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Page 42
The _sleeping sickness_ has many features of interest. In the old
slavery days it was found that the negroes from the Congo region in
the course of the voyage or after they were landed sometimes were
affected with a peculiar disease. They were lethargic, took little
notice of their surroundings, slept easily and finally passed into a
condition of somnolence in which they took no food and gradually died.
There was no extension of the disease and it was attributed to extreme
homesickness and depression. A similar disease has been known for more
than one hundred years on the west coast of Africa, and attracted a
good deal of interest and curiosity on account of the peculiar
lethargy which it produced and from which it has received the name of
"sleeping sickness." Although apparently infectious in its native
haunts, it lost the power of spreading from man upon removal to
regions where it did not prevail. At first confined to a very small
region on the Niger river, it gradually extended with the development
of trade routes and the general increase of communications which trade
brings, until it prevails in the entire Congo basin, in the British
and German possessions in East Africa, and is extending north and
south of these regions. The cause of the disease and its mode of
conveyance was discovered in 1903. The fly _Glossina palpalis_
which conveys the disease is a biting fly about the size of the common
house fly and lives chiefly in the vicinity of water. When such a fly
bites an individual who has sleeping sickness its bite can convey the
disease to monkeys, on whom the transmission experiments were made.
After biting the fly is infectious for a period of two days. After
this it is harmless, unless it again obtains a supply of living
trypanosomes. There is quite a period in which there are no symptoms
of the disease, although trypanosomes are found in the blood and in
the lymph nodes, and the individual is a source of infection. The
peculiar lethargy which has given the disease its name does not appear
until the nervous system is invaded by the parasites. It is impossible
to compute accurately the numbers of deaths from this disease--in the
region of Victoria Nyanza alone the estimates extend to hundreds of
thousands.
3. In the third mode of insect conveyance the insect does not play a
merely passive r�le, but becomes a part of the disease, itself
undergoing infection, and a period in the life cycle of the organism
takes place within it. In all these cases quite a period of time must
elapse before the insect is capable of transmitting the disease; in
malaria, which is the best type of such a disease, this period is ten
days. Malaria is due to a small protozoan, the _Plasmodium
malari�_, which was discovered by Lavaran, a French investigator,
in 1882. The organism lives within or on the surface of the red blood
corpuscles. It first appears as a very minute colorless body with
active amoeboid movements, and increases in size, attacks a succession
of corpuscles, and finally attains a size as large as or larger than a
corpuscle. The corpuscles attacked become pale by the destruction of
h�moglobin, swell up and disintegrate, the h�moglobin becoming
converted into granules of black pigment inside the parasite. Having
attained a definite size the organism forms a rosette and divides into
a number of forms similar to the smallest seen inside the corpuscles;
these small forms enter other corpuscles and the cycle again begins.
This cycle of development takes place in forty-eight hours, and
segmentation is always accompanied by a paroxysm of the disease shown
in a chill followed by fever and sweating which is due to the effect
of substances liberated by the organism at the time of segmentation. A
patient may have two crops of the parasite developing independently in
the blood, and the two periods of segmentation give a paroxysm for
each, so that the paroxysms may appear at intervals of twenty-four
hours instead of forty-eight (Fig. 20). This cycle of development may
continue for an indefinite time, and there may be such a rapid
increase in the parasites as to bring about the death of the
individual; but with him the parasite would also perish, for there
would be no way of extending the infection and providing a new crop.
The disease has been transmitted by injecting the infected blood into
a normal individual.
[Illustration: FIG. 20.--PART OF THE CYCLE OF DEVELOPMENT OF THE
ORGANISM OF MALARIA, _a-g_, Cycle of forty-eight hour development, the
period of chill coinciding with the appearance of _f_ and _g_ in the
blood. The organisms _g_, which result from segmentation, attack other
corpuscles and a new cycle begins. _h_, The male form or
microgametocyte, with the protruding and actively moving spermatozoa,
one of which is shown free. _i_ and _j_ are the macrogametes or female
forms. _k_ shows one of these in the act of being fertilized by the
entering spermatozo�n. The differentiation into male and female forms
takes place in the blood, the further development of the sexual cycle
within the mosquito.]
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