Bit & Bridle

Equine Influenza (Horse Flu)

Other Names

What is Equine Influenza?

Equine influenza is a highly contagious respiratory disease affecting horses, ponies, donkeys, and other equids. It is caused by equine influenza virus (EIV), a type A influenza virus belonging to the Orthomyxoviridae family. The disease spreads rapidly through equine populations, particularly where horses are congregated — competition venues, sales, breeding yards, and racing stables are common sites of outbreak.

Equine influenza is not a new threat — it has been circulating in horse populations since the 1960s — but the virus is antigenically variable (like human flu, it mutates regularly), which means vaccines must be updated periodically and vaccinated horses can still be susceptible to novel strains. The FEI and most governing bodies in the UK, including the BHA and British Equestrian Federation, require up-to-date influenza vaccination as a condition of competition.

What Causes Equine Influenza?

Equine influenza is caused by EIV subtypes H3N8 (the predominant strain since the 1963 epidemic) and, more recently, H7N7 (now rarely detected). Within the H3N8 subtype, two main genetic lineages — American and Eurasian — circulate, with the American lineage (specifically the Florida clade) now dominant globally, including in the UK.

How it Spreads

  • Aerosol transmission: An infected horse can expel billions of viral particles in a single cough, which remain suspended in air for up to 35 metres. This makes aerosol spread the primary transmission route in densely housed horses.
  • Direct contact: Nose-to-nose contact between horses.
  • Indirect contact (fomites): Contaminated rugs, feed buckets, grooming equipment, clothing, and hands can carry the virus between horses. The virus can survive on surfaces for up to 24 hours.
  • Wind-aided spread: Under certain conditions, aerosolised virus can travel significant distances outdoors.

Signs and Symptoms

Equine influenza has a short incubation period of 1–5 days. Clinical signs typically appear quickly and can affect multiple horses on a yard within a few days of the index case. Signs include:

  • High fever: Often the first sign, with temperatures reaching 40–41°C (104–106°F). Fever typically lasts 1–3 days but can persist longer.
  • Harsh, dry cough: Persistent and can last for several weeks, even after other signs resolve. The cough can cause significant discomfort and lasting airway inflammation.
  • Nasal discharge: Initially clear and watery, progressing to thick, yellow-green mucopurulent discharge as secondary bacterial infections develop.
  • Depression and lethargy: Affected horses are visibly dull and off their food.
  • Loss of appetite: Especially in the febrile phase.
  • Muscle stiffness and reluctance to move: Myalgia (muscle pain) is common and horses may appear stiff or uncomfortable.
  • Enlarged submandibular lymph nodes: In some cases.

In vaccinated horses exposed to a new strain, signs may be attenuated — a mild cough and mild nasal discharge without significant fever, sometimes referred to as “subclinical” influenza, which still allows the horse to shed and spread the virus.

Diagnosis

  • Clinical signs: Classic presentation in a horse or multiple horses is highly suggestive, especially in the context of recent mixing with other horses.
  • Nasopharyngeal swabs: PCR testing of swabs taken from the nasopharynx is the most sensitive and specific diagnostic method. Swabs must be taken early in the disease (within 48–72 hours of onset of signs) when viral shedding is at its peak.
  • Rapid influenza tests: Lateral flow antigen tests are available for in-field diagnosis, though their sensitivity is lower than PCR.
  • Serology: Rising antibody titres between acute and convalescent samples (2–3 weeks apart) can confirm recent infection retrospectively.

Treatment

There is no specific antiviral treatment licensed for horses with equine influenza. Management is supportive:

  • Strict rest: This is the most important treatment instruction. The horse’s respiratory tract needs time to recover. The traditional “rule of thumb” is one week’s rest for every day of fever, though this is a guideline rather than a fixed formula. Returning a horse to work too quickly risks secondary bacterial pneumonia, permanent airway inflammation, and chronic coughing.
  • NSAIDs: Phenylbutazone or flunixin meglumine to reduce fever, pain, and inflammation.
  • Antibiotics: Not indicated for uncomplicated viral influenza. However, horses that develop mucopurulent nasal discharge, persistent fever, or signs of secondary bacterial pneumonia require antibiotic therapy prescribed by a vet.
  • Fluids and nutritional support: Ensure good hydration and maintain nutrient intake. Horses with poor appetite may benefit from palatable, easily digestible feeds.
  • Environmental management: Excellent ventilation is essential. Damp, poorly ventilated stabling prolongs recovery and increases the risk of secondary respiratory complications.

Biosecurity During an Outbreak

  • Isolate all horses showing signs of respiratory illness immediately
  • Restrict yard access and prevent mixing with horses from other premises
  • Dedicate separate equipment for each group of horses
  • Disinfect clothing and footwear between groups
  • Contact your vet for guidance on whether remaining horses should receive emergency vaccination — in an outbreak situation, vaccination of at-risk horses can help limit spread even if it does not prevent all cases
  • Report outbreaks to AHT’s EquiFluNet or the Animal Health Trust surveillance network

Prevention and Vaccination

Vaccination is the primary prevention strategy and is mandatory for most competition horses in the UK. The BHA (racing), FEI (equestrian sport), and British Equestrian Federation all have specific vaccination requirements.

UK vaccination guidelines (per most governing bodies):

  • Primary course: two doses given 21–92 days apart
  • First booster: given 150–215 days after second primary dose
  • Annual boosters: within 365 days of the previous dose

Vaccines in common use in the UK include Proteqflu-Te (combined with tetanus), Equilis Prequenza, and ProteqFlu. Work with your vet to select the most current vaccine formulation, as manufacturers periodically update strains to match circulating viruses.

Vaccination does not guarantee complete protection — especially against novel or antigenically drifted strains — but significantly reduces disease severity, duration, and transmission.

When to Call the Vet

Contact your vet promptly if:

  • A horse develops a fever above 38.5°C, especially if accompanied by cough or nasal discharge
  • Multiple horses fall ill in quick succession
  • A horse fails to improve after 3–4 days despite supportive care
  • Signs of secondary pneumonia develop (persistent fever, laboured breathing, severe nasal discharge)
  • An unvaccinated foal or immunocompromised horse is affected

Frequently Asked Questions About Equine Influenza

Can vaccinated horses still get equine influenza?

Yes. Vaccination significantly reduces the risk and severity of disease but does not guarantee complete protection, particularly against strains that have antigenically drifted from vaccine strains. Vaccinated horses that do become infected typically show milder, shorter illness but can still shed and spread the virus. Maintaining up-to-date boosters with current vaccine formulations is essential.

How long should a horse rest after equine influenza?

The widely used guideline is one week of rest for every day of fever. A horse with a three-day fever should rest for a minimum of three weeks. Full return to previous workload should be gradual, not immediate. Returning to hard work before the respiratory tract has fully recovered is one of the most common causes of chronic post-influenza coughing and poor performance.

Can humans catch equine influenza?

No. Equine influenza viruses do not infect humans. There is no documented evidence of human infection with equine influenza. Horse handlers are not at risk of contracting the disease.

Is equine influenza the same as equine herpesvirus?

No — these are completely different diseases caused by different pathogens. Equine herpesvirus (EHV) can also cause respiratory disease, but it additionally causes abortion in mares and, in its most serious form (EHM), a potentially fatal neurological syndrome. They have different vaccination requirements, treatments, and biosecurity implications.