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DISTRIBUTION OF DISEASE

Geographic Distribution

Cholera has swept the world in seven great pandemic waves. From the early 1900s to 1961, cholera was virtually restricted to its endemic focus in the Indo-Pakistani subcontinent. The present pandemic, caused by V. cholerae of the El Tor biotype, began in 1958 in the Celebes Islands, Indonesia. It subsequently swept through and became endemic in the Philippines, Southeast Asia, and Africa. Outbreaks that have been more or less self-limited have occurred in the Soviet Union, Japan, Italy, Spain, northern Europe, and North America (including the United States). Cholera vibrios of the classical biotype appear to be reemerging in India and Bangladesh.

Disease Burden Estimates

The burden of disease caused by V. cholerae has been calculated only for areas in Africa and Asia where cholera is endemic. Table D-18.1 shows the estimated number of cases of the disease in Asia; Table D-18.2 presents the same information for Africa. The combined endemic disease burden for the two continents is shown in Table D-18.3.

No attempt has been made to estimate the disease burden produced by cholera epidemics or pandemics because it is very difficult to predict where and to what extent they will occur. This inability to identify a vaccine target population prevents calculation of potential health benefits that could be obtained from a vaccine. However, any vaccine developed to prevent endemic cholera could play a major role in curtailing epidemic cholera.

Traditionally, it has been assumed that cholera does not have any long-term sequelae. A recent epidemiological study in India suggests, however, that a strong association may exist between cataract development and episodes of cholera and other severe diarrheal diseases (Minassian et al., 1984). Because of the preliminary nature of this evidence, the committee chose not to include visual disability in the current disease burden estimates for cholera. Further research on this topic is warranted. Also omitted from disease burden calculations are possible adverse effects of cholera during pregnancy.

PROBABLE VACCINE TARGET POPULATION

In endemic areas, cholera occurs with greatest frequency in children between 2 and 15 years of age and in adult females. Children less than 2 years old have a relatively low incidence of the disease, particularly when breast-fed. Thus, vaccination in infancy would be appropriate, and a suitable vaccine could be incorporated into the World Health Organization Expanded Program on immunization (WHO-EPI).

In areas in which the introduction of cholera is recent (neoepidemic areas), the rates of disease are more uniform across the



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