(47)
The Indian
malaria-
carrying
species.
of work in any other village. The observer will usually find that some species
are much more abundant in houses than others, and such an observation
is of great importance. This work of catching adult Anophelines would
occupy perhaps two or three hours each morning, and the expert malari-
ologist would spend the remainder of the day in identifying the specimens
caught and in dissecting them to ascertain which of the species is carrying
malaria at the time. But, on account of the time required, the dissection of
mosquitoes with the object of finding sporozoites in the salivary glands and
zygotes in the stomach wall is beyond the scope of practical work which can
be done by medical practitioners in India unless other duties are neglected,
and the methods of dissection and examination will not be described here.
It must suffice to say that by the dissection of large numbers of different
species of Anophelines in malarious places in India it has been proved that
not all species are equally good carriers of malaria in nature, so it is
obvious that the particular species of Anopheline which is present in the
houses of a village is a very important factor in the prevalence of malaria.
Up to the present time only four species, namely, Myzomyia listoni,
Myzomyia culicifacies, Nyssorhynchus fuliginosus and Nyssorhynchus stephensi*
have been proved by dissection to be malaria-carriers in nature in India, and
it has been shown also that one species, namely Pseudomyzomyia rossii, very
probably does not act as a carrier of malaria in nature. For this reason it will
be seen that in making our "malarial survey" great attention must be paid to
the presence or absence in the houses of listoni, culicifacies, fuliginosus or
stephensi and to the character and distance of the breeding places of those
species.
In this way we shall have investigated the two chief factors that influence
the prevalence of malaria in any place, namely, (1) the prevalence of the "source
of infection," which means, practically speaking, the abundance of native
children with malaria parasites in their blood (because it has been proved
that such native children form always the chief source of infection); and (2)
the prevalence of the "carrier of infection," which means the prevalence of
the particular species of Anopheline which carries the zygote and sporozoite
stages of the parasite.
* The proof that N. stephensi is a malaria carrier in nature in India has been fur-
nished very recently by Captain Liston, I.M.S., in Bombay.