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Page 43
If a mosquito of the species _anopheles_ bites the affected
person, it obtains a large amount of blood which contains many
parasites. Within the mosquito the parasite undergoes a further
development into male and female sexual forms, which may also form in
the blood, termed respectively _microgametocyte_ and _macrogamete_.
From the microgametocyte small flagellate bodies, the male sexual
elements _microgametes_ or _spermatozoa_, develop and fertilize the
_macrogametes_; after fertilization this develops into a large body,
the _o�cyst_ which is attached to the wall of the stomach of the
mosquito. Within the o�cyst, innumerable small bodies, the
sporozoites, develop, make their way into the salivary glands and are
injected into the individual who becomes the prey of the mosquito,
and again the cycle of development begins. The presence of the
parasite within the mosquito does not constitute a disease. So far as
can be determined, life goes on in the usual way, and its duration in
the insect is not shortened.
The nature of the parasite which produces yellow fever is unknown, for
it belongs to the filterable viruses; the infectious material,
however, has been shown by inoculation to exist in the blood, and the
disease is transmitted by a mosquito of another species, the
_stegomyia_. The development cycle within this takes a period of
twelve days, which time must elapse after the mosquito has bitten
before it can transmit the disease. Here again the mutual
interdependence of knowledge is shown. Nothing could have seemed less
useful than the study of mosquitoes, the differentiation of the
different species, their mode of life, etc., and yet without this
knowledge discoveries so beneficial and of such far-reaching
importance to the whole human race as that of the cause and mode of
transmission of malaria and yellow fever would have been impossible;
for it could easily have been shown that the ordinary _culex_
mosquito played no r�le. The r�le which insects may play in the
transmission of disease was first shown by Theobald Smith in this
country, in the transmission by a tick of the disease of cattle known
as Texas fever. The infecting organism _pyrosoma bigenimum_ is a
tiny pear-shaped parasite of the red corpuscles. Smith's
investigations on the disease, published in 1893, is one of the
classics in medicine, and one of the few examples of an investigation
which has not been changed or added to by further work.
One of the most interesting methods of extension of infection, showing
on what small circumstances infection may depend, is seen in the case
of the hookworm disease, which causes such devastation in the Southern
States. The organism which produces the disease, the _Uncinaria_,
belongs to the more highly developed parasites, and is a small round
worm one-third of an inch long. The worms which inhabit the intestines
have a sharp biting mouth by which they fasten themselves to the
mucous membrane and devour the blood. The most prominent symptom of
the disease is an�mia, or loss of blood, due not only to the direct
eating of the parasite, but to bleeding from the small wounds caused
by its bite. Large numbers of eggs are produced by the parasite which
are passed out with the feces, which becomes the only infectious
material. In a city provided with water-closets and a system of
sewerage there would be no means of extension of infection. The eggs
in the feces in conditions of warmth and moisture develop into small
crawling larv� which can penetrate the skin, producing inflammation of
this, known in the region as the ground itch. The larv� enter the
circulation and are carried to the lungs, where they perforate the
capillaries and reach the inner surface; from this they pass along the
windpipe, and then by way of the gullet and stomach reach their
habitat, the small intestine. Unfortunately, the habits and poverty of
the people in every way facilitate the extension of the infection.
There is no proper disposal of the feces, few of the houses have even
a privy attached to them, and the feces are distributed in the
vicinity of the houses. This leads to contamination of the soil over
wide areas. Most of the inhabitants of the country go barefoot the
greater part of the year, and this gives ready means of contact with
the larv� which crawl over the surface of the ground. The disease is
necessarily associated with poverty and ignorance, the amount of blood
is reduced to a low point, and industry, energy and ambition fall with
the blood reduction; the schools are few and inefficient; the children
are backward, for no child can learn whose brain cells receive but a
small proportion of the necessary oxygen; and a general condition of
apathy and hopelessness prevails in the effected communities. The
control of the disease depends upon the disinfection of the feces, or
at least their disposal in some hygienic method, the wearing of shoes,
and the better education of the people, all of which conditions seem
almost hopeless of attainment. The infection is also extended by means
of the negroes who harbor the parasite, but who have acquired a high
degree of immunity to its effects and whose hygienic habits are even
worse than those of the whites. The organism was probably imported
with the negroes from Africa and is one of the legacies of slavery.
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