Deliberating the scientific evidence base for influenza transmission to raw milk consumers

Transmission of influenza A H5N1, commonly known as avian influenza or bird flu, from wild birds to cows on 1073 large US dairy farms in 17 states, and from cows to 41 dairy workers in five states, has raised concerns about limited evidence for transmission routes. Factors other than scientific evidence, particularly psychological, social, cultural, and political factors influencing different worldviews, support highly polarized risk perceptions about H5N1 in dairy cows, workers, and consumers. Of particular concern is the lack of scientific evidence to support federal warnings about the hypothesis that influenza transmits by the oral route to raw milk consumers. This review focuses on experimental evidence of disease transmission from 44 H5N1 inoculation studies conducted in primates, ferrets, cows, mice, cats, and dogs. Serious errors in extrapolation are apparent in the treatment of evidence for H5N1 in the media and some journal papers that unintentionally or intentionally amplify risk. Considerations of knowledge gaps and formal methods to bridge the gaps are introduced to motivate future risk analysis and facilitate building a coherent basis of knowledge to support development of rigorous evidence-based policies and risk messaging for H5N1.