FAQ: Epidemiology

The SIR-model is a widely used mathematical model to study infectious disease epidemiology. The model is also known as the Kermack-McKendrick model. S denotes the subpopulation of susceptible individuals, I the subpopulation of infectious individuals, and R the subpopulation of recovered (or removed) individuals. Individuals flow from te susceptible state S into state I with a rate proportional to the size of subpopulation I. Individuals flow from subpopulation I to subpopulation R with a constant rate. The SI, SIS and SEIR are closely related models, in which infectious individuals donot recover, and stay infectious (SI), or flow back to the subpopulation of susceptibles (SIS), or an extra state E is added in which individuals are infected but not infectious, i.e. latent infection (SEIR) [Anderson & May, 1992; Diekmann & Heesterbeek, 2000].

In epidemiology, the basic reproduction number R0 is defined as the expected number of secondary cases per primary case in a totally susceptible population [Diekmann & Heesterbeek, 2000]

Data on infectious diseases are usually collected using monitoring and surveillance systems, MOSS (see below). Sampling strategies depend on the goal of MOSS, e.g. random or targeted, passive or active.

Epidemy is the outbreak and spread of an (exotic) infectious disease in a formerly disease-free population. Endemy means the continued presence of an infectious disease in a population. Pandemy means an epidemic going worldwide, e.g. the Spanish Flu pandemy of 1918.

There are several definitions concerning the term surveillance, one is: “The ongoing systematic collection, collation and interpretation of accurate information about a defined animal population with respect to disease, and/or infection, closely integrated with timely dissemination of that information to those responsible for control and prevention measures” (Noordhuizen et al. 1997).

The main objectives of monitoring systems are to describe disease trends over time, thus providing veterinary authorities with the necessary data on the current disease status of a given animal population.

Since the terms surveillance and monitoring are often intermingled, the abbreviation MOSS (monitoring and surveillance system) is often used to describe the activities explained under the questions “surveillance” and “monitoring”.

Once the index case of an outbreak has been detected, it is very important to trace the infectious agent to its source from where it moved into the susceptible animal population (tracing backward) and at the same time it has to traced, where the agent has moved to after introduction into the susceptible population (tracing forward). Both activities are indispensible for the effective and rapid control of a disease outbreak.

In case of a disease outbreak epidemiological investigations are of utmost importance in order to detect route(s) of introduction and further spread of the infectious agent. The more information surrounding the outbreak is available the better the control of the epidemic can be controlled.