Grass sickness in Scotland: why is it so common? [H&H VIP]

For reasons no one really understands, Scotland has the highest prevalence of grass sickness in the world — especially in the east between Dumfries and Inverness.

It looks increasingly likely that the disease is caused by the same bacterium that causes botulism in other animals, although the mechanism by which the bacterial toxin affects the gut and nervous system is far from clear.

For years, vets have realised there must be certain factors associated with the disease, but trying to tie these down has been very difficult.

The biggest retrospective study of grass sickness cases in Scotland has just been published in the Equine Veterinary Journal (EVJ). It comprises a review of 455 cases of the disease, matched with 910 unaffected horses used as controls. All the horses were kept at grass in Scotland.

The details of the horses were recorded, together with their location and weather details at the time they became sick.

Statistical analysis showed some interesting risk factors. Scottish native breeds — the Clydesdale, Highland, Eriskay and Shetland — were more likely to suffer grass sickness than cross-breds or non-Scottish pure-breds. Disease prevalence increased towards the north.

Horses under two or more than 11 years old had a higher chance of contracting the disease, but there were no gender differences.

There appeared to be a firm link with some aspects of the weather, notably an increased incidence in areas where both minimum and maximum daily temperatures were lower and where there were more days of frost. There was a weaker association with increased hours of sunlight, but no link to rainfall.

The scientists speculate that there may be something in the way the grass grows during colder, brighter periods that increases the risk. There is, however, much work still to be done.