Possible botulism bacteria link to grass sickness?

Researchers in Britain are asking for all cases of grass sickness to be reported to help find a way to prevent the disease.

Grass sickness, which is different to staggers, has been recognised for about 100 years. It occurs throughout northern Europe - particularly in Great Britain.

In the acute form of the disease, affected animals show signs of depression and high heart rate.

The increase in heart rate is usually higher than expected for the amount of pain the horse shows. No gut sounds are audible. Horses show patchy sweating, and have difficulty eating and swallowing. Affected animals die within 1-2 days.

In the subacute form, affected animals show less dramatic signs and survive for 2-7 days.

Almost all cases are fatal, although some less severely affected animals may survive with intensive nursing.

Grass sickness results from damage to the nerves responsible for gut motility. The severity of the signs is related to the degree of damage.

The cause has not been confirmed but it looks likely that toxins from the soil-dwelling bacterium Clostridium botulinum may be involved. If so, it may be possible to produce an effective vaccine.

Researchers at the Animal Health Trust in Newmarket, working with colleagues at Liverpool and Edinburgh veterinary schools, set up the Equine Grass Sickness Surveillance Scheme in 2006 - funded by a grant from The Horse Trust.

The aim of the scheme is to produce an accurate map of where and when cases of the disease occur in the UK. This information could then be used to organise trials of a vaccine against Cl botulinum.

To achieve this they have developed two questionnaires - one relating to affected horses, the other relating to premises on which the disease has occurred since 2000.

Looking back over the records of the 1002 cases that have been reported so far, they have found:

• Grass sickness is more likely to occur in the spring. May was the month with highest risk of the disease.

• Young horses are at the highest risk of acute grass sickness, with most cases occurring in 5-6 year olds.

• Acute cases accounted for 45% of cases, and 21% were classed as subacute. All of these were fatal. Surprisingly, about half of the 34% chronic cases that were reported actually survived. Maybe this was because owners of surviving cases were more likely to report the disease.

• The risk of grass sickness was not affected by the gender of the horse. Of the reported cases, there were 46% mares, 48% geldings, and 6% stallions.

• Various breeds were affected - including Throughbred (12%), Highland (9%) and Welsh Cobs (8%). Thirty four percent of affected horses were cross-bred. The database also included three donkeys.

Georgette Kluiters has recently joined the AHT after completing a postgraduate degree in infectious disease control at the Royal Veterinary College. She is now the EGS Research Assistant, having taken over from Claire Wylie who established the initial surveillance scheme.

These preliminary results should be interpreted with caution, warns Kluiters. They include only cases reported to the scheme and do not consider the underlying horse population. "It is hoped that following the development of the National Equine Database comparisons could be made in the future," she said.

"The EGS surveillance scheme is an ongoing project. In order for our data to be representative of the disease in the UK it is vital that both owners and veterinary practices continue to report cases of the disease."

Equine Science Update