Equine influenza virus, or EIV, is an extremely contagious respiratory disease or flu affecting horses, mules, and donkeys.

EIV is characterized by fever, apathy, and lack of appetite, and causes respiratory symptoms such as coughing and nasal discharge. However, some horses infected with EIV have no observable signs of the virus.

Left untreated, the virus can lead to secondary infections and pneumonia. In most cases, influenza is not fatal, however, horses with impaired immune function may be more susceptible to negative outcomes. [1]

EIV most often affects racehorses or show horses because it is easily transmitted between horses at competitions. Influenza outbreaks are commonly linked to horses being transported to another country for competition. [1]

There are several ways to prevent the spread of equine influenza, including vaccination, good hygiene practices, and biosecurity protocols. It is important to quarantine any new horses before introducing them to a herd and to isolate horses that become sick with influenza.

Equine Influenza Virus

Equine influenza is a serious respiratory disease that can affect horses of all ages.

EIV is a member of the Orthomyxoviridae family and Influenzavirus A genus. The virus is related to but distinct from the influenza virus that affects humans and other species. [9]

There have been reports of other species, such as pigs and dogs, becoming infected with EIV during outbreaks. Some evidence suggests that humans can catch EIV, but this is rare and is unlikely to result in clinical signs. [4]

Care should be taken when handling infected horses, but you are likely not at risk of EIV if your horse has the virus. [4][5][6][7]

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Strains of EIV

There are two strains of the equine influenza virus – H7N7 (subtype 1) and H3N8 (subtype 2) – both of which are sub-types of influenza A.

The equine influenza virus is a single-stranded RNA virus, which readily mutates into different sub-strains of influenza. [9] As different variants of EIV emerge, they vary in severity and in how easily they spread. [8]

The H7N7 strain, or equi-1, was the first strain of equine influenza identified in 1956 in Europe. This strain is now considered extinct because the last outbreak of H7N7 occurred in 1979, and few cases have been reported since then.

The H3N8 strain (equi-2) was first detected in Florida in 1963 and is a greater concern for horses today. [5][8]

The H3N8 strain affects horses more severely than H7N7, putting them at greater risk of contracting a secondary infection. [9]

Pathophysiology

The virus replicates in the upper airways of the horse’s lungs, damaging cilia and epithelial cells. These are important structures within the airways that filter debris from inhaled air.

This causes the horse to develop a dry, unproductive cough and become congested. [8][11] The destruction of the epithelium in the respiratory tract also puts the horse at risk of secondary bacterial infections, including pneumonia and bronchitis.

Infected horses remain contagious for 7-10 days, during which period they shed the virus through exhaled air and nasal secretions. [10]

Most horses recover within 2-3 weeks. You can support your horse’s recovery by giving them time to rest, providing them with a well-ventilated environment, reducing exposure to dust and implementing good hygiene practices. [21]

Secondary Infections

Equine influenza is usually not fatal on its own and has a good prognosis for most horses.

However, EIV can cause the susceptible horse to develop other conditions such as secondary bacterial infections, which can lower the horse’s likelihood of survival.

Some secondary infections or conditions that have occurred in horses with EIV include: [8][17]

  • Bronchitis or bronchiolitis (inflamed bronchi and bronchioles, which are airways in the lungs)
  • Bronchointerstitial pneumonia, which is particularly prevalent and dangerous in EIV-infected foals
  • Bacterial infection and inflammation of the larynx – a structure in the trachea (windpipe)
  • Myocarditis (inflammation of the heart muscle)
  • Pleuritis (infection of the membrane covering the lungs)

Horses at the greatest risk of mortality from the equine influenza virus include: [1][11]

  • Foals, particularly at 2-3 months old when maternal antibodies are wearing off
  • Older horses
  • Horses with pre-existing health conditions

Signs & Symptoms

The equine influenza virus has an incubation period of 1-3 days. Horses are contagious during the incubation period but do not show outward signs of infection. [8]

Some horses never develop signs of the virus and are asymptomatic carriers of EIV. Asymptomatic horses may have a low viral load, may have been exposed to a mild variant of the virus, or may have antibodies from a previous infection or vaccination that suppress clinical signs. [13]

In symptomatic horses, symptoms usually become noticeable after three days and persist for 1-2 weeks. [9][10] Symptoms may include any or all of the following: [1][8][11][13][14][15]

  • Fever with a temperature greater than 102°F/38.9°C
  • Lethargy
  • Depression
  • Lack of appetite
  • Nasal discharge which is initially serous or watery, and becomes mucopurulent (a thick, opaque mucus)
  • Harsh, dry cough
  • Laboured breathing
  • Aching muscles
  • Swollen submandibular lymph nodes (located between the lower jaw bones)
  • Conjunctivitis (swollen, oozing, pink eye membrane)

In some cases, horses can also develop distal limb edema (swelling or accumulation of fluid in the lower legs) and cardiomyopathy (disorders of the heart muscle).

High fever is typically the first symptom to appear and will usually last 4-5 days, although this varies depending on the strain of the virus. Coughing may persist for longer than 2 weeks after the horse has recovered. [8]

One study reported that donkeys and mules became more ill from EIV than horses. Care should be taken to minimize the risk of transmission to your non-horse equine companions. [2]

If you notice your horse showing signs of influenza, contact your veterinarian to obtain a diagnosis.

Transmission

Equine influenza is spread through direct contact with infected horses or through contact with contaminated surfaces such as tack, equipment, feed buckets, or clothing.

The virus can spread through respiratory secretions such as mucous or nasal discharge. Inhalation of contaminated droplets expelled by coughing or sneezing is another way the virus can spread. [16]

The UC Davis School of Veterinary Medicine reports that a coughing horse can spread the virus up to 150 feet in aerosolized particles, potentially infecting an entire barn. [22]

This makes quarantine procedures very important when a case of EIV is diagnosed to prevent the virus from spreading to barn mates.

Outbreaks of EIV can occur when horses are transported long distances or between countries for competition. Horses can carry the influenza virus but exhibit no clinical signs, causing the virus to be unknowingly transmitted to fellow competitors.

If the competition is held in an area that where local horses have no antibodies against a novel strain of EIV, there is potential for an outbreak or epidemic to occur.

Furthermore, horses travelling to the competition can become infected and transmit EIV after being transported home.

Horses may remain contagious and continue to shed the virus for 7 – 10 days after the infection has resolved.

Risk Factors

Numerous factors affect each horse’s risk of contracting EIV. Knowing these risk factors will enable horse owners to take precautions to reduce their horse’s risk of infection.

Variables that increase the risk of equine influenza include: [1][14][18]

  • Exposure to recently imported horses
  • Attendance of events with many horses present, such as shows or competitions
  • Visiting vet clinics
  • Immunosuppression due to prior illness, travel, or intense training or competition
  • Age (foals and older horses are more likely to catch EIV)
  • Sex (males are at twice the risk of females)
  • Not having been EIV-vaccinated in the previous three months
  • Age of first vaccination (vaccinating before 6 months or after 18 months increases the horse risk)
  • Weather conditions (humidity below 60% combined with a 20-25oC air temperature is associated with increased risk of EIV)
  • Seasonality (higher incidence of EIV reported in colder seasons)
  • Location (horses downwind from nearby premises containing infected horses are at heightened risk)

Diagnosis

Ask your veterinarian to test your horse for equine influenza if they show clinical signs of the virus, if they are at risk because of a local outbreak, or if you plan to travel with your horse.

To detect the virus, your veterinarian will take a nasopharyngeal swab from about 25 cm inside the horse’s nose (or about 15-20 cm in a foal) and submit it for real-time PCR testing.

EIV may also be detected by immunoassay, antigen capture ELISA, or in serum from the horse’s blood. [13][15]

Horses may continue to test positive up 34 days after being infected. [13]

Treatment of EIV

If your horse is diagnosed with EIV, immediately isolate them and disinfect any equipment, tack or surfaces the horse has contacted to prevent other horses on the premises from becoming infected. [1]

Once a horse is diagnosed with EIV, the best treatment is rest and supportive care. [1][22] Do not work the horse; appropriate stall rest will lead to a faster recovery and a better long-term prognosis. [1]

In severe cases, your veterinarian may prescribe non-steroidal anti-inflammatory medications (NSAIDs) to help control your horse’s fever. Some horses may need antibiotics to address secondary infections.

You can support your horse’s recovery by ensuring they are comfortable, in a well-ventilated barn and by controlling dust in the environment. Soaking or steaming your horse’s hay can eliminate dust and support respiratory health.

Once your horse has recovered, exercise can be gradually reintroduced. It is recommended to give your horse one week off training for every day that they were febrile (had a fever).

Prognosis

Horses affected by EIV generally have a good prognosis and most make a full recovery within 2 weeks of infection if they have been rested and have not contracted secondary infections. [22]

The mortality rate is estimated between 0-30% mortality depending on several factors including: [1][2]

  • Strain of EIV
  • Horse health status
  • Level of care and nutrition
  • Whether horses were rested
  • Quarantine strategies
  • Whether horses have antibodies from vaccination or from a previous EIV infection

The mortality rate is higher in foals, horses with pre-existing health concerns, donkeys, and horses that contract a secondary bacterial infection causing pneumonia or pleuriti. [8]

Immunity

Previous exposure to EIV can protect the horse from developing severe symptoms. During the first exposure to the virus the immune system develops antibodies specific to components of the virus.

Viral proteins (called antigens) trigger the horse’s cells to produce pro-inflammatory cytokines which upregulate the horse’s immune system to fight the infection and produce antibodies. [12]

If the horse is invaded with the same strain of EIV again, the antibodies will recognize the antigen in the virus and protect the horse from being infected. [8]

The antibodies produced in response to the first exposure will protect the horse from becoming sick due to the same EIV strain for 1-5 years. However, because different strains of the virus have different antigens, the antibodies the horse has for one strain of EIV may not be able to protect the horse from other strains. [8][9]

Prevention

The best way to protect your horse from contracting equine influenza is to implement good biosecurity protocols and hygiene practices.

If you plan to travel with your horse to another country, quarantine the horse before shipping them and upon arrival at the destination.

Ensure that any new horse being introduced to your herd has been vaccinated and tested. Isolate them in their new location for two weeks.

If you or any of your property has been exposed to a horse with influenza, thoroughly disinfect your stall, clothes and equipment, so you do not transmit EIV to other horses. Use alcohol hand sanitizers and regularly wash your hands.

Minimize contact with unfamiliar horses and use your own water and feed buckets when travelling. Avoid sharing brushes or tack.

Vaccination

Keeping your horse up to date on the equine influenza vaccine is one of the best ways to protect their health. While vaccination does not always prevent your horse from getting infected with EIV, it can reduce the severity of illness and improve recovery time.

There are two main types of vaccines available with different methods of administration, including:

  • Inactivated (killed) vaccines administered by intramuscular injection
  • Modified live vaccines administered intranasally

Inactivated Vaccines

Inactivated virus vaccines consist of whole EIV virus particles that have been denatured or made inactive so they pose no risk infection or symptoms. [23]

The horse’s immune system will still detect antigens from the vaccine and produce antibodies to protect against future exposure to the virus.

However, these vaccines usually need to be administered several times in order to be effective. AAEP guidelines recommend administering three doses to horses that have not previously been vaccinated. [23]

Modified Live-Attenuated Vaccines

Modified live vaccines (MLV) are made of intact viruses that can stimulate an immune response, but have been modified in a laboratory so they cannot cause harmful effects.

These vaccines tend to be more immunogenic, meaning they do a better job of provoking the horse’s immune system to respond. MLVs provide longer lasting and stronger immunity to equine influenza. [23]

However, these vaccines are not safe for immunocompromised horses and could cause adverse effects if they are not properly administered. [23]

Limitations of Vaccines

Vaccines are an important form of protection from equine influenza, but they are not universally effective.

Antigenic drift describes the natural process that leads to the continual evolution of viruses. As a virus replicates, minor changes, or mutations, can occur in its genetic material.

Most of these mutations have no effect on the virus or its ability to infect cells. However, some mutations can confer a selective advantage, allowing the virus to spread more easily or to cause more severe disease. Over time, these advantageous variants can come to predominate within a population of viruses. [24]

This continual change is one of the reasons why it is so difficult to develop effective vaccines against viruses such as equine influenza; by the time a vaccine is developed, the circulating strains may have developed variations that current vaccines may not protect against.

The World Organization of Animal Health’s Expert Surveillance Panel on Equine Influenza Vaccine is responsible for surveying the current infectious strains of EIV worldwide and making recommendations on vaccine development. [19]

Vaccination Schedule

The American Association of Equine Practitioners (AAEP) recommends vaccinating horses annually. Competition horses at greatest risk of exposure to EIV should be vaccinated every 6 months. [1][20]

When an outbreak occurs, boosters may be recommended as soon as 3 months after the horse’s previous vaccination. Antibodies from a prior infection can protect the horse for a year or more. However, because of antigenic drift, you may wish to vaccinate your horse again after 6 months. [20]

It is also recommended to vaccinate pregnant mares several weeks prior to giving birth. This will allow the mare to develop EIV antibodies that she will then pass to her foal through colostrum when the foal nurses.

Maternal antibodies will protect the foal for approximately 6 months after the foal is born. [8]

Vaccines can protect your horse from developing serious symptoms, but they do not prevent horses from shedding the virus and infecting other horses.

Because of this, effective management practices including quarantine and decontamination procedures are very important to stop the virus from spreading. [1]

Consequences

EIV is not a reportable disease but is important to track because of its economic implications and highly infectious nature.

Horses that contract EIV require 2 weeks or more of rest to recover, preventing them from racing, showing, or training. [2]

Outbreaks of equine influenza virus are costly due to the expenses of veterinary care, the establishment of quarantine areas, vaccinations, testing of horses, banning of horse transportation, and cancellation of events to avoid the further spread of the disease. [3]

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References

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  2. Guo, Y. et al. Seroepidemiological and molecular evidence for the presence of two H3N8 equine influenza viruses in China in 1993–94. J Gen Virol. 1995. View Summary
  3. Ontario Ministry of Agriculture, Food and Rural Affairs Disease control at racetracks and horse events. OMAFRA. 2022.
  4. Xie, T. et al. A review of evidence that equine influenza viruses are zoonotic. Pathog. 2016.
  5. Tu, J. et al. Isolation and molecular characterization of equine H3N8 influenza viruses from pigs in China. Arch Virol. 2009.
  6. Crispe, E. et al. Infection of dogs with equine influenza virus: evidence for transmission from horses during the Australian outbreak. Aust Vet J. 2011. View Summary
  7. Alford, R.H. et al. Human responses to experimental infection with influenza A/Equi 2 virus. Am J Epidemiol. 1967.
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  9. Olusa, T.A.O. et al. Serologic evidence of equine H7 influenza virus in polo horses in Nigeria. Sci World J. 2010.
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  12. Bryant, N.A. et al. Isolation and characterisation of equine influenza viruses (H3N8) from Europe and North America from 2008 to 2009. Vet Microbiol. 2011. View Summary
  13. Perglione, C.O. et al. Epidemiological and virological findings during multiple outbreaks of equine influenza in South America in 2012. Influenza Other Respir Viruses. 2016. View Summary
  14. Barquero, N. et al. Risk factors for influenza infection in vaccinated racehorses: lessons from an outbreak in Newmarket, UK in 2003. Vaccine. 2007. View Summary
  15. Elton, D. et al. Facing the threat of equine influenza. Equine Vet J. 2011. View Summary
  16. Cullinane, A. et al. Equine influenza – surveillance and control. Influenza Other Respir Viruses. 2010.View Summary
  17. Britton, A.P. et al. Isolation of influenza A virus from a 7-day-old foal with bronchointerstitial pneumonia. Can Vet J. 2002. View Summary
  18. Firestone, S.M. et al. The influence of meteorology on the spread of influenza: survival analysis of an equine influenza (A/H3N8) outbreak. PLoS One. 2012. View Summary
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