Equine Influenza

What is equine influenza?

Equine influenza (EI) is a highly infectious viral respiratory disease. It is characterised by fever and coughing, which can spread rapidly among susceptible horses.

What are the signs of equine influenza?

The incubation period (that is the time between infection and the horse showing clinical signs) is 1 to 3 days, therefore your horse will rapidly start to show signs if they have become infected with EI.

Clinical signs are highly variable in severity and depend on natural and/or vaccine immune status. Fully vaccinated horses will usually show none or very few clinical signs when infected with EI, whereas young horses are very susceptible.

  • Fever up to 41oC (106oF)
  • Depression
  • Watery nasal discharge
  • Dry, non productive, harsh cough
  • Watery eyes (conjunctivitis)
  • Swollen submandibular lymph nodes
  • Filled, swollen legs

In uncomplicated cases, clinical signs usually disappear over 3 – 7 days, however coughing may continue for weeks once the larynx has been irritated and damaged by repeated coughing. Mildly affected horses usually fully recover over 2 – 3 weeks, however severely affected horses may take as long as 6 months to return to full health and performance.

Severely affected horses may also develop complications, normally due to secondary bacterial infections that invade when the immune system is compromised, there include: Pneumonia, vasculitis, myocarditis and others. These conditions can be serious, requiring intensive care in a specialist hospital facility, and may cause prolonged debilitation.

What causes equine influenza?

EI is caused by a virus in the orthomyxoviridae family, belonging to the H7N7 and H3N8 subtypes. Unfortunately, influenza viruses gradually change with time, therefore it allows the virus to evade both natural and vaccine stimulated immunity.

As EI is transmitted from horse to horse in close proximity (by coughing and inhaling live virus), this form of transmission is intensified in stabled horses, particularly in stables with shared air space. Live virus is also spread from horses to other horses via hands, clothes, grooming equipment or tack contaminated with infectious material.

How is equine influenza diagnosed?

Any horses showing the before mentioned clinical signs, particularly in non vaccinated horses, should be considered to have EI until proved or disproved by laboratory testing. Blood samples taken at the time clinical signs first appear, then again 10-14 days later, can be examined for rising antibody levels, which confirm significant challenge. There is also now a human influenza A virus antigen detection rapid test which is available at a number of equine veterinary laboratories, which will give a reliable same day result. Your vet may also advise that a naso-pharyngeal swab is taken, so the virus can be grown and types to survey strain type and to look for signs of antigenic drift.

How is equine influenza treated?

Effective antiviral drugs are currently unavailable, therefore treatment is primarily aimed at stopping exercise and providing supportive care for fever and depression, as well as improving access to clean, fresh air with low dust and mould content.

EI virus replicates in the respiratory tract lining the cells, resulting in their destruction and loss of function. Once the virus has been overcome and expelled, the lining cells require 3 weeks to recover, so it is important that infected horses are given at least 3 weeks rest (and sometimes more) from exercise to reduce the risks of secondary bacterial infection and the development of serious secondary problems.

Can I prevent my horse from getting equine influenza?

There are a number of safe and efficacious vaccines to protect against EI. It is important that vaccinations are kept up to date, as immunity will lapse when vaccination is stopped.

Prevention and control also depends upon good biosecurity measures, such as:

  • Early detection of cases
  • Isolation of cases
  • Environmental cleaning and disinfection

Useful links:
www.equiflunet.org.uk