A retrospective study of 171 cases of equine meningoencephalomyelitis in the United States, 1996-2023.

Overview

• This study analyzed 171 cases of equine meningoencephalomyelitis in the US from 1996 to 2023 to identify causes and prevalence.
• The results highlight a range of infectious causes but also a significant portion of cases where the cause remains unidentified, emphasizing diagnostic challenges.

Background and Purpose

• Equine meningoencephalomyelitis is a serious neurologic disease in horses causing inflammation of the brain and spinal cord.
• It leads to significant illness and death, but due to limited large-scale studies, the true prevalence and causative factors are not well established.
• This study aimed to retrospectively analyze a large number of cases to better understand the distribution, causes, and diagnostic difficulties of these infections in horses in the United States.

Study Design and Methodology

• Retrospective review of 171 equine meningoencephalomyelitis cases submitted between 1996 and 2023 to the New York State Animal Health Diagnostic Center at Cornell University.
• Cases were examined for histopathology and identification of infectious agents such as viruses, parasites, bacteria, and fungi.
• Statistical distribution of cases was assessed based on breed, age, sex, and infectious etiology.

Key Findings on Etiology and Prevalence

• Neuroinflammatory disease accounted for 5.4% of submitted equine pathology cases, affecting a broad range of horse breeds, ages, and sexes.
• Infectious causes identified:
• Parasitic infections in 32 cases (19%):
• Protozoa identified in 18 cases (11%).
• Metazoa identified in 14 cases (8%).
• Viral infections in 31 cases (18%):
• Equid alphaherpesvirus 1 (EqAHV1) in 12 cases (39% of viral infections).
• Eastern equine encephalitis virus in 10 cases (32%).
• West Nile virus in 5 cases (16%).
• Rabies virus in 4 cases (13%).
• Bacterial infections in 14 cases (8%).
• Fungal infections in 7 cases (4%).

Cases with Unconfirmed or Unknown Etiology

• Out of 171 cases, 87 (51%) had no definitive cause identified.
• Within these, some had suggestive but inconclusive histologic signs:
• 20 cases (23%) showed some features of protozoal disease without confirmation.
• 8 cases (9%) showed indications of EqAHV1 infection without conclusive evidence.
• 59 cases (68% of unknown etiology group) had no neuropathologic changes supporting a definitive diagnosis.

Implications and Conclusions

• The study confirmed previously known infectious agents causing equine meningoencephalomyelitis but also revealed a large number of cases where diagnosis remained uncertain.
• This highlights the limitations in current diagnostic approaches both before and after death in accurately identifying the causes of neuroinflammation in horses.
• The results emphasize the critical need for improved diagnostic testing methods, including advanced ante-mortem (before death) and post-mortem (after death) evaluations.
• Ultimately, better diagnostics can improve understanding, treatment, prevention, and management of equine neurologic diseases caused by meningoencephalomyelitis.