PSIV-18 Prevalence and concentrations of <i>Fusobacterium necrophorum subsp. necrophorum</i> in gut contents and tissues in steers with experimentally induced liver abscesses

Abstract Liver abscesses (LA) continue to be an economic challenge for the beef industry. Chronic ruminal acidosis and ruminitis contribute to LA development. Fusobacterium necrophorum subsp. necrophorum, a ruminal bacterium, is the primary causative agent. Damaged ruminal, and possibly other gut regions, become susceptible to invasion and colonization by subsp. necrophorum, which reaches the liver via portal blood to cause abscesses. We hypothesize that the LA occurrence is related to the presence and concentrations of subsp. necrophorum in gut contents and epithelial tissues. Our primary objective was to detect, isolate and quantify subsp. necrophorum in gut contents and tissues in steers with experimentally induced LA. An experimental model, which included ruminal acidosis followed by intraruminal inoculation of subsp. necrophorum alone or with Salmonella enterica serotype Lubbock, was used to assess the relationship. Steers (n = 61) from three independent studies with three treatment groups were used. Treatments included: 1) forage-based diet with intraruminal inoculation of subsp. necrophorum; 2) ruminal acidosis with intraruminal inoculation of subsp. necrophorum; and 3) ruminal acidosis with intraruminal inoculation of subsp. necrophorum and S. Lubbock. Steers were euthanized 3 wk after bacterial inoculation and liver, gut contents, and tissue samples (ruminal, ileal and colonic) from steers with LA (n = 27) and with healthy livers (n = 34) were collected and subjected to culture method and quantitative PCR to isolate, detect, and quantify subsp. necrophorum. Data were analyzed by PROC GLIMMIX of SAS after summarizing frequency of isolation in various sample types within each study for steers with and without LA and converted to a binomial proportion. Concentrations were analyzed using steer as the experimental unit. The model included the fixed effect of liver status (abscessed or not) and study as the random effect. The frequency of isolation and mean concentration of subsp. necrophorum in purulent materials of LA were 100% and 3.5 x 107 Log10 CFU/g, respectively. The frequency of isolation of subsp. necrophorum from ruminal tissues of steers with LA was greater than steers with healthy livers (35.4 vs 14.5%; P = 0.04). Likewise, the concentration of subsp. necrophorum in ruminal tissues was greater in steers with LA than with healthy livers (4.2 vs 3.6 Log10 CFU/g; P &amp;lt; 0.001). None of the ileal and colonic tissues, except one ileal tissue in each group, yielded subsp. necrophorum. The concentrations of subsp. necrophorum in ruminal, ileal, and colonic contents did not differ (P = 0.9) between steers with LA or with healthy livers. Our data suggest that the occurrence of LA is positively related to prevalence and concentration of subsp. necrophorum in ruminal tissue, and the colonization likely serves as a source of continuous flow of bacterial emboli into the portal blood to initiate LA development.