EQUINE VACCINATIONS:

By Dr. Steve Levine, member of the Vermont Veterinary Medical Association,
practicing at Northern Equine Veterinary Services in Danville, Vermont.

1. TETANUS:

Tetanus is a disease caused by a toxin produced by the bacteria, Clostridium tetani. It generally gains entrance into the body as a result of a deep or penetrating wound such as a puncture. The action of this toxin in the body of an unvaccinated animal is to inhibit the production of a special chemical necessary for the proper functioning of nerves and muscles. When the toxin is released in the horse's body it makes the muscles unable to stop their contractions resulting in prolonged, abnormal muscle firing. The horse is unable to eat or move normally, its muscles become tight and rigid and any stimulation can cause the horse to go into spasms and convulsions. Horses with tetanus will usually die.

Protection against tetanus is easy and inexpensive. Foals are generally vaccinated at about 4 months of age and an initial injection must be followed up with a booster some 2-4 weeks later. Yearly boosters are then required for the life of the horse. Horses sustaining deep puncture wounds or severe lacerations should be given immediate tetanus protection boosters even if they have previously been vaccinated. Pregnant mares should also have a tetanus booster at about 9 months of gestation so as to pass on immunity to the newborn foal.

2. EASTERN AND WESTERN ENCEPHALOMYELITIS:

These are neurologic diseases caused by viruses. The natural reservoir in the environment is birds and horses become infected by the bite of a carrier mosquito. The horse is not a natural host for this virus so it is called a "dead end" host, meaning it does not serve as a source of infection for other horses or people. The effect of the virus is on the brain and causes signs such as aimless wandering, head pressing, profound depression, convulsions and death.

Protection against encephalitis should be routine and in most cases the vaccines against tetanus and encephalitis are combined in a single injection. As with tetanus, foals are vaccinated at about 4 months of age with an initial injection and then a follow-up booster 2-4 weeks later. Yearly boosters are then given for the life of the horse. Pregnant mares should be vaccinated at about 9 months of gestation.

3. RABIES:

Rabies is a viral disease that is spread almost exclusively through the bite of an infected individual. It is always fatal and can affect people as well as animals, including horses. The reservoirs of infection in nature are primarily skunks, foxes, raccoons and bats. The incubation period of the disease is very variable as are the clinical signs. Any obvious bizarre changes in behavior accompanied by weakness and incoordination should be viewed with caution and veterinary attention secured. Foals should be vaccinated at 3-4 months of age and yearly thereafter.

4. INFLUENZA AND RHINOPNEUMONITIS:

These are both upper respiratory viral infections that are spread primarily through direct contact with infected individuals or their nasal secretions. In general they cause fever, cough and nasal discharge (snotty nose). The virus is often spread throughout a barn or stable affecting many of the horses. Most of the time the disease will run its course without complication but occasionally pneumonia or other more serious complications can occur. In addition to the most common upper respiratory form of rhinopneumonitis it may also cause abortions in pregnant mares and rarely, neurologic disease. Vaccinations against "flu" and "rhino" begin in foals at about 4 months of age and require an initial shot followed by a booster in 2-4 weeks. Depending upon the degree of exposure to other horses boosters may be given every 3-12 months. Pregnant mares should be vaccinated against rhinopneumonitis at 3, 5, 7 and 9 months of gestation.

5. POTOMAC HORSE FEVER:

This disease is rare in our area although it does occur, and can be fatal. The disease is caused by an organism called Ehrlichia risticii and is thought to be ingested while grazing or drinking in wet or marshy pastures. The clinical signs include fever, diarrhea, and laminitis and making a definitive diagnosis of PHF may be difficult. Vaccinations begin at 3-4 months of age and again require an initial dose followed by a booster in 2-4 weeks and yearly thereafter.

6. Strangles:

Strangles is a respiratory disease that is transmitted from one horse to another either by direct contact or through contaminated objects (fomites). The disease is very contagious and can rapidly spread through a stable or barn. It is characterized by fever, purulent (pus) nasal discharge and painful enlargement and abcessation of the lymph nodes under the jaw. These lymph nodes often have to be surgically drained. The disease usually responds well to antibiotic treatment once the lymph nodes have drained. Strangles can also be an ongoing problem on a farm because the organism can survive in the environment for long periods of time and continue to serve as a source of infection. Additionally, horses can become carriers of the disease and periodically shed the organism (Streptococcus equi) thereby continuing to transmit the disease to other horses. Vaccination for this disease involves either 2 intramuscular injections given 2-4 weeks apart or 2 intranasal doses also given 2-4 weeks apart. Protection from the vaccination is not complete but definitely increases the horses's resistance to the disease. Vaccination against strangles is usually done on horses residing in stables where the disease has been present in the past or travelling to areas or stables, shows, etc. where the previous vaccination histories of the other horses is unknown.

7. Equine Protozoal Myelitis:

The vaccination for this disease has only recently become available. It is given as an intramuscular injection initially and then 2-3 weeks later. The disease is caused by the organism Sarcosystis neurona which is thought to be ingested by the horse in feed contaminated by opposum feces. The natural reservoir of the organism is in wild birds. Once ingested by the horse the organism travels to the brain and spinal cord where it can cause a wide variety of signs ranging from obscure lameness to muscle wasting to severe incoordination and paralysis. This disease, also known as EPM is now the most commonly diagnosed neurologic condition in the horse. Diagnosis is made on the basis of clinical signs and blood and cerebrospinal fluid analysis. There is treatment available although it is quite expensive and not always successful.