An outbreak of the equine rhabdomyolysis syndrome in a racing yard.

The research seeks to shed light on an outbreak of muscle stiffness and poor performance among thoroughbreds in a Newmarket racing yard in 1986, presenting a possibility of an EHV-1 infection as the potential cause.

Overview of the Study

• The research centers around a case study from May to June 1986 at a flat racing yard in Newmarket. The researchers studied an outbreak among 59 thoroughbreds who displayed reduced performance and muscle stiffness.
• Over a third of the horses showed signs of muscular stiffness. 38 horses had creatine kinase (CK) activities exceeding 200 iu/litre and/or aspartate aminotransferase (AST) activities above 300 iu/litre when sampled six to eight hours post-exercise.
• The studies conducted the following season revealed significantly fewer horses with similar CK and AST activities, indicating that the outbreak was localized to the specific period in 1986.

Testing for Equine Herpesvirus

• The researchers tested plasma samples from 18 horses at the onset of the investigation and again 13 days later for antibodies to the equine herpesvirus (EHV). Ten of these horses showed a response strongly suggestive of an EHV-1 infection.
• However, further testing found no significant abnormalities in the fractional electrolyte values from seven randomly selected horses. This suggests that electrolyte imbalances were not a significant factor in the outbreak.

Muscle Biopsy Findings

• The investigation also included muscle biopsies from three horses that continued to display high enzyme activities and repeat signs of muscle stiffness.
• The biopsies revealed unusually large numbers of normally stained fibres with centrally located nuclei as well as clusters of small muscle fibres. These findings may provide further insights into the underlying muscular issues leading to the observed symptoms in the affected horses.

In conclusion, the study brings attention to the potential impact of EHV-1 infections on the health and performance of thoroughbred horses. Muscular stiffness and poor performance may be indicative of such an infection. However, more research is needed to confirm these findings and determine the full extent of the virus’s impact.