"An ounce of prevention is worth a pound of cure," is an old saying that carries a lot of truth today in protecting your horses from disease and your pocket book from treatment expenses. Vaccinations are our most effective tool in preventing disease. Thousands of dollars are invested in our horses so they will perform at optimum levels. Sickness and death not only decreases performance, but will also increase costs. This makes vaccination against debilitating diseases even more sensible.

When a horse is vaccinated, it is given a collection of weakened or dead organisms that, when introduced to the body, create immunity to disease. The organisms within the vaccine trigger the body to produce antibodies (disease-fighting proteins).

Horses develop a strong immune system by experiencing disease and infection. The body develops its own means of protecting itself once it knows exactly what it must fight. Vaccines provoke the immune system to create such protection without the actual disease developing in the body.

The living viruses and bacteria found in these vaccines are weakened to decrease the likelihood they will cause disease. Although they vary in their effectiveness, these live vaccines are more likely than killed vaccines (which contain dead organisms) to stimulate resistance. Live vaccines entail some risk because there is always the chance they will revert to full virulence and cause the very disease they protect against.

Even under the best circumstances, no vaccine is effective in every horse. While some protect nearly 99 percent of vaccinated horses, any vaccine that protects 90 percent of vaccinated horses is considered highly effective; some that are commonly used are less than 75 percent effective. It is important to remember that when giving a horse a vaccination for the first time, as in humans, boosters must be given 3-6 weeks later to establish an effective level of protection.

Few vaccines provide lifelong protection; hence, expect periodic booster shots in most cases. Having a disease always gives better protection than vaccination. For example, experiments show that three months after horses have had equine influenza, they are still immune; whereas, horses that have been vaccinated with two doses of killed-influenza are no longer immune. Unfortunately, treatment and damage caused by the disease can be costly. Horses can be re-infected with some viruses, such as equine influenza and equine rhinopneumonitis, within as little as six months after recovering from the disease. Thus, no vaccine can be expected to protect horses for longer than six months.

Rabies

Rabies is a common viral disease in Texas and every year many horse are euthanized due to this preventable disease. In Texas, horses are exposed to rabies most commonly through bats and skunks. Once clinical signs of Rabies are present, treatment is usually unsuccessful resulting in death. Clinical signs consist of unexplained lameness, leg incoordination, slobbering, lethargy and death. This vaccine is a killed virus given intramuscularly.

Tetanus

This disease is caused by the bacteria Clostridium tetani and occurs most commonly in unvaccinated horses that acquire puncture wounds or cuts. The bacterium itself does not cause the disease but releases toxins in the bloodstream that cause neurologic disease and death. Symptoms consist of lockjaw, slobbering, erect ears, and anorexia. The vaccine consists of inactivated tetanus toxin and is given intramuscularly. It is one of the most effective vaccines.

Venezuelan, Eastern and Western Encephalitis

These diseases are caused by three differing viruses transmitted by mosquitoes to horses causing neurologic disease that can lead to death. Symptoms consist of unexplained lameness, ataxia, back or neck pain, fever and depression. This vaccine is prepared from killed virus particles and is given intramuscularly.

Influenza

This respiratory disease is caused by equine influenza virus. It is highly contagious and is transmitted through the air and by direct contact. Clinical signs consist of coughing, runny nose, and fever, which can lead to pneumonia. This vaccine is comprised of killed virus particles and given intramuscularly.

Rhinopneumonitis

This disease is caused by a herpes virus that causes respiratory disease and abortion. It is a highly contagious disease transmitted through the air or by contaminated food, water, bedding, stalls and trailers. Clinical signs consist of nasal discharge, high fevers, loss of appetite and depression. It can lead to pneumonia and or cause lameness, incoordination, eye edema, and swollen legs. Also, it can cause pregnant mares to abort. This vaccine is comprised of killed virus particles given intramuscularly every six months. Pregnant mares should be vaccinated at months 5, 7 and 9 of gestation.

West Nile Virus

This deadly disease was first discovered in the United States in 1999. It is caused by a virus that is transmitted by mosquitoes. Clinical signs consist of loss of appetite, depression, muscle twitching, leg incoordination, paralysis of lip, facial muscles and tongue, and sudden change in mentation such as being "jumpy" or "just not with it". The vaccine is comprised of killed virus particles and as with any first time vaccinations, requires a booster, 3-6 weeks apart, given intramuscularly.

Equine Protozoal Myeloencephalitis

This debilitating disease is caused by a protozoa (Sarcocystis neurona) transmitted mainly by opossums. Horses ingest feed containing opossum feces infected with the protozoa. The protozoa then travel through the body to the spinal cord or brain causing neurologic disease. Clinical signs consist of ataxia, leg incoordination, muscle atrophy, and obscure lameness that progress in severity. If caught early, this is not a deadly disease, but does impair performance, as the damage is usually irreversible. The vaccine is comprised of killed protozoa particles and is given intramuscularly. Again, first time vaccinations for this disease require a booster in 3-6 weeks.

Strangles

This disease is caused by the bacteria Streptococci equi. It is most common in ages 1-5, but does affect older horses. Those horses that travel are more prone to this disease due to increased exposure. Horses with this disease can shed the organism for 10 months, whereas the disease process typically lasts 3 weeks. It is transmitted from horse to horse through the air and by direct contact. Clinical signs consist of fever, yellow nasal discharge, and lymph node swelling under the jaw resulting in abscess formation. These abscesses eventually rupture and are the defining clinical sign for Strangles. This is usually not a fatal disease and many ranches and horse farms battle with Strangles as it pops up from year to year. The vaccine consists of a live culture given intranasal (in the nostril) every 6-12 months depending on the exposure.

Courtesy of Bennet, Dwight G., 2002. "A Horse Owner's Guide to Vaccines," Western Horseman. August. pp. 84-90. Verbal permission granted.