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 Chapter 5:  Epidemiologic Investigation

 


Topic 1: Epidemiologic Investigations
Topic 2: Outbreak Investigations
Topic 3: Statistical Associations

Objective: To understand the systematic steps involved in outbreak investigations and generic epidemiologic investigations of infections.

Problem Definition: How does one decipher patterns of occurrence of diseases or health conditions in populations?

Epidemiology has two principal uses:

1. as an investigative (or diagnostic) discipline for population (herd) medicine.
2. as support for directed action against diseases i.e. control /eradication of diseases.

Topic 1: EPIDEMIOLOGIC INVESTIGATION:

This is more holistic in nature in contrast to the more reductionistic approaches of clinical diagnosis and pathology i.e. epidemiology is concerned with the total population which includes the well animals as well as the sick and dead ones and the environmental factors that interact and influence the population under study.

In epidemiologic investigations, one is concerned less with the individual patient per se than with the frequency of events of disease and death in a population along with their patterns and probabilities of occurrence. An immediate object of epidemiologic diagnosis is to identify determinants of particular disease or death frequencies, defining a web of causation using these determinants with the ultimate objective of acquiring a means of reducing these frequencies or controlling the disease in theDiagnostic uses of epidemiology have two inter-related avenues for their initial expression:

a) Intensive follow-up - a detailed, multifaceted study of all seemingly relevant aspects of a disease even; in a population unit. One of the commonest forms of intensive follow-up is outbreak investigation which is fairly standardized for some diseases.

b) Surveillance - is the opposite of intensive follow-up. This is a disease accounting process i.e. a broad, overall,organized approach to the collection, collation analysis, expression and dissemination of data about diseases in populations. e.g. of surveillance data sources - veterinary clinics, diagnostic laboratories, morbidity reports from veterinary practitioners, CDC, etc.

Areas where intensive follow-up are needed:

1. Outbreak investigations - here the veterinarian may be concerned with describing the outbreak and its causes, trace back of the primary, i.e. one tries to establish the origin of the first sick animal in the herd or possible linkages between several sick animals in different herds, e.g. a common sales yard, a farm etc. Such endeavors may relate to the protection of the remainder of the herd or other herdsin the area.
2. Pursuing reports of possible introductions into a country of an exotic infection such as with foot and mouth disease virus or of a new vector of disease etc. USDA veterinary services have specially trained teams of this type for intensive follow-up.
3. Problem herd situations - these may come up in the final stages of disease eradication efforts, e.g. in TB and Brucellosis eradication efforts, e.g. non tuberculosis cattle have been shown to react t the tuberculin test as a result of their sensitization to locally occurring species of soil mycobacteria.
4. Control of epidemiologically complex multicausal disease problems on a farm unit basis, e.g. of such diseases, mastitis, shipping fever and similar stress-related disease complexes, neonatal loss problems such neonatal calf mortality, reproductive inefficiencies, multiple parasitisms,many metabolic diseases and many disease syndromes in which opportunistic pathogens or malnutrition may play ill-defined parts.

Both under 3 and 4 above, systematic observations leading to formulation of hypothesis about possible causes,conduct of retrospective or prospective trials for positively identifying specific determinants and their effects upon the frequency of disease are necessary parts of epidemiologic investigations.

Let us examine outbreak investigations closely.

OUTBREAK INVESTIGATIONS

Topic 2: OUTBREAK INVESTIGATION

Assignment: pp. 28-32 Text

The purpose of outbreak investigations is to find out what is going on, in order to be able to take corrective action. These investigations are often carried out in response to dramatic disease events such as an acute infectious disease or intoxication. However, the methods applied in outbreak investigations are basically the same as those used in other forms of intensive follow-up.

The approach then is one of problem solving viz.

1. Define the problem - (identify and define the problem)
2. Implement solutions or corrective actions based on the specific case in question.

An outbreak ( or short term epidemic is a series of events clustered in time and in space. The events usually are new cases of disease occurring at a higher frequency rate than what is normally expected. Outbreak investigation is a systematic procedure to identify causes and sources of epidemics. It should answer the following questions:

What is the problem?
Can something be done about it?
Can further occurrences be prevented?

Outbreak investigation relies on the premise that the cases of a disease are not distributed randomly in a population, but rather occur in certain patterns. The investigator attempts to discover this pattern. Once identified, the pattern will lead the investigator to hypothesize on its determinants (causes). Two major types of patterns can be distinguished: temporal patterns, and spatial patterns.

Temporal patterns. One can represent the temporal pattern of an outbreak in graph form. The frequency of cases on the ordinate (y-axis) is plotted against time on the abscissa (x-axis). Such a graph is termed an epidemic histogram or epidemic curve; its shape may reveal information about the nature of the epidemic, e.g. point epidemic vs. propagated epidemic.

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