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Editorial Information

David Rendle BVSc MVM CertEM(IntMed) DipECEIM MRCVS



Equine Influenza virus is one of the most contagious diseases that affects horses and in susceptible populations it can be devastating. We are often complacent about the disease because we expect horses to be vaccinated; however the proportion of horses vaccinated in the UK (around 30%) falls well below that which is required to prevent a major disease outbreak (around 70%). Should a major outbreak occur with a new virus strain then unvaccinated horses will suffer severe and potentially fatal disease and vaccinated horses are also likely to be affected. Multiple small outbreaks occur every year in the UK and major outbreaks have occurred before and may well occur again. Should it happen then equine sport will grind to a halt and the economic consequences will be potentially huge; in 2007, a billion dollars was spent in Australia eradicating influenza after it entered the country for the first time.


The influenza viruses are all members of the Orthomyxoviridae family of viruses. They are classified into different types and subtypes based on the proteins that coat their surface. The most important proteins are haemagglutinin (H) and neuraminidase (N) and the type of each protein gives each subtype its name. The influenza virus that affects horses is known as H3N8 and is specific to horses although on very rare occasions dogs have been infected. The equine virus does not pose a risk to people. Horses also used to suffer from an influenza virus known as H7N1 but this is considered to have died out. Avian or bird flu which crossed to people in 2004 is known as H5N1 and has not spread to horses.  The proteins on the surface of influenza viruses change frequently enabling the virus to evade the immune system of the host it is trying to infect. When mutation occurs the virus may have the ability to overcome the defences of vaccinated horses and outbreaks of disease occur. When a completely new protein develops then a major outbreak is likely to occur.

Clinical signs

Following infection the incubation period (the time taken for clinical signs to develop) is between 3 and 5 days. The severity of the disease is very variable depending upon the strain of the virus, the horse's immune system and the horse's vaccination status. Once infection occurs the virus attacks the lining of the upper airways and prevents the normal drainage of secretions from the lungs. Fluid may collect in the lower airways and may become infected by bacteria even giving rise to pneumonia. A cough and snotty nose often develop and the horse usually develops a very high temperature (103° to 105°F).

fig 1

Figure 1:  A high temperature may be an early indication of influenza infection and is an important way of detecting new cases.

Affected horses usually lose their appetite and become depressed, they may also have muscle soreness and be reluctant to move. Illness may last for 2 to 10 days but complete recovery takes much longer and horses remain capable of spreading disease throughout the period during which they are sick. In vaccinated horses there will hopefully be no clinical signs but some horses will have a cough and clear nasal discharge.

fig 2

Figure 2:  A nasal discharge is a typical sign of influenza infection and may become thicker and whiter if secondary bacterial infection develops.

fig 3

Figures 3 (above) and 4 (below):  The lining of the respiratory tract becomes damaged causing an accumulation of mucus in the trachea (3) which can be sampled to determine whether bacterial infection is also present (4).

fig 4

Vaccinated horses may be capable of spreading the virus even if they show very mild signs themselves. The severity of signs in vaccinated horses (and the amount of virus they spread) depends upon the similarity between the strain of virus used in the vaccine and the strain responsible for the infection and also the length of time between vaccination and infection.  In rare cases influenza may affect other body systems and cases of heart disease and meningitis have been reported. In foals, donkeys and mules the disease is more severe and frequently fatal.


Influenza infection may be suspected from clinical signs but confirmation either requires detection of the virus or detection of changes in the horse's antibody levels to the virus. The virus can be detected from swabs collected from the back of the throat but this has to be done in the first few days of infection.

In an outbreak it is important to collect the virus to determine where it may have spread from and to determine how it may be mutating. Changes in influenza viruses are monitored by The Animal Health Trust in Newmarket and reported to international surveillance panels allowing vaccines to be updated appropriately.

fig 5

Figure 5:  Swabs can be collected from the back of the throat and analysed in the laboratory to demonstrate that influenza is present.

Changes in antibody levels in blood confirm that influenza infection has occurred but do not give any information on the virus itself. Two blood samples around two weeks apart may be necessary to show an increasing level of antibodies if antibody levels on the first sample are low.

fig 6

Figure 6:  Blood samples can be used to show changing antibody levels in response to infection.


There are no anti-viral drugs that are proven to benefit horses with influenza so treatment is essentially supportive. Anti-inflammatory drugs are important to bring temperature back to normal and also help reduce muscle soreness, improve attitude and encourage eating and drinking. Some horses may stop drinking and supplementary oral or intravenous fluids may therefore be necessary.  If secondary bacterial infection occurs antibiotics may be required and some vets will give them early in severe cases of disease to prevent secondary infection. Drugs that dilate the airways and break-up mucus may also be administered but it is questionable how beneficial they are. Following infection it takes 1-2 months for the lining of the respiratory tract to repair and it is important that horses are rested throughout this time. It has been suggested that for every day the horse is sick with influenza and has a temperature it should be given a week off to recover in order to prevent the risk of long-term lung damage or heart disease.

Disease Control and Prevention

After the first signs of disease, horses remain infective throughout the period that they are sick. The virus is spread in respiratory fluids that can be passed from horse to horse by nose-to-nose contact, coughing or on people or equipment.

In an outbreak or suspected outbreak it is essential that infected horses and suspect cases are isolated rigorously and veterinary advice is sought immediately. It is worth monitoring the rectal temperatures of horses which have been in-contact with suspect cases as this may be the first warning sign that the disease has spread. Horses with high temperatures should also be isolated immediately.

Vaccination is essential for the prevention of disease spread; without it the disease spreads rapidly and is very difficult to control. When the disease spread to a non-vaccinated population of horses in Australia one billion dollars was spent enforcing movement restrictions and ring vaccination to control the disease.  We are often complacent in the UK but in reality only 30% of horses are vaccinated and experts suggest that 70% of horses need to be vaccinated to prevent an outbreak. Furthermore, some of the vaccines used in the UK are very out of date. Some vaccines are also better than others and those which more closely mimic normal infection offer greater protection. Whilst it is worth enquiring as to whether your vet is using the most up-to-date vaccines, vaccination with any vaccine is far better than not vaccinating.

Frequency of vaccination is also important especially in younger animals. Whilst it may seem as though we vaccinate too often, in reality the opposite is true as immunity can be short-lived against an ever-changing virus. Following vaccination, immunity peaks at around 2 weeks and then wanes over time so some horses are at considerable risk if the intervals between vaccination are too long. It is important that the primary course consists of two initial vaccinations 4-6 weeks apart and then a third vaccination around 6 months later. Some of the sporting regulatory bodies are more flexible than this but deviation from the recommended interval compromises the degree of protection. After the primary course it is recommended that horses are vaccinated at least every 12 months. Horses vaccinated every six months (as required by the FEI) are better protected than those vaccinated once per year.

Vaccine technology has advanced greatly in recent decades and current vaccines are extremely safe; it is exceedingly rare for reactions to occur at the vaccination site or for horses to be "off-colour" after vaccination.  There used to be a reluctance to vaccinate because of the perception that horses would be set-back by vaccination or would loose training days. With the current vaccines this is unjustified and failure to vaccinate places the individual horse as well as the equine population as a whole at increased risk.

Summary of key learning points

  • Influenza is caused by a virus that is specific to horses
  • The virus mutates frequently which may enable it to avoid the immune system and cause infection in vaccinated horses
  • In foals and non-vaccinated horses disease may be severe and even fatal
  • Affected horses should be isolated - the disease is highly contagious
  • There is no direct treatment. Supportive and symptomatic treatment care may include anti-inflammatories, fluids and antibiotics
  • Vaccination is vital in protecting individual animals and preventing the spread of disease

To optimise protection use a vaccine that mimics natural infection and vaccinate every 6 months after an initial course of 3 vaccinations at 0 weeks, 4-6 weeks and 6 months

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