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Equine Influenza

Equine influenza is the most potentially damaging of the respiratory viruses that occurs in UK equines.

The disease symptoms in non-immune animals include high fever, coughing and nasal discharge and, in cases where secondary bacteria are involved, bronchitis and pneumonia can result. Young foals and elderly animals, particularly those with pre-existing lung disease, can suffer fatal pneumonias. Unvaccinated animals may suffer permanent lung damage and those that are very badly infected may never regain their previous athletic performance. It is a highly contagious disease and one subclinically infected  animal can introduce the infection to and cause a severe disease outbreak in unvaccinated populations. Natural immunity following infection lasts on average for no more than 1 year.

Influenza outbreaks in unvaccinated or poorly vaccinated populations can disrupt equine sporting activities for weeks or months and it was for this reason that the British Horseracing Authority introduced mandatory vaccination of all racehorses and those using racecourse premises in 1981. Since that time, outbreaks have not caused such severe widespread disruption to the vaccinated populations.

Modern vaccines available in the UK can provide significant protection against infection and disease with equine influenza viruses, largely through stimulating circulating antibodies. However, vaccines must be given at appropriate intervals, often shorter than those recommended by vaccine manufacturers in the case of younger animals, and must contain epidemiologically relevant strains in order to remain effective against different circulating viral lineages. Virological investigation of disease outbreaks is essential for surveillance and for ensuring that vaccines continue to be regularly updated.

Pregnant mares should be fully vaccinated and boosted with influenza (and tetanus) 4-6 weeks prefoaling, to ensure that the young foal can be protected through maternally derived antibody in colostrum.

The side effects from equine influenza vaccination are minimal. The documented evidence for hundreds of thousands of vaccinations is that considerably less than 1 in a thousand horses have any adverse response.

If your horse is showing signs of flu then call us as soon as possible. If we also suspect flu, then we will be able to test a swab from inside your horse’s nose. The best time to do this test is a couple of days after symptoms are first seen.

Horses shed the most virus in the first few days of infection, so the sooner we take a sample, the more likely we are to get a positive test result. Paired blood samples can also useful for making a diagnosis, especially if your horse is not sampled for several days after it first shows signs of infection.

It takes between 50-100 days for the lining of the respiratory tract to fully recover after a horse has been infected with flu. During this period they should not undergo any stress or strenuous exercise as they will be predisposed to developing other respiratory infections. A good rule of thumb is for every day that they had a temperature they will need at least a week off from exercise.


Tetanus is a bacterial disease that can affect most animals. Horses are particularly susceptible because of their environment and tendency to incur injuries. Sadly, infected horses and ponies usually die or require euthanasia.

Tetanus is caused by the bacterium Clostridium tetanii which can be found in soil and droppings just about everywhere. It survives in the environment for long periods of time. It enters the body through wounds, particularly puncture wounds if the wound is dirty. Puncture wounds on the sole of the foot are common sites of infection. Infection can be acquired via the intestines, after eating contaminated soil or droppings, through gastric or intestinal ulcers. In foals, infection can occur via the umbilicus (navel). The tetanus bacteria do not need oxygen (they are classified as ‘anaerobic’ bacteria) and multiply rapidly in the damaged tissues at the site of the injury. They produce a toxin (tetanus toxin) and it is this potent neurotoxin that causes the classical signs of tetanus.

Tetanus toxin attacks nerves controlling the muscles of the body. This causes progressively worsening muscular stiffness and spasm. The affected horse will become stiff and have difficulty moving and eating. The third eyelid (membrana nictitans, a membrane which can be easily seen at the inner corner of the eye) starts to protrude across the eye, particularly if the horse is startled. The tail is often held out straight and the horse develops an anxious expression because of facial muscle spasm. Any stimulus such as loud sound, bright light or touch can exacerbate the signs. The horse may sweat. In advanced cases the horse will collapse with spasms, convulsions and death from respiratory failure.

Most cases of tetanus ultimately result in death of the affected animal. If diagnosed early, treatment is aimed at destroying the bacteria so that no more toxin is produced and reducing the effects of the toxin that has already been produced. Large doses of antibiotics, usually penicillin, are used in conjunction with Tetanus Antitoxin injected usually intravenously and intramuscularly. If the horse is able to eat, food should be offered at a height where it can easily reach it. In severe cases, slinging may be necessary. Intravenous fluids and/or catheterization of the bladder may also be necessary. Unfortunately, the chances of recovery are extremely poor if such measures are necessary and euthanasia on humane grounds is usually more appropriate.

Tetanus is an easily preventable disease. Vaccination with ‘tetanus toxoid’ should be used for all horses and ponies. The initial course consists of two injections given approximately four to six weeks apart followed by boosters at two-year intervals.

Foals should start a course of vaccine from 3-6 months of age. As protection before this age we recommend that foals receive tetanus antitoxin soon after birth. They will also receive some protection via colostrum if the mare is well vaccinated.

Good first aid can help prevent tetanus. Wounds should be cleaned as soon as they occur and steps should be taken to encourage drainage of deeper injuries. Paddocks, stables and stable yards should be kept safe, clean and clear of dangerous items such as old tractor parts, corrugated iron sheets and building materials that may cause injury.

Equine Herpes Virus (EHV)

Equine Herpes Virus is a common virus that occurs in horse populations worldwide. The two most common strains are EHV-1, which causes abortion, respiratory disease and paralysis, and EHV-4, which usually causes respiratory disease only but can occasionally cause abortion.

EHV abortion can occur from two weeks to several months following infection with the virus. It usually occurs in late pregnancy (from eight months onwards) but can happen as early as four months. Respiratory disease caused by EHV is most common in weaned foals and yearlings, often in autumn and winter. However, older horses can succumb and are more likely than younger ones to transmit the virus without showing signs of infection.

Vaccination of all breeding stock, under veterinary direction, raises the level of protection against EHV and is believed to be advantageous in preventing abortion storms although vaccination will not necessarily provide total protection.

It is recommended that a Herpes Virus Vaccine, licensed for use as an aid in the prevention of both abortion and respiratory disease caused by EHV-1 and EHV-4, is used for breeding stock.

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