Adult house flies (Musca domestica L.) and other environmental sources of enteric bacteria at Kansas beef cattle operations

House flies are coprophagous, feeding upon and developing in manure and consequently congregating at animal production settings where manure is available. Flies acquire and disseminate enteric bacteria from manure and disseminate microbes to animals, feed, water, and surrounding communities, making them important to human and animal health. The goal of this study was to elucidate variables contributing to bacterial carriage by house flies in beef cattle operations. House flies and environmental samples (manure, feed, and trough water) were collected over five dates in 2020 from four Kansas beef producers. Bacteria were enumerated from samples on MacConkey agar to select for Gram-negative and enteric bacteria. Then, plates with 1-75 colony-forming units were replica plated via stamping to test for resistance to florfenicol, tetracycline, and enrofloxacin. Within each site, and overall, female flies carried a higher bacterial abundance than males. Although collection site and date did not significantly impact overall bacterial abundances in flies, abundance was affected by collection site for males at one site and for both males and females at another site. Gram-negative bacterial abundance in environmental samples varied among beef operations. Flies carried the greatest number of triple-resistant isolates and had the greatest variability in morphotypes, with many similar morphotypes observed in environmental samples. Although confirmation of species identity is pending, we infer that flies acquired and transmitted bacteria between cattle manure, feed, and water tanks. Therefore, fly control should be implemented to reduce the dissemination of antimicrobial-resistant bacteria at confined cattle operations.