Background: The epidemiology of equine influenza (EI) in the United Kingdom (UK) has not been systematically described since the 2019 epidemic. Objectives: 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 UK were collated. Outbreaks were ≥1 case on a premises within four-weeks. Monthly outbreak counts were analysed using negative binomial regression with year, calendar-quarter and ordered quartiles of one-month lagged Irish exports to the UK for various equid commodity codes. A subset of traceable Q4-2022 sales-related outbreaks were mapped. Results: Epidemiological data were available for 149 cases on 126 premises. Outbreaks displayed a repeatable late-year pattern: Q4 (October–December) accounted for 52% (65/126), with a 3.25-fold higher per-month rate than the rest of the year. Over 75% of premises (95/126) reported a new arrival within ≤2 weeks, mostly the arrival being the index case; 56% (28/50) of index new-arrival cases with recorded origin came from Ireland. Q4 incidence exceeded Q1 (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 of ‘equids other than pure-bred breeding animals’, improved fit and 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). ‘Total equid’ and ‘pure-bred breeding’ imports from Ireland were not consistently associated. Main limitations: Under-ascertainment; UK-wide exposure data versus GB-only outcomes; COVID-19 suppression of movements/testing in 2020. Conclusions: UK EI is characterised by a notable last-quarter 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 horses prior to arrival, post-arrival quarantine and strengthened biosecurity during transport.
