaccinations as those “that protect from diseases that are endemic to a region, those with potential public health significance, required by law, virulent/highly infectious, and/or those posing a risk of severe disease. Core vaccines have clearly demonstrated efficacy and safety, and thus exhibit a high enough level of patient benefit and low enough level of risk to justify their use in the majority of patients.”  The following equine vaccines meet these criteria and are identified as ‘core’ in these guidelines.

  • Eastern/Western Equine EncephalomyelitisClick to Expand

    In the United States, equine encephalitides for which vaccines are available include eastern equine encephalomyelitis (EEE), western equine encephalomyelitis (WEE), Venezuelan equine encephalomyelitis (VEE) and West Nile Virus encephalomyelitis. The distribution of EEE has historically been restricted to the eastern, southeastern and some southern states (but disease incidence is also reported in the upper Midwestern states of Ohio, Michigan and Wisconsin). Outbreaks of WEE have been recorded in the western and mid-western states. Variants of WEE have caused sporadic cases in the northeast and southeast, most notably Florida. VEE occurs in South and Central America but has not been diagnosed in the United States for more than 35 years. The availability of licensed vaccine products combined with an inability to completely eliminate risk of exposure justifies immunization against EEE and WEE as core prophylaxis for all horses residing in or traveling to North America and any other geographic areas where EEE and/or WEE is endemic.

    Transmission of EEE/WEE/VEE is by mosquitoes, and infrequently by other bloodsucking insects, to horses from wild birds or rodents, which serve as natural reservoirs for these viruses. Human beings are also susceptible to these diseases when the virus is transmitted to them by infected mosquitoes; however, horse-to-horse or horse-to-human transmission by mosquitoes is highly unlikely, because the amount of virus in the blood of horses affected by EEE or WEE is small. The viremia that occurs with VEE is higher and direct horse-to-horse or horse-to-human transmission is possible. Of these 3 encephalidites, WEE has the lowest mortality (approx. 50%). Eastern equine encephalomyelitis is the most virulent for horses, with mortality approaching 90%. Epidemiological evidence indicates that young horses are particularly susceptible to disease caused by EEE. Venezuelan equine encephalomyelitis can also be lethal, however some horses develop subclinical infections which result in lasting immunity.

    The risk of exposure and geographic distribution of EEE and WEE vary from year-to-year with changes in distribution of insect vectors and reservoirs important in the natural ecology of the virus. EEE activity in mosquito and birds, and resultant disease in humans and equids, continues to cause concern along the East Coast and demonstrates northward encroachment. WEE has caused minimal disease in horses in the last two decades; however, the virus continues to be detected in mosquitoes and birds throughout the Western states. In addition, variants that cause clinical disease in equids have been detected in the eastern U.S.

    VEE is a reportable foreign animal disease. Epidemics of VEE occur when the virus undergoes genetic change and develops greater virulence in avian and mammalian hosts. These viral variants are able to multiply to high levels in the horse and then the horse becomes a reservoir in these outbreaks. Areas of Southern Texas, California, Louisiana, Mississippi, Alabama and the West Coast of Florida are likely at the most  risk for natural VEE encroachment from central and south America.  Vaccination against VEE is controversial because:

    1) Vaccination against a foreign animal disease may confound testing in the event of an outbreak. However, vaccination against EEE/WEE will also cause cross-reaction in VEE antibody testing.

    2) Experimental and field data have demonstrated that previous infection with either EEE or WEE may provide cross protection against VEE. It is therefore possible that EEE/WEE bivalent vaccination may provide partial cross-protection, although it is likely to be less than that following infection.

    3) A conditionally available modified live (MLV) vaccine has been released during previous outbreaks. Should an outbreak occur it is likely that this highly attenuated MLV would be released, and this MLV provides superior protection against transmissible viremia.

    In summary, horses that receive annual EEE/WEE vaccines may be partially protected against VEE infection. In the event of an outbreak, the availability of and vaccination with the highly effective MLV product would induce rapid, complete immunity while allowing for accurate surveillance before VEE specific vaccination.  The use of killed VEE vaccine should be performed only in high risk areas of the US, at the discretion of the attending veterinarian, or when necessary, with input from state agriculture officials. The use of the inactivated VEE vaccine is less effective that the MLV VEE vaccine, and will blunt responses to the MLV VEE vaccine.

    Vaccines:

    EEE/WEE vaccines currently available are formalin inactivated adjuvanted whole virus products. Early testing of bivalent (EEE/WEE) vaccines was performed by intracranial challenge with either EEE and WEE; the formalin inactivated preparations were shown to be highly efficacious protecting against clinical disease.

    Currently, the only available VEE vaccines are killed products; a MLV would be conditionally released in the face of an outbreak.

    Vaccination Schedules EEE/WEE:

    Adult horses previously vaccinated against EEE/WEE: Annual revaccination must be completed prior to vector season in the spring. In animals of high risk or with limited immunity, more frequent vaccination or appropriately timed vaccination is recommended in order to induce protective immunity during periods of likely exposure. In areas where mosquitoes are active year-round, many veterinarians elect to vaccinate horses at 6 month intervals to ensure uniform protection throughout the year, although this practice is not specifically recommended by manufacturers of vaccines.

    Adult horses, previously unvaccinated against EEE/WEE or of unknown vaccinal history: Administer a primary series of 2 doses with a 4 to 6 week interval between doses. Revaccinate prior to the onset of the next vector season and annually thereafter.

    Pregnant mares, previously vaccinated against EEE/WEE: Vaccinate 4 to 6 weeks before foaling.

    Pregnant mares, unvaccinated or having unknown vaccinal history: Immediately begin a 2-dose primary series with a 4 week interval between doses. Booster at 4 to 6 weeks before foaling or prior to the onset of the next vector season—whichever occurs first.

    Foals of mares vaccinated against EEE/WEE in the pre-partum period: 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 to12 months of age prior to the onset of the next mosquito season.

    In the southeastern U.S., due to earlier seasonal disease risk, vaccination may be started at 2 to 3 months of age. When initiating vaccinations in younger foals, a series of 4 primary doses should be administered, with a 4 week interval between the first and second doses and a 4 week interval between the second and third doses. The fourth dose should be administered at 10 to 12 months of age prior to the onset of the next mosquito season.

    Foals of unvaccinated mares or having unknown vaccinal history:  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. 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.  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 lifelong 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.

  • RabiesClick to Expand

    Rabies is an infrequently encountered neurologic disease of equids.  While the incidence of rabies in horses is low, the disease is invariably fatal and has considerable public health significance.  It is recommended that rabies vaccine be a core vaccine for all equids.

    Exposure occurs through the bite of an infected (rabid) animal, typically a wildlife source such as raccoon, fox, skunk, or bat. Bites to horses occur most often on the muzzle, face, and lower limbs. The virus migrates via nerves to the brain where it initiates rapidly progressive, invariably fatal encephalitis.

    Vaccines:

    Three vaccines are licensed for rabies prophylaxis in horses. All are inactivated tissue culture derived products. The vaccines are given by intramuscular injection and appear to be safe. Rabies is an excellent immunogen and these vaccines induce a strong serologic response after a single dose.

    Challenge studies demonstrating efficacy are required for licensing of all rabies vaccines (including those labeled for use in equids in the USA), published results are available for the most recently licensed equine Rabies vaccine. The challenge studies are conducted by the vaccine manufacturers as outlined in the Code of Federal Regulations (CFR) from the United States Department of Agriculture.

    Vaccination Schedules:

    (Veterinarians should read the label for each specific product recommendation.)

    Adult horses previously vaccinated against rabies:  Annual revaccination.

    Adult horses previously unvaccinated against rabies or having unknown vaccinal history:  Administer a single primary dose. Revaccinate annually.

    Pregnant mares, previously vaccinated against rabies:  Vaccinate 4 to 6 weeks before foaling. Alternatively, veterinarians may recommend that mares be vaccinated with rabies vaccine before breeding. Duration of immunity is such that antibodies to rabies virus are maintained at sufficient levels in mares vaccinated prior to breeding as to provide passive immunity through colostrum to the foal. Administration of rabies vaccine prior to breeding of the mare reduces the number and type of vaccines given in the period prior to foaling.

    Pregnant mares, previously unvaccinated or of unknown vaccinal history:  Vaccinate 4 to 6 weeks before foaling.

    Foals of mares vaccinated against rabies:  Administer a primary 2 dose series. The first dose of vaccine should be administered no earlier than 6 months of age. The second dose should be given 4 to 6 weeks later. Revaccinate annually thereafter. This schedule avoids maternally-derived antibody (MDA) interference with induction of a serologic response in the foal.

    Foals of mares NOT vaccinated against rabies: Administer according to label directions.  The first dose of vaccine should be administered at 3 to 4 months of age.  Revaccinate annually thereafter.

    Foals of mares of unknown vaccinal history - Follow one of two rational options:

    1.  Assume the mare to be antibody-positive and follow the above recommendations for foals from mares known to be vaccinated against rabies, i.e. the first dose starting at 6 months of age followed by second dose 4 - 6 weeks later.  Revaccinate annually thereafter.

    2.  Document the rabies antibody status of the foal by testing serum collected from the foal at 24 hours of age or older, or from the dam during the peri-parturient period.*  If the foal of mare is rabies antibody-negative, follow the above recommendations for foals of mares known not to be vaccinated against rabies.  If the foal or mare is rabies antibody-positive, follow recommendations for foals of mares known to be vaccinated against rabies.

    *Testing for rabies antibodies using the rapid fluorescence focus inhibition test (RFFIT) is available through the Kansas State Veterinary Diagnostic Laboratory, Mosier Hall O-245, 1800 Denison Avenue, Manhattan, KS 66506-5601.  Further information is available at www.vet.k-state.edu.

    Horses exposed* to confirmed rabid animal

    Horse currently vaccinated against rabies with one of the USDA-approved rabies vaccines:  Immediate revaccination by a licensed veterinarian and observation (as directed by public health officials) for 45 days for development of clinical signs of rabies.

    Unvaccinated horse: Contact public health officials immediately as they will have established requirements and conditions for the monitoring and/or disposition of exposed, unvaccinated animals. These officials will dictate what options are available for the exposed horse. (These options may include isolation and immediate post-exposure immunization of the horse).

    Alternatively, the horse can be euthanatized immediately. If the owner is unwilling to have this done, then the horse should be closely monitored under veterinary supervision for 6 months (per approval from the appropriate public health officials).

    *Rabies exposure and transmission occur only when the virus is introduced into bite wounds, into open cuts in skin, or onto mucous membranes from saliva or other potentially infectious material such as neural tissue.

  • Tetanus Click to Expand
    All horses are at risk of development of tetanus, an often fatal disease caused by a potent neurotoxin elaborated by the anaerobic, spore-forming bacterium, Clostridium tetani. Tetanus toxoid is a core equine vaccine and is indicated in the immunization program for all horses.

    Clostridium tetani organisms are present in the intestinal tract and feces of horses, other animals and humans, and are abundant as well as ubiquitous in soil. Spores of Cl. tetani survive in the environment for many years, resulting in an ever-present risk of exposure of horses and people on equine facilities. Tetanus is not a contagious disease but is the result of Cl. tetani infection of puncture wounds (particularly those involving the foot or muscle), open lacerations, surgical incisions, exposed tissues such as the umbilicus of foals and reproductive tract of the postpartum mare (especially in the event of trauma or retained placenta ).

    Vaccines

    Vaccines currently available are formalin-inactivated, adjuvanted toxoids. Tetanus toxoid is a potent antigen that rapidly induces strong serological responses. Circulating antibody is able to mediate complete protection against tetanus. It is generally accepted that tetanus toxoid administered per manufacturer recommendations is both safe and effective.

    A 6-month study comparing serologic responses of equids to commercial vaccines demonstrated significant IgG response for the duration of the study. The end point for antibody persistence was not explored and may potentially be longer than the 6 months stated in the study. Extending the revaccination interval beyond the manufacturer’s recommendation for annual revaccination is not advisable due to a veterinarian’s liability if label recommendations are not followed.

    There are no challenge studies that have been published to document the onset or duration of immunity induced by tetanus toxoid products available for use in horses in the USA. Conclusions regarding the efficacy of products used in the USA are based on serologic response in laboratory animals and field experience. This may be accepted as evidence of vaccine efficacy as antibody alone can mediate protection. Tetanus has rarely been documented in vaccinated horses in the USA, illustrating the variability of response of equids to any biologic product. Note: Survival of horses with tetanus was strongly associated with previous vaccination.

    Vaccination Schedules

    Adult horses, previously vaccinated against tetanus: Vaccinate annually. Horses that sustain a wound or undergo surgery 6 or more months after their previous tetanus booster should be revaccinated with tetanus toxoid immediately at the time of injury or surgery.  Note: The severity of the wound does not predict the risk for development of tetanus. Superficial wounds have resulted in clinical tetanus in horses.

    Adult horses, previously unvaccinated against tetanus, or of unknown vaccinal history: Administer a primary 2-dose series of tetanus toxoid with a 4- to 6-week interval between doses. Protective concentrations of immunoglobulin are usually attained within 14 days of the second dose of vaccine. Vaccinate annually thereafter.

    Tetanus antitoxin is indicated to provide passive immunity in situations where the horse is at risk of tetanus infection and has not been immunized according to labeled recommendations for tetanus. If the veterinarian determines that administration of tetanus antitoxin is indicated, then it should be administered in one site and the initial dose of a priming series of tetanus toxoid vaccinations in a distant muscular site. The rare, but fatal, risk of Theiler’s disease consequent to the use of tetanus antitoxin needs to be taken into consideration when determining if use is indicated.

    Pregnant mares previously vaccinated against tetanus: Vaccinate annually 4 to 6 weeks before foaling, both to protect the mare should foaling-induced trauma or retained placenta occur and to enhance concentrations of colostral immunoglobulins.

    Pregnant mares unvaccinated against tetanus or of unknown vaccinal history: Administer a 2-dose primary series of tetanus toxoid with a 4 to 6 week interval between doses. Revaccinate 4 to 6 weeks before foaling.

    Foals of mares vaccinated against tetanus in the pre-partum period: Administer a primary 3-dose series of tetanus toxoid 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.

    Foals of unvaccinated mares or mares of unknown vaccinal history: Administer a primary 3-dose series of toxoid beginning at 1-4 months of age with 4-week intervals between doses. Serologic data indicates that a 3-dose initial series produces a more consistent anamnestic response in all foals, regardless of the age at which the series is initiated.  Tetanus antitoxin is indicated to provide passive immunity in situations where a foal is born to a non-vaccinated mare and is at risk of tetanus infection.(SeeTetanus antitoxin above.)

    Horses having been naturally infected with tetanus and recovered:  Revaccinate annually.

  • West Nile VirusClick to Expand

    West Nile virus (WNV) is the leading cause of arbovirus encephalitis in horses and humans in the United States. Since 1999, over 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 and 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 as 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.

    There are three challenge models that 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 in the atlanto-occipital space) challenge model 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; 2) an aid in reduction of viremia, encephalitis and clinical disease; 3) an aid in prevention of disease, viremia, and encephalitis; or 4) an 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 adjuvant. Label 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 (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.