Respiratory and Gastrointestinal Microbiome Shifts in Livestock Exposed to High-Ammonia Environments: A Systematic Review.

Environmental NH3 is a major pollutant in intensive livestock production, but its effects on host-associated microbiota remain insufficiently characterized. This systematic review evaluated the impact of environmental NH3 exposure on the respiratory and gastrointestinal microbiome of poultry, swine, and cattle. Following PRISMA 2020 guidelines, PubMed, Web of Science, and Scopus were searched, identifying 175 records. After duplicate removal and screening, eight studies were included: five in broiler chickens and three in pigs, while no eligible bovine studies were identified. NH3 exposure was associated with alterations in microbial diversity and composition, accompanied by epithelial injury, inflammatory activation, oxidative stress, and impaired production outcomes. Evidence was strongest in broilers, where both respiratory and gastrointestinal microbial communities were affected. In pigs, gastrointestinal microbiome and metabolic alterations were demonstrated, whereas evidence for respiratory microbiome involvement was more limited. Microbiota depletion and transplantation experiments in broilers further indicated that intestinal dysbiosis may contribute to respiratory injury through TLR4/MyD88/NF-κB signaling, supporting a potential gut-lung interaction. Overall, environmental NH3 represents an important ecological stressor affecting livestock microbiota, although standardized longitudinal and mechanistic studies, particularly in pigs and cattle, are needed to establish causality and species-specific responses.