The surge of highly pathogenic avian influenza A (H5N1) infection in animals across the United States since 2024 has renewed pandemic concern, making public perception a critical determinant of effective risk communication. We analyzed 42,957 Reddit comments (January 2024–December 2025) to characterize the patterns in online discourse during the outbreak. Topic modeling identified themes of public health, pandemic comparisons, and food safety. Sentiment analysis identified fear as the dominant emotional response, while anger was the most event-reactive, correlating with reported cases in cats, poultry, and wild mammals. Autoregressive Integrated Moving Average with Exogenous Regressors (ARIMAX) models with blocked cross-validation identified reported cases in domestic cats and dairy cattle as predictors of weekly Reddit engagement, reducing forecasting error by 31% over baseline. Mammalian cases thus were associated with early-warning signals for public reaction during the 2024–2025 outbreak, offering a one-week window for proactive risk communication.