RISK FACTORS ASSOCIATED WITH CULLING AGE IN DAIRY CATTLE: APPLICATIONS OF FRAILTY MODELS

Culling decisions for dairy cattle are an important component of dairy herd management. To investigate risk factors for culling, farms (clusters) constitute the sampling units. Therefore, we believe that ages-at-culling may be correlated within farms. The score test on the null hypothesis of no extra-variation in survival data was not supported by age-at-culling data collected from 72 dairy farms from the province of Ontario, Canada. To correct for the intraherd correlation, three modelling approaches were used to fit the data: Population-Averaged (PA) , cluster-specific (CS), and Random Effects Models (RAEM). The modelling approaches are described and compared using the dairy cow culling data

The efficacy of antibiotics to prevent collibacilosis in broiler poultry: A protocol for a systematic review and network meta-analysis.

Antibiotics are used in broiler poultry production both for the prevention and treatment of infectious diseases. However, antibiotic use is a driver of antibiotic resistance. The World Health Organization has published numerous reports urging all stakeholders concerned with both food-producing animals and humans to establish recommended steps to enhance the prudent use of antimicrobials (WHO, 2015). Similarly, the World Animal Health Organization has also published recommendations and position statements regarding prudent use and risk management related to antimicrobial use in animals (OIE, 2017). Colibacillosis is an important bacterial pathogen of poultry, and a costly disease for the industry resulting in multimillion dollar losses annually through morbidity, mortality or carcass condemnation at slaughter. Colibacillosis refers to any localized or systemic infection caused entirely or partly by the organism avian pathogenic Escherichia coli (APEC). These bacteria may be isolated as the sole pathogen or contribute to a disease complex with mixed viral and bacterial infections (Guabirara and Schouler, 2015). Two main disease processes important in the broiler industry are early mortality and cellulitis. Early mortality is defined by chicks under a week of age experiencing a higher than normal percentage of deaths in a flock. Early mortality can be caused by many things, for example chilling, overheating, or dehydration, however E.coli infection, or colibacillosis, is one of the main culprits. Colibacillosis can present with omphalitis, yolk sacculitis, enteritis, pasty vents, pericarditis, perihepatitis, polyserositis, congested lungs, splenomegaly and darkened proventriculus or any these combinations (Guabiraba and Schouler, 2015; Geetha and Palanivel, 2018). Many chicks succumb to an early and severe infection or are culled due to excessive morbidity. Antibiotics are typically used to reduce early mortality (Chauvin et al., 2005; Dziva and Stevens, 2008). Those with severe infection are unlikely to survive, however appropriate treatment reduces transmission between birds and improves the suitability of those with a mild infection. Not every labelled drug for E.coli is efficacious, resistance is common (Kabir, 2010) and effectiveness can vary from flock to flock, even within a flock, with more than one strain and more than one treatment. Understanding the efficacy of antibiotics used to prevent colibacillosis in broiler chickens is essential to optimizing their use; ineffective antibiotics should not be used for prevention or, if there are multiple efficacious antibiotics, their importance to human medicine should be considered when making decisions on antibiotic use. Systematic reviews of randomized controlled trials, and network meta-analysis to provide input on relative antibiotic efficacy, will yield the highest level of evidence for efficacy of treatments under field conditions (Sargeant and O’Connor, 2014)

Levels of Evidence, Quality Assessment, and Risk of Bias: Evaluating the Internal Validity of Primary Research

Clinical decisions in human and veterinary medicine should be based on the best available evidence. The results of primary research are an important component of that evidence base. Regardless of whether assessing studies for clinical case management, developing clinical practice guidelines, or performing systematic reviews, evidence from primary research should be evaluated for internal validity i.e., whether the results are free from bias (reflect the truth). Three broad approaches to evaluating internal validity are available: evaluating the potential for bias in a body of literature based on the study designs employed (levels of evidence), evaluating whether key study design features associated with the potential for bias were employed (quality assessment), and applying a judgement as to whether design elements of a study were likely to result in biased results given the specific context of the study (risk of bias assessment). The level of evidence framework for assessing internal validity assumes that internal validity can be determined based on the study design alone, and thus makes the strongest assumptions. Risk of bias assessments involve an evaluation of the potential for bias in the context of a specific study, and thus involve the least assumptions about internal validity. Quality assessment sits somewhere between the assumptions of these two. Because risk of bias assessment involves the least assumptions, this approach should be used to assess internal validity where possible. However, risk of bias instruments are not available for all study designs, some clinical questions may be addressed using multiple study designs, and some instruments that include an evaluation of internal validity also include additional components (e.g., evaluation of comprehensiveness of reporting, assessments of feasibility or an evaluation of external validity). Therefore, it may be necessary to embed questions related to risk of bias within existing quality assessment instruments. In this article, we overview the approaches to evaluating internal validity, highlight the current complexities, and propose ideas for approaching assessments of internal validity