malarial patients is derived from red blood corpuscles broken
down by the malarial parasites; many of these, in turn, are
devoured by leucocytes, which in malarial blood are usually
greatly increased in number, and frequently contain much pigment,
which they have obtained either directly from the fluid
plasma or from the pigmented parasitic organism. The work
recently carried out by Bruce on the tsetse-fly parasite, by A. J.
Smith on Texas fever, and by W. S. Thayer and Hewitson on the
blood parasites of birds, has opened up the way for the further
study of the malarial parasites outside the human body. There
can be no doubt as to the close relation of the multiplication and
sporulation of the malarial parasite with the ague paroxysm:
the anaemia results from the breaking down of blood corpuscles.
Toxic substances are present in the blood during the setting
free of the spores; of this we have proof in the increased toxicity
of the urine during the paroxysmal stages of the disease; moreover
necrotic areas, similar to those found in acute toxic fevers
produced by other micro-organisms, are met with. It is well to
bear in mind that the accumulation of debris of parasites and
corpuscles in the capillaries may be an additional factor in this
necrosis, especially when to this is added the impairment of
nutrition necessarily involved by the impoverished condition of
the malarial blood. It is interesting to note that, although,
as pointed out by Nuttall, the Italian and Tirolese peasantry
have long been firmly of the opinion that malaria is transmitted
through the mosquito, and although the American, Dr Josiah
Nott, in 1848 referred to malaria as if the mosquito theory had
already been advanced, little attention was given to this
question by most observers. Still earlier, Rasori (in 1846) had
stated that “for many years I have held the opinion that intermittent
fevers are produced by parasites, which renew the
paroxysm by the act of their reproduction, which recurs more or
less rapidly according to the variety of the species”; and this
appears to be the first well-authenticated reference to this subject.
Nuttall, who gives an excellent summary of the literature on the
mosquito hypothesis of malaria, assigns to King the honour of
again drawing attention to this question. Laveran in 1891,
Koch in 1892, Manson in 1894, Bignami and Mendini in 1896,
and Grassi in 1898, all turned their attention to this hypothesis.
Manson, basing his hypothesis upon what he had observed as
regards the transmission of Filaria by the mosquito, suggested a
series of experiments to Major Ronald Ross. These were carried
out in 1895, when it was found that in mosquitoes that had taken
up blood containing amoeboid parasites, crescents, which were
first described as cells, appeared in the stomach-wall after four
or five days; these contained a number of stationary vacuoles
and pigment granules, ten to twenty in number, bunched together
or distributed in lines. Grassi, Bignami and Bastianelli
confirm and supplement Ross’s observations; they find that
Anopheles claviger, taking the blood from a patient suffering
from malaria, soon develops haemosporidia in the intestine.
These parasites are then found between the muscular fibres of
the stomach; they increase in size, become pigmented, and more
and more vacuolated, until they project into the body-cavity.
On the sixth day these large spheres contain an enormous
number of minute bodies, refractive droplets like fat, and a
diminishing amount of pigment. On the seventh day numerous
filaments, arranged in rows around several foci, are seen. They
are very delicate, are stained with difficulty, and appear to be
perfectly independent of each other, though grouped within a
capsule. After the capsule has ruptured, these thread-like
“sporozooites,” escaping into the body-cavity, gradually make
their way to and accumulate in the cells or tubules of the salivary
glands, whence their passage through the proboscis into the
human blood is easily understood.
Thus two phases or cycles of existence have been demonstrated—one within the human body, the second in the mosquito. That within the human body appears to be capable of going on almost indefinitely as long as the patient lives, but that in the mosquito appears to be an offshoot or an intermediate stage. The minute Development of the Malarial Parasite. specks of protoplasm, the amoebulae, which have already been described as occurring in the red blood corpuscles of the higher animals, increase in size, take up blood pigment, probably from the red corpuscles, and then become developed into sporocytes or gametocytes. The sporocyte is the form which, remaining in the body, ultimately breaks up, as already seen, into a series of minute spores or amoebulae, which in turn go through the same cycle again, increasing in size and forming spores, and so on indefinitely. Gametocytes (the true sexual form) are in certain species, to outward appearance, very similar to the sporocyte, but in others they assume the crescentic shape, and can thus be recognized. The male cell resembles the female cell very closely, except that the protoplasm is hyaline and homogeneous-looking, whilst that of the female cell is granular. It has already been noted that when the blood is withdrawn from the body certain of the malarial parasites become flagellated. These flagella may be looked upon as sperm elements, which, forming in the male gametocyte, are extruded from that cell, and, once set free, seek out the granular female gametocytes. A single flagellum becomes attached to a small projection that appears on the female cell; it then makes its way into the protoplasm of the female cell, in which rapid streaming movements are then developed. In certain species the female cell is somewhat elongated, and may be peculiarly constricted. It becomes motile, and appears to have the power of piercing the tissues. In this way the first stages of development in the mosquito are passed. The gametocytes, taken along with the blood into the stomach of this insect, pass through the various phases above mentioned, though the zygote form of the human malarial parasite has not yet been traced. In the blood of a patient bitten by an infected mosquito the ordinary malarial parasite may be demonstrated without any difficulty at the end of a week or ten days, and the cycle recommences.
This theory, now no longer a hypothesis, in which the mosquito acts as an intermediary host for one stage of the parasite and transmits the parasite to man, affords an explanation of many apparently anomalous conditions associated with the transmission of malaria, whilst it harmonizes with many facts which, though frequently observed, were very difficult of explanation. Malaria was supposed to be associated with watery exhalations and with the fall of dew, but a wall or a row of trees was seemingly quite sufficient to prevent the passage of infection. It was met with on wet soils, on broken ground, in marshes, swamps and jungles; on the other hand, it was supposed to be due to the poisonous exhalations from rocks. All this is now explained by the fact that these are the positions in which mosquitoes occur: wherever there are stagnant pools, even of a temporary nature, mosquitoes may breed. It has been observed that although the malarial “miasma” never produces any ill effects in patients living at more than a few feet from the surface of the ground, malaria may be found at a height of from 7000 to 9000 ft. above sea-level; and the fact that a belt of trees or a wall will stop the passage of the poison is readily explicable on the mosquito theory. These insects are incapable, owing to their limited power of flight, of rising more than a few feet from the ground, and cannot make their way through a belt of trees of even moderate thickness. Broken ground, such as is found in connexion with railway cuttings and canals, may be a focus from which malaria may spread. In such broken ground pools are of common occurrence, and afford the conditions for the development of the mosquito, and infected tools used in one area may easily convey the ova to another. All these facts afford further support of this theory. The conditions of climate under which malaria is most rife are those which are most suitable for the development of the mosquito. The protection afforded by fires, the recognized value of mosquito curtains, the simultaneous disappearance of Anopheles and malaria on the complete draining of a neighbourhood, the coincidence of malaria and mosquitoes, and the protection afforded by large expanses of water near walls and trees are also important in this connexion.
The mosquitoes specially associated with the transmission of malaria in the human subject belong apparently to the genus
