Prevalence and concentrations of major bacterial pathogens in liver abscesses and ruminal and colonic epithelial tissues in relation to abscess severity in feedlot cattle.

Liver abscesses (LA) in cattle are commonly categorized as mild or severe, based on the size and number of abscesses, and only the latter has significant economic impact. Fusobacterium necrophorum subsp. necrophorum, F. necrophorum subsp. funduliforme, Trueperella pyogenes, and Salmonella enterica are implicated in LA. Our objectives were to compare the prevalence and concentrations of these bacterial pathogens in purulent materials of mild (n = 134) or severe (n = 140) LA, and in matched ruminal (n = 94 and n = 88) and colonic (n = 52 and n = 69) epithelial tissues of feedlot cattle collected at slaughter in three independent studies. Samples were homogenized, and methods of detection included culture and quantitative PCR (qPCR) to determine prevalence and concentrations. Data were analyzed using a generalized linear mixed-effects model with severity, mild or severe, as a fixed effect, and the feedlot study as a random effect. Nearly all LA (90.9-98.9%) contained either or both subspecies of F. necrophorum and the prevalence was greater (P = 0.04) in severe LA (96.7%) compared to mild LA (90.9%) with qPCR and tended to be greater (P = 0.09) with the culture method (97% vs. 92.7%). The subsp. funduliforme, not associated with subsp. necrophorum, was prevalent in 15-21% of LA and did not differ (P > 0.4) in prevalence between mild and severe LA. Concentrations of F. necrophorum were in the range of 6.9-7.3 log10 CFU per gram of purulent material and were not different (P > 0.14) between mild or severe abscesses. The prevalence of T. pyogenes, detected by the culture method, was greater (P < 0.01) in severe (42.1%) compared to mild (26.1%) abscesses, but the prevalence based on qPCR only tended to be greater (P = 0.06) in severe LA (32.5%) compared to mild abscesses (22.2%). The prevalence and concentration of S. enterica were not affected (P > 0.13) by LA severity. The co-occurrences of the subsp. necrophorum and T. pyogenes and subsp. necrophorum and S. enterica were greater (P = 0.03) in severe compared to mild LA. Prevalence and concentrations of the four bacterial pathogens in ruminal and colonic epithelial tissues were not affected by LA severity. In conclusion, the prevalence of subsp. necrophorum and T. pyogenes were greater in severe compared to mild LA, but the concentrations were not different, and the greater frequency of association of the two species illustrates pathogenic synergy, possibly contributing to the severity of LA.