Equine influenza

What is Equine influenza?

Equine influenza (EI) is a highly contagious though rarely fatal respiratory disease of horses, donkeys and mules and other equidae. The disease has been recorded throughout history, and when horses were the main draft animals, outbreaks of EI crippled the economy. Nowadays outbreaks still have a severe impact on the horse industry.

EI is caused by two subtypes of influenza A viruses: H7N7 and H3N8, of the family Orthomyxoviridae. They are related to but distinct from the viruses that cause human and avian influenza.

EI is anWOAH-listed disease and must be reported to the WOAH as indicated in its Terrestrial Animal Health Code.

Transmission and spread

Highly contagious, EI is spread by contact with infected animals, which in coughing excrete the virus. In fact animals can begin to excrete the virus as they develop a fever before showing clinical signs. It can also be spread by mechanical transmission of the virus on clothing, equipment, brushes etc carried by people working with horses.

Once introduced into an area with a susceptible population, the disease, with an incubation period of only one to three days, spreads quickly and is capable of causing explosive outbreaks. Crowding and transportation are factors that favour the spread of EI.

Public health risk

There is little risk to public health. In experimental settings the virus has shown the ability to infect humans, and a few people in contact with infected horses developed antibodies to equine influenza viruses, but no humans exposed to the virus have become ill.

Clinical signs

In fully susceptible animals, clinical signs include fever and a harsh dry cough followed by a nasal discharge. Depression, loss of appetite, muscle pain and weakness are frequently observed. The clinical signs generally abate within a few days, but complications due to secondary infections are common. While most animals recover in two weeks, the cough may continue longer and it may take as much as six months for some horses to regain their full ability. If animals are not rested adequately, the clinical course is prolonged.

While the disease is rarely fatal, complications such as pneumonia are common, causing long term debility of horses, and death can occur due to pneumonia, especially in foals.

Diagnostic

Clinical signs are suggestive of EI, but definitive diagnosis is by serology or isolation of the virus according to procedures in the WOAH Manual of Diagnostic Tests and Vaccines for Terrestrial Animals.

Prevention and control

Vaccination is practiced in most countries. However, due to the variability of the strains of virus in circulation, and the difficulty in matching the vaccine strain to the strains of virus in circulation, vaccination does not always prevent infection although it can reduce the severity of the disease and speed recovery times. Vaccines are produced according to the guidelines in Chapter 2.5.7 of the WOAH . The WOAH also convenes an Expert Surveillance Panel on Equine Influenza Vaccine that examines the strains of virus in circulation making recommendations on which strains should be included in the vaccines.

When the disease appears, efforts are placed on movement control and isolation of infected horses. The virus is easily killed by common disinfectants, so thorough cleaning and disinfection is part of biosecurity measures in responding to the disease.

Since the disease is most often introduced by an infected animal, isolation of new entries to a farm or stable is paramount to preventing the introduction of disease to a premise.

For movement of horses across international boundaries the WOAH  sets the standards by which countries should control the import of horses.

Geographical distribution

The disease is entrenched in most of the world, with the exceptions of Australia (where an important outbreak occurred in 2007), New Zealand, and Iceland.

Other reference

• The Center for Food Security and Public Health, Iowa State University
• Merck Veterinary Manual