Further delineation of the etiology of liver abscesses in cattle and indication of hindgut as a potential source of pathogens

Liver abscesses (LA) in feedlot cattle are polymicrobial infections, initiated primarily by the translocations of the two subspecies of <i>Fusobacterium necrophorum, necrophorum</i> and <i>funduliforme,</i> and secondarily by <i>Trueperella pyogenes</i> and <i>Salmonella enterica</i> from the rumen into the liver via portal blood. The hindgut has not been investigated as a potential source of LA pathogens. Our objectives were to analyze LA and matched ruminal and colonic epithelial tissues (<i>n</i> = 96), collected at a commercial beef processor immediately after evisceration, to determine culture-based prevalence of pathogens, specifically, <i>Escherichia coli</i>, <i>Klebsiella pneumoniae</i>, <i>Pseudomonas aeruginosa</i>, and <i>Bacteroides fragilis,</i> which have not been targeted before, in addition to <i>F. necrophorum, T. pyogenes,</i> and <i>S. enterica,</i> and quantitative PCR-based prevalence and concentrations of the <i>F. necrophorum</i> subspecies. The subsp. <i>necrophorum</i> was the most dominant species in LA detected by both methods, and concentrations were in the range of 7.0 log₁₀ colony forming units (CFU). Fusobacterial concentrations were higher in ruminal epithelial than in colonic epithelial tissues, but colonic epithelial tissues yielded more subsp. <i>necrophorum</i> isolates than ruminal epithelial tissues. Of the targeted bacterial species, <i>E. coli</i> was the most dominant, and none of the tissues was positive for <i>B. fragilis</i>. A majority of the isolations of <i>E. coli</i> were after enrichment, suggesting the concentrations were too low to contribute to the abscess development. The data reaffirm that LA are polymicrobial infections, <i>F. necrophorum,</i> specifically <i>necrophorum,</i> is the most dominant species prevalent, and the hindgut is likely to be a source, in addition to the rumen, of pathogens that cause LA.IMPORTANCELiver abscesses (LA) in beef cattle are initiated by entry of bacterial pathogens from the rumen via portal blood. The two most frequently isolated pathogens are the two subspecies of <i>Fusobacterium necrophorum,</i> followed by <i>Trueperella pyogenes</i> and <i>Salmonella enterica</i>. Our objectives were to determine the prevalence of pathogens that have been reported sporadically and have not been targeted before. Liver abscesses and matched ruminal epithelium and colonic epithelial tissues, collected from feedlot cattle at slaughter, were analyzed by culture and PCR methods. The subsp. <i>necrophorum</i> was the most dominant bacterium in liver abscesses. Colonic epithelial tissues yielded more subsp. <i>necrophorum</i> than ruminal epithelial tissues. Although <i>Escherichia coli</i> was the second most prevalent species, a majority of the isolates were obtained after enrichment, indicating low concentrations. Our data reaffirm that <i>F. necrophorum</i> is the dominant species prevalent in LA, and the hindgut is likely to be another source of pathogens that cause LA.