Risk factors for bovine respiratory disease in Australian feedlot cattle: Use of a causal diagram-informed approach to estimate effects of animal mixing and movements before feedlot entry

Abstract

A nationwide longitudinal study was conducted to investigate risk factors for bovine respiratory disease (BRD) in cattle in Australian feedlots. After induction (processing), cattle were placed in feedlot pens (cohorts) and monitored for occurrence of BRD over the first 50 days on feed. Data from a national cattle movement database were used to derive variables describing mixing of animals with cattle from other farms, numbers of animals in groups before arrival at the feedlot, exposure of animals to saleyards before arrival at the feedlot, and the timing and duration of the animal's move to the vicinity of the feedlot. Total and direct effects for each risk factor were estimated using a causal diagram-informed process to determine covariates to include in four-level Bayesian logistic regression models. Mixing, group size and timing of the animal's move to the feedlot were important predictors of BRD. Animals not mixed with cattle from other farms prior to 12 days before induction and then exposed to a high level of mixing (≥4 groups of animals mixed) had the highest risk of developing BRD (OR 3.7) compared to animals mixed at least 4 weeks before induction with less than 4 groups forming the cohort. Animals in groups formed at least 13 days before induction comprising 100 or more (OR 0.5) or 50-99 (OR 0.8) were at reduced risk compared to those in groups of less than 50 cattle. Animals moved to the vicinity of the feedlot at least 27 days before induction were at reduced risk (OR 0.4) compared to cattle undergoing short-haul transportation (<6h) to the feedlot within a day of induction, while those experiencing longer transportation durations (6h or more) within a day of induction were at slightly increased risk (OR 1.2). Knowledge of these risk factors could potentially be used to inform management decisions to reduce the risk of BRD in feedlot cattle.

Associations between animal characteristic and environmental risk factors and bovine respiratory disease in Australian feedlot cattle

“…The NBRDI study design and study population have been described elsewhere (Hay et al, 2014). Induction refers to processing or preparing the animal for placement "on feed" (i.e.…”

“…Each animal's induction date was defined as "day 0". Data detailing animal-level lifetime movement history were obtained from the National Livestock Identification System (NLIS) database and used to derive variables describing group structure at time points of interest prior to day 0 (Hay et al, 2014). Thus, the group that an animal was part of 13 days prior to induction ("group-13") was determined from the property (i.e.…”

“…Further data were supplied for animals that were hospitalised or died during the observation period; data from hospitalised animals were used to derive the case definition as described elsewhere (Hay et al, 2014). Briefly, all animals with diagnoses indicating respiratory system involvement at first examination in the hospital crush (i.e.…”

“…In capturing animal-level data from this heterogeneous population, the National Bovine Respiratory Disease Initiative (NBRDI) was well placed to address many of the limitations of previous research (Hay et al, 2014). The project aimed to identify and quantify the effects of putative risk factors on the occurrence of BRD in Australian feedlot cattle.…”

Explanation and Elaboration Document for the STROBE‐Vet Statement: Strengthening the Reporting of Observational Studies in Epidemiology—Veterinary Extension

“…As part of the post hoc assessment, authors should consider whether the variables were confounders or variables with other relationships, such as collider or intervening variables. These other variables can also introduce bias into the association between a different independent variable and the outcome and be detected by algorithm‐based approaches 42, 43, 45, 46…”