Avian influenza or “bird flu” reached Britain’s shores in October last year after it emerged that imported birds had been found with the virus at a quarantine centre in Essex.

The threat of a human pandemic continues to hit the headlines, as scientists have confirmed the virus in poultry within the European Union, Africa and elsewhere, but what danger does it pose to our horses?

The science

Influenza viruses are divided into three categories: A, B and C. Type A infects both people and animals, including birds and horses, and has been responsible for pandemics. Currently, only type A influenza viruses are known to infect horses.

The avian influenza currently making the news is the type H5N1, while other types such as H3N8 or H7N7 usually cause equine influenza. Ordinarily, different species have their own flu viruses; for instance, human flu does not spread to horses.

Yet on rare and unpredictable occasions, a flu virus from one animal species crosses into a different host. When a flu virus infects the host to which it has become adapted, it causes illness but is not usually life-threatening. Indeed, it is not in the interests of the virus to kill all its hosts.

By causing relatively mild symptoms, the virus allows the infected host to spread it to others. But when it spreads to a species to which it has not become adapted, the flu virus can be fatal, as the new host has no immunity.

The theory

In theory, there is no need to panic; horses are not likely to contract bird flu. When closely examined, equine receptors are subtly different from those of a bird, which means that although the virus will infect birds relatively easily, it would be very unlikely to infect horses.

High standards of stable management in the UK mean that, in general, horses do not come into prolonged contact with birds, nor are they frequently in situations that may lead to contraction of the disease, such as drinking out of a pond that may be home to infected birds. Many recently reported human cases of bird flu are in poultry workers or those in close contact with birds.

However, there is a precedent for such infection occurring. In 1989, a new equine influenza virus was discovered in China. Genetic research suggested that this had originated from birds. Since then, there have been no further reports of this virus in horses anywhere. This suggests that it is not impossible for horses to contract a virus from birds.

Interestingly, the Respiratory Research medical journal has published today that scientists in China have stopped mice from dying from the H5N1 strain by injecting them with equine antibodies. This breakthrough is hoped to offer a potential lifeline if the virus manages to cross species to humans. The findings also suggest that horses may carry some natural immunity to the virus.

  • Read more about equine and avian flu in the current issue of Horse & Hound (23 March, ’06)
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