Using epidemiologic principles to conduct a disease investigation (Proceedings)
When planning a disease outbreak investigation, it is very helpful to know beforehand the major risk factors associated with the disease to be investigated. This knowledge can be gained through, for example, reading the scientific literature, conversations with experts, and/or personal experience. A pathway model can be helpful to visualize the interrelationship between risk factors and their effects on each other.
A suggested set of disease investigation steps are presented below. Each is recommended to be conducted in the order given.
Confirm the diagnosisPhysical examination and laboratory findings are critical pieces of data to use to decide if the disease in question is likely to be present.
Develop a case definition
Herd and management data should be collected (e.g. questionnaire) and reviewed to decide what constitutes a case of disease "X". A case could be established based on clinical information, some operational (e.g. treatment response) or statistical (e.g.'normal' within 2 std. dev. of age-specific mean) parameter, or based on some other piece of information.
Find new cases
Surveillance should be changed from a passive nature to one of active surveillance.
Plot epidemic curve
The importance of plotting an epidemic curve is to prove that an epidemic exists, i.e. starting with the index case, determine that a disease is occurring at a level which exceeds its usual frequency (endemic level) over some period of time. Typically, the number of cases of disease is plotted on the Y axis and time of disease occurrence is plotted on the X axis.
The shape of the curve and the time scale depend on the incubation period of the disease, the infectivity of the agent, the proportion of susceptible animals in the population, and the distance between animals (animal density).
Several types of epidemic curves can occur. A common source (point-source) epidemic is one in which all cases are infected from a source that is common to all individuals (e.g. toxin). Typically, all cases of a point-source epidemic occur within one incubation period of the causal agent.
In contrast, a propagating epidemic is one that is caused by an infectious agent in which initial cases (primary or index case) shed the agent and subsequently infect susceptible individuals (secondary cases). The time interval between peaks of successive temporal clusters of cases reflects the incubation period of the causal agent. If the period between subsequent peaks of the epidemic curve is less than the most common incubation period then it is difficult to differentiate between a propagating epidemic and a series of point-source epidemics.
Review cases and non-cases
It is important to view cases and non-cases in a temporal (time) and spatial (place) context.
Formulate a hypothesis about the epidemic
The aforementioned steps should help establish a hypothesis as to the likely disease agent responsible for the disease outbreak in questions. A farm event chart and temporal chart can aid in identifying its mode of transmission, source of infective material, and reservoir host.
A farm events chart is a written record of farm-wide events listed by their date of occurrence, e.g. entry of animals onto premises, feed changes, whole herd vaccination, previous episodes of illness, and any herd-wide management changes. If at all possible, go back at least six months prior to start of the problem. These events should be compared to risk factors from your pathway model.
A temporal chart lists the farm events in chronological order with respect to time. A graph of these events allows juxtaposition of their occurrence with the pattern of disease occurrence (epidemic curve).
Institute temporary control measures
The control measures that are instituted should be based on your working hypothesis for the causative agent (and likely causative risk factors) suspected to be present in the diseased animals. Any or all of these control measures may change as more information is gathered.