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Equine flu update: where are we now July 2019

Earlier this year a group of experts from the European College of Equine Internal Medicine, the FEI, the BHA and the OIE (World organisation for animal health) came together as a round table to discuss the current flu outbreak. A webinar of this event was then made available for equine vets to watch and gain an insight into the thoughts of these experts.

Flu is endemic in the UK and Europe. This means that it is always present and circulating at low levels. In the UK there are an estimated 460,000 leisure / pleasure horses. It is thought that 60% of these are un-vaccinated . 

From 2010 to 2017 all uk outbreaks have been caused by a strain known as American Florida Clade 2. The difference with the current outbreak (Late 2018/early 2019) is that cases involve American Florida Clade 1. Not only is a different virus strain isolated, but the symptoms of what is essentially a seasonal cold have been more severe than those seen previously. Horses often have a temperature, feel very unwell, have nasal discharge and a cough. As with any virus there are longer term impacts on performance, which broadly speaking are more significant following more severe symptoms.

Every year an equine group from the OIE (the animal equivalent of the World Health Organisation, consisting of representatives from surveillance labs in, among others, the UK, Germany and America) meet to disucss disease activity and vaccine relevance. They track EI activity over the past year, and take an overview of viruses isolated, vaccine breakdown etc. We may not get a full picture of the current outbreak therefore until retrospectively.

In the UK there are 3 licenced vaccines, all of which have passed the rigorous thresholds of the licencing procedure.

Each vaccine has 2 main components:

  1. Antigenic material relating to EI virus
  2. Associated means of promoting engagement with the horse’s immune system

A bit of virus/vaccine science!

Virus and vaccine science is a complex alchemy and even the big cheeses on the panel of experts don’t know it all! Vaccines stimulate the body’s immune system to create defences that are specific to a particular disease. They do this in a number of ways, some of which are poorly understood.

  1. The Equine influenza virus has surface glycoproteins (antigen). The vaccine contains part of these glycoproteins that are presented to the horse’s immune system, resulting in specific antibodies and other defence mechanisms being prepared by the horse, so that when it comes into contact with real virus, it’s immune system recognises it and deals with it, thus preventing clinical disease.
  2. Vaccine technology: the way that the vaccine presents this antigen to the horse’s immune system varies. The 3 available vaccines in the UK use 3 different types of vaccine technology. In the round table discussion on equine influenza vaccination it was postulated that having a population of horses vaccinated using different technologies is probably advantageous with respect to disease in the population.
  3. Cross protection: this is a phenomenon whereby a vaccine contains a strain different to the challenge strain, but still affords protection against clinical disease.

Various factors affect vaccine efficiency. Mutation of the virus’ surface glycoprotein is one of these. It is therefore slightly different when it meets the immune system of the horse. In horse’s this mutation process, or antigenic drift, is relatively slow, and therefore vaccines stay relevant for longer without needing to be updated. The human influenza virus mutates very rapidly, and is able to evade vaccination regimes readily, unless vaccines are updated very regularly. Human vaccines are therefore updated every year. 

The reality in the equine world is that all of the 3 influenza vaccines available are out of date with respect to the virus surface protein they represent. Updates are complex and fraught with regulation, registration and authorisation processes. The most recent update that took place took 3 years to put in place and cost 4 million euros. 

This might sound worrying, but in the UK there is no concern regarding performance of any of the 3 available vaccines and all have passed the rigorous thresholds of the licencing procedure. This is probably due to the phenomenon of cross protection mentioned above. So although they are technically out of date with respect to the precise protein on the equine flu virus, they still provide protection against disease. If this were not the case we would see far more cases of flu in vaccinated horses.

The panel also discussed the fact that most horse’s in the UK have received a mixture of different vaccine types throughout their lifetimes, and sometimes even different types as their primary course. One study showed that protection against flu virus was as good in horses vaccinated with multiple brands. It would be prudent to ensure that the first 2 vaccines a horse has are the same type though. One of the panel commented that it is probably a good thing from an immunity point of view that the UK population of horses is vaccinated with slightly different strains using different technology.

 Although there have been some cases of flu in vaccinated horses in the current outbreak, the majority of cases have been in unvaccinated horses. Furthermore, the cases seen in vaccinated horses have been much milder. 

Why have there been outbreaks in vaccinated horses?

During the current outbreak there have been cases of clinical disease in horses vaccinated with the relevant strains. There have been breakdowns in horse’s vaccinated with all 3 of the currently available vaccine products. The disease in vaccinated horses has been much milder than those in non-vaccinated horses, and it is understood that most of the vaccinated horses that had disease had been vaccinated in the 6-12 month window.

It should be noted that theofficial requirement of the vaccine is to reduce the severity and duration of disease but not produce complete sterile immunity.

It has been well reported in various studies that levels of influenza antibody in the blood are a good marker of protection against the virus. These antibodies fall following vaccination, and they do so more rapidly after a 6 month period. This fact informed the FEI schedules for vaccination and at FEI events boosters must have been given within 6 months and 21 days of arriving on site. In the current outbreak reducing the booster interval from 12 months to 6 months is thought to have rapidly improved the overall level of protection within the population.

Many vets are therefore recommending that any horses regularly mixing with others are best advised to vaccinate every 6 months. This advice may alter going forward as experts discover more about the nature of the current outbreak.

What are the adverse effects of vaccination?

As vaccination involves injecting a substance foreign to the body to trigger an immune response, it is not surprising that in a small number of cases there are obvious signs of an adverse reaction. These are usually mild, take the form of a sore neck or slight temperature, and are usually managed with rest and anti-inflammatories for a few days. Occasionally more severe signs occur, with rare reports of abscesses at the injection site. For this reason vets prefer to vaccinate clean dry horses to minimise the risk. Some owners have reported reduction in performance levels for short periods following vaccination. It is therefore sensible to vaccinate horses during quieter periods and to allow several rest or quiet days following vaccination.

What are the current regulations?

The first thing to point out is that many of the current regulations from governing bodies such as the FEI and BHA are not consistent with the vaccine data sheet recommendations. For example, requirement for horses to have had a booster vaccination within 6 months of a competition is technically off label use, despite there being good evidence and clinical data to back it up. Many of the regulations came into force decades ago, after the 1979 outbreak. There is a strong case for all competition governing bodies and vaccine manufacturer guidelines to come into line. It is also true that the current regulator (OIE) requirements and vaccine company recommendations are not fully aligned.

Different governing bodies are changing all the time, especially in the climate of an outbreak, so it is vital that horse owners keep abreast of the requirements of their own discipline. Websites such as the BEF, BHA and FEI are updates regularly. It should also be noted that some venues such as racecourses may insist on adherence to rules that are different to the bodies of horses using them. For example dressage and pony club ponies on camps on the racecourse must adhere to BHA rules with respect to vaccination.

The following  requirements were correct on 31.5.19.


Horses must have been vaccinated within 6 months +21 days of arriving at the event. Must not have been vaccinated within 7 days of arriving at an event venue.*


Horses must have been vaccinated within the last eight months, currently with a one month grace period, so technically 9 months. This grace period will be removed as of 1/1/20*

British eventing:

Horses must have had a booster within 6 months of the event. Vaccination must not be given on day of event.*

British dressage:

Boosters no more than 1 year apartno vaccine within 7 days of competing.

They strongly recommend 6 monthly boosters*

British showjumping:

Boosters must be no more than 1 year apart*

Pony club:

For all competitions at area level and above – Boosters must be no more than 1 year apart but 6 monthly boosters are recommended. For rallies/training- vaccination requirement depends on the venue specification*

Riding club:

Boosters must be no more than 1 year apart, no vaccine on any of the 7 days prior to competition*

*Correct on the 31/05/19. Subject to change – check with regulatory body before competing. Also variation may exist between venues and so it is advisable to check before competing.

**As per individual regulatory organisations.

When should you vaccinate foals?

Foals should not be vaccinated prior to 6 months of age as they will still have immunity from the mare before this time and it may also be that their own immune system is not fully matured to enable response to vaccine, and this will interfere with vaccine efficacy.

Practical biosecurity

Instigating a “red/amber/green” system for cases with disease/in contacts/non contacts is a sensible approach, and further details of this can be found on the animal health trust website.


The “equibiosafe” free app has useful information regarding biosecurity and also has a vaccine compliance calculator.

The equine influenza virus is spread by aerosol, so the question of how far in contacts should be isolated is a difficult one. It is thought that 300 metres is ideal, but often impractical to achieve. 25 metres is better than nothing, and implementing basic hygiene measures such as washing hands and use of overalls and virucidal foot dips for handlers of infected cases will minimise spread.

Infected cases should be kept comfortable in a clean non dusty environment and rested for several weeks after symptoms have disappeared.


BEF: British Equine Federation

FEI: Fédération Equestre Internationale

BHA: British Horseracing Authority

OIE: World Organisation for Animal Health

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