Results of an epidemiological study of recurrence of equine grass sickness on affected premises

Emma Hedderson*, Richard Newton & James Wood Animal Health Trust, Newmarket, *Imperial College, Wye Campus *** Footnote added April 2006 ***

Although it is recognised that equine grass sickness (EGS) recurs on premises, the factors that relate to this have never been previously investigated. Therefore, a questionnaire-based epidemiological study was conducted by the Animal Health Trust to identify factors associated with recurrence of EGS on affected premises. It is hoped that the results of this study might be useful in modifying management practices in order to delay the recurrence of EGS on previously affected premises and may provide better insights into causal aspects of the disease.

Information on premises, horses, previous EGS cases and pasture and horse management practices were collected by a postal questionnaire sent to 509 premises between October1999 and December 2001. Questionnaires were returned from 305 premises (60% return rate) and data on the history of disease were used to determine the number of cases occurring during a given period for each premises. Appropriate statistical methods (Poisson regression) were used to examine which factors were significantly associated with the recurrence of EGS.

Results are presented as ‘relative risks’ (RR), which are the ratios of the ‘risk’ of recurrence during the given period among categories of a factor (e.g. livery yard, stud farm, rented field) compared to a baseline comparison category (e.g. farm). As the RR of a comparison group is always 1.0 (i.e. ratio of risk to itself), the RRs for the other categories indicate how much more or less likely they are to suffer recurrence of disease. Using this method, RRs >1.0 indicate an increased risk of recurrence and RRs <1.0 a decreased risk (i.e. a protective effect).

Several significant risk factors were identified for the recurrence of EGS on premises and these included i) number of horses on the premises, ii) type of premises, iii) soil type, iv) method of droppings removal, v) other domestic species, vi) pasture cutting and vii) younger horses present on premises.

Figure 1 shows an increasing risk of recurrence with increasing numbers of horses on premises. However, this is not unexpected as horses and not premises suffer EGS and, therefore, as the numbers of horses increases, so the risk of recurrence of the disease would be expected to increase.

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Figure 1: Relative risks of recurrence according to increasing numbers of horses on premises

There was also an increased risk of recurrence on studs and livery/riding establishments compared to farms, but as both these types of premises had significantly higher numbers of horses this did explain some of this difference. In addition, studs and livery/riding establishments tend to have a higher turnover of new animals and studs in particular have younger horses, both recognised risk factors for EGS.

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Figure 2: Relative risks of recurrence according to different soil types

There was significantly increased risk of recurrence on premises with sand or loam soils and decreased risk on chalk compared with clay (Figure 2). These findings are important in the context of the theory that EGS is caused a by toxico-infection with Clostridium botulinum, a soil-borne bacteria (as reported in the Winter 2001 edition of Equine News) and that the risk of disease increases when there is increased exposure of horses to soil during grazing. Any form of soil disturbance may increase the risk of grass contamination thereby bringing the bacterium into contact with grazing horses more frequently. Soil disturbance and turnover may be related to soil exposure by overgrazing or the action of soil inhabitants, such as earthworms or moles, which are able to burrow more freely through sand and loam soils compared to clay and chalk.

Premises that removed droppings mechanically were at a significantly increased risk of recurrence than those premises that did not remove droppings and those that removed droppings manually were actually less likely to suffer a recurrence of EGS (Figure 3). It is proposed that removing droppings mechanically, in particular by paddock sweepers, causes soil disturbance and contamination of grass with soil.

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Figure 3: Relative risks of recurrence according to methods of droppings removal

The presence of domestic birds or fowl on premises increased the risk of recurrence of EGS compared with those establishments that did not have domestic birds (Figure 4). This finding corresponds with previous observations that EGS recurs in areas where there is intensive rearing of game birds. In addition, birds have long been recognised as a source of botulism and the importation of ‘guano’ (a fertiliser of avian origin) from south America in the mid to late 19th Century to improve Scottish agricultural land was soon followed by the first reports of EGS in this part of Britain.

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Figure 4: Relative risks of recurrence according to other domestic species on premises

The presence of ruminants on premises reduced the risk of recurrence by half compared with premises that did not have these domestic animals. In addition, cutting of grass used for grazing was also associated with a reduced risk of recurrence. The reduced risk of recurrence associated with both grazing of ruminants and grass cutting may simply relate to reduced grazing by horses but may also be associated with removal of toxic material by co-grazers and encouragement of more even grazing by grass cutting, thereby reducing overgrazing and exposure to soil.

Consistent with several previous epidemiological studies of EGS, this study also found an increased risk of disease with presence of younger horses on premises.

In conclusion, this study has identified important factors related to the recurrence of EGS on premises and future studies are warranted to assess the effect of specific interventions on reducing the frequency of disease on premises.

Acknowledgements: We are extremely grateful for the time and effort taken by all horse owners in completing and returning questionnaires. The support of the Home of Rest for Horses and The Equine Grass Sickness Fund is gratefully acknowledged.

Footnote added April 2006

Whilst we didn’t differentiate types of mechanical faeces removal in our study, our working hypothesis is that paddock sweepers specifically increase the risk of recurrence of EGS on previously affected paddocks. We believe that this is due to soil disturbance and contamination of grass as well as possibly dissemination over the pasture of aetiological factors/agent present in faeces and/or soil. In contrast, we believe that if the soil remains undisturbed, the vacuum faeces removal may actually be protective as it is much more akin to the manual faeces removal, which our study did find to be protective against the recurrence of EGS.