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Equine veterinary journal2025; 58(2); 434-443; doi: 10.1111/evj.70117

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

Abstract: 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. Objective: To describe clinical signs, clinicopathological abnormalities, and outcomes in horses naturally infected with EE. Methods: 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 cells/L; range 0.15-9.21 × 10 cells/L), thrombocytopenia (median 67.5 × 10 cells/L; range 3-303 × 10 cells/L), and leukopenia (median 5.44 × 10 cells/L; range 2.08-18.07 × 10 cells/L). Conclusions: Retrospective study design with a small number of cases and many of these evaluated at differing times after infection. Conclusions: 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.
© 2025 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
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Publication Date: 2025-11-14 PubMed ID: 41235818PubMed Central: PMC12892390DOI: 10.1111/evj.70117Google Scholar: Lookup
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  • Journal Article

Summary

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Overview

  • This study investigated the clinical signs, blood abnormalities, and outcomes in horses naturally infected with equine encephalosis virus (EEV), a virus related to African horse sickness and Bluetongue virus and transmitted by midges.
  • It aimed to characterize how horses present when infected and highlight the importance of recognizing EE as a differential diagnosis in regions where the virus and its vectors exist or may expand to due to climate change.

Background

  • Equine encephalosis (EE) is caused by the equine encephalosis virus (EEV), an Orbivirus from the Sedoreoviridae family.
  • EEV is closely related to African horse sickness virus (AHS) and Bluetongue virus (BTV), which are also transmitted by Culicoides midges.
  • While donkeys and zebras can carry the virus without showing symptoms, only horses develop clinical disease.
  • The distribution of Culicoides midges is dependent on climate, with known circulation in Southern Africa, the Middle East, and India.
  • Climate change and global warming could facilitate the geographical expansion of Culicoides, potentially introducing the virus to new regions such as Europe.

Objective

  • To describe in detail the clinical signs, laboratory blood test abnormalities (clinicopathology), and outcomes in horses naturally infected with EEV.

Methods

  • A retrospective, descriptive, observational study design was used.
  • Data were collected from the Onderstepoort Veterinary Academic Hospital’s clinical database for the years 2013 to 2023.
  • Horses diagnosed with EE during this period were identified, and their clinical histories, examination findings, and laboratory blood test results were analyzed.
  • Both clinical signs and clinicopathological parameters were documented to characterize the disease presentation.

Results

  • A total of 25 horses with EE infection were included in the study.
  • The majority of cases occurred seasonally, primarily between February and April, aligning with peak vector activity periods.
  • Common clinical signs included:
    • Pyrexia (fever) in 100% of cases, with an average max temperature of 39.3°C (normal horse temperature is roughly 37.5-38.5°C).
    • Tachycardia (increased heart rate) in 64% of horses, with median maximum heart rate of 52 beats per minute (normal resting rate is ~28-40 bpm).
    • Tachypnoea (increased respiratory rate) in 76% of cases, median max respiratory rate 24 breaths per minute (normal ~8-15 bpm).
  • Common clinicopathological abnormalities observed within 24 hours of presentation included:
    • Lymphopenia (reduced lymphocytes) median 1.17 ×10^9 cells/L (normal range varies but typically higher in healthy horses).
    • Thrombocytopenia (reduced platelets) median 67.5 ×10^9 cells/L.
    • Leukopenia (reduced white blood cells) median 5.44 ×10^9 cells/L.

Conclusions

  • Despite the retrospective design and relatively small sample size, the study provides valuable insights into the clinical presentation of EE in horses.
  • Fever, increased heart rate, and increased respiratory rate are the most consistent clinical signs and should raise suspicion for EE in appropriate geographical and seasonal contexts.
  • Hematological evaluation is important in diagnosing and assessing EE, with a characteristic pattern of reduced white blood cells, lymphocytes, and platelets.
  • Clinicians should include equine encephalosis as a key differential diagnosis when horses present with fever and hematologic abnormalities in areas where EEV vectors exist or could spread.
  • The potential geographic expansion of the virus due to climate change underscores the importance of surveillance and awareness.

Cite This Article

APA
Piketh G, Viljoen A, Eberhardt C. (2025). Clinical signs, clinical pathology and outcomes in horses infected naturally with equine encephalosis virus. Equine Vet J, 58(2), 434-443. https://doi.org/10.1111/evj.70117

Publication

Equine veterinary journal
ISSN: 2042-3306
NlmUniqueID: 0173320
Country: United States
Language: English
Volume: 58
Issue: 2
Pages: 434-443

Researcher Affiliations

Piketh, Graeme
  • Department of Companion Animal Clinical Studies, Faculty of Veterinary Science, University of Pretoria, Onderstepoort, South Africa.
Viljoen, Adrienne
  • Department of Companion Animal Clinical Studies, Faculty of Veterinary Science, University of Pretoria, Onderstepoort, South Africa.
Eberhardt, Christina
  • Department of Companion Animal Clinical Studies, Faculty of Veterinary Science, University of Pretoria, Onderstepoort, South Africa.

MeSH Terms

  • Animals
  • Horses
  • Horse Diseases / virology
  • Horse Diseases / pathology
  • Retrospective Studies
  • Orbivirus
  • Male
  • Female

Conflict of Interest Statement

The authors declare no conflicts of interest.

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