Grass sickness, as its name suggests, is strongly associated with grazing but there have been a few cases in animals with no access to pasture. In these rare cases, hay has been implicated as the source of the causal agent. Although most cases have been at grass full-time or during the day, the disease can affect horses which have only a few minutes’ access to grass daily. Giving supplementary feeding in the form of concentrates does not have a protective effect, but hay feeding reduced the risk factor in one study.
It is well recognised that certain premises, or even fields within single premises are associated with the occurrence of grass sickness cases. Animals which have been on affected premises for less than 2 months are more likely to develop the disease. Commonly, only one animal is affected at a time but ‘outbreaks’ of the disease with several cases in a period of a few weeks are not infrequent.
There is no clear association with type of pasture (new ley, permanent pasture, hill grazing, clean or ‘horse-sick’ pasture) but recent evidence suggests that high nitrogen content of soil and soil disturbance may be risk factors. While it was previously thought that grass sickness was more common in pastures with a high clover content, recent studies indicate that it can also occur on pastures with no clover. Thus clover is not the sole cause of the disease, and at worst may be a trigger for a bacterium such as Clostridium botulinum.
Other suggested risk factors include increased numbers of horses on the pasture, mechanical droppings removal and presence of domesticated birds on fields. Stress appears to be a factor in predisposing to the disease and a significant number of animals have a history of recent stress including recent purchase, mixing with strange horses, travelling a long distance, breaking and castration. Animals in good to fat condition also appear to be predisposed.
Many horse owners have firm opinions about the type of weather prevailing when grass sickness cases occur. In a survey of weather conditions in the two weeks preceding multiple-case outbreaks, it was found that cool, dry weather with a temperature between 7 and 11°C was recorded in a statistically significant number of instances. This may partly explain the higher incidence of the disease in the eastern side of Britain where such conditions are more prevalent.
Results of two surveys suggest that the risk of developing grass sickness is slightly higher in horses which are wormed more frequently with certain types of wormers. However, it should be emphasised that the consequences of not worming can be very serious or even fatal and it is not suggested that owners should decrease their use of wormers. There is also no indication that wormers themselves contain the toxin that causes grass sickness.
CAUSAL AGENT
The cause of grass sickness is unknown despite almost 100 years of investigation. Many potential causes have been examined over the years including poisonous plants, chemicals, bacteria, viruses, insects and metabolic upsets. A common suggestion by horse owners is that mineral or vitamin deficiencies may be the cause. None have any proven link with the disease, although selenium deficiency, which results in reduced levels of protective antioxidants in the body, may have some role to play. Grass sickness does not appear to be contagious and the type of damage to the nervous system suggests that a toxic substance is likely to be involved. The currently favoured theory under investigation is the possible involvement of Clostridium botulinum, a soil-associated bacterium.
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