Experimental design and treatment arrangements

The data used for this study were previously collected (Kayser et al., 2018b) to evaluate the effects of live yeast (LY) (Saccharomyces cerevisiae boulardii strain I-1079 at 25 g/hd/d; ProTernative Advantage, Lallemand Animal Nutrition) supplementation on animal performance prior to and post MH challenge. Therefore, steers were stratified by herd origin, initial BW, MH titer dilution, exit velocity (objective measure of animal temperament; Olson et al., 2019), and prestudy ADG, then a random number generator was used to assign steers to one of four treatments (9 hd/treatment) arranged in a 2 × 2 factorial array. Supplementation with live yeast, however, did not affect the SPC procedures sensitivity, specificity, or accuracy (Kayser et al., 2018b). Therefore, the effect of dietary yeast treatment was not considered in the current analysis, only two treatment groups were evaluated: steers inoculated with MH (n = 18) or PBS (n = 18).

The MH inoculum was prepared as described by Mosier et al. (1995). Briefly, MH serotype A1 was grown on trypticase soy agar containing 5% sheep blood for 18 h at 37°C in 7% CO₂. Colonies were inoculated into brain-heart infusion broth and incubated for 16–18 h at 37°C with aeration. The bacteria were then centrifuged at 3,000 × g for 15 min at 4°C and washed twice with PBS. After the second wash, the bacteria were centrifuged as before and the pellet was resuspended in PBS at a final concentration of 1.2–1.4 × 10⁹ CFU/10-mL dose. After preparation, the inoculum was placed on ice in a dark cooler and transported to the site of inoculation (approximately 17 km).

Steers were fed for a total 56 d, and baseline data collection began on day −28 d prior to MH inoculation on day 0. To avoid any chance of PBS animals receiving MH via contamination of the instruments used for inoculation, the PBS treatment group was inoculated prior to the MH treatment group. The inoculations were performed with an endoscope as described by Theurer et al. (2013). Steers were restrained in a standard hydraulic squeeze chute that allowed more specific restraint of the head. An endoscope 1 m in length was inserted into the ventral meatus of one nostril and passed into the trachea to the level of the right apical lung lobe bronchi allowing visualization of the opening. A sterile bronchoalveolar lavage tube was inserted into the endoscope portal and passed until the tip of the lavage tube was visible emerging from the endoscope. Thereafter, the lavage tube was advanced another 1–2 cm into the opening of the right apical lung lobe bronchi. Once in place, steers in the PBS treatment group were administered 10 mL of PBS followed by a 60 mL flush of PBS for a total of 70 mL. Following the treatment of all the PBS animals, the endoscope was disinfected with chlorhexidine solution and rinsed with saline. Subsequently, steers (n = 18) in the MH treatment groups were challenged with 10 mL of MH serotype A1 at 1.2–1.4 × 10⁹ CFU/mL followed by 60 mL of PBS for a total of 70 mL. Following this procedure, steers in both treatment groups were observed for adverse effects of the challenge procedure.

Throughout the 56-d study, all animals were group housed in four pens at Texas A&M University’s Beef Cattle Systems Research Center in College Station, TX. The MH- and PBS-challenged steers were comingled within pen and there were equal number of steers from each treatment housed together. During the study, steers were offered a growing diet ad libitum, which was provided twice daily at 0700 and 1600 hours.