More than a century after Equine Grass Sickness (EGS) first appeared, with devastating results, livery yards in Scotland are involved in a trial which may lead to an exciting breakthrough in combating the disease. If it's successful, a vaccine could be available within the next few years.

The pilot trial now underway is based on the theory that EGS is caused by a micro-organism in the soil called Clostridium botulinum. But today's researchers aren't the first to have had C.botulinum in their sights.

Ironically, the current trial mirrors the work of a scientist who claimed he had found a way to prevent the illness as early as 1922. Dr JF Tocher was convinced that grass sickness was linked to botulism, and claimed he could prove it.

But Tocher's theory was cast aside for 80 years after his findings were discredited, and research funding went to alternative lines of inquiry.

The first EGS cases in Britain were reported in Angus in the early 1900s. It spread rapidly throughout Perthshire, into Aberdeenshire and beyond. The illness damages the nervous system, paralysing gut and giving rise to a series of distressing symptoms. It was given the name Grass Sickness because most cases appeared when horses were turned onto summer grazing.

In the early years of the 20th century, the disease had a horrific effect on the working horse population, particularly amongst young Clydesdales. Many hundreds died every year, especially along the east coast.

The problem was so great that Scottish politicians described it as a national crisis. In 1918, The Highland and Agricultural Society of Scotland began to fund its own research into the disease. The scientist they commissioned was Dr John Fowler Tocher of Aberdeen, who was the Society's chemist.

Over the next few years, Tocher and his colleagues carried out a series of field and laboratory experiments. They quickly ruled out the widespread view that a poisonous plant was bringing horses down with grass sickness.

The Aberdeen team became convinced that the horses were being affected by a toxin produced by a bacterium they called bacillus botulinus, which is found in the soil. They claimed that they had been able to find this organism in the spleens of horses which had died from EGS. Furthermore, they could produce the symptoms of EGS in healthy horses by injecting them with the botulinus toxin.

In 1922 and 1923, the team carried out inoculation trials involving more than 2000 horses in the worst affected areas of Scotland. Tocher injected half the horses in his experiment with a serum prepared from bacillus botulinus. As a scientific control, half the horses went without vaccination.

Tocher published results which claimed to show that the inoculated horses had a much lower level of EGS infection. In 1923, 9.3% of the unvaccinated horses in his trial died from grass sickness. In contrast, of the horses which had two inoculations, only 2.3% died.

But Tocher was not the only researcher who was working on EGS. And not everyone agreed with his conclusions.

Professor Sydney Gaiger was the chief investigator of the newly formed Animal Diseases Research Association in Scotland, now known as the Moredun Foundation. He challenged Tocher's scientific methods and his conclusions. In several public lectures, he said the botulism theory was incorrect. At that time, Prof Gaiger believed that grass sickness was a type of meningitis which attacked the brain.

Veterinary opinion in Scotland sided with Prof Gaiger. At a meeting in Perth in 1923, vets agreed unanimously that they would only use Tocher's serum at the specific request of clients, and would not guarantee that horses could be protected from grass sickness. They passed a resolution urging that the ADRA should urgently investigate the disease.

Tocher continued to claim the benefits of his vaccination, and adverts for the serum appeared for several more years. But funding for EGS research in Scotland was now directed by both government and the Highland and Agricultural Society to the ADRA. The botulism theory, so widely reported in the early 1920s, sank without trace in the public debate.

For almost 80 years, grass sickness baffled scientists, while continuing to take a terrible toll on Scotland's horses. Theories ranging from poisonous plants, to insect bites and viruses were proposed, and rejected.

It was not until the very end of the 20th century that there was renewed scientific interest in the link with botulism. Now it has become the preferred theory again, with contemporary research backing up Tocher's early efforts.

n See next week's Scottish Horse special for more information on the inoculation trial being undertaken in Aberdeenshire, run by the Animal Health Trust in Newmarket and the Dick Vet in Edinburgh.