Quantification of major bacterial pathogens in liver abscesses and matched ruminal and colonic epithelial tissues of feedlot cattle.

Liver abscesses (LA) in cattle are a polymicrobial infection, and the major bacterial pathogens associated are as follows: Fusobacterium necrophorum subsp. necrophorum (FNN), F. necrophorum subsp. funduliforme (FNF), Trueperella pyogenes (TP), and Salmonella enterica (SE). In polymicrobial infections, the contributions of individual species are difficult to assess. We hypothesized that species abundance in abscesses may be indicative of their contributions. Therefore, the objective was to develop a 4-plex quantitative PCR (qPCR) assay to determine prevalence and concentrations of the pathogens in LA (n = 384) and matched ruminal (RT; n = 374) and colonic epithelial (CT; n = 256) tissues. Fusobacterium necrophorum, either FNN or FNF, was detected in 85.9% of LA by qPCR, which was slightly lower than the culture-based prevalence (89.1%). Only 16.9% of LA were positive for FNF with no FNN. The concentrations of FNN or FNF were 7 to 7.5 log10 CFU/g. The qPCR assay identified more (P < 0.01) samples (29.2%) as positive for TP than the culture method (16.7%). The mean concentration of TP was 5.9 log10 CFU/g. None of the LA was positive for SE by the qPCR assay. The prevalence of FN was greater (P < 0.01) in RT than CT (73.2% vs. 16%). The concentrations were in the 4 to 5 log10 CFU/g. The low concentrations of TP suggest that it is unlikely to be the primary etiologic agent. Although SE was detected by culture method, it was not quantifiable, which suggests that the species does not contribute to the development of LA.IMPORTANCELiver abscesses (LA) in cattle are a polymicrobial infection, and four bacterial pathogens are implicated: Fusobacterium necrophorum subsp. necrophorum (FNN) and subsp. funduliforme (FNF), Trueperella pyogenes (TP), and Salmonella enterica (SE). In mixed infections, the species abundance may be indicative of their contributions. Our objective was to develop a quantitative PCR assay to determine prevalence and concentrations of the pathogens in LA (n = 384) and matched ruminal (RT; n = 374) and colonic tissues (CT; n = 256). The dominant species in LA was FNN with a mean concentration of 7.0 log10 CFU/g. The subsp. FNF without FNN was prevalent in a small number of LA, with a mean concentration of 7.0 log10CFU. The concentration of TP was 5.0 log₁₀ CFU, which suggests that it is unlikely to be the primary etiologic agent. Although S. enterica was prevalent in LA, none was quantifiable, which suggests that it is not contributing to LA.