83
places of N. stephensi exist and where adult insects can be caught fairly easily
the index of malarial infection is very high and in Dhobi Talao where breeding
places are also numerous both the spleen index and parasite index show
malaria to be exceedingly prevalent. In B ward the portion most affected
with malaria adjoins the New Dock enclosure where many breeding places of
N. stephensi occur. In South Fort, Colaba and Market Sections breeding
places of N. stephensi are more widely scattered and malaria is present in
lessened amount. In Dongri, Umarkhadi, Khara Talao, Kumbarwada and the
greater portion of. Bhuleshwar, breeding places are not very numerous but it is
possible that they existed there in greater number previously, especially in areas
which have been demolished by the Improvement Trust. In Fanaswadi
Section, Girgaum, a portion of Bhuleshwar, Chowpati and Khetwadi, there are
a number of breeding places but they are rather widely scattered and with few
exceptions appear to harbour comparatively scanty numbers of N. stephensi.
Further to the north with the exception of the neighbourhood of the reservoirs
at Malabar Hill and Bhandarwada, the number of malaria-carrying anopheles,
as judged by the occurrence of their breeding places, diminishes very greatly ;
and in the extreme north N. stephensi is absent and only a few breeding
places of M. listoni and M. culicifacies exist. The number of the latter
mosquitoes too appears to be very limited as their larvæ are always found in
very scanty numbers.
217. Character of Breeding Places of Malaria-carriers in relation to
the prevalence of Malaria in Bombay.—In the Chapter on mosquitoes it was
stated that the commonest breeding places of malaria-carrying mosquitoes were
open wells ; and in particular 65 per cent. of the breeding places favoured be
N. stephensi were of this character. Of these wells, the ones situated inside dwells
ing-houses are usually the most prolific producers of N. stephensi, whereas those
in the open, although frequently infected, often show only scanty numbers of
larvæ. Pools of water do not appear to be greatly favoured by N. stephensi
and other malaria-carrying anopheles in Bombay ; and with the exception of
the surface pools which occur in the New Dock Works and some of the
neighbouring yards, which are prolific breeding places daring the monsoons
larvæ are never very plentiful in them. But tin-pots, tubs and other small
collections of water appear to be very suitable breeding places, for when the
larvæ of N. stephensi has been found in such situations it has generally been
present in considerable numbers. Cisterns vary very greatly as regards their
output of N. stephensi. In the monsoon they often produce considerable
numbers but, when the weather is warm and dry, the water in cisterns whose
sides are exposed to the sun often becomes quite hot and this does not appear
to be favourable to mosquito larvæ.
The existence of favourable breeding places of the malaria-carriers is of
course directly responsible for their numbers, and in this way a more or less
direct relationship can be traced between the character of breeding places and
the prevalence of malaria. As has already been pointed out, the existence of
swamps and large numbers of pools, the lack of drainage, and the continuance
of rice cultivation in the north of the island does not appear to increase
malaria to any appreciable extent, as shown by the low spleen and parasite
indices observed there.
218. Rainfall and Malaria in Bombay.—The malarious season in Bombay
begins shortly after the commencement of the rains and continues for a certain
period after they have ceased. Cases of malaria usually show an increase in July
and infected anopheles are met with in considerable numbers in that month.
It is difficult to trace any relation between the amount of rainfall and the
prevalence of malaria. There is little evidence to show that years of excessive
rain have been specially malarious and in 1907 which had a rainfall of 100
inches, although there was a considerable amount of malaria, there was not
nearly as much as in 1908, which had a short rainfall of only 51 inches. In both
1909 and 1910, although the rainfall exceeded that of 1908, malaria was much
less than in 1908. The explanation of this want of relation between the
amount of rainfall and prevalence of malaria is to be sought in the fact that the
majority of the breeding places of the malaria-carrying anopheles of. Bombay