Limitations of bacterial culture, viral PCR, and tulathromycin susceptibility test methods from upper respiratory tract samples in predicting the outcome of tulathromycin control or treatment of bovine respiratory disease in high risk feeder heifers

A cross-sectional prospective cohort study to correlate BRD clinical outcomes for tulathromycin metaphylaxis/treatment for bovine respiratory disease (BRD) with the results of bacterial culture and tulathromycin susceptibility from isolates of deep nasopharyngeal swabs (DNS) as well as viral polymerase chain reaction (PCR) results from nasal swabs revealed poor correlation of bacterial culture and tulathromycin susceptibility with response to tulathromycin metaphylaxis or treatment. 1031 heifers, assumed to be at high-risk (>40% expected BRD morbidity rates), were procured and transported to a research feedlot in Texas. Isolation rates from DNS collected on arrival and at first treatment respectively were: Mannheimia haemolytica (10.9% & 34.1%); Pasteurella multocida (10.4% & 7.4%); Mycoplasma bovis (1.0% & 36.6%); and Histophilus somni (0.7% & 6.3%). Prevalence of BRD viral nucleic acid on nasal swabs collected at first treatment were: PI-3V (34.1%); BVDV (26.3%); BoHV-1 (10.8%); and BRSV (54.1%). Increased relative risk of treatment failure was associated with positive viral PCR results, PI-3V (1.2644), BVDV (1.3917), BHV-1 (1.5156), and BRSV (1.3474) from nasal swabs collected at first pull and culture of M. haemolytica (1.2284) from DNS collected at arrival; however, no other statistically predictable risk of treatment outcomes were measured from DNS for bacterial isolation or tulathromycin susceptibility for M. haemolytica or P. multocida at arrival or first treatment. Predictive values of bacterial culture and tulathromycin susceptibility were substantially lower than the 85% level expected with susceptibility testing. These results indicate tulathromycin susceptibility testing of isolates of M. haemolytica or P. multocida from DNS collected on arrival or at first pull unreliably predict clinical efficacy of tulathromycin for BRD control or treatment most likely due to impacts of unpredictable risk factors and other viral and/or bacterial BRD comorbidities.