First highly pathogenic avian influenza outbreak in a commercial farm in Brazil: outbreak timeline, control actions, risk analysis, and transmission modeling

Abstract: On May 15, 2025, Brazil reported its first highly pathogenic avian influenza (HPAI) outbreak in a commercial poultry breeder farm in Montenegro, Rio Grande do Sul. This study presents the outbreak timeline, control measures, along with spatial risk assessment and epidemiological model used to simulate detection delays. The transmission model considered Susceptible Exposed Infected Recovered Dead farm statuses to simulate within farm and between farm dynamics under 3 day, 5 day, and 10 day detection delays. The single infected commercial farm lost 15,650 birds, with 92% mortality due to HPAI, and additional culling of the remaining birds on Day 5 post-notification to the state animal health officials. Based on the mortality and outbreak response data, the introduction likely occurred 3 10 days before its official detection. Our field investigations suggested that wild birds were the most likely source of introduction, although biosecurity breaches could not be ruled out. Control measures implemented included movement restrictions and a control zone, from which 4,197 vehicles were inspected upon entry. Risk analysis classified 64.4% of municipalities as low risk, 35.0% as medium risk, and 0.6% as high risk. Our HPAI disease simulation results showed that the number of secondary infections would increase from a median of 4 farms (IQR 2 5) with a 3 day delay to 6 (IQR 3 22) and 34 (IQR 12 47) farms with 5 day and 10 day delays, respectively. The rapid veterinary response eliminated the outbreak within 32 days of detection, highlighting the critical role of early detection and prompt response.