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

BACKGROUND

The epidemiology of equine influenza (EI) in the United Kingdom has not been systematically described since the 2019 epidemic.

OBJECTIVES

To summarise UK EI surveillance (2020-2024), quantify outbreak seasonality and assess movement-related sources.

STUDY

DESIGN

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).

MAIN LIMITATIONS: 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.