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are little influenced by rain. It may be remarked also that in 1909-1910 many
of the breeding places in the Dockyard, etc., which might be so influenced were
treated with larvacides.
Distribution of rainfall probably exerts some influence upon malaria in
Bombay. It was observed in July 1910 that during a long break in the
rains a larger proportion of sporozoit-carrying anopheles was met with than
at any other time, whereas during a period of nearly continuous rain the
proportion of sporozoit-bearing anopheles was greatly reduced. It appears
probable that the hot sunny weather combined with high atmospheric humidity
that usually characterises the breaks in the rains is favourable to infected
anopheles, whereas in periods of continuous rain many of the insects perish,
being killed possibly by exposure to rain when attempting to lay their eggs.
219. Temperature and Malaria in Bombay,—(1) Heat. High tem-
perature is favourable to the occurrence of malaria especially when associated
with rain ; and exposure to a high temperature appears to promote both the
onset of symptoms of an infection in man and the occurrences of relapses. In
Bombay relapses of malaria often occur at the hottest time of the year and the
period of new infection coincides with a period of slightly lower but almost
uniform high temperature. High temperature also shortens the larval and pupal
stage of anopheles and is favourable to the increase of mosquitoes in the presence
of suitable condition of humidity, etc. It also quickens the development of
sporozoits in infected mosquitoes.
220. ( 2) Cold. Observation has shown that the parasite of malaria will not
develop in the mosquito at a temperature below 60° Fahrenheit ; but the temper-
ature in Bombay rarely falls as low as this and never remain below 65° Fah.
for more than a few hours, so that as far as temperature is concerned infection
of anopheles can go on all through the year. The onset of the colder weather
in Bombay, as in other malarious countries, is often the cause of sudden small
outbreaks of malaria due to the maturing of infections contracted at an earlier
period ; and the occurrence of relapses is also favoured by cold. Infections which
have remained latent often appear when the diurnal variation of the temperature
increases suddenly, owing possibly to the fact that the resisting power of
infected persons may have been slightly reduced by the change. In Bombay
whenever the variation between the night and clay temperature exceeds 10 or
12 degrees cases of malaria increase in number, a fact which is very apparent
each October.
221. Atmospheric Humidity and Malaria in Bombay.—The malarious
season of Bombay coincides with the period of highest and most uniform
atmospheric humidity, and it is possible that this is due partly to the fact that
excessive humidity is favourable to insect life and partly because it acts as a
depressing influence upon man. When the humidity of the atmosphere rises,
mosquitoes become very active and bite more readily than at other times and
it is probable that this condition conduces to their longevity also. There is the
further possibility that atmospheric humidity may influence the development
of malaria parasites in the body of the mosquito. Infected anopheles appear to
become numerous in Bombay only during periods when atmospheric humidity
is uniformly high and in periods when it is low or when there is a great diurnal
variation, infected anopheles tend to disappear.
Section 2.
222. Conditions affecting malaria by their influence upon man.—The
conditions which increase malaria by influencing man are those which tend to
predispose him to infection or to bring about relapses in persons already infect-
ed. In either case the result in a malaria-infected community will be the same,
namely, an increase in the total number of parasites present. But although the
various factors which may come into play in this connexion are important it
must be pointed out that they have nothing to do with the origin of malaria ;
they can only modify it when it is present either by increasing or reducing
its severity.
223. Social conditions in relation to malaria in Bombay.—It has often
been observed in malarious places that communities of low social status suffer
more severely than those of higher standing and the reason for this is easily
understood, because all parasitic diseases tend to occur more frequently and to