Clinical signs, clinical pathology and outcomes in horses infected naturally with equine encephalosis virus

Background Equine encephalosis (EE) is caused by an Orbivirus from the family Sedoreoviridae and is thus similar to African horse sickness (AHS) and Bluetongue viruses (BTV). These viruses are transmitted by Culicoides midges. Equine encephalosis can infect horses, donkeys and zebras sub‐clinically while only horses develop clinical disease. The vector's distribution is climate‐dependent with evidence for circulation in Southern Africa, the Middle East and India. Global warming could facilitate the expansion of this distribution and consequently the potential spread into Europe should not be overlooked. Objectives To describe clinical signs, clinicopathological abnormalities, and outcomes in horses naturally infected with EE. Study Design A retrospective, descriptive, observational study. Methods Data were obtained from the Onderstepoort Veterinary Academic Hospital's clinical database to identify cases with EE from 2013 to 2023. Data including the history, clinical signs and clinicopathology were analysed. Results Equine encephalosis cases predominantly occurred from February to April. Twenty‐five horses were included. Throughout the disease, 25 (100.0%) horses had pyrexia (mean maximum temperature 39.3°C; SD 0.86°C), 16 (64.0%) horses had tachycardia (median maximum heart rate 52/min; range 36–100/min), 19 (76.0%) horses had tachypnoea (median maximum respiratory rate 24/min; range 12–60/min). Within 24 h of presentation, horses predominantly displayed lymphopenia (median 1.17 × 10 9 cells/L; range 0.15–9.21 × 10 9 cells/L), thrombocytopenia (median 67.5 × 10 9 cells/L; range 3–303 × 10 9 cells/L), and leukopenia (median 5.44 × 10 9 cells/L; range 2.08–18.07 × 10 9 cells/L). Main Limitations Retrospective study design with a small number of cases and many of these evaluated at differing times after infection. Conclusion Pyrexia, tachycardia and tachypnoea are the most common clinical signs associated with EE. Haematological evaluation appears valuable in EE cases, with leukopenia, lymphopenia, and thrombocytopenia commonly observed. Equine encephalosis is a relevant differential diagnosis for other infectious diseases in horses in geographical regions where EEV and Culicoides vectors are potentially present.