Equine influenza is endemic in the UK and is a major cause of respiratory disease in horse populations around the world.

Below is information to help vets when faced with a potential influenza outbreak, including details of our Surveillance Scheme which provides free advice and diagnostic testing for equine flu.

Why is equine flu such a problem?

Equine influenza is endemic in the UK and is a major cause of respiratory disease in horse populations around the world. It has been responsible for substantial economic losses in performance horses, with outbreaks such as the one in Australia in 2007 costing the racing industry up to a billion Australian dollars.

Register for EIV Surveillance Scheme

Register now

The equine influenza virus

Equine influenza is a major cause of respiratory disease in horse populations around the world. Current viruses belong to the subtype H3N8 and, like human influenza viruses, undergo antigenic drift due to the gradual accumulation of mutations in the surface glycoproteins. In particular, changes in the viral haemagglutinin (HA) help the virus avoid immunity acquired from previous infection or vaccination. As a result, vaccine strains need to be updated regularly. Current OIE recommendations (available to download below) are to include a representative strain from each of the two sublineages of EIV, Florida Clade 1 and Florida Clade 2.

Cartoon showing influenza virus structure. Image kindly supplied by Dr Paul Digard
Structure of influenza HA showing amino acid  differences between Florida clade 1 and  Florida clade 2 viruses

Clinical signs

In unvaccinated horses the classic signs of equine influenza include a harsh dry coughpyrexia and labored breathing. This is typically accompanied by lethargy, depression and a loss of appetite. Horses may also develop a mild serous or mucoid nasal discharge. The infection will usually spread very rapidly through a naive horse population with close to 100% infection rate.

Horses that have only partial protection, e.g. due to irregular vaccination or the use of outdated vaccine strains, will typically show signs of milder non-specific respiratory disease.


Damage to the epithelium and cilia of the upper respiratory tract caused by the virus leaves horses with increased susceptibility to secondary opportunistic infections. The development of secondary bacterial infections is a very important complication of equine influenza. Prolonged pyrexia, lethargy and malaise are usually accompanied by a more profuse mucopurulent/purulent nasal discharge.

In compromised horses this can lead to the development of pneumonias and even death. This increased susceptibility typically lasts for 50-100 days post infection. It is critical to recognise this and not over stress or over work the horse during this period as they run a higher risk of developing complications.

Actions during an outbreak


Obtaining a positive diagnosis for flu can be very helpful in deciding how to manage the yard and instigating appropriate isolation procedures, as well as how to best manage the individual animal. It is also important to know if a horse has suffered from equine influenza as it will affect the length of time required for recovery post infection, in order to prevent secondary complications.

To request sampling packs please register here.

Barrier nursing

If you suspect a horse has influenza, it is important to isolate the individual with appropriate barrier nursing in place. The virus spreads extremely quickly, so if it is appropriate it is sensible to isolate all in contacts as well. The virus can survive for limited periods outside of the host, so fomite transmission via clothing, tack and other inanimate objects such as brushes is also possible and must be considered when it comes to nursing methods. The virus is fairly labile so normal detergents are sufficient to kill it.

The virus can be spread easily and very quickly from horse to horse, with close to a 100% infection rate to be expected in a group of unvaccinated individuals. Most horses exposed to the virus will show signs within a period of 1 to 5 days.


Which horses need to be sampled?

  • Any unvaccinated horse with typical influenza signs
  • Vaccinated horses with non-specific respiratory signs
  • Unvaccinated in-contact horses

Which samples

Both nasopharyngeal swabs and paired bloods not less then 14 days apart can be used for diagnosis.

When to sample

Nasopharyngeal swabs: to maximize the chances of obtaining sufficient virus samples need to be taken no longer than 2-3 days after clinical signs first appear. If it is too late to swab the affected horse it can still be fruitful to take swabs from in contact animals as shedding begins before clinical signs.

Paired plain bloods: can be taken further into the disease process because they can still pick up antibody change even if virus excretion is too low to detect with a swab. To obtain a meaningful diagnosis two samples not less than two weeks apart are always required.

Surveillance scheme

Monitoring virus changes

Our surveillance scheme was set up to monitor genetic and antigenic changes in equine influenza viruses circulating in the UK. It is sponsored by the HBLB and provides free advice and free diagnostic testing for equine influenza to all practices registered with the scheme. It also provides a means for us to communicate rapidly with veterinary practitioners in the face of an outbreak.

The information that we collect from nasal swabs and paired blood samples allows us to compare currently circulating viruses with those used in commercial vaccines. This data is used to determine whether current strain recommendations should be updated or not.

Vaccine strains

Official recommendations for vaccine strains are made annually by the OIE (World Organisation for Animal Health). For more information about current recommendations please click here

If you would like to register for the scheme, please click here.  We will send you free sampling packs & submission forms.

Equine influenza in dogs

In the UK there have been two confirmed outbreaks of H3N8 equine influenza in foxhounds, retrospectively diagnosed by the AHT. The cases occurred between 2002-2003, when there were large numbers of horses infected with equine influenza across the UK.

It is likely that the hounds contracted equine influenza from infected horses when they travelled together in close proximity in a horse lorry. The enclosed airspace of the lorry would have provided ideal conditions for the virus to be transmitted from the horses to the hounds.

Equine influenza has also been diagnosed in dogs in Australia following the large outbreak in horses in 2007. Most of those horses had no protection and could therefore shed large quantities of virus. Dogs kept in close contact with infected horses seroconverted to equine influenza.

In the USA equine influenza has also infected dogs, but with a very different outcome. In 2004 greyhounds in Florida first showed signs of acute respiratory disease, with a 10% mortality rate. The disease spread rapidly between states and has now adapted to transmit efficiently in the dog. H3N8 canine influenza has not been reported anywhere outside the US, despite being present in the country for several years.

Clinical signs include:

•    Persistent harsh cough (despite treatment with antibiotics)
•    Nasal discharge
•    Fever
•    Increased respiratory rate and effort
•    Rapid spread within a group of dogs

These signs usually appear two to five days after exposure to the virus. As canine influenza has not been diagnosed in the UK, there is no vaccine available for it.

At the AHT we are able to screen for equine influenza (and canine influenza) in dogs as part of our panel of canine respiratory diseases, or as individual tests.

Supported by Horserace Betting Levy Board