Equine herpesvirus 1 myeloencephalopathy incidence and case fatality rates in the USA

Equine herpesvirus 1 (EHV-1) myeloencephalopathy (EHM) has substantial animal welfare and economic consequences. Despite extensive research, population-level estimates of EHM incidence and case fatality rates remain poorly characterized.

Describe the burden of EHM and estimate EHM attack and case fatality rates in the United States.

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A retrospective observational study was conducted using surveillance data collected by the Equine Disease Communication Center (EDCC) in the United States during 2019-2023. Regression models were used to estimate the mean annual EHM incidence and to quantify attack and fatality rates. The incidence and potential seasonality of EHM by age category were also assessed.

Surveillance data showed that isolated EHM cases predominated, and the estimated mean annual incidence in the United States was 6.7 per million horses (95% CI, 4.9-8.9). The attack rate was 4.3% (95% CI, 3.5-5.4) and the case fatality rate was 49.5% (95% CI, 40.5-60.5). In the United States, older horses (>19 years) had higher EHM incidence than younger horses. Cases were more frequent from January to June, peaking in March.

Equine herpesvirus 1 myeloencephalopathy is an uncommon clinical manifestation of EHV-1 infection, but the high case fatality rate emphasizes the clinical relevance of this condition.