The disease particularly affects younger horses, with a large proportion of cases being between three and seven years old, however it can however affect horses of any age.
The peak time of year for cases is April, May and June.
Horses are at increased risk if premises have been previously affected. This risk decreases over time.
Other factors that can increase the risk of grass sickness include:
- Mechanical dropping removal/harrowing
- A recent change in pasture/paddock
- High stocking rates and intensive grazing
- Soil contamination of a pasture
- Contamination of the pasture by domestic birds
- A prolonged period of cool, dry weather
The disease has been categorised into three different types: acute, sub acute and chronic. These essentially describe the speed of onset and the severity of the disease. For acute and sub acute cases, the fatality rate approaches 100%.
In acute cases, which are characterised by sudden onset severe clinical signs, often including colic, the horses are often euthanised or die within 48 hours.
For sub acute cases, which may not initially present with colic, the course of the disease can last up to a week. The symptoms in chronic cases develop more slowly and although they can still lose condition very quickly, treatment can sometimes be successful.
The outward signs of the disease are caused by a disruption throughout the nervous system. The disease particularly affects a set of nerves called the autonomic nerves. These tell the parts of the body, that you can’t tell what to do, what to do! For example you tell your fingers what to do and they will move, but you cannot tell your guts or your heart what to do. So the effects of the damage to the autonomic nervous system include a slowing down or irregular movements in the guts, a speeding up of the heart (with some horse standing there quietly with twice or even three times the normal heart rate) and patchy sweating as the nerves controlling sweating are affected.
Some of the other signs that you may see in a horse with grass sickness include:
- Lack of appetite
- Muscle fasiculations
- Drooling of saliva
- Dry nostrils
- Tucked up appearance
- Narrow stance
- Small dry droppings
The cause of grass sickness has been a subject of much debate with many different ideas having been suggested since the first case was recorded. Current research suggests that a bacteria Clostridium botulinum is involved, and that it is the toxins produced by these bacteria that are affecting the nervous system.
Horses are particularly susceptible to clostridial diseases and the most obvious example of this is tetanus, which is caused by Clostridium tetani. High levels of the botulinum toxin have been found in the guts of affected horses and a high level of immunity to these bacteria is protective against grass sickness.
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