First Equine Herpes Myeloencephalopathy (EHM) Outbreak in Chile

Equine herpes myeloencephalopathy (EHM) is a neurological disease in horses caused by Equine herpesvirus type 1 (EHV-1). It is a serious condition, often resulting in severe neurological deficits, and can be fatal. In this report, we discuss the first outbreak of EHM in Chile (clinical signs, epidemiology, diagnosis, and control measures). A total of 567 polo horses were kept at the facility: 58.4% mares, 41.6% geldings, and stallions. Mean age was 9.7 ± 0.132 years. Thirteen horses showed clinical signs of EHV-1, eight had fevers (1.4%), and eleven developed EHM (1.9%). Only six of the eleven EHM cases had high fevers (>39.5 °C). Mortality was 0.35% (index case and one additional case) and morbidity reached 3.35% (19/567). PCR testing was performed on 214 horses (37.7%); 13 tested positive (6.1%) and 201 negative (93.9%). Of PCR-positive horses, 53.8% showed clinical signs. Notably, 2.9% of PCR-negative horses also presented signs compatible with EHV-1. CT values among positive cases ranged from 30 to 39. This first EHM outbreak in Chile showed low morbidity and mortality. Rapid implementation of strict quarantine and biosecurity effectively limited spread. Further data on viral prevalence and types are needed to improve preparedness.