Mannheimia haemolytica in feedlot cattle: prevalence of recovery and associations with antimicrobial use, resistance, and health outcomes.

BACKGROUND: Mannheimia haemolytica is an important etiological agent in bovine respiratory disease. OBJECTIVES: Explore risk factors for recovery of susceptible and resistant M. haemolytica in feedlot cattle and explore associations with health outcomes. ANIMALS: Cattle (n = 5,498) from 4 feedlots sampled at arrival and later in feeding period. METHODS: Susceptibility of M. haemolytica isolates tested for 21 antimicrobials. Records of antimicrobial use and health events analyzed using multivariable regression. RESULTS: M. haemolytica recovered from 29% of cattle (1,596/5,498), 13.1% at arrival (95% CI, 12.3-14.1%), and 19.8% at second sampling (95% CI, 18.7-20.9%). Nearly half of study cattle received antimicrobial drugs (AMDs) parenterally, mostly as metaphylactic treatment at arrival. Individual parenteral AMD exposures were associated with decreased recovery of M. haemolytica (OR, 0.2; 95% CI, 0.02-1.2), whereas exposure in penmates was associated with increased recovery (OR, 1.5; 95% CI, 1.05-2.2). Most isolates were pan-susceptible (87.8%; 95% CI, 87.0-89.4%). AMD exposures were not associated with resistance to any single drug. Multiply-resistant isolates were rare (5.9%; 95% CI, 5.1-6.9%), but AMD exposures in pen mates were associated with increased odds of recovering multiply-resistant M. haemolytica (OR, 23.9; 95% CI, 8.4-68.3). Cattle positive for M. haemolytica on arrival were more likely to become ill within 10 days (OR, 1.7; 95% CI, 1.1-2.4). CONCLUSIONS AND CLINICAL IMPORTANCE: Resistance generally was rare in M. haemolytica. Antimicrobial drug exposures in penmates increased the risk of isolating susceptible and multiply-resistant M. haemolytica, a finding that could be explained by contagious spread.

Effect of fibrolytic enzymes added to a Andropogon gayanus grass silage-concentrate diet on rumen fermentation in batch cultures and the artificial rumen (Rusitec).

In vitro batch cultures were used to screen four fibrolytic enzyme mixtures at two dosages added to a 60 : 40 silage : concentrate diet containing the C(4) tropical grass Andropogon gayanus grass ensiled at two maturities - vegetative stage (VS) and flowering stage (FS). Based on these studies, one enzyme mixture was selected to treat the same diets and evaluate its impact on fermentation using an artificial rumen (Rusitec). In vitro batch cultures were conducted as a completely randomized design with two runs, four replicates per run and 12 treatments in a factorial arrangement (four enzyme mixtures×three doses). Enzyme additives (E1, E2, E3 and E4) were commercial products and contained a range of endoglucanase, exoglucanase and xylanase activities. Enzymes were added to the complete diet 2 h before incubation at 0, 2 and 4 µl/g of dry matter (DM). Gas production (GP) was measured after 3, 6, 12, 24 and 48 h of incubation. Disappearance of DM (DMD), NDF (NDFD) and ADF (ADFD) were determined after 24 and 48 h. For all four enzyme mixtures, a dosage effect (P>0.05) DM, N, NDF or ADF disappearance after 48 h of incubation nor daily ammonia-N, volatile fatty acids or CH(4) production. However, enzyme application increased (P<0.05) microbial N production in feed particle-associated (loosely-associated) and silage feed particle-bound (firmly associated) fractions. With A. gayanus silage diets, degradation may not be limited by microbial colonization, but rather by the ability of fibrolytic enzymes to degrade plant cell walls within this recalcitrant forage.