Equine influenza: the ever evolving story - Veterinary Practice
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Equine influenza: the ever evolving story

ALASDAIR KING traces the development of both the disease and vaccines

WITH the increased movement of horses, all vets, including those caring for horses involved in the UK’s Olympic team, must be particularly vigilant for signs of equine influenza, a highly contagious respiratory disease.

Vaccinated horses exposed to the virus may experience little if any loss of performance, but unvaccinated horses can be severely ill and in young or stressed horses fatalities may even occur.

Understanding the virus

Due to a short incubation period, equine influenza can spread rapidly, aided in part by the increased movement of horses around the world.

When infected, a horse will produce antibodies to fight against the virus in an attempt to protect itself against disease, but there are limitations because it can take days for the body to mount a sufficient response to the challenge. This is why vaccination is required, to prompt the body into producing an immune response before exposure to the virus.

The flu virus has two surface antigens, the haemagglutinin protein (HA) and the neuraminidase protein (NA). The virus uses the HA protein to enter and infect healthy cells where it replicates, and uses the NA to exit the cell, resulting in viral shedding. The NA is also involved in disease severity and penetration through mucus, allowing it to reach the epithelial cells, leading to infection.

The HA protein is constantly changing. Alterations in the gene sequence allow new variants to “escape” immunity – known as antigenic drift. This occurs when changes in the surface glycoproteins take place through a misreading of the genetic code during replication. Antigenic drift may result in significant new viruses capable of spreading disease to vulnerable horses.

As antigens change it increases the chance of the virus escaping the immune system as the antibodies may not efficiently recognise the new antigens. The HA antigen changes more quickly than the NA, so when attempting to improve immunity against changing strains, vaccines which also contain the NA will have a higher ability to resist antigenic drift. The World Health Organisation 2004 report on developing influenza vaccines supports the role of HA and NA within a vaccine. It states that when present in high quantities, antibody against NA can prevent infection.

With the history of sporadic outbreaks around the world, coupled with the changing nature of the virus, it is especially important to focus on the choice of vaccine in competitive yards.


Thought to date back to the 18th century, the equine flu virus was first isolated in 1955 in Czechoslovakia. Identified as Prague/56, this was a Type 1 virus that primarily affected young horses. A significant and major change emerged a few years later – Miami/63 – which was the first identified Type II virus that affected all ages and arrived in the UK in 1965.

It is the Type II virus that has branched into a European and American lineage, and it is the American lineage which has predominated in recent outbreaks, although European-like strains are still circulating and changing.

The 2003 outbreak in Newmarket with the Newmarket/03 strain affected more than 21 racing yards and was particularly virulent. While vaccinated horses in Newmarket showed few signs, unvaccinated horses were severely ill and some developed meningoencephalitis.

A further outbreak occurred that year in South Africa, a few years after mandatory vaccination was abandoned following a 15-year absence of the virus.

If there remains any doubt about the importance of vaccination, the 2007 outbreaks in Australia and Japan are a harsh reminder of the vulnerability of unvaccinated horses, and the costs. The Australian outbreak affected more than 10,000 yards and cost the industry more than £200 million.

Whilst Newmarket/03, South Africa/03 and Australia/07 were all from the American lineage, currently considered the greatest threat, it is impossible to gauge if this will be the case in the future. It is therefore essential that where horses are vaccinated, the vaccine administered protects against strains we already know, and those we think may emerge in the near future.


The Australian outbreak best illustrates what can happen in an unvaccinated market. In the UK where the equine population now exceeds 1.35 million (BETA, 2006), only an estimated 35- 40% are vaccinated against equine influenza.

As trusted professionals, vets are in a strong position to encourage clients to maintain the vaccination programme of their horses.

Vaccinated horses with influenza appear to display reduced symptoms and faster recovery rates, if affected at all. Importantly, a vaccine which contains both HA and NA components will help reduce the risk of infection and shedding to other horses of current strains and provides peace of mind in the face of changing strains.

The improvement of flu vaccines should ideally also lead to improved safety profiles, reducing the occurrence of abscesses at the injection site. Vaccines, which are developed without chemical adjuvants avoid the formation of a depot at the injection site and thus a focus for infection and are therefore another step in improving overall safety.

Whilst vets cannot control the emergence of new strains, we can manage equine flu by choosing a modern vaccine with known efficacy against the latest strains, and which offers both HA and NA components to protect against the next unexpected strain

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