An observational study of the temporal and spatial patterns of Marek's-disease-associated leukosis condemnation of young chickens in the United States of America

Abstract

Marek’s disease, a disease primarily affecting immature chickens, is a worldwide problem that has on at least three occasions threatened the poultry industry in the United States. A rich dataset to study the epidemiology of this disease is available because the United States Department of Agriculture has required mandatory inspections of all commercially sold poultry of significant scale since the mid-20th century with over 99% of all chickens inspected. This dataset includes monthly totals aggregated by state since 1961 of the number of “young chickens” inspected and the number with “leukosis”, a condemnation category that is almost always associated with Marek’s disease in this category of birds. The objective of this study was to analyze temporal and spatial patterns in this condemnation data to gain insight into the ecology and epidemiology of the causative virus. We extracted visual patterns in the data using seasonal trend decomposition, and we tested for statistical significance using extended linear modeling techniques. The analysis confirmed previous findings that there are differences in leukosis condemnation rates between states, across years, and within years. The analysis also revealed several patterns not previously highlighted, including spatial and temporal autocorrelations in leukosis condemnation, changes to the amplitude of seasonality over time, and increasing within-year variation in condemnation rate over time. These patterns suggest that locally shared farm practices, virus transmission between farms, or viral persistence may be important to understanding the dynamics of the disease. We also discuss the plausibility of other potential explanations for these patterns.

Modeling Marek's disease virus transmission: A framework for evaluating the impact of farming practices and evolution

“…4 and 10). The historical decline in MDV-associated condemnation is strongly correlated with declines in rearing durations, making it difficult to tease apart the relative impact that rearing duration and other changes in biosecurity and vaccine administration have had on disease control (Kennedy et al, 2015b). Nevertheless, our model shows that rearing duration is one of several factor that can potentially be manipulated to control problems with disease should they reappear.…”

“…Yet the ecology of MDV is poorly understood. This is perhaps most clearly exemplified by the conventional wisdom that the virus is ubiquitously found on industrialized poultry farms (Office International des-Epizooties, 2010; Dunn, 2013), despite recent surveillance data suggesting that the virus may not be present on a large fraction of farms (Groves et al, 2008; Wajid et al, 2013; Walkden-Brown et al, 2013; Bettridge et al, 2014; Kennedy et al, 2015b, 2017; Ralapanawe et al, 2015). …”

“…This condition can be caused by various diseases, but in chickens reared for meat, leukosis is almost exclusively caused by MD (Sharma, 1985). Current rates of condemnation due to leukosis are extremely low (Kennedy et al, 2015b), but future reductions in MD vaccine efficacy due to virus evolution might cause leukosis rates to increase, as was documented with the erosion of vaccine efficacy in the past (Witter, 1996). A method to relate changes in farming practices to changes in risk of condemnation would therefore be a useful tool should virus evolution continue along the trajectory of the past.…”

Industry-wide surveillance of Marek’s disease virus on commercial poultry farms

“…Collection date was fit as two continuous factors, one the sine and one the cosine of 2π/365 times the calendar day that a sample was collected, to capture seasonal variation (28). Bird age was fit as a categorical factor using a spline with knots at cohort ages of 21, 42, 100, and 315 days (20,21).…”

“…Clinical disease and production losses coupled with these observations motivated near-universal vaccination in commercial poultry farming in the United States and many other nations. More recently, virus prevalence has been inferred from condemnation data (16,28,35,45), but the reliability of these methods are limited by changes in condemnation rates that may occur irrespective of virus dynamics (28).…”