Flying under the radar: Are there unexpected outcomes of antibiotic use in livestock production?

On a global scale, antibiotics are administered extensively to livestock for therapeutic, prophylactic, and/or growth-enhancing purposes, especially in animals kept in confined animal feeding operations (CAFOs) like feedlots. Administered antibiotics are excreted in animal waste (urine and manure) which accumulates in the CAFO environment. This waste, rich in antibiotics and other chemicals used to manage disease and pests serves as a developmental substrate for house flies (Musca domestica L.). The impact that high levels of antibiotic residues in manure have on house fly biology, particularly larval development, has not been well described. In this study, we determined the effects of two antibiotics commonly used in beef cattle production (tetracycline and enrofloxacin) on house fly larval development. Both antibiotics were administered to the larval substrate (separately and in combination) at low, medium, and high concentrations: 4.4, 44, and 440mg/kg for tetracycline; 7, 7, and 70mg/kg for enrofloxacin. Dose dependent effects on house fly larval development were observed including slowed larval time to pupation, lower pupation rate, reduced pupal mass, and reduced eclosion rate. Our results indicate that antibiotic residues in animal production systems impact house fly larval development. Future studies are aimed at determining whether antibiotics potentially select flies for tolerance to chemical stressors, which may have important impacts on managing these pests.