Due to the severity of tetanus and the risk of exposure, all horses should be vaccinated for this disease.  The vaccines that are on the market today are very safe and offer great protection.  The incidence of adverse reactions to the tetanus vaccine is only about 1 in 250,000 and most of the reactions were mild.  Most of the tetanus vaccine available for horses comes in combination with other vaccinations.  The most common combination is tetanus with eastern and western encephalitis.  Though the tetanus vaccine is one of the best available for efficacy (how well the vaccine protects your horse against disease) no vaccine can prevent disease 100% of the time. 

Horses should be vaccinated for tetanus yearly.  Weanlings should be vaccinated at 4-6 months and then given a booster one month after the first vaccination and one month after that for a total of three shots.

Eastern and Western Encephalomyelitis (EEE and WEE)

These diseases are commonly called "sleeping sickness".  The western version is found throughout North America.  The eastern variety is found only in the east and southeast.  There is another type of encephalomyelitis, the Venezuelan variety, which has not been seen in the United States since the early 1970's (1).  They are caused by a virus that is most often carried by mosquitoes.  The mosquitoes get the virus from birds and rodents and transmit it to horses (and sometimes people) through the mosquito's bite.  It is very rare that EEE or WEE is transmitted from horse to horse.  (2)

Symptoms of these diseases vary widely.  The virus attacks the brain and spinal cord of the horse.  Early signs can include fever, depression, and appetite loss.  As the disease progresses, horses may begin to stagger when they walk and show signs of paralysis.  About 50 percent of horses infected with WEE and about 90 percent of horses infected with EEE will die from the disease.

The vaccines available for EEE and WEE are very safe and work well to prevent disease.  Adverse reactions were only noted in 1 out of 101,000 doses (1).  All horses should be vaccinated for EEE and WEE in the spring before the mosquitoes come out.  Weanlings should be vaccinated for EEE and WEE at 3-4 months and given two boosters.

West Nile Virus

West Nile Virus is a mosquito-borne virus that can cause encephalitis, or inflammation of the brain.  The mosquitoes acquire the virus from birds and then can pass it on to other animals including horses and people.                  

Infected horses do not usually have a fever.  They can show signs such as listlessness, stumbling, muscle twitching, weakness, partial paralysis and death.  In 1999 and 2000 a total of 85 horses showed signs of West Nile Virus infection.  Of those, 32 died or were euthanized.  That equates to a mortality rate of 38%.  It is, however, likely that many horses recover from infection without clinical illness.  You must remember that the mortality rates that are reported are only for horses that developed clinical signs, not for all the horses that were infected with the Virus.

There are a couple of vaccines available for West Nile.  Both available vaccines are muscle injections.  The one that we use here at Alpine Veterinary Clinic has an efficacy of  90% after one year (5).  Horses that were vaccinated and given the booster in one month were exposed to the virus one year later and 90% of the horses did not have any signs of the disease.  The vaccine has also proven itself to be very safe.  Horses should be vaccinated at least two weeks before mosquito season.  If your horse has not been vaccinated for West Nile before, he will need a booster one month after the first dose.

West Nile is a reportable disease. In our practice vicinity, there have not been any reports of confirmed disease in Lincoln or Teton counties (6). However, Sublette,  Sweetwater, and Fremont counties have recorded positive cases as well as most other counties in Wyoming.  Idaho and Colorado have both had many cases of West Nile reported.

Equine Influenza

Influenza ("the flu") is one of the most common respiratory diseases in the horse.  Influenza is caused by a very contagious virus.  The virus can be passed through the air over distances as far as 30 yards by coughing or snorting.  Horses get influenza from other horses.  Horses that are not in contact with very many other horses have a low risk of contracting the flu virus.   

Symptoms of flu in the horse are a dry cough, runny nose, fever, depression and loss of appetite.  With proper care most horses recover from the flu within 10 days.  Some, however, may show signs of illness for weeks, especially if put back to work too soon.

In the past, the efficacies of flu vaccines were mediocre at best.  The flu virus in horses mutates frequently, similar to the flu virus in people.  Recently there have been some new vaccines developed.  The newer vaccines are much better at preventing disease then the ones of the past.  There are two types of flu vaccines available.  One is an injection that goes in the muscle.  This type of flu vaccine is often part of a combination vaccine.  Adverse reactions in these combinations are about 1 in 21,000 (1).  The muscle vaccination has a 73% efficacy rate for decreasing the severity or preventing the disease.   That means that the vaccine will not necessarily prevent your horse from getting the flu, but is likely to decrease the severity of the disease.  Your horse may have less of a fever and less of a snotty nose if vaccinated.  The immunity from the vaccine only lasts 3-4 months.

The other new vaccine available is an intranasal product.  This vaccine is squirted up your horse's nose instead of injected into his muscle.  This vaccine has a 95% efficacy rating for decreased fever and viral shedding. (4)  That means that your horse will be much less likely to have clinical signs and will also be much less likely to spread the disease to his pasture-mates if he is vaccinated with this vaccine.  The intranasal product will not protect the foal of a vaccinated mare, so your pregnant mare should have the injectable product so that the unborn foal will benefit from the vaccine.  The immunity from this vaccine lasts about 6 months.

A series of three doses one month apart is recommended for the first vaccination of a yearling. Our practice does not recommend vaccinating weanlings for influenza.  Adult horses should get the injectable product every 3-4 months or the intranasal product every 6 months.  If you decide to vaccinate your horse for flu you need to booster the vaccine often enough, or your horse will not be getting the maximum benefit of the vaccine. 

Rhinopneumonitis

Rhinopneumonitis (or "rhino" for short) has two different virus types that cause two different types of disease.  Equine herpes virus 1 (EHV-1) and equine herpes virus 4 (EHV-4) both cause respiratory tract problems.  EHV-1 can also cause abortion, foal death and paralysis.  Infected horses can be lethargic, feverish, have nasal discharge, or a cough.  Rhino is spread from horse to horse through the air, by direct contact, or on buckets or watering troughs.  There have been reports that most foals are infected with EHV in the first few months of life with no obvious adverse effects. Most horses are likely carriers of Equine Herpes Virus. (3)

Vaccines that are available for Rhino come as a modified live virus or as a killed virus.  The killed virus is used for pregnant mares to aid in preventing abortions.  There is a 1 in 2600 chance of adverse reactions for the killed product.  The live virus product comes in many combinations that range in safety.  Most of the combinations have a rate of reactions in the 1 in 5000 to 9000 range.  The efficacies of rhino vaccines are poor at best.  In the vast majority of studies, the vaccine did not protect horses from the disease.  The vaccine may reduce the severity of the disease.  Horses immune systems do respond to the vaccine, but when they are exposed to the disease, they still become infected (3).

If you decide to vaccinate your horse for rhino, you need to booster the vaccine every 3-4 months year around.  For pregnant mares you should use the killed product at 5, 7, and 9 months of gestation.

Streptococcus Equi (Strangles) (Distemper)

Strangles is caused by a bacterium called Streptococcus equi.  The bacterium is transmitted from horse to horse in nasal secretions and pus.  The strep equi bug is very tough and can be carried on feed buckets, tack, brushes, and even your boots.  It can survive in the environment for up to 8 weeks (3).  Horses that do not seem sick can carry the disease for long periods of time.  Horses that get strangles can have a fever, a snotty nose and swollen lymph nodes under their jaw that may burst open and drain pus.  The disease got the name strangles from the swelling of these lymph nodes.  They have been known to swell enough the cut off the airway of very young horses.  The vast majority of horses recover from strangles in a couple weeks with no complications.  There are a few horses (3-10%) that can develop very serious complications that can cause death (3).  Horses that recover from strangles are unlikely to get the disease again in their lifetime (2).                  

There are three vaccines available for strangles.  Two of them are intramuscular shots and one is an intranasal product like the flu vaccine.  The strangles vaccines have the highest risk of side effects of any of the other commercially available equine vaccines.  The side effects range from muscle soreness at the site of injection to abscesses, signs of the disease itself and even a condition called purpura hemorrhagica that can cause death.  The intranasal product works better than the muscle vaccines, but none of the vaccines will prevent disease.  All of the vaccines have been shown to decrease the severity of the disease.  The intranasal product should not be injected into the muscle as this will cause a severe abscess. 

If your horse has just recovered from strangles or if there is a chance of recent exposure, he/she should not get the strangles vaccine.  In these situations the vaccine could potentially cause severe immune reactions or other complications that can lead to death.  In most cases, a horse that has recovered from strangles won't need any future strep vaccinations. If you decide to vaccinate your horse for strangles he should have a booster at least yearly.  Weanlings should be vaccinated four times one month apart with the injectable product or twice with the intranasal product beginning at 6 months of age.

At the Alpine Veterinary Clinic, we don't routinely recommend vaccinating for Strep equi, nor will we use the above mentioned intranasal product.

Potomac Horse Fever

Potomac Horse Fever (PHF) is caused by a parasite called Neorichettsia risticii which is carried in freshwater snails.  The parasite is then passed from the snail to any of 17 species of insects, such as dragonflies, caddisflies, mayflies, damselflies and stoneflies.  It is believed that horses become infected by accidentally eating the flies.  Most infected horses live close to freshwater streams or rivers where there are both snails and the flies that can carry the parasite.  Horses start to show symptoms 7-10 days after eating the infected flies.  Infected horses first show signs of depression and a high fever.  These signs can then progress to loss of appetite, colic and severe diarrhea.  Sometimes the horse's legs and head will swell and they can develop life threatening laminitis (founder).  After a horse recovers from the disease he may be immune for up to 20 months (nicuvet.com).  There have been reports of PHF cases in the southwest corner of Wyoming near the Hams Fork.

There are 6 known strains of PHF all of which cause similar disease.  Until the summer of 2004 the only vaccines available contained only one strain.  Because of this, they simply did not work against the disease.  There is a new vaccination that came out in 2004 that has been reported to protect 78% of  horses from all clinical signs of PHF except, in most cases, fever (22% had no clinical signs, 56% developed fever only, 22% developed fever and other signs of disease).  Only 50% of horses are fully protected at six months after vaccination, and protection decreases to 33% after nine months. The protection from the vaccine is incomplete at best (nicuvet.com).  Even though horses may get antibodies from the vaccine, they may not be protected from natural infection (7).  If you decide to vaccinate for PHF, horses should get a booster about every 4 months.

References

1.                  Wilson JH.  Vaccine Efficacy and Controversies.  AAEP Proceedings, Vol.51, 2005.

2.                  The American Association of Equine Practitioners.  Immunizations. Bayer Corporation, 1998.

3.                  Townsend HGG.  The Role of Vaccines and Their Efficacy in the Control of Infectious Respiratory Disease of the Horse.  AAEP Proceedings, Vol. 46, 2000.

4.                  www.fluavert.com.

5.                  Long MT et al.  Safety and Efficacy of a Live Attenuated West Nile Chimera Vaccine in Horses With Experimentally Induced West Nile Virus Clinical Disease.  AAEP Proceedings, Vol. 51, 2005.

6.                  Creekmore T. Vector-Borne Disease Coordinator.  Wyoming State Veterinary Lab. 2005.

7.                  Madigan J and Pusterla N.  Life Cycle of Potomac Horse Fever Implications for Diagnosis, Treatment, and Control: A Review.  AAEP Proceedings, Vol. 51, 2005.