Characterization of Mannheimia haemolytica in beef calves via nasopharyngeal culture and pulsed-field gel electrophoresis.

Mannheimia haemolytica is a major bacterial component of bovine respiratory disease (BRD); unfortunately, very little is known about M. haemolytica transmission dynamics among cattle. Identifying potential variation in M. haemolytica populations over time and induction of nasopharyngeal colonization and subsequent shedding are 2 areas where knowledge is lacking. In our study, 2 separate loads of 20 mixed-origin, male calves were purchased through an order buyer on different dates. Deep nasopharyngeal cultures (NPC) were performed on all calves on arrival and, if M. haemolytica-negative, a second screening culture was obtained. Calves that were negative on 2 initial NPCs (NEG; n = 4) were subsequently challenged with a previously isolated field strain of M. haemolytica in both the upper and lower respiratory tract, individually housed, and then monitored for M. haemolytica shedding via NPCs at 0.5, 1, 3, 5, 7, and 9 days postchallenge. Naturally M. haemolytica-positive calves (2 per load) were kept for additional daily cultures (POS; n = 4). Individual calf M. haemolytica status for both the POS and NEG groups was inconsistent between study days. Additionally, pulsed-field gel electrophoresis performed on isolates from the positive cultures showed that the NEG calves did not shed the M. haemolytica challenge strain, but rather 2 distinct clusters of M. haemolytica were shared among POS and NEG calves regardless of their initial status. Although sample sizes were small, these findings illustrate how variable the results of a single nasopharyngeal swab can be and the challenges of using an individual culture to truly represent animal M. haemolytica status.