Carriage of bacteria and tetracycline resistance by adult house flies (Musca domestica L.) at confined cattle operations

Adult house flies harbor human and animal pathogens, including antimicrobial resistant (AMR) bacteria, which are acquired from interactions with microbe-rich substrates like cattle manure. Female flies inspect these substrates closely for possible oviposition sites which influences their potential to acquire bacteria. House flies were collected biweekly from beef feedlots in three Kansas counties (Marion, Riley, and Washington) across five dates in 2019. Bacteria from male and female flies were enumerated on non-selective (TSA, overall bacteria) and selective media (VRBA, coliforms). A subset of morphologically-distinct coliforms were cultured onto tetracycline infused agar to test for antimicrobial resistance. Fly sex significantly influenced the abundance of bacteria and coliforms from flies at feedlots. Collection date significantly influenced the abundance of bacteria and coliforms in flies from some of the feedlots, but this influence varied by site and within fly sex. However, neither fly sex nor collection site influenced the prevalence of house flies carrying tetracycline-resistant coliforms. On average 89% of male and 83% of female flies carried at least one tetracycline-resistant coliform, while 74% of male and 64% of female flies carried two or more resistant coliforms. Because flies were significant reservoirs of bacteria, including those that were AMR, implementing fly control may help reduce bacterial dissemination and AMR prevalence at feedlots.