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Equine veterinary journal2026; doi: 10.1002/evj.70156

What happened after the epidemic? Equine influenza surveillance sheds light on sources and seasonal risk in the United Kingdom.

Abstract: The epidemiology of equine influenza (EI) in the United Kingdom has not been systematically described since the 2019 epidemic. Objective: To summarise UK EI surveillance (2020-2024), quantify outbreak seasonality and assess movement-related sources. Methods: Retrospective observational analysis of national surveillance and horse importation data. Methods: Epidemiological data for laboratory-confirmed EI cases in the United Kingdom were collated. Outbreaks (EI-infected premises) were defined as one or more laboratory-confirmed cases on the same premises within a 4-week period. Monthly outbreak counts were analysed using negative binomial regression with year, calendar-quarter and ordered quartiles of 1-month lagged Irish exports to the United Kingdom by equid commodity code. A subset of Q4-2022 sales-related EI outbreaks were mapped. Results: Epidemiological data were available for 149 cases on 126 premises. Outbreaks displayed a repeatable late-year pattern: Q4 (October to December) accounted for 52% (65/126), with a 3.25-fold higher per-month rate than the rest of the year. Over 75% (95/126) of premises reported a new arrival within ≤2 weeks; 56% (28/50) of index new-arrival cases with recorded origin came from Ireland. Q4 incidence exceeded Q1 (incidence rate ratio [IRR] 6.9, p < 0.001) and years 2021-2024 incidence exceeded 2020 (IRRs 4.5-5.6, p < 0.001). Adding lagged Irish imports other than pure-bred breeding animals, improved fit, attenuated the Q4 effect (IRR = 3.9, p < 0.001) and identified higher import quartiles as predictors (quartile-3: IRR 4.5, p < 0.001; quartile-4: IRR 3.7, p < 0.001). Conclusions: Under-ascertainment, UK-wide exposure data versus Great Britain-only outcomes, COVID-19 suppression of movements/testing in 2020. Conclusions: EI in the United Kingdom in 2020-2024 was characterised by a notable October to December risk window and strong links to horse movements. Trade in non-pure-bred horses aligns with outbreak timing and partly explains the seasonal excess. Control measures should prioritise vaccination of new arrivals, post-arrival quarantine and strengthened biosecurity during transport. Unassigned: Die Epidemiologie der Pferdegrippe, Equinen Infuluenza (EI), im Vereinigten Königreich (UK) wurde seit der Epidemie von 2019 nicht systematisch beschrieben. Unassigned: Zusammenfassung der EI‐Surveillance im Vereinigten Königreich (2020–2024), Quantifizierung der saisonalen Ausbrüche und Bewertung der mit Transporten verbundenen Ursachen. Methods: Retrospektive Beobachtungsanalyse nationaler Überwachungs‐ und Pferdeimportdaten. Methods: Es wurden epidemiologische Daten zu laborbestätigten EI‐Fällen im Vereinigten Königreich zusammengestellt. Ausbrüche (mit EI infizierte Betriebe) wurden als ein oder mehrere laborbestätigte Fälle in demselben Betrieb innerhalb eines Zeitraums von vier Wochen definiert. Die monatlichen Ausbruchszahlen wurden unter Verwendung einer negativen binomialen Regression mit Jahr, Kalenderquartal und geordneten Quartilen der um einen Monat verzögerten irischen Exporte in das Vereinigte Königreich nach Equiden‐Zolltarifnummern analysiert. Eine Untergruppe der verkaufsbezogenen EI‐Ausbrüche im vierten Quartal 2022 wurde kartografisch dargestellt. Results: Epidemiologische Daten lagen für 149 Fälle in 126 Einrichtungen vor. Die Ausbrüche zeigten ein wiederkehrendes Muster zum Jahresende: 52% (65/126) entfielen auf das 4. Quartal (Oktober bis Dezember), wobei die monatliche Rate 3,25‐mal höher war als im Rest des Jahres. Über 75% (95/126) der Einrichtungen meldeten innerhalb von ≤2 Wochen einen Neuzugang; 56% (28/50) der Indexfälle von Neuzugängen mit dokumentierter Herkunft stammten aus Irland. Die Inzidenz im vierten Quartal übertraf die des ersten Quartals (Inzidenzratenverhältnis [IRR] 6,9, p < 0,001) und die Inzidenz in den Jahren 2021–2024 übertraf die von 2020 (IRRs 4,5–5,6, p < 0,001). Durch Hinzufügen von verzögerten Importen aus Irland, die keine reinrassigen Zuchttiere waren, verbesserte sich die Anpassung, der Q4‐Effekt wurde abgeschwächt (IRR = 3,9, p < 0,001) und höhere Importquartile wurden als Prädiktoren identifiziert (Quartil 3: IRR 4,5, p < 0,001; Quartil 4: IRR 3,7, p < 0,001). HAUPTEINSCHRÄNKUNGEN: Untererfassung; britische Expositionsdaten im Vergleich zu Ergebnissen nur für Großbritannien; COVID‐19‐bedingte Einschränkung von Bewegungen/Tests im Jahr 2020. Unassigned: Die EI im Vereinigten Königreich in den Jahren 2020–2024 war durch einen auffälligen Risiko‐Zeitraum von Oktober bis Dezember und einen starken Zusammenhang mit Pferdetransporten gekennzeichnet. Der Handel mit nicht reinrassigen Pferden deckt sich mit dem Zeitpunkt des Ausbruchs und erklärt teilweise den saisonalen Anstieg. Kontrollmaßnahmen sollten die Impfung von Neuankömmlingen, die Quarantäne nach der Ankunft und eine verstärkte Biosicherheit während des Transports priorisieren.
© 2026 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
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Publication Date: 2026-03-05 PubMed ID: 41787702DOI: 10.1002/evj.70156Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This study analyzed equine influenza (EI) surveillance data in the United Kingdom from 2020 to 2024 to identify patterns in outbreak timing and the role of horse movements, especially imports from Ireland, in disease spread.
  • The research found a strong seasonal peak in outbreaks during October to December, significantly linked to recent arrivals of horses, particularly non-pure-bred imports from Ireland, informing recommendations for targeted control measures.

Background and Objectives

  • Equine influenza is a contagious respiratory disease affecting horses, with notable outbreaks causing economic and animal health impacts.
  • After a major EI epidemic in 2019, systematic data on the disease’s epidemiology in the UK had not been reported.
  • The study aimed to summarize national EI surveillance between 2020 and 2024:
    • Quantify the seasonality of EI outbreaks across the UK.
    • Assess the influence of horse movements and imports, especially from Ireland, as sources of outbreaks.

Methods

  • The research was a retrospective observational analysis using two main datasets:
    • National surveillance data of laboratory-confirmed EI cases in the UK.
    • Records of horse importation, with a focus on equid commodity codes to categorize horses.
  • Outbreaks were defined as one or more confirmed EI cases on the same premises within a 4-week period.
  • Statistical analysis involved:
    • Negative binomial regression to analyze monthly counts of outbreaks.
    • Covariates included calendar year, calendar quarter, and quartiles of 1-month lagged Irish exports by horse type.
  • An additional mapping analysis was done for outbreaks linked to sales in the last quarter of 2022.

Key Results

  • A total of 149 EI cases were recorded on 126 premises across the UK during 2020-2024.
  • Seasonality:
    • 52% of outbreaks occurred in the fourth quarter (October-December).
    • The rate of outbreaks per month during Q4 was 3.25 times higher than other quarters.
  • Horse movements as outbreak sources:
    • Over 75% of affected premises reported receiving a new horse arrival within two weeks prior to outbreak detection.
    • Of index (first) new arrival cases with known origin, 56% came from Ireland.
  • Temporal trends:
    • Incidence in Q4 was significantly higher compared to Q1 (incidence rate ratio [IRR] 6.9, highly significant).
    • Years 2021 to 2024 showed higher outbreak incidences than 2020 (IRRs 4.5 to 5.6).
  • Role of Irish imports:
    • Including lagged import data for non-pure-bred horses from Ireland improved model fit and lessened the strength of the Q4 seasonal effect (IRR reduced to 3.9).
    • Higher import quartiles were strong predictors of outbreak risk (Quartile 3 IRR 4.5; Quartile 4 IRR 3.7, p < 0.001).

Limitations

  • Potential under-reporting or under-ascertainment of EI cases and outbreaks.
  • Difference in exposure data at UK-wide level versus case outcome data limited to Great Britain only (England, Scotland, Wales), possibly affecting interpretation.
  • Restrictions and disruptions caused by the COVID-19 pandemic in 2020 may have limited horse movements and testing, influencing data quality and outbreak frequency that year.

Conclusions and Implications

  • EI outbreaks in the UK from 2020-2024 exhibit a distinct seasonal risk period in the last quarter of the year, roughly October to December.
  • This increased late-year risk is strongly associated with horse movements, particularly imports of non-pure-bred horses from Ireland.
  • Trade and movements of these horses align well with the timing of outbreaks, explaining much of the seasonal excess in cases.
  • Recommended control measures based on these insights include:
    • Prioritizing vaccination for newly arriving horses to reduce susceptibility.
    • Implementing post-arrival quarantine protocols to limit disease spread from new arrivals.
    • Enhancing biosecurity standards and practices, especially during transport.
  • These targeted interventions could effectively reduce the occurrence and impact of future EI outbreaks in the UK.

Cite This Article

APA
Whitlock F, Grewar J, Newton R. (2026). What happened after the epidemic? Equine influenza surveillance sheds light on sources and seasonal risk in the United Kingdom. Equine Vet J. https://doi.org/10.1002/evj.70156

Publication

Equine veterinary journal
ISSN: 2042-3306
NlmUniqueID: 0173320
Country: United States
Language: English

Researcher Affiliations

Whitlock, Fleur
  • Equine Infectious Disease Surveillance (EIDS), Department of Veterinary Medicine, University of Cambridge, Cambridge, UK.
Grewar, John
  • jDATA (Pty) Ltd, Sandbaai, South Africa.
Newton, Richard
  • Equine Infectious Disease Surveillance (EIDS), Department of Veterinary Medicine, University of Cambridge, Cambridge, UK.

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