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

Overview

• This study estimates the incidence and severity of equine herpesvirus 1 myeloencephalopathy (EHM) in horses across the United States using surveillance data from 2019 to 2023.
• The findings highlight that while EHM is relatively rare, it has a high fatality rate, particularly affecting older horses and peaking in the first half of the year.

Background

• Equine herpesvirus 1 (EHV-1) is a virus that can cause various diseases in horses, including respiratory illness, abortion, and neurological disease known as equine herpesvirus 1 myeloencephalopathy (EHM).
• EHM affects the nervous system of horses, leading to severe clinical signs and significant welfare and economic impacts.
• Despite knowing the virus and its effects, broader population-level data on how often EHM occurs (incidence) and how often it results in death (case fatality rate) were not well defined prior to this study.

Objectives

• To describe the burden of EHM by estimating how frequently it occurs among U.S. horses.
• To determine the attack rate, which is the proportion of exposed horses that develop EHM.
• To calculate the case fatality rate, meaning the proportion of horses with EHM that die from the disease.
• To assess whether incidence varied by horse age and time of year, identifying any potential seasonal trends.

Methods

• The study was retrospective and observational, evaluating already collected data rather than conducting an experimental approach.
• Data came from the Equine Disease Communication Center (EDCC), a surveillance system in the U.S. that tracks equine diseases.
• Data covered the years 2019 to 2023.
• Statistical regression models were used for estimating the mean annual incidence rate, attack rates, and case fatality rates.
• Analyses also considered age categories (with particular attention to horses over 19 years old) and seasonal occurrence patterns.

Results

• Isolated cases of EHM were more common than outbreaks affecting multiple horses.
• The estimated average annual incidence rate of EHM was 6.7 cases per million horses, indicating that it is a relatively uncommon condition.
• The attack rate was 4.3%, showing that about 4 in 100 horses exposed to EHV-1 developed EHM.
• The case fatality rate was very high at 49.5%, meaning that nearly half of horses diagnosed with EHM died as a result.
• Older horses (>19 years) had a higher incidence of EHM compared to younger horses, suggesting increased susceptibility with age.
• Cases were more frequent between January and June, with a peak in March, indicating possible seasonal influence on disease occurrence.

Conclusions

• EHM is a rare but severe neurological consequence of EHV-1 infection among horses in the U.S.
• The high mortality associated with EHM highlights the need for vigilance and improved prevention and management strategies in affected horses, especially older animals.
• The seasonal peak may guide timing for monitoring and implementing biosecurity measures to reduce EHM spread during higher-risk months.
• The study provides important epidemiological benchmarks for veterinarians, researchers, and equine industry stakeholders to better understand and combat EHM.