First highly pathogenic avian influenza outbreak in a commercial poultry 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 intra-farm dynamics (Susceptible-Exposed-Infected-Recovered/Dead birds) and between-farm dynamics, via local transmission and farm-to-farm bird movements, at 3-days, 5-days, and 10-days 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 Official Veterinary Services. 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 4197 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- 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.