The yearly variations in plague in India in relation to climate: Forecasting epidemics

Publisher Website
Google Scholar
Abstract
It has long been known that plague in India, and elsewhere, has a definite seasonal incidence year after year in the same area, although the months of maximum prevalence vary in different provinces. E. H. Hankin as early as 1905 pointed out the disappearance of dog fleas in the hot weather in Agra, and he suggested that a similar decline in the rat fleas might account for the great fall in plague in the United Provinces of India during the hot season. The Extensive and valuable Reports on Plague Investigations in India, published in the Journal of Hygiene from 1906 to 1917, contain numerous data on the subject, and they bring out the unfavourable effects of high temperature and low humidity on the duration of life of rat fleas separated from their hosts, and on their rate of multiplication, and they thus accounted for the seasonal incidence of the disease in different areas of India. Further, the work of Bacot in England and of unnamed workers in Poona on the bionomics of fleas showed that relative humidities below 50° to 55° F. and high temperatures are unfavourable in England, and that in Poona, on the Deccan plateau of Western India, the life of fleas was five times as great in August, with an 8 a. m. relative humidity of 80 per cent., as with one of 45 per cent. in April and early May, when plague, is at a minimum in that area, where the temperatures are moderate and play much less part than humidity in controlling the plague season. Moreover, studies of the seasonal incidence of plague in different parts of India by the same investigators, working under the Advisory Committee nominated by the India Office, the Royal Society and the Lister Institute, showed that a temperature of 85° F. caused a more rapid disappearance of plague bacilli from the stomach of fleas, with reduced power of infecting animals, as compared with a temperature of 70° F., and with the low mean temperature of 50° F. a large number of the infected rats died before plague bacilli appeared in their blood, so that their fleas had no chance of becoming infected. Rat fleas were also more numerous during the height of the plague season, so that climatic conditions clearly acted by reducing the number of infected fleas. The incidence of plague in different areas of India was also reported on by R. St. John Brooks (1) in relation to temperature and saturation deficiency, the latter term indicating "the difference between the actual tension of aqueous vapour present in the atmosphere at the temperature in question and the tension of aqueous vapour that would be present in a saturated atmosphere at the same temperature." It is thus a measure of the drying capacity of the air, and Brooks concluded that a degree of dryness indicated by a saturation deficiency of 0·300 inch (measured in terms of mercury) suffices to prevent plague continuing to be epidemic, if the mean temperature is also over 80° F., and that a high saturation deficiency rapidly brings an epidemic to an end even with a temperature below 80° F., but an epidemic may commence and increase with a temperature over 80° F.., if the saturation deficiency is below 0·300. Moreover, in certain places, including Java and Mauritius, with constantly favourable temperature and saturation deficiency, the disease may be prevalent indifferently at any season of the year.

This publication has 1 reference indexed in Scilit: