294 Award Talk: Longitudinal assessment of Fusobacterium necrophorum and Salmonella enterica concentration and prevalence in relation to feedlot cattle health and management

Abstract

Our objective was to longitudinally assess the prevalence of Fusobacterium and Salmonella in relation to feedlot cattle health and management. In Exp. 1, crossbred steers (n = 227; BW = 353 ± 39.6 kg) were transported to a research feedlot and fed a high-concentrate diet. Nasal, ruminal fluid, and fecal samples were aseptically collected following feedlot arrival (d 5), 1 wk after adaptation to a finishing diet (d 35), and the day before harvest (study end). Livers were collected at harvest, and cattle were assigned into either control or liver abscess (LA) groups based on liver abscess prevalence. Overall LA prevalence was 18.7%. The concentration and prevalence of Salmonella decreased in ruminal fluid and increased in feces with days on feed (P < 0.01). Conversely, ruminal fluid prevalence of F. necrophorum subsp. necrophorum and F. varium increased with days on feed (P < 0.01). Fusobacterium abundance in ruminal fluid and feces was not indicative of LA development, except for F. varium being more abundant (P < 0.01) in the ruminal fluid of steers with LA. Abundance of subsp. necrophorum was greater in abscessed liver tissue than in healthy tissue (P = 0.03), but no other differences in bacterial abundance or prevalence were observed in livers. These data suggest Fusobacterium and Salmonella were prevalent throughout the gastrointestinal tract and affected by days on feed, but their prevalence and abundance were generally not indicative of LA formation. In Exp. 2, crossbred beef steers (n = 232; BW = 213 kg ± 6.3 kg) from 2 sources were used in a generalized complete block design. Experimental treatments and steers were randomly assigned to pen within source block. Treatments included a negative control (s.c. injection with sterile saline; 0M); metaphylaxis with tulathromycin applied at random to 33 or 66% of steers (33M and 66M), respectively; and conventional metaphylaxis applied to 100% of the steers (100M; positive control). Body weights, whole blood samples, and fecal samples were collected on d 0, 14, and 35. The percentage of steers treated for bovine respiratory disease (BRD) once was 108% and 62% greater on average for 0M and 33M, respectively, than for 66M and 100M steers (P = 0.01). Growth performance from d 0 to 35 did not differ among treatments (P ≥ 0.63). Eosinophil percentage was greatest for 100M and least for 0M and 66M (P = 0.01), while day affected most complete blood cell count variables (P < 0.01). Fecal Salmonella concentrations and prevalence (P ≥ 0.40) did not differ among treatments but increased from d 0 to 14 (P < 0.01). These data suggest that metaphylaxis can be administered to 66% of high-risk beef steers without increasing BRD morbidity, total antimicrobial use, or pathogen shedding.