Clustering of equine grass sickness cases in the United Kingdom: a study considering the effect of position-dependent reporting on the space-time K-function.

Equine grass sickness (EGS) is a largely fatal, pasture-associated dysautonomia. Although the aetiology of this disease is unknown, there is increasing evidence that Clostridium botulinum type C plays an important role in this condition. The disease is widespread in the United Kingdom, with the highest incidence believed to occur in Scotland. EGS also shows strong seasonal variation (most cases are reported between April and July). Data from histologically confirmed cases of EGS from England and Wales in 1999 and 2000 were collected from UK veterinary diagnostic centres. The data did not represent a complete census of cases, and the proportion of all cases reported to the centres would have varied in space and, independently, in time. We consider the variable reporting of this condition and the appropriateness of the space-time K-function when exploring the spatial-temporal properties of a 'thinned' point process. We conclude that such position-dependent under-reporting of EGS does not invalidate the Monte Carlo test for space-time interaction, and find strong evidence for space time clustering of EGS cases (P < 0.001). This may be attributed to contagious or other spatially and temporally localized processes such as local climate and/or pasture management practices.

Equine grass sickness is associated with low antibody levels to Clostridium botulinum: a matched case-control study.

REASONS FOR PERFORMING STUDY: Equine grass sickness is a high mortality disease which, despite many years of investigation, is of unknown aetiology. Recent findings indicating that the disease is associated with Clostridium botulinum require support from an epidemiological study that recognises and controls for potential confounders, e.g. age, time of year and premises. HYPOTHESIS: EGS is associated with low antibody levels to C. botulinum antigens. METHODS: A matched case-control study was conducted. Data were collected from 66 histologically confirmed cases of EGS and 132 premises-matched control horses. The probability of EGS in horses was modelled using conditional logistic regression. RESULTS: EGS was significantly associated (age-adjusted P < 0.005) with low antibody levels to each of 3 clostridial antigens; C. botulinum type C and C. novyi type A surface antigens and a C. botulinum type C toxin complex toxoid. These serological risk factors for EGS remained highly significant when entered into multivariable models. This study also identified new horse-level risk factors for EGS; feeding hay or haylage was associated with a decreased risk of disease, change of feed type or quantity during the 14 days prior to disease was associated with increased risk, and the use of an ivermectin anthelmintic at both the ultimate and penultimate treatments was also associated with a significantly increased risk of EGS. CONCLUSIONS: This study provides strong support for the role of C. botulinum in the aetiology of EGS and identifies managemental risk factors for the disease. POTENTIAL RELEVANCE: Increasing anticlostridial antibody levels by vaccination and appropriate managemental interventions may decrease the risk of EGS occurring.

Why are certain premises at increased risk of equine grass sickness? A matched case-control study.

REASONS FOR PERFORMING STUDY: Equine grass sickness (EGS) occurs repeatedly on certain premises over time. Few studies have sought, or identified, the determinants of this phenomenon in order to inform advice on disease prevention strategies. HYPOTHESIS: Premises-level risk factors are important determinants of whether EGS occurs. METHODS: A matched case-control study was undertaken. Sixty premises giving rise to one or more histologically confirmed case of EGS and 120 time-matched control premises were sampled. Data were collected on pasture management, soil nutrient content, pasture nutrient content and local weather conditions for 2 weeks prior to the onset of disease. Data were analysed by conditional logistic regression. RESULTS: Multivariable modelling identified an association between EGS and increased soil nitrogen content, pasture disturbance and previous occurrence of EGS on the premises. None of the meteorological variables recorded in this study were significantly associated with EGS occurrence. No relationship between certain management practices (e.g. harrowing, fertilisation, reseeding) and the risk of EGS was detected. CONCLUSIONS AND POTENTIAL RELEVANCE: This information is useful in understanding the causal pathway of EGS and may be used in the formulation of evidence-based disease avoidance strategies.

Epidemiology of equine grass sickness: a literature review (1909-1999).

The geographical spread of grass sickness between 1909 and 1999, particularly in England and Wales, is described, and the experimental investigations to identify a causal agent are summarised. The epidemiological techniques used to investigate grass sickness vary from clinical observations, to more advanced methods such as case-control studies using logistic regression analyses. Several risk factors for grass sickness have been reported consistently (age, time of year and recent movement to new pasture or premises) and several others have been reported for which the findings remain inconsistent (weather, pasture type, breed, supplementary feeding and use of anthelmintics).

The effect of sodium heparin on equine articular cartilage.

This study compared the effect of sodium heparin and gentamicin sulphate on equine articular cartilage (AC) explants in order to investigate the possible use of sodium heparin in the treatment of infectious arthritis. Six concentrations of sodium heparin and gentamicin sulphate were tested. The supernatant and explant digest were assayed for glycosaminoglycan (GAG) content with the dimethyl-methylene blue assay and the per cent loss of GAG was calculated. A significant (P< 0.001) increase in percentage GAG loss was noted for the sodium heparin groups when compared to the control, whilst no significant increase was found among the treatment groups (P =0.782). For gentamicin, no significant difference in percentage GAG loss was found between the control and three of the five treatment groups (P =0.667). The percentage GAG loss in the sodium heparin treated AC explants was greater than for any of the gentamicin-treated AC explants. It can be concluded that sodium heparin sulphate stimulates an increase in GAG release from equine articular cartilage explants, though no firm conclusions can be drawn on its use in treating equine infectious arthritis.