Hank Lee, DVM

So many of life’s chores are repeated based solely on history, without evaluating the relevance of the action in today’s dynamic world. Often the final outcome is the same, it is the route that got us there that has changed. Probably nothing fits this application as well as does the term "modern medicine". In this field, it seems that our basis and reasoning for diagnosing and treating changes daily. It forces us to constantly evaluate the reasons we do things, or more particularly the reason we did this in this specific case.

My purpose here is to update the public knowledge base on equine vaccination programs. I want to give some incite to the reasoning we should or should not inoculate against certain equine diseases.

No vaccination prevents disease in all cases. Every animal is an individual; there is a tremendous amount of variation among individuals in terms of vaccine response. All vaccination protocols, including those used in humans, are based on promoting the public good. That is, if all healthy horses in a given population are properly vaccinated against a disease for which there is an effective vaccine available, then the likelihood of that disease reaching epidemic proportions is very unlikely. On the other extreme, if one unhealthy horse is vaccinated in the same way, and placed under stress in a population of horses in which the disease is endemic (everywhere), that animal is much more likely to become ill. Keep in mind, disease prevention must be carried out on many fronts. Vaccination is but a single component in a complex arrangement of defenses we must put in place. In this article I will briefly describe the vaccine portion.

How do vaccines work? There are many different vaccine technologies being used today. They all share the same common premise. That is they introduce some nonpathogenic (non disease-causing) portion of the disease into the horse and encourage the horse’s immune system to build antibodies against it. It often takes a series of vaccine administrations to get this accomplished. If all goes well, if and when the horse contacts the actual disease, its body will react promptly to neutralize the disease causing agent before it has a chance to replicate and severely affect the body.

The body’s immune system has a finite memory. If a vaccine or disease is not introduced to the body at regular intervals, the body no longer considers it a threat, stops building antibodies against it, and concentrates its energies on something more pressing. This is the reason vaccines must be boostered.

Some vaccines are more effective at preventing their target disease than others. It is accurate to state that no vaccine eliminates disease. They effectively reduce the incidence and severity of disease to a level we can more easily manage. Just because your horse has been properly vaccinated does not mean it is completely immune to the disease. Vaccines are not perfect; remember, they are only a component of a much bigger plan.

What do we vaccinate against? The American Veterinary Medical Association 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." There are five diseases that meet these criteria in equine medicine. They are Eastern Equine Encephalitis, Western Equine Encephalitis, West Nile Virus, Tetanus, and Rabies. Every horse that is forced to live under human control should be vaccinated against these diseases. Let me briefly explain the reasons below.

Eastern/Western Equine Encephalitis Virus: As veterinarians we often consider both EEE and WEE to be the same disease process. They are vaccinated against in combination in the same vaccine, have very similar clinical signs, disease pathways, and treatment conditions, and for the most part differ only in that they are caused by similar but different viruses. EEE is the more common and more clinically important of the two and has been here in the Southeast forever, and will remain. We see horses die from this every year. Greater than 90% of the horses that contract EEE die. It causes severe neurologic disease that often culminates in violent thrashing, seizures and agonizing death. The virus is present in bird populations and is spread to humans and horses via the mosquito. Horses and humans are considered dead-end hosts for this disease. This means that we do not get enough virus replication in our body to allow it to spread. You and your horse can catch EEE from the same mosquito; it is highly unlikely that you would catch it from your horse.

The vaccine is very effective at preventing disease, with little incidence of side effects. All available EEE/WEE vaccines are labeled for once yearly dosing following a properly administered initial series. On the Gulf Coast, we vaccinate twice yearly due to the high incidence of the disease and its ever present source, the mosquito.

West Nile Virus: This is the leading cause of viral encephalitis in both people and horses. There have been approximately 25,000 equine cases of this disease in the US in the last decade. The disease process is much like EEE except that it is only 30-40% fatal. For those horses that survive, there is usually a transient period of moderate to severe neurologic disease and convalescence. These cases can be very expensive to treat due the time-consuming care they require. Post-survival, many of these horses exhibit permanent neurologic defects. It, too, is a mosquito-borne disease in which human and horses are considered dead-end hosts.

All available WNV vaccines are labeled for once yearly dosing, after a properly administered initial series. Again, in my practice along the Gulf Coast we vaccinate every six months for reasons stated before. In past years, the state of Florida has recommended more frequent vaccinations. Due to the lack of support for dosing more frequently than semiannually, we have never instituted that in our practice with no recognized ill effect. Follow the recommendations of your veterinarian.

Tetanus: This disease, often called "lockjaw", is caused by a toxin produced by the bacterium Clostridum tetani. The organism is ubiquitous (present everywhere) in the soil in the form of a highly stable spore. It is also present in high concentrations in the feces and gastrointestinal tract of horses and other animals. Horses are very susceptible to this disease. Commonly they are affected through administration of the organism into lacerations, punctures, foal navels and retained fetal membranes (retained placentas) in mares. Affected animals often exhibit a straight-legged sawhorse stance with raised and prominent third-eyelids, then progress to recumbency and most often death, even with treatment.

All tetanus vaccine formulations in this country are in the form of a toxoid, hence the familiar term "tetanus toxoid" which is interchangeable with "tetanus shot". A toxoid is a vaccine that induces an immune response against a toxin, thereby blocking the toxin’s ill effects on the body. Following a properly administered initial series of vaccinations, it is recommended that horses be vaccinated once yearly. That is unless the horse suffers a penetrating wound or laceration, at which case it should immediately receive a tetanus toxoid if it has been greater than 6 months since last vaccination.

Rabies: You can catch this from your horse. We live in an area that is considered endemic for rabies. Rabies kills. Rabies has a highly variable clinical appearance and is impossible to distinguish from other neurologic diseases (i.e., EEE, ENV, WNV, Etc., Etc.) until your horse has died a horrible violent death, its head has been cut off, and a pathologist has evaluated brain sections.

Vaccination against rabies in very effective, cheap, and considered a must if you live with a horse. Once yearly vaccination is recommended for all currently available equine Rabies vaccines. Check local laws before purchasing or administering this vaccine. Due to the public health significance of Rabies, many states tightly control the distribution of this product. In Alabama, only licensed veterinarians are legally permitted to possess or administer Rabies vaccine, other states differ. Discuss this with your veterinarian. Remember, rabies kills.

The other equine diseases for which vaccinations are available in this country are categorized as risk-based. These diseases are either highly variable in their distribution, not cost-effective for wide-spread vaccine administration, or vaccination carries inherent risks that may not outweigh the potential benefits.

It is beyond the scope of this article to describe these diseases in detail. Diseases for which there is an available vaccine that fall into this category are listed below along with a brief description of each. Have your veterinarian help you perform a risk-benefit analysis for your particular situation.

Anthrax: Anthrax is a rapidly fatal disease that is only present in certain geographic areas in the United States. Only animals in those areas are typically vaccinated.

Botulism: Like Tetanus, Botulism is caused by a spore forming bacteria called Clostridium botulinum. It is found in the soil in certain areas, and in decaying plant matter. You may recall last year’s outbreak at one of Florida’s equine reproduction facilities that resulted in several deaths. It is highly fatal.

Equine Herpesvirus (Rhinopneumonitis): The two serovars of equine herpesvirus that we commonly vaccinate against are EHV-1 and EHV-4.

EHV-1 is most commonly associated with late-term abortions in mares. In fact, it is the most common infectious cause of abortion in mares. A mutation of EHV-1 has been implicated in recent outbreaks of neurologic disease in horses in the United States.

EHV-4 causes upper respiratory infections in young horses most commonly.

It is strongly recommended that horses subjected to the stresses of travel and intermingling with other horses should be vaccinated against these diseases at least twice yearly. Pregnant mares should be vaccinated at months 3, 5, 7, and 9 of gestation. Non-published data suggests that vaccination with the pregnant mare vaccine protects horses from the neurologic strain.

Equine Viral Arteritis: This disease is caused by Equine Arteritis Virus (EAV). It most commonly presents as abortion in mares and some deaths among infected foals. It is transmitted via respiratory secretions and more commonly as a venereal disease spread by infected stallions. Vaccination programs generally include only breeding animals.

Equine Influenza: EI is one of the most common respiratory infections in horses worldwide. It is extremely contagious and spreads rapidly through horse populations. Young horses (<5 years) are most affected, with older horses showing more natural resistance. Any horse subjected to the stresses of travel is susceptible to infection. For this reason it is recommended that traveling horses or horses intermingled with outside horses be vaccinated at least twice yearly.

You may remember this disease having completely shut down Australia’s equine industry last year, costing billions of dollars.

Potomac Horse Fever: This disease has been recognized in certain geographic regions of the United States and Canada, concentrating in the Eastern US. It has anecdotally been reported as far South as Birmingham, AL.

Potomac Horse Fever causes diarrhea (which can be severe), fever, colic, and laminitis. Vaccination has not proven to be effective in field conditions. However, we do vaccinate horses in our practice that are traveling to the Eastern United States during the summer months.

Rotoviral Diarrhea: Rotoviral diarrhea is a highly contagious disease of nursing foals. Its prevalence is centered around the highly-concentrated breeding areas of this country. Brood mares traveling to or from these places or other areas known to have rotoviral diarrhea should be vaccinated.

Strangles: Strangles is a highly contagious upper respiratory disease caused by the bacterium Streptococcus equi. It is characterized by fever, purulent nasal discharge, cough, lymph node swelling and abscessation, and commonly rupture and open drainage of lymph nodes in the throatlatch area.

Some associated side effects of the disease can be severe. Included in these is guttural pouch empyema, purpura hemorrhagica and associated vasculitis, and "bastard strangles" which is abscessation of lymph nodes in places other than the throatlatch (i.e, thorax or abdomen).

Horses subject to exposure to strangles should be vaccinated. Vaccinate twice yearly with any of the currently available injectable vaccines, or once yearly with the modified-live intranasal vaccine.

Venezuelan Equine Encephalitis: VEE is very similar to EEE and WNV in its clinical signs and disease processes, with a few very important differences. VEE is a foreign animal disease (it is not present in this country) that occasionally creeps in from Central America. Human and horses are not dead-end hosts; this disease can be transmitted between the species.

Vaccination against VEE is controversial for reasons listed below. It is for these reasons our practice does not recommend and actually discourages VEE vaccination outside of horses traveling to very high risk areas.

Many factors not mentioned in this article can influence your veterinarian’s decisions when forming a vaccine protocol for your horse. Please take the time to discuss this subject with him or her. You will find that even without saying, your veterinarian has likely predetermined what your horse needs based on his or her knowledge of the horse’s lifestyle.