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.

Background : This is the first EHV-1 outbreak with EHM in Chile. Objectives : Describe clinical, epidemiological and outcome data from an EHM outbreak in South America. Study design : Case Report, retrospective outbreak data analysis. Methods : Clinical and diagnostic data were analyzed. EHV-1 was detected by qPCR (EHV-1-Glycoprotein-B). Frequency tables were used for descriptive analysis of categorical variables and the chi-square test and Cramer’s V to test relationships between them. Results : A total of 567 Polo Horses were kept in the Polo operation, 58.4% were mares and 41.6% geldings and stallions. Mean age was 9.7±0.132 years. 13 horses showed clinical signs compatible with EHV-1 infection, 8 had fever (1.4%) and 11 developed EHM (1.9%). Only 6/11 horses developed EHM and high fevers (>39.5C). The mortality rate was 0.35% (index case and case 1) while morbidity 3.35% (19/567 horses). PCR analysis was performed in 214 horses (37.7%), 201 were negative for EHV-1 (93.9%) and 13 positive (6.1%). 7 horses with a positive PCR presented clinical signs (53.8%), while 6 with a negative PCR result did show clinical signs compatible with EHV-1 infection (2.9%). CT values of positive PCR results ranged from 30 to 39. Main limitations : Limited number of horses were diagnostically analyzed (PCR and/or serology analysis). Viral isolation and sequencing have not yet been achieved. Conclusions : This is the first EHM outbreak reported in Chile showing low morbidity and mortality rates. Viral spread was successfully controlled with prompt and strict quarantine and biosecurity measures. More information on viral prevalence and type during the outbreak and in the Chilean horse population is required to further understand the outbreak and prepare for the impact of future ones.