Endemicity

An endemic disease implies that there is a constant rate of infection occurring in the community. As new individuals are born, they become infected, are cured (including self-cure), retain the infection for life or become immune. Prevalence rates will measure the level of endemicity as it applies to the community. Incidence rates will measure change in the level of infection over a period of time.

While it is useful to compare prevalence from one community to another, on more

Table 2.1. Food-specific attack rates and the relative risks of eating different foods (meal eaten by 152 persons).

Ate Did not eat

Food item Sick Well Attack rate (%) Sick Well Attack rate (%) Relative risk

Rice

115

28

80.4

45

4

55.5

1.4

Potatoes

111

31

78.2

9

1

90.0

0.9

Fish

93

22

80.9

17

30

18.9

4.3

Beans

101

29

77.7

16

6

72.7

1.1

Coconut

86

22

79.6

24

20

54.5

1.5

Bananas

109

32

77.3

10

1

90.9

0.8

careful investigation, it will be found that within a community, prevalence rates can also vary. These areas of increased prevalence within a community are called foci. Two types of foci occur:

• host focality, where some individuals have more severe infection than others, e.g. worm load in schistosomiasis;

• geographical focality, where certain localities have a higher prevalence rate than others. Malaria exhibits geographical focality.

These concepts are important in control strategy. When a control method is applied equally to a community, then the overall decrease in disease will leave the foci to maintain infection. However, if the foci are identified and treated, then the infectious source is contained (Fig. 2.8).

Incidence rates show change in the endemicity either upwards, downwards or remaining the same. A decreasing incidence will indicate that the disease may be dying out, especially if control measures have been used. Incidence rates often show a seasonal pattern (Fig. 1.7) and threshold levels that take into account this seasonal variation can be set to give early warning of the disease becoming epidemic.

The Focality Endemic Disease
Fig. 2.8. The focality of endemic disease. (a) A universally homogenous prevalence rate is measured in an area. (b) Once control measures have been implemented, foci of persistent transmission are revealed.
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