Reference | Benedict_2015_PLoONE (10363)

Briefly, study cattle were procured and managed at four commercial beef feedlots in south-central Alberta. Cattle were sourced from across Canada through the auction market system, and entered the feedlots at a range of weights (typically 225–400 kg), age classes, frame sizes and sexes. Cattle were processed at the time of arrival to feedlots to examine and treat ill cattle, and to administer standardized preventive and prophylactic treatments. Feedlots employed production practices typical for large feedlots located throughout western Canada and the U.S. Based upon these factors and historical patterns of illness in cattle of different types and sources, veterinary consultants (FHMS) classified arriving cattle according to perceived risk for developing bovine respiratory disease (BRD; very low risk to very high risk), and these classifications were used in assignment of prevention and treatment protocols. During arrival processing, all cattle received a hormonal implant, topical anthelmintic, and vaccines against selected pathogens (e.g., clostridial diseases, and cattle classified as having a very high risk of BRD received M. haemolytica anti-leukotoxin vaccine). Additionally, cattle classified as having high risk of developing BRD, or any that were exhibiting signs of systemic illness or fever also received metaphylactic or therapeutic AMDs during initial processing; lower risk cattle and cattle without clinical BRD were not treated with AMDs at arrival (S1 Table). Metaphylactic and therapeutic treatment protocols differed by risk status; cattle in higher-risk categories received drugs shown to have greater efficacy for prevention and treatment of respiratory disease [20–23]. Cattle were fed a diet that met or exceeded the National Research Council requirements for beef cattle [24] until reaching a body weight of 550–650 kg, at which time they were sent to slaughter, typically 120–250 days after arrival in the feedlot. After initial processing, cattle were grouped in pens for housing through the feeding period. Most pen populations remained intact throughout the feeding period but a minority were subsequently split or merged with animals from other pens prior to harvest to facilitate marketing of similarly sized cattle at the time of harvest (Fig 1). The health of cattle was evaluated daily by trained feedlot personnel, and animals deemed to be sick were treated under the supervision of veterinarians from FHMS using standardized protocols (S1 Table). Additionally, all cattle received AMDs in feed as prophylaxis for liver abscesses. Cattle were enrolled from September 2007 to January 2010 using two-stage random sampling. As cattle arrived at the feedlots, 30% of all newly formed pens were randomly selected for inclusion, and approximately 10% of all cattle housed in selected pens were randomly enrolled. Data regarding exposure of individual cattle to AMDs were recorded at each feedlot. Exposures were recorded for cattle enrolled in the study as well as their pen-mates. These data included the unique animal identification, the AMD product used, dose, route of administration, and the date administered. Ionophore and coccidiostat exposures were not evaluated in this study.

Samples were collected as cattle arrived at the feedlot, and then at a second time point later in the feeding period when cattle were rehandled for routine production practices (e.g., placement of hormone implants). To facilitate analysis, the time of sampling relative to the arrival at feedlots was categorized (arrival or 0 days on feed [DOF], 33–75 DOF, 76–120 DOF, or >120DOF). Fecal samples were collected per rectum, and swab samples were collected from deep in the nasopharynx using 22 cm guarded swabs [13, 20]. Swabs were processed to recover M.haemolytica [25] and fecal samples were held until culture status for M. haemolytica was determined. As a standard protocol for this surveillance project, fecal samples from individual cattle were only processed to recover NTSEC isolates if M.haemolytica was cultured from that animal’s nasopharyngeal swab (Fig 1). Because the majority of isolates were obtained from cattle that were culture-positive for M. haemolytica, isolates included in this study from the first sampling were not necessarily recovered from cattle that provided isolates obtained from at the second sampling.

AST Method: Multiple Methods

Reference explicitly reports AST breakpoints: True

Reference reports using a MIC table: False