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Equine influenza general information

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

China, Japan and Australia suffered large-scale epidemics in 2007 that are reported to have involved tens of thousands of horses. Previously the disease had not been reported in Australia and their equine population had not been vaccinated, they were therefore immunologically highly susceptible to infection.

In UK, Europe and North America, the disease is endemic and horse racing and performance horse regulators insist upon influenza vaccination to help prevent disruption to their events. These organisations rules often exclude horses that are not fully and properly vaccinated from entering licenced and performance premises.

What causes Equine influenza?

EI is caused by an Orthomyxovirus, belonging to the H7N7 and H3N8 subtypes (the latter is the currently circulating subtype). There are many other influenza viral subtypes that cause infections in other animal species, including humans. Unfortunately, influenza viruses gradually change with time (antigenic drift), allowing the virus to evade both natural and vaccine-stimulated immunity, so influenza vaccines for all species need to be regularly updated to maintain full efficacy. Influenza viruses are generally species-specific, i.e. only infect their specific target animal species, and equine influenza is not known to infect humans, nor human influenza virus infect horses.

EI virus is transmitted from horse to horse, in close proximity, by coughing and inhaling aerosolised 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 people to horses via hands, clothes, grooming equipment or tack contaminated with infectious material. Overcrowding of horses, poor stable biosecurity, poor hygiene, feeding and water supply conditions and the transport of horses are all predisposing risk factors for spread of infection.

What are the symptoms of Equine influenza?

The incubation period (time between infection and appearance of symptoms) for EI is usually 1 to 3 days.

Clinical signs in horses are highly variable in severity, depending upon natural and/or vaccinal immune status. Fully specifically vaccinated horses usually show no or very mild signs of EI. Young horses are most susceptible.

A high fever of up to 41°C (106°F), depression, a watery nasal discharge, watery eyes (conjunctivitis), filled (swollen) legs and a non-productive, harsh dry cough, sometimes with swollen glands under the throat (submandibular lymph nodes) are typical of acute infection. Widespread coughing in stabled, susceptible horses a few days after the introduction of an infected horse is the typical scenario. In uncomplicated cases, clinical signs usually abate over 3-7 days, although coughing may persist over weeks. Mildly affected horses usually recover over 2-3 weeks but severely affected ones may take at least 6 months to recover full health and performance potential.

Severely affected horses may develop complications with, most commonly, secondary bacterial infections of the throat and lungs (pneumonia) and less commonly problems associated with blood vessels (vasculitis), muscles (myositis) and the heart (myocarditis). These conditions can be serious, requiring intensive care and treatment and may cause prolonged debilitation.

What other infections may look like Equine influenza?

Equine Herpesvirus respiratory infection causes fever and initially a watery nasal discharge, but a harsh dry repetitive coughing is usually the ‘tell-tale’ clinical sign of EI. Many other viruses can cause usually relatively mild upper respiratory disease in horses and ‘strangles’, (Streptococcus equi) is always an important diagnosis to exclude in horses with fever and throat gland swellings.

How is Equine influenza diagnosed?

Sudden-onset fever, depression, watery nasal discharge and repeated harsh dry coughing, particularly in non- vaccinated horses, should be considered EI until proved or disproved by laboratory testing. There is 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. Blood samples taken at the time of first clinical signs and then 10-14 days later can be examined for significant rising antibody levels (seroconversion), which confirm significant challenge. Your veterinary surgeon will recommend that he or she sends nasopharyngeal swabs from acutely infected horses to the Animal Health Trust (AHT), which runs a World Organisation for Animal Health (OIE) reference laboratory for EI, so that the virus can be grown and typed, to survey strain type and to look for signs of viral antigenic drift.

Can Equine influenza be treated?

Effective equine antiviral drugs currently remain unavailable and so the most important first steps are to stop exercising the horse(s) and to provide supportive care for fever and depression, and to improve access to clean fresh air with low dust and mould content. In some cases, your veterinary surgeon may wish to give medication to reduce fever and to stimulate appetite but in most cases the fever should be allowed to run its course and the horse to overcome the infection by natural defence mechanisms. Where fever persists and the nasal discharge changes to cloudy/yellow, i.e. pus, suggesting secondary bacterial infection, treatment with appropriate antibiotic medication may be indicated.

EI virus replicates (multiplies itself) within the infected horse’s respiratory tract lining cells, resulting in their destruction and loss of their normal 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, depending upon their individual needs) from exercise to reduce the risks of secondary bacterial infection and the development of serious lung, heart, muscle and blood vessel complications.

How can Equine Influenza be prevented?

Your veterinary surgeon will advise you of the most appropriate vaccine for use with your horses. It is important that your horses are vaccinated with a schedule as recommended by the manufacturer of the vaccine used and fits with the requirements of the regulators of your horse’s performance intentions.

Prevention and control also depends upon good biosecurity protocols for equine stables and populations. Early detection of cases, their isolation, followed by environmental cleaning and disinfection is essential. All equine contacts with actively infected cases should be traced, quarantined and appropriately tested until evidence of transmission has been found and monitored, tested and treated, as necessary. As routine policy, all new horses arriving at stables should undergo an appropriate period of quarantine, during which they are monitored for the development of clinical signs of infectious and contagious diseases, including EI. If symptoms develop, the affected horses should be specifically examined, tested and treated in isolation, by their veterinary surgeon.

No horse with confirmed or suspected EI or direct equine contacts of confirmed or suspected EI cases should be taken to sales, equine shows or performance events, for their own health and welfare and to prevent the risk of transmission to others.

All those who enjoy and work with horses, particularly those who run racing, performance and pleasure riding stables or studfarms, should, for the health, welfare and continuing performance of their and their patron’s horses and for the benefit of their ongoing businesses, make sure that all their resident horses are fully and properly vaccinated, at least against EI and tetanus, and that all incoming horses are similarly vaccinated. All equine vaccinations should be verified by veterinary entries in properly identified and registered passports.


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