The epidemiological investigation

Collecting information on the cause and method of transmission utilizes the three pillars of epidemiology - persons, place and time. Information should be collected from as many angles and from as wide a field as possible. The more pointers there are to a method of transmission, the stronger will be the case.

It will generally not be possible to complete a detailed epidemiological investigation before starting some control methods, for example, if it is diarrhoeal disease, then emergency boiling of drinking water can be started. However, a full investigation must be made and completed, as quite often different factors come to light. A full investigation will help prevent a recurrence.

The method used in an epidemiological search is as follows:

1. Look for a common event that is shared by all the cases.

2. Study exceptions to see if there are rational explanations.

3. Base these findings on the population at risk.

4. Elucidate changes that have occurred in the environment, which may have favoured the outbreak.

5. Make a hypothesis of cause, route of transmission and method of control.

Ideally, information should be collected on every case, but this might be scant or absent in the first few cases. However, it is important to investigate these first cases thoroughly so that the start of the epidemic can be accurately fixed and an epidemic curve drawn (see Chapter 2). If it is a very large epidemic, then it might be preferable to take a sample of cases and study these in detail, but some record of the total number of cases will always be required.

Information on persons should be available, and sex and age classification can readily give an indication as to the cause of the epidemic. If it is just children who are involved, then it is a common disease in which adults have obtained immunity such as measles. If there are more cases in one sex than another, then this might indicate a division of duties such as women (who are the main collectors of water) succumbing to cholera in larger numbers than men.

The address of each case should be plotted on a map and a note made of the most affected areas and whether there is any clustering. Look for associations, such as rivers, breeding places of vectors, forests that might harbour reservoir animals, or any other feature that the nature of the disease indicates to be important. If maps are not available, then constructing a simple sketch map might be necessary, especially if it is a very well-defined epidemic. Typhoid cases often occur in communities, so the houses of individual victims will need to be identified on a sketch map of the village or town. Any clustering or association of cases might lead to the carrier from which the epidemic started. Exceptional cases can often provide definitive evidence of an association such as the visit by a person resident in a different area, which subsequently becomes infected.

All calculations, such as the morbidity and mortality rates, must be done on the population at risk. Normally, this is the population of the entire area, district, region or country, but in a very localized epidemic, the population of the village, town or group of villages might give a better estimate. Population figures are available from census data, malaria control programmes and often collected by the village authorities. Otherwise, a sample needs to be taken of the number of occupants in a random number of houses, followed by counting of all the houses in the area and multiplied by the average house occupancy.

There is normally a reason why an epidemic has occurred at a particular period in time. Diarrhoeal diseases often start at the beginning of the rainy season and influenza is more common during winter months. Religious gatherings or other large collections of people provide ideal conditions for the transmission of disease. If there are strong indicators, then these can be used in future surveillance and to initiate preventive action.

A working hypothesis is established as soon as possible so that emergency control action can be commenced, but a detailed investigation must be completed. Search back within the maximum and minimum incubation period from the first case or cases using other indicators gained from person and place data. Laboratory confirmation of cases might give a different pattern from clinical assessment, especially where several medical staff are involved. If available, samples might need to be taken from a suspect cause, such as a food item, or from the environment, such as a river used for drinking water, which will all take time to be analysed. However, a negative result will not necessarily alter the hypothesis; the specimen may have been collected too late or from the wrong place. It is the strength of association of all the different pieces of evidence that should be used to decide on the cause.

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