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Vet's Corner




IAN MOORE, D.V.M.,B.Sc.Agr.

Atlantic Veterinary College
University Of P.E.I.









Vaccinations - What's Necessary In Our Region

As in most other domesticated species in the world, as well as in humans, vaccination is an important part of health management and disease prevention. Prevention and protection from disease is almost always far superior than treatment of the problem.

Disease verses protection question can be affected by many factors. The actual immunity or defense mechanisms of the host (in this case, the horse), how strong is it? Nutrition and body conditions affect the host's ability to resist disease. Age, previous infections, presence of other disease and of course, whether or not vaccinations have been given for specific problems all have a say on host immunity. Environment also plays a role in several ways. Specific diseases exist in certain areas or certain types of environments, contact with other horses and sanitation or management conditions. The type or pathogen involved is another contributing factor. How virulent (how infectious) and the actual number of disease organisms present are important.

When considering types of vaccination for your horse, one must consider all of the following. The risk of exposure, chance of infection and the consequences of infection are important to know. The safety and efficacy of a particular vaccine are other considerations.

Passive immunity is obtained by foals in the colostrum from mares, and this gives foals protection from everything the mother has antibodies against for the first three months of life. Active immunity occurs when a disease organism or a vaccine challenges a horse's immune system. In a good preventive health management program, active vaccination of horses should begin at about three months of age. Killed vaccines are most commonly used, but some live or modified live vaccines are needed as well. Protection occurs two weeks after vaccination and boosters are needed four weeks after the first vaccine in young horses.

Here in the Maritimes, horses have different needs than in other areas of North America and the following are some of the more important diseases they should be protected from.

Tetanus or lock jaw most common in humans and horses, is caused by clostridium tetani. These spores live in the soil and feces and this is a worldwide problem. Infection most commonly occurs by contamination of wounds or punctures to hooves. Treatment is usually futile and death is usually the end result. The tetanus toxoid vaccine has been available since the 1930's, and is very effective in preventing the disease. Aged horses need annual vaccination, while foals can start three months, followed by four and 12 month boosters.

Rabies is a virus that invades the brain of all species in the world. Frequently, rabies is transmitted by wildlife bites or aerosol spray from animals like foxes, skunks, racoons or bats. Rabies is 100 per cent fatal and poses a serious risk to people. In southwestern Ontario, rabies hits the horse population from 60- 100 times per year. On P.E.I., there have only been two reported cases of the dumb form of rabies in foxes about five to six years ago, while Nova Scotia has reported several forms of bat rabies. New Brunswick has recently presented racoon rabies near the U.S. border. Rabies vaccinations are readily available, but to date, no efficacy studies have been done on horses. The risk is low here in our region, but if you have ever been involved with rabies in horses as I have, then I would strongly recommend annual vaccination. Young horses should start at five months and one year of age.

The respiratory viruses are constantly sticking their heads up in all of North America, usually here just before some big stake races. Late spring or early summer, early fall just after the annual yearling sales and late fall, after the Harrisburg Sale are the usual time of epidemics here at Maritime tracks. Equine influenza is the most common form isolated here, but equine herpes virus (rhinoneumonitis) also occurs here. Mixed infections are not uncommon.

Equine influenza causes mild to moderate lower respiratory disease, which usually lasts 5-14 days. The "virus", as it is affectionately called, is often not the major problem, but rather the secondary complications that occur. These would include bacterial tracheobronchitis (lung infection), pharyngeal lymphoid hyperplasia (throat infection or pimples), allergic bronchitis, pneumonia, pleuritis, displacement of soft palate or exercise-induced pulmonary hemorrhage (lung bleeder). Flu virus is transmitted by direct contact and aerosol which is exacerbated by the transient horse population moving all over the place to race, etc. Horses less than three years of age are almost always the only ones affected. Aged horses rarely would obtain this virus. Obviously, a great loss of training and racing time happens and often whole stables or even whole racetracks are affected.

Traditional vaccines available offer only short term or incomplete protection primarily because of strain differences. If given every two months to two and three-year-olds at high risk, they do however reduce the frequency, severity and duration of the disease. A newly released intra nasal vaccine called Flu Avert, is a modified live vaccine called Flu Avert, is a modified live vaccine delivered through the nose and supposedly, a single dose will have greater than six months protection.

Equine Herpes Virus-1 and EHV-4(Rhinoneumonitis) not only causes respiratory disease with similar signs in young horses, but it also can cause abortion in mares (often abortion storms) or a paralytic form can cause temporary or permanent paralysis in horses. Risk factors are similar to influenza, but EHV is an upper respiratory problem. Some of the secondary problems listed for influenza would be the same for EHV. EHV-1 is the cause of abortion, and vaccination at three, five, seven and nine months of pregnancy with a killed vaccine, like Pneumobort- K, offers good protection. EHV-1 and EHV-4 cause respiratory problems and, again, vaccines offer only short term or incomplete protection and should be given bi-monthly. This vaccine can cause a mild transient fever for a day, or two.

The four conditions listed above are the most common problems vaccinated for in the Maritimes, but there are other considerations, especially if your horse is being transported to certain areas of the United States.

Eastern and Western Equine Encephalomyelitis is commonly reported in the southern U.S., but is uncommon in Canada. Annual spring vaccinations are usually effective.

Strangles is a highly contagious bacterial infection that affects upper respiratory tracts and often other areas of the body, primarily in weanlings and yearlings. Death does occur in 5-10 per cent of reported cases. This bacteria, streptococcus equi, can survive in environments for six months, and is transmitted by nasal discharge and ruptured abscesses, and can become endemic on some breeding farms. There are no definitive studies concluded yet, but anecdotal evidence suggests that both the intramuscular and intra nasal vaccines are ineffective, and the latter can possibly induce disease on a farm.

Potomac horse fever (PHF) can cause sever diarrhea and death, and has occured in the American east coast and midwest, as well as in Ontario and Quebec, but has not yet been reported in the Maritimes. An Ehrlichial bacteria is the causitive agent, but transmission is unknown. Tick may be involved. Recent studies suggests that currently available vaccines are not effective and since the risk of disease is low, vaccination is not recommended.

In the future, increased use of modified live vaccines is likely to occur along with the use of new adjuvants or carriers. DNA vaccine studies are currently underway in horses and these may offer better protection for some of the diseases that currently are difficult to vaccinate successfully for. The following summary table would apply to Maritime horses only.