Strangles vaccine on the horizon

Strangles has been the scourge of the horse world since medieval times, but a four-year research project at Cambridge University Veterinary School could lead to the development of a safe, effective vaccine. Preventing or even eradicating this highly infectious disease will probably still take many years, but for the first time it is now a realistic goal.

Dr Josh Slater, senior lecturer in equine medicine at Cambridge, says that strangles is one of the most common infectious diseases in horses in the UK.

“A lot of people think it is something that happened in ‘the old days’, but it’s very much a current problem,” he says.

Strangles is caused by the bacterium Streptococcus equi (S. equi). It can affect all equines of any age and it only takes one carrier to bring the infection on to the yard.

Research at the Animal Health Trust and at Cambridge has given scientists a much greater understanding of how strangles is caused and why some seemingly healthy animals can be carriers. The latest work has identified previously unknown virulence genes of S equi.

“We have identified several new genes we know are implicated in the virulence of the bacterium,” says Dr Slater.

In 2005, when the current project ends, these genes will have been characterised in detail and could form the basis of a new vaccine, although getting it on the market depends on further funding.

Developing vaccines for bacterial infections is much more difficult than creating them for viral infections such as flu, because bacteria are much more complicated than a virus. The usual way to make a vaccine is to take the bacterium, kill it and inject it back into the animal — but this hasn’t worked with strangles.

The researchers believe a vaccine is a realistic goal, because horses with strangles do develop immunity for many years; however, it would have to be a more sophisticated vaccine, such as a live attenuated vaccine in which the disease-causing parts have been removed.

“To do this, you have to understand the process at gene level and understand which genes do what,” he says.

Before the Cambridge and Newmarket projects this would have been impossible; now it is becoming a reality.

Another problem with strangles is that some seemingly healthy horses are long-term carriers, with bacteria and pus in the guttural pouch. AHT researchers found that the strangles organism migrated into the lymph nodes in the head, then released toxins which the immune system couldn’t cope with.

It is now known that another bacterium closely related to S. equi, Streptococcus zooepidemicus, is naturally found in the airway of all horses and causes disease in some, but not in others. Because this behaviour contrasts markedly with that of S. equi, comparing the two types of bacteria should give more information about S. equi.

These latest scientific advances are obviously exciting, but Dr Slater says that parts of the horse world still need to take strangles more seriously — without becoming hysterical about it. He stresses that it is important to get rid of the stigma attached to the condition, so that more owners report possible cases — and that it is a bacterial infection rather than a sign of bad management.

This article was published in full in the 11 September issue of Horse & Hound. To purchase a back issue (tel: 020 8532 3628).