The nasopharyngeal microbiota of beef cattle before and after transport to a feedlot

Holman, Devin B.; Timsit, Edouard; Amat, Samat; Abbott, D. Wade; Buret, Andre G.; Alexander, Trevor W.

In terms of membership, most striking was the enrichment of Mycoplasma in day 14 samples (Figs. 2 and 3 ). At this time point, four animals actually had a NP microbiota that was comprised of greater than 63% Mycoplasma. We have previously observed an increase in Mycoplasma 60 d after feedlot entry [1] indicating that the feedlot environment, and its associated stressors, may provide conditions that allow for the proliferation of Mycoplasma in the nasopharynx. Based on the present study, it would appear that changes in the NP microbiota that allow Mycoplasma to proliferate occur within the first 14 d of feedlot placement. Further classification of the most relatively abundant OTUs identified as Mycoplasma revealed that they were evenly split between the species M. bovirhinis and M. dispar. Although M. bovirhinis and M. dispar are isolated from diseased as well as healthy cattle, M. bovis is the primary Mycoplasma sp. associated with BRD [47, 48] . Because of its importance in chronic pneumonia, it would be of interest to determine whether M. bovis is similarly capable of rapid proliferation and occupying a larger relative proportion of the NP microbiota, should it be present to colonize the nasopharynx at feedlot entry. In a previous study, the prevalence of M. bovis in bronchoalveolar lavage fluid increased from 1.7% at feedlot entry to 72.2% 15 days after mixing and placement of cattle in a commercial feedlot [49] . Although the authors did not report on relative abundance, it is apparent that M. bovis is capable of spreading amongst cattle when a source of this opportunistic pathogen is available. Pasteurella spp., which were enriched in the nasopharynx of cattle prior to transport to the feedlot, were identified by BLASTn to be P. multocida, a bacterial species commonly associated with BRD. Despite the presence of these species, and also M. haemolytica at a lower relative abundance, the cattle all remained healthy throughout the study, thereby confirming that Mycoplasma spp., P. multocida, and M. haemolytica are natural inhabitants of the NP tract. This observation is further supported by the presence of these bacteria in the cattle prior to shipping to the feedlot, as we used cattle from a closed and disease-free herd that has been maintained in this state for over 30 years. It is of particular interest that transportation to a feedlot did not result in proliferation of P. multocida and M. haemolytica, the latter being the opportunistic pathogen most often associated with BRD cases. In contrast to this, increased shedding of M. haemolytica has previously been described as a result from stress associated with cattle transportation, although in that study, the distance travelled was 1,600 km and the authors used recently weaned calves [50] which may have increased predisposition to pathogen growth.