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Environmental sampling for characterization of Mannheimia haemolytica shedding by feedlot cattle
Crosby, W.B., Valeris-Chacin, R...
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Objective
Control and prevention of Bovine Respiratory Disease (BRD) is a major reason for treatment of cattle with antimicrobial drugs. However, the prevalence of antimicrobial resistance (AMR) is increasing in Mannheimia haemolytica and other BRD pathogens which may limit treatment efficacy. Investigation of AMR impacts on BRD requires recovery and characterization of M. haemolytica by costly and time consuming sampling of individual cattle. Identifying group sampling methods that are comparable to individual sampling could facilitate surveillance and research. The objective of this study was to compare detection of M. haemolytica in cattle via individual sampling using culture and quantitative real-time PCR (qPCR) of nasopharyngeal swabs (NPS) with detection from group sampling methods - water bow swabs, ropes hung on pens, and pooled DNA from NPS.
Materials and Methods
Cattle housed in 10 pens located at 3 commercial feedlots in Texas were sampled at 10-22 days on feed. Ten animals from each pen (n=100) were randomly selected and NPS were obtained using rayon swabs for bacterial culture and DNA extraction. Five 1-cm diameter polyester ropes were hung from each study pen for 24 hours. Water bowls from each pen were swabbed at 3 locations in the bowl: the bottom, the waterline, and the top. qPCR for the leukotoxin D gene of M. haemolytica was performed in triplicate using DNA from individual NPS (n=10 animals/pen), pools of NPS (n=3 pools/pen), ropes (n=3/pen), and water bowls (n=3/pen). Individual NPS (n=10 animals/pen), ropes (n=2/pen), and water bowls (n=3/pen) were cultured to identify M. haemolytica. Mean Ct for each sample type was compared by Kruskal-Wallis analysis of variance on ranks, with post-hoc Dunn test using Benjamini-Hochberg correction.
Results
M. haemolytica was only cultured from individual NPS, with animal within pen prevalence ranging from 0-50% of animals sampled (median=10 %). qPCR identified M. haemolytica in 41% of individual NPS (n=41), 60% of pooled NPS samples (n=18), 76% of rope samples (n=23), and 78 % of water bowls (n=14 - only 18 samples had sufficient DNA for testing). Overall, ope samples had a lower mean Ct than water bowls, NPS pools, and individual NPS (26.5 vs 30.1, 31.1, 34.9, respectively; p 0.05).
Conclusions
These results support the use of group sampling to characterize group prevalence of M. haemolytica via qPCR. Culture of M. haemolytica did not reliably identify the agent in group sampling methods.
Keywords: Real-time PCR, BRD, bacterial culture.
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