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

Equine meningoencephalomyelitis is an important cause of morbidity and mortality and is associated with a wide variety of infectious etiologies. Because of the lack of large retrospective studies, the prevalence and incidence of these diseases are unknown. Here we describe 171 cases of meningoencephalomyelitis in horses submitted to the Section of Anatomic Pathology at the New York State Animal Health Diagnostic Center (Cornell University, Ithaca, NY, USA) from 1996-2023. Neuroinflammatory disease was identified in 5.4% of submitted horses with a wide breed, age, and sex distribution. A parasitic cause was identified in 32 (19%) cases, with protozoa in 18 (11%) cases and metazoa in 14 (8%) cases. A viral cause was identified in 31 (18%) cases, corresponding to infection by equid alphaherpesvirus 1 (EqAHV1; 12 of 31, 39%), eastern equine encephalitis virus (10 of 31; 32%), West Nile virus (5 of 31; 16%), and rabies virus (4 of 31; 13%), followed by 14 bacterial (8%) cases and 7 fungal (4%) cases. Of the remaining 87 of 171 (51%) cases, 20 (23%) had some histologic features, although not conclusive, of protozoal disease, and 8 (9%) of EqAHV1 infection. However, 59 (68%) cases did not have any neuropathologic changes that would support a definitive diagnosis. Although we found the expected causes of equine meningoencephalomyelitis in our study, the large number of cases with unknown etiologic diagnoses highlights the challenges of definitively proving causes of neuroinflammation in the horse and supports the need for improved ante- and postmortem testing.