HORSE VACCINATION

Vaccines have an important role in preventing diseases in many species, including horses. They also have had an important role in eradicating the disease. If you think back to the Equine Influenza outbreak in 2007 vaccination had an important role in preventing the spread of the disease and eradicating the disease from Australia.

Vaccines are medications that contain either a small portion of the causative agent of the disease that it is designed to prevent (for example the Hendra Virus vaccine contains a small portion of the Hendra virus called the G glycoprotein) or an inactivated form of the causative agent of the disease it is designed to prevent. When the vaccine is administered the horse’s immune system responds by making antibodies against the disease. In the event a vaccinated horse is exposed to the disease it should be able to mount a rapid immune response and not develop the disease. Most vaccines require an initial course of 2 or 3 vaccinations followed by periodic booster vaccinations. It is important to realize vaccination does not cause immediate protection against a disease, so vaccination after exposure to a disease does not mean your horse will be protected against the disease.

What diseases do we commonly vaccinate against?

Hendra is caused by a virus. The virus has bats as its natural reservoir, and occasionally for reasons not completely understood but thought to be related to stress the virus appears to ‘spill over into other species such as horses. Dogs, ferrets, and cats are also known to be affected by Hendra virus but in these species it may not cause symptoms. Antibodies against Hendra virus have been found in 4 species of fruit bats along the east coast as far south as Melbourne, and also along a significant portion of the west coast of Australia. At this stage Hendra virus has been diagnosed in horses as far south as Scone in NSW however there is concern that cases may be found further south (as there are infected bats further south).

Hendra virus will usually cause death over a short time period in horses. The symptoms of Hendra virus varies with respiratory, colic and neurological symptoms all being found in affected horses. Hendra virus particles are found in most tissues of affected horses including respiratory and salivary secretions, urine and feces.  Affected horses may also act as a reservoir of infection for humans.

A vaccine has been developed to prevent Hendra virus infection in horses. It was released under a minor permit in Australia in late 2012 and has since achieved full registration with the Avpma. The vaccine must be administered by a veterinarian. There are strict requirements with regards to the timing of vaccination booster injections, and there are also strict requirements regarding the recording of vaccinations in a database managed by the developer of the vaccine. All horses receiving the Hendra virus vaccine must have a microchip implanted.

The initial vaccination course is 2 vaccinations 21-42 days apart. A booster vaccination is required 6 months after the second Hendra virus vaccination. Subsequent boosters will be required at 12-month intervals.

Vaccination is highly recommended for all horses who may have exposure to fruit bats or horses who come in contact with horses who may have exposure to fruit bats. (e.g. all horses who attend shows, events, pony clubs etc along the east or west coast of Australia where fruit bats are found)

Tetanus is caused by a bacteria called Clostridium tetani. The spores are passed in the feces of animals including horses and infect the ground. The disease occurs if the spores enter a wound and multiply. The bacteria cause muscle spasms in affected animals. Horses are very sensitive to the effect of tetanus. Affected horses usually adopt a ‘sawhorse’ stance with the ears pricked, the head elevated and the tail held out stiffly, if they are able to walk they have a stiff-legged gait. Affected horses may not be able to eat due to muscle spasms. Death is usually due to respiratory paralysis. Treatment for affected horses is difficult, time-consuming, and expensive, and many horses die regardless of attempts at treatment.

The initial course is 2 vaccinations 4 weeks apart, with the first booster vaccination 12 months later and additional vaccinations every 1-3 years thereafter. Vaccination is highly recommended for all horses.

Strangles are caused by a bacteria called Streptococcus equi subsp equi. The bacteria cause abscesses, usually in the lymph nodes in the region of the guttural pouches. As the abscesses grow swelling of the throat region is seen. If the abscesses become large enough they have the potential to block off the airway at the level of the throat (hence the name ‘strangles’) and death may occur due to respiratory failure. The abscesses eventually burst to cause the discharge of purulent material containing bacteria into the environment which becomes a source of infection to other horses. The bacteria may remain active in water or damp areas for several months. Strangles is highly contagious in-contact horses. It may also be spread by objects such as feed buckets and water containers, or be taken to other properties on people’s boots or clothes.

There are several potential sequellae to strangle infection

  • some horses may develop ‘bastard strangles’, which is the formation of abscesses in areas other than the lymph nodes of the throat region. If the abscess develops in an area such as the muscles of the neck it may be possible to treat the condition by lancing and draining the abscess, however, if the abscess occurs in a cavity such as an abdomen it can be very difficult to treat
  • some horses may develop stone-like objects called chondroid (basically a dried-up ball of pus) in their guttural pouches which may remain as a source of infection for other horses
  • a small number of horses may become asymptomatic carriers of the disease i.e. the horse shows no clinical signs of disease but the bacteria may be found on a sampling of the pharynx or guttural pouch and the horse may become a source of infection for other horses

The vaccination program involves 3 vaccinations approximately 2 weeks apart followed by booster vaccinations every 6-12 months. Vaccination is highly recommended for all horses who come in contact with other horses eg show, pony club, competition horses, horses on agistment, horses sent to stud.

Tetanus and strangles vaccine comes as a combined product referred to as the ‘2 in 1’ vaccine which is convenient for booster vaccinations.

It is a myth that vaccinating against strangles can cause strangles. In the unlikely event an abscess was to form at the site of vaccination it is highly unlikely to be caused by the bacteria that causes strangles.

The herpes viruses are a family of viruses found in many animals. In horses, a vaccine is available against EHV 1 and 4. These strains are most commonly associated with upper respiratory tract infections (‘colds’) however they are on occasions associated with abortion in broodmares and paralysis in horses. Most horses are affected by the respiratory form of EHV 1,4 during their lifetime. If EHV 1 causes abortion the fetus and fetal fluids become a source of infection for in-contact mares and a so-called abortion storm may occur. Outbreaks of neurological herpes are less common but may occur. Neurological herpes causes weakness of the hind limbs, bladder paralysis, and other symptoms. A fever is often found. Treatment for neurological herpes is difficult and involves intense nursing care. The spread of disease is most commonly by contact with respiratory secretions, however, the virus may also be spread by contact with feed bowls, hands, etc.

The vaccination protocol varies with the use of the horse. It is strongly recommended in pregnant mares going away to studs or agistment properties, plus for some competition horses and horses being sent away to multi horse facilities for breaking in, sales, etc. The vaccine reduces the spread and severity of respiratory disease in horses and aids in the control of equine herpes virus abortion in mares.

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