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Equine Flu

Equine Influenza is endemic in the population of horses in the United Kingdom and Mainland Europe

The virus circulates at low levels and is diagnosed from time to time in all parts of the continent.

Both vaccinated and unvaccinated animals can be affected, but vaccinated animals may show only mild clinical signs as opposed to the high temperature, depression, harsh cough with laboured breathing, nasal discharge, lethargy, depression and loss of appetite seen in unvaccinated animals.

Historically, in the United Kingdom, two or three outbreaks have been reported in small numbers of animals over the last 4 years. In November-December 2015 equine influenza was diagnosed in Staffordshire, Cleveland and Northamptonshire, all in one or two individual unvaccinated horses, two of which were recent imports from Spain. In 2016 it was diagnosed in Stirlingshire and Lanarkshire in September and October; in 2017 in Hampshire and Kent in June, and again in Essex in October. Last year two outbreaks were recorded: in February in East Lothian and Oxfordshire in April. All these aforementioned cases were in unvaccinated animals.

Due to the geographical spread of these cases and even taking into account that the virus can spread by the airborne route for up to 30km in optimum conditions it would seem obvious that there may be many undiagnosed low-level cases acting to spread the virus. Personal conversation with a recognised specialist in veterinary immunology at an International conference last year confirmed that both he and most Industry specialists are of this opinion. It is likely that mild cases have gone undiagnosed in vaccinated sports and leisure horses as only minor signs are seen and a non- specific virus is attributed to the symptoms. Veterinary surgeons may not necessarily be alerted to mild clinical signs and therefore not elect to test for influenza.

Since the end of last year, there has been an increase in equine influenza cases reported in Europe. Multiple cases were reported in France in December 2018 and in January 2019 cases have been diagnosed in France, Belgium, Germany, Ireland and the Netherlands.  In these outbreaks, cases have been seen in both vaccinated and unvaccinated animals. A link has been established between the Irish outbreak and that in Northern Europe and links between those in France and Belgium.

The initial case diagnosed in the UK was on 2nd January 2019 in an unvaccinated 5-year-old in Essex which had recently been acquired. The following day an unvaccinated horse at a yard in Cheshire was diagnosed and on 7th January an 11-year-old unvaccinated horse in Derbyshire was confirmed with the disease and ten days later on 17th January, another unvaccinated equine, a 12-year-old Shetland pony was diagnosed. It is widely acknowledged that unvaccinated horses shed more virus and thus are a far greater risk for in contacts.

The first vaccinated case was a non-thoroughbred 5-year-old in Leicestershire which was confirmed with equine influenza on 30th January. Another unvaccinated case was seen on 1st February in a recently imported Irish pony at premises in Yorkshire. Two larger outbreaks were diagnosed in Suffolk on 4th February, one in six unvaccinated non-thoroughbreds and another in eight 2-year-old thoroughbreds which were vaccinated and showing relatively mild symptoms. All had been vaccinated with either of the two brands of vaccine containing Florida Clade 1 which is the recommended vaccine strain but which has been demonstrated to be responsible for the first four cases in the current outbreak, (The others have yet to be typed). Florida Clade 1 has not been seen in the United Kingdom since 2011.

On 6th February three cases were diagnosed in National Hunt horses in Cheshire as a result of which racing was closed down until 13th February, pending further review and subject to there being no further cases. Trainers whose horses could potentially have been in contact with the affected stable (i.e. those with runners at the same meetings) will not be allowed to declare horses to race until all animals in their stable have been shown to be free of the virus. At present, the BEF have not advised cancelling sport horse events but are advising vigilance and caution. The general advice is that horses who have not been vaccinated within the last six months should receive an extra booster vaccination.

By 8th February therefore, we had seen nine outbreaks in the five weeks of 2019 as opposed to the two to three in preceding years. Three of these involved vaccinated animals and animals where the Florida Clade 1 was included in the vaccine. There is some speculative discussion amongst equine veterinary surgeons about whether this might be due to the failure of the administration in the regime suggested by the British Horseracing Board and adopted by most other National Governing Bodies. Of the two vaccines available which contain Florida Clade 1; one manufacturer advises the second vaccination at 4 weeks and the other at 4-6 weeks. Both advise that the third vaccination of the course be given at 5 months after the first booster as it is known that there is a dip in immunity at five months following a primary course in naïve (previously unvaccinated) animals. These regimes differ greatly from the prescribed 21-92 days followed by the third vaccination at 150-215 days. Although annual boosters are recommended it is known that immunity begins to tail off after six months and for this reason, the FEI has long adopted the policy that horses must have been vaccinated within six months +/- 3 weeks of competition in addition to the annual booster. Personally, I would advocate six monthly boosters in competition animals as these mixes more readily and there are stresses involved with travel and competition which could negatively influence the body’s immune defence mechanisms.

After testing thousands of samples from all the horses in yards where runners could have been in contact with the original cases diagnosed in Cheshire, and finding only two infected sites, racing was given to go ahead to restart on 13th February. The British Horseracing Board has however adopted an interim policy that only horses which have received a vaccination within 6 months can run. Trainers have to monitor the health status of all horses very closely and sign a declaration of health for each runner, including its temperature as recorded before departing for the races.

Horses which have been vaccinated with the recommended strain and where this has been administered according to the manufacturer’s data sheet recommendations should be at the least risk of contracting the infection

Dr GA Ashman MA Vet.MB MRCVS

Meanwhile, equine influenza has continued to be diagnosed in non -racehorses UK wide. 14 more outbreaks have been reported, of which only one case with clinical signs occurred in a vaccinated horse. This animal was bright, alert and eating, just presenting with a mild cough and clear nasal discharge. No treatment was required.  Four horses were diagnosed as having the virus as part of the tracings of in contacts from the original case in racing.  None of these horses displayed any clinical signs.

This contrasts drastically with the majority of cases which are occurring in unvaccinated horses. Nearly all of these had pyrexia (temperature), profuse discharges ranging from clear to mucopurulent and varying degrees of lethargy and inappetence. One outbreak was recorded in a yard where strangles had been diagnosed last month and was already on voluntary lockdown which illustrates that this is a very contagious disease. Only about half of 35 in contacts are vaccinated and therefore at high risk. In another yard of 24 horses 16 tested positive on a swab. Only 5 of these 24 horses has been vaccinated and 3 were positive but with one showing mild clinical signs and no symptoms in the other two. Of the other 13 unvaccinated positives, all were coughing with a high temperature and nasal discharge with 3 horses quite severely affected.

It is hoped that the rapid response by the BHB to the case in the racing stable will prevent the virus from spreading further within the racing industry.  Amongst the general population, it is clear that unvaccinated horses and those newly arrived on the premises are most affected. In the 14 outbreaks since 9th February, 5 were in newly imported horses and one yard had a new horse arrive within the preceding two weeks. All horse owners and veterinarians are now being vigilant and as predicted more cases are being detected. However, even allowing for this it is clear that this year is extraordinary in respect to the amount of equine influenza circulating in Northern Europe.  There is also an indication of increased virus activity in the USA; the virus has yet to be typed but epidemiologists are keen to understand if there is any link between these and the European cases. It is clear that the movement of horses between premises and international movement particularly of unvaccinated animals has disseminated the virus in the UK and Europe.  As of 14th February, 15 individual counties in England and Scotland have been affected 7 of which have had more than one outbreak.

Horses which have been vaccinated with the recommended strain and where this has been administered according to the manufacturer’s data sheet recommendations should be at the least risk of contracting the infection. Vaccination within six months of competition is now mandatory for racehorses as well as FEI horses and recommended for all horses competing at affiliated events nationally.

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