Outbreak of equine herpesvirus myeloencephalopathy in France: a clinical and molecular investigation.

This article presents a detailed investigation of a severe outbreak of Equid herpesvirus 1 (EHV-1)-associated myeloencephalopathy (EHM), a neurological disease affecting horses, in a riding school in France in 2009. By sharing clinical, virological, and molecular data, the researchers underscore the importance of taking into account even low viral loads as potential risk factors for neurological manifestations of this disease.

Background to the Study

• The disease being studied here is Equid herpesvirus 1 (EHV-1)-associated myeloencephalopathy (EHM), a condition that affects the central nervous system of horses. The interest in this disease has been growing, but there’s a lack of detailed epidemiological data, especially related to large outbreaks.
• The study delves into a severe outbreak that happened in a riding school in France in July 2009, affecting 7 of the 66 horses aged between 12 and 22 years.

Epidemiology and Clinical Presentation

• The researchers noted that EHM morbidity was 10.6% (7 out of 66 horses), mortality was 7.5% (5 out of 66 horses), and the case fatality rate was quite high at 71.4% (5 out of 7 cases).
• The telltale sign of the disease was that the horses systematically developed a fever within two days before the severe neurological symptoms set in.

Diagnostic Methods and Findings

• To support the diagnosis of EHM, the researchers employed various techniques, including real-time PCR and virus culture for EHV-1 detection, and SNP-PCR for viral strain characterization.
• In all available samples of blood and nasal swabs collected from the horses, EHV-1 was detected by PCR. Only the neuropathogenic strain of the virus (G(2254)) was isolated during the outbreak.
• The researchers used Cycle threshold (Ct) values as indicators of the viral load. The Ct values among the six sampled horses ranged between 26.0 and 37.0. Interestingly, they also noted that the amount of virus in the samples wasn’t always directly proportional to the severity of the observed clinical signs.

Conclusions of the Study

• With this study, the researchers have provided a detailed description of a severe, though temporally limited and geographically restricted, outbreak of EHM.
• The most important takeaway is the strong suggestion that even low viral loads of EHV-1 should be considered as potential risk factors for the neurological manifestations of this disease. This may have significant implications for future surveillance and management strategies for the control of EHM.