Concepts of causation
Investigating associations between risk factors (i.e. animal factors, environmental factors, or pathogen factors) and a particular outcome of interest (i.e. the occurrence of respiratory disease in weaned calves) are the core of epidemiologic studies. Although it may seem straightforward to determine the cause of disease or decreased production on a livestock operation, understanding all of the contributing factors isn’t always easy or even possible. Many problems of disease or decreased production on livestock operations are complex, dynamic problems that have many interacting causes. These problems often develop as the result of human decisions in previous weeks, months, or even years. These problems often do not arise overnight (although producers may notice them suddenly), and as a result, cannot often be resolved overnight. This article will discuss concepts related to causation of disease and decreased production on livestock operations.
In epidemiology (i.e. the study of disease in populations), models of causation help describe the role of specific factors (e.g. bacteria, nutritional stress, environment, commingling, etc.) in the occurrence of disease. One of these models is called the component-cause model. This model is based on the idea that each condition (the term condition is used broadly to describe any state of ill health or decreased production in a livestock production system) results from necessary and sufficient causes. Necessary causes are factors without which disease cannot result. For example, bovine anaplasmosis cannot occur without the presence of the bacteria Anaplasma marginale. Sufficient causes are factors that when present, always result in disease. Single sufficient causes are actually very rare, and there are few factors that can be considered sufficient causes by themselves. More often, several component causes combine to create a sufficient cause. Component causes may be factors that are present simultaneously or follow each other sequentially in time.
For an example of component causes of disease, let’s consider bovine respiratory disease (BRD). BRD does not have a single sufficient cause of disease. Rather, several component causes of disease combine to form a number of sufficient causes of disease. These component causes often include the presence of a respiratory virus (e.g. infectious bovine rhinotracheitis, IBR, virus), the presence of a respiratory bacteria (Mannheimia hemolytica), and environmental stressors such as weaning, commingling, transportation, nutritional stress, etc. When these component causes occur together (either simultaneously or sequentially, depending on the disease) a sufficient cause of BRD results. In this example, M. hemolytica serves as a component cause.
When investigating component causes of a particular disease, it must be noted that identifying etiologic agents (e.g. M. hemolytica) may offer less leverage over the problem than other component causes. For example, M. hemolytica can be found in the respiratory tract of normal, healthy cattle. Finding M. hemolytica in an investigation of the cause of an outbreak of BRD may not be as useful as discovering that calves were commingled from multiple sources. Thus, improving biosecurity and biocontainment to prevent commingling would likely do more to prevent future outbreaks than testing calves for M. hemolytica.