Association of dry cow therapy with the antimicrobial susceptibility of fecal coliform bacteria in dairy cows.

The prophylactic use of intramammary antimicrobial drugs at the end of lactation in dairy cows, known as dry cow therapy (DCT), is widely practiced in US dairy herds. This extremely common use of high-dose, slow-release antimicrobials may influence the ecology of bacterial flora on dairy farms. We investigated the association between the antimicrobial used for intramammary DCT and the relative number of fecal coliform bacteria with reduced susceptibility to three antimicrobial drugs in dairy cattle. Most probable number (MPN) data were estimated from 463 individual fecal samples collected from lactating cows in 15 dairy herds in Ohio, USA. These data were used to calculate the relative number of fecal coliform bacteria with reduced susceptibility to cephalothin, streptomycin, and tetracycline for individual cow samples. The farms included in this project were classified based on DCT, with 8 farms using a cephalosporin-based product and the remaining 7 using a penicillin/streptomycin therapy. Results of a linear mixed model indicate that herds using a cephalosporin DCT had higher (P<0.01) relative numbers of fecal coliform bacteria with reduced susceptibility to cephalothin and streptomycin compared to those using a penicillin/streptomycin intramammary therapy. Relative numbers of fecal coliform bacteria with reduced susceptibility to tetracycline was not associated with DCT. These results suggest that high-dose slow-release antimicrobials applied locally in the udder to populations of dairy cows might influence the antimicrobial susceptibility of the enteric flora. However, the potential animal and public health implications of this result are not clear.