The Importance of Equine Vaccinations

By Lauren Lamb, DVM, MS

Vaccinating horses is crucial to overall health maintenance and essential to responsible ownership. It is important to protect the horses against diseases they could be exposed to whether they travel or stay at the stable or farm.

The number of diseases a horse is exposed to is directly proportional to the number of vaccinations necessary to ensure adequate protection. Horses that live a more isolated lifestyle will be exposed to a lower number of diseases and will need to only receive the core vaccinations. On the other hand, a horse that is showing and traveling is exposed to a greater number of diseases. These horses need to receive the core vaccinations plus additional vaccinations due to increased risk.

Another consideration when deciding which vaccines to administer to a horse is treatment costs versus vaccination costs and following boosters.

A good example of this scenario would be the rabies vaccination, which is relatively cheap. A rabies vaccinated horse is not 100 percent protected from the disease. However, the risk of the horse developing the disease if exposed is significantly decreased. The flip side of the coin would be if you did not vaccinate your horse for rabies, which leaves them at significant risk of developing the disease if exposed.  This is compounded by the fact that there is no treatment for a horse with rabies.

Vaccines can be divided in to four large groups based on how they are made and how they work when administered: live vaccine, modified live vaccine, recombinant vaccine and killed vaccine.  They all contain antigens, parts of the disease-causing organism, that will train the immune system, but they will not cause a clinical disease in a horse.

Live and modified live vaccines are capable of inciting an immune response most like the natural occurring disease without causing the horse to develop the disease. Because they cause such a strong immune response, a booster vaccine does not need to be administered as frequently as the recombinant and killed vaccine. It is very important to administer live and modified live vaccines as directed on the label. Failure to follow labeled directions can result in significant complications or decreased efficacy of the vaccine.

Recombinant and killed vaccines incite an immune response that is sufficient to protect the horse from disease, but less like the natural disease compared to the live and modified live vaccine. Recombinant and killed vaccines require more frequent booster vaccinations, due to the less natural response to these vaccines. Deciding which vaccines to use depends on what vaccines are available and a local veterinarian’s recommendation.

All vaccines are given to a horse with the goal of the horse mounting an immune response to the vaccine. This immune response trains the horse’s immune system to recognize certain characteristics of the disease-causing organism, which in most cases is a bacteria or virus. The horse’s immune system can then react in a timely and aggressive manner that will rid the body of the disease. Without the prior vaccination, the horse’s immune system may not react to the inciting organism in a timely manner, allowing the disease to be established in the horse.

The American Association of Equine Practitioners established guidelines and recommendations for vaccinating horses depending on the level of exposure to diseases, age and pregnancy status.

Eastern and Western Equine Encephalitis, West Nile Virus, Rabies and Tetanus are the core vaccines that each horse living in North America should receive. The core vaccines are based on the high risk of any horse living in North America being exposed to one of these disease. These vaccines provide a very high level of protection but not 100 percent.

Eastern Equine Encephalitis, Western Equine Encephalitis and West Nile Virus are all viruses that affect the horse’s central nervous system. They cause clinical signs of dullness, depression, muscle fasciculation or ataxia. They are all spread by mosquitoes and are seen more commonly in the summer months when the mosquitoes are more prevalent.

Eastern Equine Encephalitis is more commonly seen in the eastern portions of North America and WEE is seen in the western portion of North America. West Nile Virus can be seen in all regions of North America and is the leading cause of encephalitis cases in horses. Horses that have WNV, EEE or WEE have a fair to poor prognosis.

Rabies is a virus that a horse contracts by being bit by another rabid animal. The virus migrates from the bite wound up the peripheral nerves and attacks the horse’s central nervous system. As stated before, no treatment currently exists for rabies.

Tetanus is a disease caused by a neurotoxin that is produced by Clostridium Tetani bacteria. This bacterium is found everywhere in the environment. Once the bacteria gains access to a wound, it will replicate and release the neurotoxin into the horse’s body. If a horse contracts tetanus, there is a guarded prognosis for recovery.

Risk-based vaccines are vaccines that are administered based on the horse’s level of exposure to a disease. Three commonly used vaccines in our practice are Strangles, Equine Influenza, and Equine Herpes.

Strangles vaccine works against the Streptococcus equi subspecies equi bacteria and helps prevent or limit the strangles disease in a horse. It is usually administered to young horses in environments known to have the strangles bacteria present.

Equine Influenza and Equine Herpes are both respiratory viruses that can be spread by aerosolized respiratory droplets. They are extremely contagious and are seen in locations with a high number of horses from many different locations including large horse shows. Consulting a local veterinarian about which vaccines would be prudent given its level of exposure is recommended.

The frequency that a horse should be vaccinated depends on several factors including age, vaccination history and pregnancy status. In general, EEE, WEE and WNV vaccines should be administered in the spring before the mosquito season starts. Most veterinarians will administer the remainder of the core vaccines at this time. Risk-based vaccines may need to be given more frequently depending on the level of risk that the horse is exposed to.

The bottom line is, consult with your veterinarian and develop a vaccination program that works for you and your horse.

For more information on equine vaccination, visit www.aaep.org. The AAEP website provides horse owners with detailed information about core and risk-based vaccines.

This article originally appeared in the January 2016 issue of Oklahoma Farm & Ranch.