Macrolides are the first-line treatment against bovine respiratory disease (BRD), and are also used to treat
infections in humans. The
macrolide,
tylosin phosphate, is often included in the diet of cattle as a preventative for
liver abscesses in many regions of the world outside of Europe. This study investigated the effects of administering
macrolides to beef cattle either systemically through a single
subcutaneous injection (therapeutic) or continuously in-feed (subtherapeutic), on the prevalence and antimicrobial resistance of Mannheimia haemolytica and Enterococcus spp. isolated from the nasopharynx and faeces, respectively. Nasopharyngeal and faecal samples were collected weekly over 28 days from untreated beef steers and from steers injected once with
tilmicosin or
tulathromycin or continuously fed
tylosin phosphate at dosages recommended by manufacturers.
Tilmicosin and
tulathromycin were effective in lowering (P < 0.05) the prevalence of M. haemolytica, whereas subtherapeutic
tylosin had no effect. M. haemolytica isolated from control- and
macrolide-treated animals were susceptible to
macrolides as well as to other
antibiotics. Major bacteria co-isolated with M. haemolytica from the nasopharynx included Pasteurella multocida, Staphylococcus spp., Acinetobacter spp., Escherichia coli and Bacillus spp. With the exception of M. haemolytica and P. multocida,
erythromycin resistance was frequently found in other isolated species. Both methods of
macrolide administration increased (P < 0.05) the proportion of
erythromycin resistant enterococci within the population, which was comprised almost exclusively of Enterococcus hirae.
Injectable macrolides impacted both respiratory and enteric microbes, whereas orally administered
macrolides only influenced enteric bacteria.