“Flu vaccination” is the bane of many horse openers’ lives. Making sure your horses are up to date for travelling or competition, or even staying away, can be a nightmare! So is it really necessary?Or is it just a box-ticking exercise? Vet Avice O’Connor investigates the medical need for the influenza vaccine in horses.

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What is equine influenza, and is it serious?

Equine influenza is a highly infectious respiratory disease of horses. Affected animals typically show clinical signs within as little as one day of exposure to the virus, although it can be up to five days after exposure. The most commonly seen symptoms are high temperature, cough, nasal discharge, lethargy and loss of appetite. An increase in temperature (above 38.5C) is often the first symptom. Signs of illness can last for 3-6 weeks. Many horses will need a prolonged period of recovery before they can resume athletic work. It is recommended as a general rule that most horses should be rested for one week for each day of illness to minimise the risk of damage to the heart or lungs. 

Once a horse is infected, they are very contagious

Influenza can spread through a yard or competition venue with speed. In populations that are unvaccinated and with no history of infection, up to 100% of horses can be affected. Like many other respiratory infections, the virus is spread by coughing and by environmental contamination. It is very easy for the virus to be transmitted on contaminated clothing or surfaces. This means that any people working with, or around infected horses can easily facilitate viral spread. 

Some affected animals can suffer secondary complications such as pneumonia or lower limb swelling. These are more concerning in older or very young horses, or in those with a previous history of respiratory disease and can be fatal, although fortunately this is rare. 

Vaccination

Without doubt, vaccination is the most effective way of reducing the risk of the disease. However, the percentage of horses vaccinated in the UK may be as low as 30%. (Based on vaccine sales and the horse population). The majority of horses vaccinated tend to be those in work or competing, with retired or companion animals less likely to be vaccinated regularly. 

However, there are some key points to consider:

Vaccination does not guarantee complete immunity

Just like in humans, the flu virus can mutate over time, and as such some vaccines may confer greater levels of immunity than others depending on the circulating strain. A variety of vaccines are commercially available, with slightly different strains. Even if the vaccine does not contain the current circulating strain, it may provide some degree of reduction of clinical signs; but this is not guaranteed. An outbreak of Equine Influenza in the UK in 2019 highlighted the need for greater numbers of horses to be vaccinated to achieve herd immunity. And also prompted changes in the recommended interval between vaccination in high-risk animals. 

In this outbreak, even vaccinated horses suffered from clinical signs. Albeit generally with less severity and shorter duration than those of their unvaccinated cohorts. But, the outbreak was of sufficient severity to necessitate a shutdown of racing for six days, and the animal welfare and the economic implications of an outbreak can be severe. 

How often to vaccinate?

Following the 2019 outbreak, veterinary surgeons now often recommend a booster vaccination every six months to ensure greater immunity levels.

However, the vaccination schedules, or timing of each vaccination in the course, vary depending on the governing body of the individual discipline. One point worth noting is that some guidelines are not consistent with those of the manufacturer, which may lead to lower rates of immunity. If you are concerned, it is always advisable to discuss the vaccination schedule and timings with your vet. They will be able to advise on the best approach taking your circumstances into account. 

How safe is it?

It is true that adverse reactions can occur after vaccination. However, most are mild, with localised muscle stiffness or swelling at the site of injection, lethargy, or a fever most common. Less commonly, an abscess can form at the site of injection. These adverse reactions are very uncommon and are usually short lived. If you are concerned, please discuss with your veterinary surgeon. The vaccination may be able to be timed to make sure that if any reaction does occur, it will be at a time of minimal disruption. 

So should I vaccinate my horse?

There is no government legislation that requires horses to be vaccinated, so this decision tends to be down to you, based on your circumstances. However, if a horse is competing, they must be vaccinated to comply with specific regulations. This can also include specific venues such as racecourses who will only allow vaccinated animals onto the premises; even if not competing. These rules will vary depending on the regulatory body. If in doubt, please consult your veterinary surgeon or the specific authority. Guidelines do change occasionally. So it is the owner’s responsibility to keep up to date with any changes as otherwise they risk being turned away from competition. Many yard owners will also insist that horses are vaccinated based on the manufacturer’s guidelines so that all animals on the yard are as protected as possible. 

When surveyed, most owners who were reluctant to vaccinate were so because of concerns about unnecessary vaccination, adverse reactions, lack of efficacy or the cost. 

So what happens if your horse is retired or never leaves the property? 

These horses are at lower risk of contracting influenza. But it is rare for horses to exist without contact with other horses or with people who have contact with horses who are at risk. It is possible for the virus to be carried to your horse by visitors to your yard or by nearby horses. Even though your horse may not leave the yard, others may do so. Unfortunately, as the virus can be spread in the air, even horses that are not in immediate nose to nose contact may pose a risk. Not vaccinating not only puts the individual horse at risk, but also risks the health of any other in contact animals. 

Cost can be a significant factor in deciding whether to vaccinate. While this is understandable, particularly in today’s climate of economic uncertainty, one should be aware that if the horse or pony does contract influenza, this will undoubtedly be more expensive in the long run. 

In general, veterinary recommendations are that all horses are vaccinated against Equine influenza at 6 monthly intervals. Always discuss your individual case with your veterinary surgeon who will be able to tailor their advice to the needs of your animal. 

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