Vaccine Recommendations for Your Horse

[By Douglas Novick, DVM]


Vaccines Given Once a Year

Equine Viral Arteritis
Potomac Horse Fever

Vaccines Given Two Times a Year

Influenza Given up to 4 times per year
Rhinopneumonitis Given up to 4 times per year
West Nile Virus Given up to 2 times per year


Special Considerations

Horses going to frequent competitions or at racetrack settings should be vaccinated for Influenza and Rhinopneumonitis every 3 months. In some situations you will need to make that every 2 months. Pregnant Mares should be vaccinated against Rhinopneumonitis at months 5, 7 and 9 to prevent late term abortions. They should also be vaccinated at months 10 for Encephalitis, Tetanus, and Flu along with vaccinations for all others used in your program. In the Mid-Atlantic States and Kentucky Botulism should be one of the additional vaccines included. This is done to boost the anti-bodies transferred to the foal from the mare. These antibodies are the only protection a foal has against these diseases for the first few months of life. Foals should be vaccinated for all diseases in your program at 2 months, 3 months and 6 months. Follow the adult program after that time.

Eastern and Western Encephalomyelitis:

These are two similar but distinct viral diseases of the brain. The Eastern virus has mortality in the 75% to 90% region. The Western virus is on the order of 50% fatal. Those few horses that do recover can have permanent neurologic damage. The diseases are transmitted by mosquito bite.

Vaccines are very effective and widespread. However, each year outbreaks of both these diseases occur. Having seen cases in an outbreak in New Jersey several years ago, I can tell you that it is tragic to see a horse deteriorate over a week's time. It will slowly lose muscular control and eventually go down, unable to rise. When you know that a simple vaccination could have prevented such a tragic end, you become and advocate for aggressive vaccination

All horses should be vaccinated against these two viruses once a year.

Venezuelan Encephalomyelitis:

Similar to the eastern and western viruses, the Venezuelan strain is currently not affecting the United States. There was a scare in 1994 when several horses in southern Mexico came down with the disease. Movement of horses from Mexico was banned for a time. The outbreak was contained in southern Mexico and did not appear to move north. So far cases of Venezuelan Encephalomyelitis have not been seen in the United States since at least 1980, if not earlier.


Tetanus is a disease caused by the toxin of the bacteria Clostridium tetani. Of all animals, horses are among the most susceptible. The bacteria are always around and generally gain access to an animal via a contaminated cut. The toxin is released causing massive contraction without relaxation of muscles. One of the lay terms for the disease is "lock jaw" because there is contraction of the muscles that allow the horse to chew food. They lock the jaw closed. The horse can not eat or drink.

Vaccination is very effective and should be done once a year.


This is an upper respiratory viral disease that presents as a snotty nose, cough and fever. Transmission from horse to horse is by touch and aerosol from coughs and sneezes. It is highly contagious. In unvaccinated barns over 50% of the horses will come down with the disease. Although mostly self-limiting, Influenza will breakdown the defense systems of the respiratory tract. This opens up the lungs and bronchi to secondary bacterial infections. It is this risk of secondary bacterial infections that cause veterinarians to put many infected horses on antibiotics. The stress of transportation has been linked to an increased risk of both catching the virus and of getting secondary bacterial infections following viral infections.

The vaccines are not 100% effective and have a maximum effectiveness of only 3 to 4 months. My current recommendation is that all horses be vaccinated two times each year. For horses going to several horse shows and horses at racetracks, owners should have them vaccinated every three or four months.


This is a viral disease that causes three different forms of disease. This upper respiratory tract disease is virtually identical and indistinguishable from the disease caused by Influenza virus. The course of the disease and vaccination protocols for both viruses are the same. All horses should be vaccinated at least two times per year. Horses at greater risk should be vaccinated 3 to 4 times per year.

Rhinopneumonitis also has two other forms. One causes a neurologic disease that is typified by a gradual loss of control of muscles starting in the hind end. The other form of the disease causes late term abortions in pregnant mares. For this reason all pregnant mares should be vaccinated against Rhinopneumonitis at 5, 7, and 9 months of pregnancy.

In the last three years work has been done to separate the strain of the virus that cause abortions from the strain which causes the upper respiratory disease. Although there are some cross protections, vaccines made to protect against one strain do not provide as much protection against the other strain.

West Nile Virus

West Nile Virus is a viral brain infection that came to the United States in 1999. It causes severe neurological symptoms and kills over 30% of horses that contract it.  Mosquitoes spread it with birds being the natural host for the disease. In 2002 it spread across to the country with nearly every state except California affected. As of August 20, 2003 it reached California. It is well established in most of my practice area.  Vaccination has proven to be highly effective at preventing this disease in horses.

Potomac Horse Fever:

This is a disease caused by a special type of organism called and Ehrilichia. Although other diseases caused by similar organisms, Lyme Disease, Rocky Mountain Spotted Fever etc. are proven to be transmitted by ticks, that has not yet been proven in the case of Potomac Horses Fever. The disease presents with a variety of clinical signs. The classic cases have acute diarrhea that can be life threatening along with fever. Many of the cases will founder during the course of the disease. Although named for the Potomac River Valley back in Maryland where it was first described, the disease is present throughout the United States and in California. Because of the suspected mode of transmission, it does not caused widespread disease on affected farms. Instead, a small number of horses are usually affected. The incidence in varies from state to state. Outbreaks occur each year.

All horses in areas where outbreaks have occurred should be vaccinated. This is especially true for horses at boarding stables. Horses going to frequent competitions should also be vaccinated. This is particularly true in Mid-Atlantic States and California.


Strangles is a bacterial disease caused by Streptococcus Equi. As the name implies, the disease in its worst form can literally strangle a horse by compressing its trachea. Fortunately most cases never get that far. Strangles generally starts out looking like Influenza or Rhinopneumonitis with a snotty nose and fever. The lymph nodes either under the jaw or behind the larynx then enlarge, fill with pus and abscess. Although rarely fatal, it can lead to mortality. Strangles can also lead to pneumonia. It is highly contagious and can be picked up from contact, aerosol or even from bacteria in the ground. Outbreaks will affect the majority of horses on affected farms.

Horses that are stabled at a barn where Strangles has occurred in the previous three years should be vaccinated. A horse going to frequent horse shows or to breeding farms should also be vaccinated. The reason more horses are not vaccinated for Strangles is that of all the vaccines, this one gives the most reactions. More so than other vaccines you will see sore necks, a couple days of fever or poor appetite following vaccinations for Strangles. There is also controversy about the effectiveness of the vaccines commercially available. My own experience is that no vaccinated animal in my practice has contracted Strangles in the face of an outbreak. The vaccine is should be administered once yearly.


Rabies is a deadly viral infection of neurologic tissue. It is contracted from the bite of an infected animal and the virus then travels up the nervous system into the brain. It is virtually 100% fatal and can be transmitted to humans from affected horses. The most prominent sign in the affected horses is an inability to swallow called dysphasia. Fortunately, it is extremely rare in horses. On the East Coast rabies has been gradually moving up from the Mid-Atlantic area to New England in wild animals. In California, the incidence of disease in wild animals is small but real. In domestic animals, the cases occur only very rarely.

Owners whose horses are exposed to wildlife should consider vaccinating against Rabies. The vaccine is given once a year.


Botulism is a disease cause by toxin release from the bacteria Clostridium botulinum. The toxin blocks signal transmission in nerves and quickly causes paralysis. It is a significant problem for foals in the Mid-Atlantic states and Kentucky by causing Shaker Foal Syndrome.

In those areas horses should be vaccinated annually for botulism and pregnant mares should receive a booster 2 to 4 weeks prior to foaling.

Equine Viral Arteritis (EVA)

EVA is a disease causing massive inflammation of the arteries throughout the horse. Clinical signs vary but can include fever, poor appetite along with depression, swelling of the limbs, head, genitalia, and trunk. It can also present like other respiratory viruses and cause abortion in pregnant mares.

Vaccinations are strictly regulated because vaccinations will cause your horse to be positive for an antibody test for the disease. Any horse that is positive to an antibody test for EVA will find it difficult if not impossible to be moved to another country. Outbreaks have occurred in Kentucky as well as other Midwestern states.

Vaccination is done once a year. It is only given in areas where an outbreak has occurred and is controlled by the US department of Agriculture.

Dr. Douglas Novick is an equine veterinarian practicing in the San Francisco Bay Area. His practice is limited to the treatment of horses with special interests in equine lameness, equine dentistry and reproduction. He is also the first veterinarian in Northern California to implant horses with ID Microchips with optional freeze brands as a method of preventing horse theft.

See more at

(C) 2004 Douglas Novick, DVM- This article is copyrighted. It is licensed for personal use only. Any re-use, duplication, re-transmission via electronic or other means without the expressed written permission of the author, Douglas Novick, is strictly forbidden.