Many horseowners do their own vaccinations, but you need to know what you’re giving, how often to give it, why you’re giving it, and what to do if there’s a reaction.

In the year 1798, Edward Jenner discovered that deliberately injecting healthy patients with material from a patient with cow-pox could protect them from getting smallpox. Unfortunately, they often got very ill, and his contention that this odd practice could protect against smallpox was not widely accepted.

In 1879, Louis Pasteur discovered that injecting chickens will an old, weakened culture of the bacteria that causes fowl cholera protected them. A Pasteur Anthrax vaccine followed in 1881. 

In 1885, Pasteur successfully prevented rabies after vaccinating animals with a vaccine made from dried nervous system tissue from dogs that had rabies, and saved the life of a nine-year-old boy bitten by a rabid animal by giving him a series of injections of the rabies vaccine—basically the same protocol that is still used today.

Building on these discoveries, many researchers began to come out with techniques for vaccines to protect from a wide variety of diseases. The pros far outweighed the cons, but side effects and reactions to these still crude vaccines were significant. With the advent of killed vaccines, serious side effects dropped considerably. Unfortunately, the protection provided by the killed vaccines is often inferior.

Human vaccine development moved on to uncover better ways to change organisms so that they were still alive but had little or no potential to produce disease and still generated a strong immune response. The next big breakthrough was to inoculate with only key pieces of the DNA of an organism. One of these technologies employs “chimeras,” which take pieces of the DNA from organisms you’re interested in protecting against, incorporating them into the DNA of another organism that will not produce disease in the target species. The “carrier” organism takes this DNA into the cells and generates a strong immune response.

While many modern, successful modified live or genetic-material vaccines made their way into small-animal medicine, for a long time most vaccines available for horses were the killed vaccines, which provide only limited protection for many diseases.

A notable exception was Pfizer’s Rhinomune, a modified live EHV-1 vaccine that was recently shown in one experiment to provide superior protection against the neuropathogenic strain of EHV-1/Herpes, aka Rhinopneumonitis. Two other innovative vaccines are the intranasal influenza and intranasal strangles, which use modified live organisms and stimulate protective immunity where the horse needs it the most, along his respiratory tract.

Most exciting of all are three new vaccines within the past two years. Merial’s Recombitek West Nile vaccine uses canarypox virus to carry West Nile DNA into the cells. Merial has also introduced a Recombitek Influenza vaccine, offering strong protection on a bodywide basis.

The newest addition is Intervet’s PreveNile, another chimera DNA vaccine that requires only one injection and begins to protect in as short a time as two weeks. We’re impressed with these manufacturers for working to bring equine vaccines into the 21st century.

Staying On Top of Influenza

The bird-flu scare and recent episodes where equine flu has jumped species and infected dogs has influenza on many people’s minds. As you may know, the flu strains used in human vaccines changes periodically, based on patterns of activity and mutation that have been observed over many years. This has allowed scientists to fairly accurately predict in advance what strains are likely to cause problems. This isn’t possible yet for equine flu, but scientists around the world do keep a close eye on this virus. We’ve been spared a major equine flu epidemic in this country in recent years, but problems have surfaced in Europe, with current vaccines providing inadequate protection. Based on strain typings from those outbreaks, recommendations have been issued to vaccine manufacturers to drop the EIV-A1 strains and concentrate on specific strains of EIV-A2. Merial’s new Recombitek Influenza vaccine was developed using the EIV-A2 (aka H3N8) strain that was responsible for the 2003 outbreak in the United Kingdom. This strain is related to EIV-A2 viruses of the “Florida lineage,” an earlier version of which is the Kentucky 97 (KY 97) strain found in the other currently available vaccines. No flu vaccine is 100% protective, but when horses are exposed to strains closely resembling those in the vaccine they are very effective in eliminating or greatly reducing both symptoms and the amount of virus shed. If manufacturers are willing to change their vaccine strains to match the most current information, we will have the best tools possible to fight this disease.

Choosing Vaccinations
It’s neither necessary nor advisable to vaccinate your horse against each and every disease out there. Many things should enter into your choice of vaccines, particularly the risk of exposure. Virtually anyone would think it’s ridiculous to vaccinate a horse in this country for a disease only present on some remote island, but it’s equally silly to vaccinate for American diseases the horse is unlikely to be exposed to. For example, horses living in the Arizona desert don’t have to worry about Potomac Horse Fever.

Your horse’s level of exposure to insect vectors or horses carrying diseases that spread by direct contact is also important to consider.

If your horse does a lot of traveling, or lives with horses that do, you might want to consider something like an influenza vaccine, but if he’s basically isolated from horses that move around a good bit, the risk of exposure is extremely low.

A history of bad reactions to vaccines may also influence your decision, and check out our list of when you should never vaccinate. In some instances, the effectiveness of an individual vaccine is so questionable it makes little sense to even consider it. Veterinarians are becoming much more sensitive to issues concerning vaccination and less likely to inundate every horse with each vaccine out there.

Bottom Line
Based on published studies, we think the following vaccines are stand-outs:

Rhinopneumonitis/Herpes: Rhinomune.

Routine vaccination of adult, nonbreeding stock is usually unnecessary and is currently controversial because of the suspicion that this practice may have increased the prevalence of the neuropathogenic form. However, if you do need to vaccinate your horse because of high risk of exposure, especially the neuropathogenic form, this is the only vaccine that may provide some protection from that strain.

Influenza: Recombitek Influenza. FluAver I.N. Calvenza EIV.

Strangles: Pinnacle I.N.

While some problems have occurred with local vaccine reactions that mimic strangles, the protection from this vaccine against the disease is strong and the local reactions aren’t as serious as local or systemic reactions to the intramuscular vaccine.

West Nile: PreveNile. Recombitek West Nile.

Know What Your Horse Needs: Don’t Do More Harm Than Good

Don’t vaccinate horses that: • Already show symptoms of an infectious disease. • Are recently known to have been directly exposed to one and may still be in the incubation period. • Horses in poor physical/nutritional condition. Weigh benefits vs. risks with: • Foals less than three months old, because of the likelihood that maternal antibodies will inactivate the vaccine. An exception might be made to this rule in the case of an outbreak of a disease where it is reasonably certain that the mare has also never been exposed. • Horses with a history of severe vaccine reactions. Vaccinations in this group should generally be limited to only the life-threatening diseases, and titers can help determine if the horse even needs these vaccines. Risk of reactions may also be further reduced by avoiding brands known to have caused a problem in the past, spreading out vaccinations, avoiding multi-ingredient vaccines, and pretreatment with anti-inflammatories and possibly antihistamines.

Equine Diseases with Available Vaccines
TETANUS: This is a must for all horses. Horses are extremely susceptible to tetanus, and the organism is present throughout the environment, even in their own manure. It multiplies in deep wounds, producing a toxin that causes a rigid contracture of the muscles. Fatality rate is virtually 100%. The vaccine is effective, with low rate of vaccine reactions. Vaccine-sensitive horses can have a blood titer taken to determine if they need a booster.

EEE, WEE, VEE or Eastern, Western And Venezuelan Viral Encephalitis: These diseases have high death rates. The Western form is found only on the West Coast. The Eastern is found east of the Mississippi River, and the Venezuelan form occasionally turns up in states bordering Mexico. The disease is transmitted by mosquitoes and cases primarily occur in coastal or swampy areas although it does turn up more inland on occasion. Always vaccinate well in advance of mosquito season.

RABIES: Extremely rare in horses but very deadly. Pastured horses in rabies-endemic areas most at risk. Talk to your vet about titers to monitor vaccinated horses and time repeat vaccinations. Fevers and local vaccination site reactions common.

WEST NILE VIRUS: Not as deadly as the other encephalitis viruses but fatal in approximately 30% of the cases. Horses that may have already been exposed can be titered before deciding on vaccination. Always vaccinate well in advance of mosquito season.

EPM or Equine Protozoal Myeloencephalitis: Although a vaccine is available, its effectiveness has never been proven and many knowledgeable vets are against using this vaccine.

POTOMAC HORSE FEVER: While the available vaccine has been demonstrated to induce an antibody response, it performed poorly under real-life conditions by published reports.

RHINO or Rhinopneumonitis: Incomplete protection, at best, against the respiratory and abortion forms of this disease. Adult horses are highly resistant to respiratory infections caused by Herpes anyway, and most carry the virus. Vaccination is most advisable on breeding farms, where there are large numbers of susceptible young horses and mares at risk of abortion. Current vaccines provide no protection against the neuropathogenic mutation of this virus, with the possible exception of Pfizer’s modified live vaccine, Rhinomune.

INFLUENZA: Much more serious respiratory infection than rhinopneumonitis and may strike any age group. Intranasal vaccine avoids injection-site reactions and provides strong protection. Killed intramuscular vaccines, although less than 100% effective, also provide some protection with Calvenza’s vaccine proven in challenge trials. Vaccination-site reactions and fevers fairly common with these vaccines but may vary by brand. The newest addition is Recombitek Influenza, with recombinant DNA/”chimera” technology and produced using a strain currently isolated from a serious flu outbreak.

Put It To Use
• Ask your vet for the name of the vaccine and the lot number. • If the horse has a reaction, avoid that brand in the future. • Vaccinate for what you truly need. • Avoid vaccinating during times of high stress, such as traveling, competing or moving the horse to a new barn. • Time your vaccines for optimal effectiveness rather than lump them together to save a barn-call charge.

STRANGLES: This highly infectious bacterial respiratory disease can cause significant problems, with death in about 10 to 20% of the cases.  Recovered horses may be carriers. Intramuscular vaccines provide poor protection and a high rate of injection-site reactions, even systemic reactions. The intranasal vaccine is more effective, with a small but real risk of causing a strangles-like vaccine reaction. Horses previously exposed are usually immune for life. Vaccination usually only routinely used for horses on premises known to have a problem, or on high-traffic breeding farms.

BOTULISM: Like tetanus, botulism involves a bacterial toxin that interferes with normal muscle function, in this case causing a progressive paralysis. Mild cases may recover, but most horses die. There is an antitoxin available, but it is not 100% effective and it is expensive. The organism is in the environment and animals’ digestive tracts in an inactive form. It begins to multiply and produce toxin under very specific conditions of low-oxygen tension and alkaline pH. 

On rare occasions, botulism may develop after a wound is contaminated, but the vast majority of cases come from the horse eating something contaminated with the toxin. Fermented feeds like silage and the interior of large round hay bales are the most common source. It may also grow in smaller bales when small animals are killed and baled into the hay, or may contaminate water sources. Farm outbreaks in horses on turnout have occurred, probably when weather conditions favored growth of the organism under rotting vegetation. Fortunately, the disease is relatively rare, but some people consider the vaccine cheap prevention.

LYME DISEASE: There is no FDA-approved vaccine for horses, and the human Lyme vaccine has been taken off the market, but there is still one available for dogs. Most experts and veterinary schools feel strongly that this vaccine should not be used because of the potential for irreversible serious side effects in animals that already have or had exposure to the Lyme organism.

EVA or Equine Viral Arteritis: Equine viral arteritis is similar to the Rhinopneumonitis virus in that it can cause respiratory disease and abortion. Infected stallions become carriers and spread the virus in their semen. Although endemic in this country, outbreaks tend to be sporadic. When they do occur, abortions early in pregnancy, fevers, respiratory disease and limb edema are the common signs. The only available vaccine is a live virus, which vaccinated horses will shed for three weeks and which can cause symptoms in horses exposed to them, especially foals or pregnant mares. For these reasons, and because many other countries prohibit antibody-positive horses from entering, the vaccine is rarely used.