The Conditions Influencing the Incidence and Spread of Cholera in India

Abstract

This is an interesting essay written with the author's usual care. The numerous charts and tables are of considerable value. Conclusions.-" (1) In the 1817-19 and subsequent cholera epidemics in India, up to those from 1859 to 1871 described by Bryden and Cornish, the disease appears to have spread from its home in Lower Bengal over North-Western, Central and Southern India in a series of waves of two to four years' duration at somewhat irregular intervals, the endemic area, according to Bryden, being limited to Bengal and West Assam. " (2) Since 1877 the monthly cholera mortality for every district in India has been recorded, furnishing far more detailed information than the army and gaol figures of Bryden's time; but they have not hitherto been utilized for a comprehensive study of the incidence and spread of cholera in India, such as is attempted in this paper. " (3) A study of the average monthly cholera incidence and rainfall, temperature and humidity in a diagram and four maps of the seasonal incidence in forty-five divisions of India, shows: (a) No uniform relationship with rainfall, as the disease during the south-west monsoon is at its maximum in most parts of India, but at its minimum in Lower Bengal; but it shows. (b) a regular great decline or disappearance of the disease in all parts of India when the absolute humidity falls to or below 0.400, such great dryness of the atmosphere preventing the epidemic prevalence of the disease. The months in which cholera first shows a great increase after the winter quiescent period in North-West and Central India are those in which the absolute humidity first rises to over 0.400, the seasonal increase in most parts of India being thus explained quite irrespectively of any spread from Bengal. " (4) The average annual incidence of cholera is highest in Assam, Lower Bengal, Bihar and the eastern sub-Himalayan divisions of the United Provinces of Northern India, and in South-east Madras; all areas with few or no months in absolute humidity below 0.400, and consequent continued prevalence of the disease throughout the year. " (5) The present endemic areas, as shown by the disease never having been absent for a single year in three recent decades, include the areas of high incidence, just mentioned, of Bengal, the United Provinces and Madras, together with the low-lying west coast of Bombay, with the constant absolute humidity of over 0.400, so the endemic areas are now far more extensive and scattered than the parts of Assam and Bengal indicated by Bryden as late as 1869. " (6) The epidemic areas, in which severe outbreaks occur frequently after a year or two of complete absence of the disease, include the south and west of the United Provinces, all the Punjab, the Sind, Gujerat and Deccan divisions of Bombay, and the whole of the Central Provinces. The spread of epidemics in these areas has been studied by means of curves of the annual incidence in forty-five divisions for forty-five years, and in as many yearly maps of the distribution and months of the first annual increase, maximum prevalence and decline for each year. " (7) A map showing the number of epidemic rises in each area, and those in which a rise occurred in the same years in contiguous areas, shows a larger number of epidemics in the United Provinces than in Lower Bengal, so they could not all have originated from Bryden's Bengal endemic area. The studies of the forty-five years' maps clearly show that a number of the increases originated in the endemic area of the United Provinces above described, from which they spread over the Punjab with decreasing intensity in proportion to the distance of the divisions from the United Provinces, and the dryness of their climate. Similarly, it is shown that the Central Provinces in recent decades were sometimes invaded from the east, from the southern Orissa divisions of Bengal, occasionally from the north, from the United Provinces, and frequently also from the west from the Deccan divisions, contrary to Bryden's conclusion that cholera always spreads from Bengal to the north-west over the United Provinces, or to the south-west over the Central Provinces to Bombay, with the monsoon winds. The facts on which he based his theory are now explained by the effect of low absolute humidity in inhibiting epidemics described in this paper. " (8) The three most severe epidemics of modern times are considered in the light of the foregoing data, and in connexion with the meteorological conditions associated with them. The diffusion of the 1875-77 epidemic is mapped out and shown to have spread, largely through pilgrims, from separate endemic foci, north-west from Bengal and the United Provinces, east and south from Bombay, and north from Ceylon or Southern Madras. In each area of very high incidence the epidemic was associated with great deficiency of the previous rainfall, accompanied by drought, by bad water supplies, and often by famine. The epidemic of 1891-92 was spread mainly in the first year by a rarely occurring great Ganges pilgrimage, aided by deficient rains, and in the second year by continued deficient rainfall, and especially by the Hardwar pilgrims. The most severe epidemic of all, in 1900, was once more associated with very exceptional failure of both the monsoon and the succeeding winter rains over very large areas of India. A table is also given of the epidemic prevalence in every other of the forty-five years in which the total cholera mortality in India was much over the average. Out of twenty-five affected areas, in no less than twenty-four previous greater or less deficiency of the rains preceded the cholera exacerbations, and in the remaining area in the United Provinces in 1894 very exceptionally high humidity throughout the winter months was followed by a unique early recrudescence of the disease culminating in an epidemic. " (9) By watching the climatic conditions influencing the seasonal and annual incidence of cholera in any area to which attention is now drawn, increased or epidemic prevalence should usually be foreseen in time to enable steps to be taken to lessen its spread by pilgrims and other travellers, by means of inoculating them against the disease before attending religious and other gatherings in cholera-infected districts. The Punjab, Sind, Gujerat and Deccan divisions of. Bombay and the Central Provinces, so liable to invasion by epidemics, have largely in their own hands this simple means of lessening their cholera mortality. The sanitation, and especially the provision of a pure water supply, in all important pilgrim centres, should be a first charge on imperial and provincial revenues under reliable sanitary administration." J. H. Tull Walsh.