4
of new infections is therefore ordinarily balanced by the number of cures and the so-called " endemic index " of the city remains low, although every year numbers of people continue to contract the disease. In 1900, Christophers found the spleen index of Calcutta to be zero. A spleen census is now being taken but is not yet complete. So far it has been found that although in several parts the splenic figure is zero, certain localities show a spleen index as high as 4 per cent. Personal inquiries in such cases generally show that the cases found have been imported from the malarious parts of Bengal and not contracted inside the city. But the suburbs of Calcutta show varying spleen indices. Parts of Tollygunge give figures from 4 to 10 per cent. averaging 5 per cent. Kidderpore 2 per cent. Entally nil, Sibpur 2 per cent. The figures for the central parts of Calcutta are not yet complete. During recent years, medical men in Calcutta have observed that malaria is to some extent prevalent in the city, and that people who have never left Calcutta have contracted a virulent type of malaria. But this malaria, though appreciable, has not been large enough to attract the attention of the public. Many medical men believe that malaria has been increasing during the last few years, and malarial fever is certainly very common in the suburbs at present, although the spleen index is low. The malaria parasite is therefore always present, and we now see that there is a great risk of the anopheline factor being increased. So long as the present conditions are allowed to exist and in these circumstances, Calcutta runs the risk, sooner or later, of being overrun by malaria. The city is developing rapidly, and this development is leading on the one hand to the increase of potential breeding places for A. stephensi in all directions, and on the other to the influx of workers from many places, a proportion of whom are almost certain to be already infected with malaria. If this sort of thing continues we may suddenly find as the result of an epidemic of malaria as they did in Bombay a few years ago, that Calcutta has become malarious almost before any one is aware of the fact. Two points require to be emphasized: firstly, a more complete mosquito survey of Calcutta is essential and, secondly, necessary measures for the control of A. stephensi breeding in Calcutta should be carried out, supported by adequate legal powers.
With regard to A. stephensi, a scheme on the lines of that suggested for Bombay in 1911 by Dr. C. A. Bentley * would appear to be advisable for combating malaria in the city. For the other species a proper system of tank-cleaning, reclamation, oiling, etc., may have to be adopted, and these steps will have to be continued indefinitely and without relaxation. Otherwise, matters will soon regain their original condition. It is probable that something of this kind has happened in Bombay to account for the epidemic of malaria recently reported during September and October of this year (1919).
The breeding places of Stegomyia and the extent of their prevalence in Calcutta, though surveyed, have been omitted from this paper and will be communicated at a later date.
Habits of the mosquitoes.-I have already mentioned the breeding of Anopheles stephensi in cisterns, but it is not possible to say whether it prefers filtered water to unfiltered water; as present observations show, it occurs equally in both, though to a varying extent. In cisterns in which the unfiltered water has remained for some time, it appears to occur more profusely. But this increase is due not so much to the cleanness of the water as to its non-disturbance. Algæ appear in filtered water more rapidly than in unfiltered water, and this is significant as these algæ and the minute organisms that abound and feed upon them that form the favourite food of mosquito larvæ; and this explains the greater tendency shown by cisterns with filtered and settled unfiltered water to harbour anopheles larvæ. In the rural areas of Bengal it has been found that the entrance of silt-laden water into tanks, pools, drains and other water-courses tends to diminish the number of malaria carriers. We do not as yet know the explanation of this phenomenon; and we now find that A. stephensi appears to be an exception. The presence of silt in the water does not prevent its breeding, as it is found to breed readily in the unfiltered-water cisterns.
There also appear to be some variations in the number of larvæ found in the same cistern at different periods, I am not in a position to conjecture
* C. A. Bentley. Ibid.