PS12-8. Prevalence and Distribution of Major Bacterial Pathogens in Liver Abscesses of Beef-on-dairy Cattle.

Abstract Liver abscesses represent a significant economic issue for the beef cattle industry, accounting for about 67% of liver abnormalities in slaughtered cattle in the U. S. The primary causative agents, believed to originate from the rumen, include Fusobacterium necrophorum (subspecies necrophorum and funduliforme), Trueperella pyogenes, and Salmonella enterica. Prevalence ranges 10–20%, reducing carcass value by up to $38 per animal for severe cases. This study aimed to determine the prevalence and distribution of major bacterial pathogens associated with liver abscesses in beef-on-dairy cattle and to assess associations with liver abscess score, sex, and feedyard-level clustering. A total of 219 liver abscesses were collected at slaughter from two feedyards, consisting of 15 and 21 lots, respectively. Abscesses with intact capsules were sampled and processed for bacterial isolations and identifications under anaerobic, aerobic, and with 5% CO2 using enrichment and selective media. Pathogen prevalence was evaluated by culture and quantitative PCR (qPCR) methods. Data analyses were performed using R (version ≥ 4.5.2), with liver abscess samples as the unit of analysis. Descriptive statistics summarized pathogen prevalence as proportions and stratified by liver abscess severity (A+, A, A−), sex (heifer vs. steer), feedyard, and lot. Associations between pathogen prevalence and categorical predictors were assessed using Pearson’s chi-square or Fisher’s exact tests. Logistic regression models with a binomial distribution and logit link were fitted for each pathogen to evaluate main effects of liver abscess score and sex, their interaction, and potential clustering by lot nested within feedyard. Patterns of pathogen co-occurrence were summarized descriptively, and heatmaps visualized within-score prevalence of individual pathogens and multi-pathogen combinations. Statistical significance was evaluated at α = 0.05. Fusobacterium necrophorum subsp. necrophorum was the most frequently detected pathogen (culture: 90.4%; qPCR: 92.7%), followed by T. pyogenes (28.3%; 53.0%), subsp. funduliforme (19.2%; 57.1%), and S. enterica (3.7%; < 1 %). Culture-based T. pyogenes prevalence was higher in steers than heifers (39.3% vs. 20.8%; p = 0.004), while S. enterica was detected only in heifers (6.2% vs. 0%; p = 0.022). Liver abscess score was not significantly associated with culture or qPCR prevalence of any pathogen. Co-occurrence analyses indicated that single-pathogen infections dominated, while multi-pathogen abscesses were rare. The qPCR-based detection revealed higher prevalence of subsp. funduliforme and T. pyogenes compared with culture. Logistic regression confirmed sex-specific differences for T. pyogenes and S. enterica, whereas liver abscess score was not a significant predictor. Together, these results provide a characterization of pathogen prevalence in beef-on-dairy liver abscesses, highlighting sex-related differences for select pathogens, and establish a foundation for interpreting biological mechanisms and management-level risk factors. This information may inform targeted intervention strategies to reduce liver abscess incidence and associated economic losses in beef-on-dairy production systems.