Keeping Horses Healthy with Proper Vaccine Protocols

Beth Davis, DVM, PhD, DACVIM

Professor, Equine Internal Medicine

It is spring and this is a great time to get your horse up to date on annual health needs like dental floatation, vaccinations and fecal egg count to determine parasite burdens. When it comes time to determine which vaccines to administer to your horse, this can be a bit of a daunting task. The great news is that there are some outstanding resources for horse owners to help make well informed decisions. Naturally, talking with your veterinarian is a perfect place to start! If we can be of any assistance please feel free to call the equine clinicians the VHC Large Animal Desk at 785-532-5700.

Recommendations made by veterinarians in the field are established by the American Association of Equine Practitioners (AAEP). Specifically, horse owners can go to www.aaep.org and follow the link to Vaccine Guidelines to learn more information about vaccines and protocols. Vaccine manufacturers are stringent with regard to vaccine preparation regarding both safety and efficacy (protection from disease). Veterinarians that administer vaccines document exact details regarding the site of vaccine administration in the medical record. Additionally, they also provide horse owners with this information as well as expectations following vaccination. Typically horses are rested or only have light exercise for 1-2 days following vaccination. Vaccine sites should be closely monitored for soreness, swelling or heat. Although mild swelling in the area may occur, marked pain or swelling is uncommon and may indicate an adverse reaction. If any abnormalities are noted, such as injection site pain or fever the veterinarian should be contacted immediately.

Most vaccine protocols are initiated in the first year of life once maternal antibodies from colostrum have waned. On average equine vaccines are started at six months of age. The primary series includes 3 vaccines, the first one at 6 months of age, second dose 3-4 weeks after the first one and a third one approximately 60-90 days later. At 1 year of age the yearling can receive a booster vaccine for each one they have been primed for in the initial series which will provide an amnestic (memory) response to provide protection for the remainder of the year (Figure 1). From an immunologic perspective, if we only administer an initial vaccine without providing the additional booster vaccines we are unlikely to induce a pronounced memory response. If a memory response is not induced heightened immunity will not be present to protect the horse for a sustained period of time.

AAEP recommendations divide vaccines into Core and Risk-Based vaccines. Core vaccines have been determined to be essential for all horses, the diseases that are vaccinated against are serious to the point of potentially leading to fatal disease or may pose serious risk to humans if exposed to infected horses (zoonotic potential). Those pathogens that may cause fatal disease in horses include: Eastern equine encephalomyelitis (EEE), western equine encephalomyelitis (WEE), west Nile virus (WNV), and tetanus. The pathogen of zoonotic concern is rabies. The overall number of rabies positive cases among animals in Kansas were higher in 2015 than they have been in several years, so this is an important reminder that rabies is present in our local environment and horses need to be well vaccinated, even if they do not live outdoors at all times (Figure 2).

Core Vaccines:

Equine encephalomyelitis (sleeping sickness): inactivated vaccines are currently available for eastern equine encephalomyelitis (EEE) and western equine encephalomyelitis (WEE). Horses residing in southern Texas may also benefit from Venezuela equine encephalomyelitis (VEE) vaccination. The initial vaccine series should include a series of 3 vaccines at 3-6 week intervals. Depending on the geographic location, age and health status of the individual horse booster vaccines should be administered 1-2 times annually.

Encephalitic viruses are transmitted by mosquitoes that feed on infected reservoir hosts, such as certain types of birds. The mosquito acts as a vector to transmit the viral disease to horses. It is for this reason that it is ideal to booster vaccinate horses during the spring months so that they have optimal antibody levels in circulation at the time of peak exposure, which typically coincides with high mosquito levels in the warm summer months. In areas where there is an extended warm season or high risk individual it is often ideal to consider a booster vaccine again in July or August to ensure adequate antibody levels through the entire warm season while risk remains present.

West Nile Virus (WNV): protection against WNV induced encephalomyelitis, similar to other vector-mediated diseases requiring strict attention to control of insects as well as an effective vaccine program.

Recommendations include vaccination with a primary series at an approximate 4 week interval followed by a 12-month booster.

Tetanus: Clostridium tetani is a bacterial species that is present in large numbers in the soil and specifically in the equine environment which originates from equine feces. Therefore, due to the frequency of this bacterium in the equine environment optimal tetanus immunization protocols should be maintained for all horses. After an initial series in the first year of life all horses should be vaccinated with an annual booster.

Rabies: Rabies is a serious viral pathogen that causes fatal illness in animals and humans in many parts of the world. In North America the application of vaccine protocols substantially limits the number of cases, particularly among domesticated mammals. However, using the Kansas State Veterinary Diagnostic Laboratory (KSVDL) data as a point of reference, it is apparent that rabies remains a continued threat among wildlife populations, particularly in bats and skunks (Figure 2). Since horses often live in an outdoor environment, they have the potential to be exposed to skunks. For this reason as well as the zoonotic potential of this virus, the AAEP has included equine rabies vaccination among core vaccine protocols. All horses should be vaccinated against rabies on an annual basis. In Kansas, the law states that rabies vaccination must be performed by a licensed veterinarian.

Vaccines for pregnant broodmares: Other considerations for vaccine administration include how to manage pregnant broodmares. Healthy broodmares should be well vaccinated prior to breeding, once in foal they should be vaccinated to protect against diseases that may cause abortion (see below for risk based vaccine examples). Then during the stage approximately 30 days before expected foaling date mares should receive a booster vaccine against routine pathogens that they have been previously vaccinated against. This will allow for the mare to increase her antibody levels in colostrum, providing the foal will optimal immunity for the first several months of life until foal vaccines are initiated at approximately 6 months of age.

Risk Based Vaccines:

The interested reader is encouraged to visit the www.AAEP.org website to learn about all available risk-based vaccine options and through work with their veterinarian determine the optimal protocol for their horses.

Equine respiratory viruses: EHV-1, EHV-4 and equine influenza

EHV-1 is associated with respiratory disease (rhinopneumonitis), abortion, and neurologic disease. EHV-4 is most commonly associated with respiratory disease (rhinopneumonitis). All horses on breeding and training farms should be regularly vaccinated for rhinopneumonitis (EHV-1/4).

Additionally, vaccination against EHV-1 is recommended for use in pregnant mares to aid in the prevention of abortigenic EHV-1 and prevention of EHV-1 / 4 induced respiratory disease (rhinopneumonitis) in foals, weanlings, yearlings, young performance and show horses that are included in the high risk group for viral exposure. Well planned vaccine protocols are an important component of maintaining horse health against EHV-1 and EH-4 associated disease.

Similar to previous comments, an initial series of 3 vaccines is recommended in the first year of life, using a 4 to 6 week interval between doses. When administering the initial series in foals, vaccination should be initiated at 4 to 6 months of age with a 4 to 6 week interval between the first 2 doses and the third dose being given at 10 to 12 months of age. Booster vaccines are recommended to maintain an adequate immunity at approximately 6 month intervals. Pregnant brood mares should be vaccinated at 5, 7 and 9 months of gestation with an inactivated product to protect against abortion (high antigen load). Many clinicians also utilize a 3 month vaccine. In order to provide adequate colostral antibody protection a booster vaccine with an inactivated EHV-1/4 vaccine should be administered 4 to 6 weeks before expected foaling. Horses maintained on breeding farms such as barren mares, stallions and teaser stallions should be vaccinated at the beginning of the breeding season with 6 month booster vaccines administered. Adult horses should be vaccinated every 6 to 12 months, young horses (high risk, travel, commingling) will be optimally protected with booster vaccines every 6 months.

Equine Influenza: equine influenza is one of the most common viral pathogens of the equine respiratory tract. Equine influenza rarely circulates among equine populations; rather it is introduced by an infected individual. Since this virus is rapidly removed by the immune system in horses, appropriate quarantine protocols (14 day quarantine upon arrival) and effective vaccine strategies can substantially limit the potential for disease outbreak. Therefore, all horses on breeding and training facilities should be regularly vaccinated for equine influenza. Risk factors that have been identified to be associated with disease include: 1) young horses, 1 to 5 years, 2) ineffective serum concentrations of influenza specific antibodies, 3) high risk environment such as frequent contact with large numbers of horses.

Influenza is rapidly spread among groups of horses through the transmission of virus containing fluids such respiratory secretions from infected individuals (coughing). Vaccination with an approved vaccine with known efficacy should be selected for use as an aid to prevent disease. Similar to other vaccine protocols, careful consultation with an equine veterinarian can aid in establishment of an optimal vaccine program for your horse. In the first year of life, once maternal antibodies have waned vaccination with a series of 3 vaccines is ideal. Subsequently adult horses that have previously been immunized and are at risk for exposure can be effectively vaccinated 1-2 times annually, depending on risk of exposure.

Selected references:

Davis EG, Bello NM, Bryan AJ, et al. Characterization of immune responses in healthy foals when a multivalent vaccine protocol was initiated at age 90 or 120 days. Equine Vet J, 2015; 47(6):667-674.

Davis EG, Vaccine Programs, In Robinson’s Equine Medicine, 7^th Ed., Editors: Sprayberry and Robinson. Elsevier Saunders, St. Louis, MO. 2015, pp. 196-200.

Tizard IR, The Use of Vaccines, Veterinary Immunology, 9^th Ed. Editor: Tizard. Elsevier Saunders, St. Louis, MO. 2013, pp. 272-282.