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Why should we vaccinate against equine influenza

THE THREAT of equine infectious anaemia – or swamp fever – was discussed in the last issue of Scottish Horse.

With a second case being identified in Devon as the magazine went to press, vets remain on alert.

However, it is arguably another, less exotic, viral disease that poses a greater threat to the health of our horses and the finances of equestrian industry. That disease is equine influenza.

An outbreak of 'flu in Eastern Australia in 2006 cost the industry and taxpayers close to £1bn during a six-month period when there were no competitive or commercial equine activities.

Equine influenza is highly contagious viral disease. When it strikes a susceptible population of horses, nearly 100% will be infected within several days.

Symptoms are very similar to those suffered by humans with 'flu, including a high temperature, harsh dry cough, lethargy and loss of appetite.

Equine 'flu is rarely fatal but can compromise immunity leading, to secondary bacterial infections and fatigue that can last for up to six months.

Could equine influenza wreak havoc in the UK on the scale that it did in Australia? While the answer is probably 'no', there have already been outbreaks of equine influenza this autumn in Essex and Durham, as well as most recently, Roxburghshire.

The reason why these have not been catastrophic is that, in contrast to Australia, many horses in the UK are vaccinated against 'flu.

Vaccination is recommended by bodies such as the British Horse Society and is required by the British Horseracing Authority and other organisers of competitive activities like British Eventing.

Horses should be vaccinated from five months of age, with an initial course of two injections typically four-six weeks apart followed by a first booster five-seven months later. Vaccination is then done annually in order to provide protection from the disease.

However, despite this, there have been outbreaks of equine 'flu in Europe in horses that have been regularly vaccinated. Investigation of these cases by the World Organisation for Animal Health (OIE) revealed that they were caused by a strain of influenza virus not included in the vaccines used.

This type of virus – referred to as clade 2 – belongs to a family of influenza virus that was first isolated in Florida. Viruses belonging to this family have been identified in the UK and Ireland, including in the recent outbreak in Essex.

Vaccine manufacturers are being urged by the OIE and the European Medicines Agency to update their vaccines to include this strain and are actively doing so. But this process takes time as vaccines need to be developed and then tested for safety as well as efficacy.

In the meantime there is evidence that certain of the currently available vaccines offer some protection against clade 2 influenza viruses. This means that vaccinated horses will suffer less severe symptoms and pose less of a threat in terms of the amount of virus that they can transmit to other horses.

Additional protection can be achieved by ensuring that all horses on a yard are vaccinated and not just those that require vaccination for racing or competition.

It is also important to continue vaccinating older animals as they, along with those that are injured or suffering from conditions such as Cushing's, can have lower immunity.

The International Equestrian Federation (FEI) currently requires that horses competing in its international events are vaccinated against 'flu every six months or at least within the six months preceding the event. This is because of concerns that stress from travelling and competing can reduce immunity to equine influenza.

While owners of FEI-registered horses may grumble about the cost of additional boosters, there is scientific justification for the FEI's position.

Research from the Irish Equine Centre, published last year in the journal Vaccine, showed that antibodies following vaccination decline in some animals to levels at six months that offer negligible protection.

Full protection against 'flu is fundamental to the work being done by the FEI in collaboration with the OIE to simplify movement of horses around the world. This involves the creation and surveillance of a group of high health status horses that are able to travel more freely within and between continents than is presently allowable.

So, to answer the question of whether we need to continue vaccinating horses against 'flu, the answer is undoubtedly yes.

Cost is not a reason to stop; the price of dealing with equine influenza on either an individual or yard basis is likely to be many times greater than that of the vaccines.

Possible concerns over side effects from 'flu vaccines are also not valid. The current vaccines are among the most technologically advanced and safest products available for any species. They require little more than 24 hours off work following vaccination and the actual incidence of adverse events is extremely low.

If, however, your horse has been unfortunate and suffered malaise, a slight fever or mild 'flu-like symptoms following vaccination, then discuss with your vet using a different type of vaccine on the next occasion.

It is only with vaccination will we have any defence against current and newly emerging equine influenza viruses and thus be protected from the disruption and financial losses caused by this disease.