Balancing Risk and Vaccination

Aside from careful management and limiting exposure to disease, vaccination is one of the most practical and cost-effective means for reducing infectious disease incidence in horses. Various pharmaceutical companies offer dozens of equine vaccines against a variety of diseases, and they frequently adjust vaccine types and availability. These factors and many others can make decisions about vaccinations confusing, so it’s not surprising that horse owners and managers often turn to their veterinarian for advice about vaccine selection and scheduling.

In the case of the four “core vaccines” recommended by the American Association of Equine Practitioners (AAEP), the decision is easy. All horses in North America should be vaccinated for tetanus, West Nile virus, rabies, and Eastern/­Western equine encephalomyelitis annually. But deciding whether to have your veterinarian also administer so-called “risk-based vaccines” is not as easy. You should base these decisions on a balance between the benefits and costs of vaccinating for that particular disease versus the risk of not vaccinating. This balance depends on a host of factors specific to your region, your horses, the vaccine in question, and your management practices. Your veterinarian is equipped to help you determine the factors to consider in your specific situation and to formulate an effective and reasonable vaccination plan for all your horses. 
Infectious diseases in horses are caused mostly by bacterial and viral organisms. These organisms, or infectious agents, are transmitted to horses in many different ways specific to the agents. Modes of transmission might include direct contact between horses, insect bites, and contact with contaminated equipment, among many others. Whether a horse becomes infected in a given situation depends on several factors, such as his susceptibility to disease, the dose of infecting organism he receives, and the organism’s ­characteristics. 
Vaccination prompts the body to produce antibodies (proteins that bind directly to the agent and help the body get rid of it) against a specific infectious agent. Along with keeping horses in excellent general health and implementing biosecurity measures, vaccination is an effective and convenient way to boost immunity against a specific agent.
The Risk-Based Vaccines
Risk-based vaccines are those that veterinarians recommend in specific circumstances, when the risk of acquiring the disease in question exceeds any disadvantage of using the vaccine. Risk-based vaccine use varies regionally, from population to population within an area, or among horses within a given population. Your veterinarian knows the status of regional diseases of concern and should be up-to-date on information about new vaccines and pertinent state and federal regulations. Taking into ­consideration each horse’s history, management, and use, he or she can help you formulate a customized vaccination plan. 
So why not just vaccinate every horse with every vaccine? Most vaccines are not very expensive, but cost can still be a factor for many horse owners, especially those with large numbers of horses to vaccinate. In addition, there is always a ­possibility—albeit a small one—of adverse reactions to vaccination, some of which are life-threatening. This risk might increase as more vaccines are used ­simultaneously. 
For the purposes of this article, common risk-based vaccines include those for strangles, influenza, equine ­herpesviruses-1/4, equine viral arteritis, Potomac horse fever, rotavirus, botulism, and anthrax.
Strangles is a respiratory disease caused by the bacterium Streptococcus equi. The classic clinical sign of strangles is abscess formation behind and beneath the horse’s jaw. The name “strangles” itself comes from these large swellings, which rarely are severe enough to cause difficulty in swallowing and breathing. Along with the swellings, the disease typically starts with nasal discharge, fever, loss of appetite, lethargy, and coughing. Strangles is highly contagious among at-risk horses—particularly young horses and adult and older horses without history of prior exposure to the organism or the vaccine. S. equi is easily transmitted by indirect contact (via “fomites”: handlers’ hands, shared tack and equipment, shared water sources, etc.) with nasal secretions and organism-containing abscess drainage.
Preventing this disease with vaccination has proven difficult. Veterinarians most commonly use a ­modified-live intranasal vaccine; they consider this product to be somewhat effective, but it has been faulted for having a ­higher-than-acceptable adverse reaction rate. Other strangles vaccines are intramuscular, but they might cause abscess ­formation at the injection site. 
As is the approach with other risk-based vaccines, veterinarians usually recommend strangles immunization when the risk of infection outweighs the risk of vaccine side effects. Be sure you take steps to reduce horses’ exposure to the organism. Recommended management includes quarantine of any new horse before introducing him into the rest of your herd and of any horses showing signs of the disease. 
This common virus typically causes high fever, cough, and nasal discharge, along with other clinical signs. It is highly contagious and spreads rapidly when coughing horses expel it, either via aerosol or nasal secretions. As with all influenza viruses, the equine version is constantly changing its form via antigenic drift (changes in the virus’ genetics make current vaccines not work as well or at all), which makes it difficult for manufacturers to develop vaccines targeting the most current virus strain.
Horses in contact with others (especially show horses and horses at stables with transient populations) are at greater risk for contracting the virus. The usual recommendation is to vaccinate these horses at least twice annually using one of several recently developed vaccines that cover the more current virus strains. The intranasal vaccine has been shown safe and effective, but several of the newer intramuscular vaccines are also considered effective. 
Potent toxins produced by the soil-dwelling bacterium Clostridium botulinum cause botulism. In horses this disease takes several forms, involving different types of toxins. Most commonly, C. botulinum toxins block transmission of nerve ­impulses, resulting in signs of weakness that progress toward complete paralysis and death. Botulism causes several syndromes with very different origins. For instance, the toxin can arise in the digestive tracts of foals that have ingested the organism; be released into the bloodstream from wounds where the organism has grown; or be ingested by horses in fermented feed or feed containing animal carcasses.1
The only approved botulism vaccine is designed to protect against the Type B toxin, which is associated with shaker foal syndrome. This is seen fairly commonly in foals in Kentucky and the mid-Atlantic states. Thus, horse owners in this geographic region might consider using this vaccine with their veterinarian’s direction. 
There are no licensed vaccines available for preventing botulism due to C. botulinum Type C or the other toxin types.
Equine Herpesvirus (EHV)
Also known as rhinopneumonitis, herpesviruses can cause respiratory problems, neurologic signs, or abortion in horses. The most common of these viruses areEHV-1 and EHV-4. In their chapter on infectious disease in the 4th edition of Large Animal Internal Medicine, David Wilson, DVM, MS, BVMS, MRCVS, professor of medicine and epidemiology at the University of California (UC), Davis, and Nicola Pusterla, DVM, PhD, Dipl. ACVIM, associate professor in the Department of Medicine and Epidemiology at UC Davis, state that “EHV-1 and EHV-4 are spread by direct and indirect contact with nasal secretions, by (contact with) aerosolized secretions from infected coughing horses, and, in the case of EHV-1, by (contact with) aborted fetuses, fetal fluids, and placentae associated with ­abortions.”
One unique characteristic of these viruses is they can lie dormant and undetected in infected horses and cause signs of disease when stress or other factors compromise immunity.1 This makes disease control challenging and helps explain how cases can appear in closed herds.
In general, vaccination does not prevent infection, according to the AAEP, but certain EHV-1/4 vaccines might help reduce signs of respiratory disease and viral shedding in infected animals. These should be given at least twice annually to all horses that will be in contact with other horses off the farm (e.g., show horses and those that travel frequently). Unfortunately, vaccination does not prevent the neurologic form of EHV-1. But due to its effect on viral shedding, it might reduceEHV-1’s spread, thus reducing overall neurologic disease incidence. Pregnant mares should be vaccinated as directed with a vaccine licensed to protect against the abortive form of the disease.  
Reduced shedding and spread is enough reason to vaccinate horses against EHV, even if direct protection against the disease is questionable. The vaccine likely won’t benefit older and isolated horses. 
Equine Viral Arteritis (EVA)
This respiratory and venereal disease is caused by the equine arteritis virus (EAV) and is transmitted either via aerosol transmission and direct contact with infective respiratory secretions or through breeding with fresh, cooled, or frozen semen. While it’s typically not life-threatening to otherwise healthy adult horses, EAV can cause abortion in pregnant mares and, rarely, death in young foals exposed to the virus. Colts and breeding stallions can become long-term carriers in which they show no signs of disease but harbor the virus in their bodies and transmit it in their ­semen. 
The indications for vaccination against EVA have been to protect stallions against infection and subsequent development of the carrier state; to protect mares before they are bred with EAV-infective semen; and to prevent outbreaks in nonbreeding populations of horses.
In planning an EVA vaccination program, consult state and/or federal animal health officials to ensure your plan is in compliance with your state’s EVA control program, if one exists. 
Rotaviral Diarrhea
Equine rotavirus is a major contagious cause of foal diarrhea and, according to the AAEP, it has been documented to cause 50% or more of foal diarrhea cases in some areas. Mortality usually is low but the virus can infect many foals on a premises. Rotavirus is transmitted when foals ingest feces containing the organism.1
Vaccinating a pregnant mare several times during pregnancy increases her antibodies to the virus; this immunity is then passed on to the foal in her ­colostrum (first milk) when he nurses, helping prevent the disease, as evidenced in vaccine field trials. There is likely less benefit in vaccinating the young foal.
Veterinarians recommend using this vaccine on breeding farms, especially in regions where the disease is more common and on those farms that have had prior outbreaks.1
Potomac Horse Fever (PHF)
This disease manifests in horses as life-­threatening diarrhea and is more common in the eastern United States, although it has now been diagnosed in many states and some foreign countries, according to the AAEP. Wilson and Pusterla state that “PHF is seasonal, occurring between late spring and early fall in temperate areas, with most cases in July, August, and September at the onset of hot weather.”
The causative agent is associated with water and relies on a complex lifecycle involving snails and aquatic insects. There is disagreement in veterinary literature about the available vaccine’s efficacy, although it is considered fairly safe, and if given frequently (at four- to six-month intervals) might help protect against disease.1 A reasonable approach might be to vaccinate in geographic areas where the risk is higher. 
Anthrax is a rapidly fatal blood disease caused by the bacterium Bacillus anthracis. It is typically only found in certain geographic areas with moist, alkaline soil. The ­intramuscular vaccine is thought to be effective but can cause mild to severe ­vaccination reactions. Because of this, veterinarians recommend using the vaccine only in regions where risk of infection outweighs the risk of ­vaccination.1
Take-Home Message
There are many conflicting opinions, and even conflicting scientific research, concerning the science behind the ­described diseases and the risk-based vaccines that protect against them. Therefore, discuss these vaccines with your ­veterinarian, who is trained to interpret the scientific research and apply that to your individual horses and your specific needs.
1. Wilson WD, Pusterla N. Equine Vaccination and Infectious Disease Control, in Smith, BP, ed, Large Animal Internal Medicine, 4th edition, pp. 1557-1587, 2009. 

About the Author

Doug Thal, DVM, Dipl. ABVP

Doug Thal, DVM, Dipl. ABVP, is an equine veterinarian with 18 years experience in clinical practice. Thal Equine ( is his full-service equine hospital near Santa Fe, N.M.

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