Handbook of Medical Entomology Part 22

11. Various writers have maintained that malaria is arrested by canvas curtains, gauze veils and mosquito nets and have recommended the rise of mosquito curtains, "through which malaria can seldom or never pa.s.s." It can hardly be conceived that these intercept marsh-air but they certainly do protect from mosquitoes.

12. Malaria spares no age, but it affects infants much less frequently than adults, because young infants are usually carefully housed and protected from mosquito inoculation.

Correlated with the miasmatic theory was the belief that some animal or vegetable organism which lived in marshes, produced malaria, and frequent searches were made for it. Salisbury (1862) thought this causative organism to be an alga, of the genus _Palmella_; others attributed it to certain fungi or bacteria.

In 1880, the French physician, Laveran, working in Algeria, discovered an amboid organism in the blood of malarial patients and definitely established the parasitic nature of this disease. Pigmented granules had been noted by Meckel as long ago as 1847, in the spleen and blood of a patient who had died of malaria, and his observations had been repeatedly verified, but the granules had been regarded as degeneration products, and the fact that they occurred in the body of a foreign organism had been overlooked.

Soon after the discovery of the parasites in the blood, Gerhardt (1884) succeeded in transferring the disease to healthy individuals by inoculation of malarious blood, and thus proved that it is a true infection. This was verified by numerous experimenters and it was found that inoculation with a very minute quant.i.ty of the diseased blood would not only produce malaria but the particular type of disease.

Laveran traced out the life cycle of the malarial parasite as it occurs in man. The details as we now know them and as they are ill.u.s.trated by the accompanying figure 125, are as follows:

The infecting organism or _sporozoite_, is introduced into the circulation, penetrates a red blood corpuscle, and forms the amboid _schizont_. This lives at the expense of the corpuscle and as it develops there are deposited in its body scattered black or reddish black particles. These are generally called melanin granules, but are much better referred to as haemozoin, as they are not related to melanin. The haemozoin is the most conspicuous part of the parasite, a feature of advantage in diagnosing from unstained preparations.

[Ill.u.s.tration: 125. Life cycle of the malaria parasite. Adapted from Leuckart's chart, by Miss Anna Stryke.]

As the schizont matures, its nucleus breaks up into a number of daughter nuclei, each with a rounded ma.s.s of protoplasm about it, and finally the corpuscles are broken down and these rounded bodies are liberated in the plasma as _merozoites_. These merozoites infect new corpuscles and thus the as.e.xual cycle is continued. The malarial paroxysm is coincident with sporulation.

As early as Laveran's time it was known that under conditions not yet determined there are to be found in the blood of malarious patients another phase of the parasite, differing in form according to the type of the disease. In the pernicious type these appear as large, crescent-shaped organisms which have commonly been called "crescents."

We now know that these are s.e.xual forms.

When the parasite became known there immediately arose speculations as to the way in which it was transferred from man to man. It was thought by some that in nature it occurred as a free-living amba, and that it gained access to man through being taken up with impure water.

However, numerous attempts to infect healthy persons by having them drink or inhale marsh water, or by injecting it into their circulation resulted in failure, and influenced by Leuckart's and Melnikoff's work on _Dipylidium_, that of Fedtschenko on _Dracunculus_, and more especially by that of Manson on _Filaria_, search was made for some insect which might transfer the parasite.

Laveran had early suggested that the role of carrier might be played by the mosquito, but Manson first clearly formulated the hypothesis, and it was largely due to his suggestions that Ross in India, undertook to solve the problem. With no knowledge of the form or of the appearance in this stage, or of the species of mosquito concerned, Ross spent almost two and a half years of the most arduous work in the search and finally in August, 1897, seventeen years after the discovery of the parasite in man, he obtained his first definite clue. In dissecting a "dappled-winged mosquito," "every cell was searched and to my intense disappointment nothing whatever was found, until I came to the insect's stomach. Here, however, just as I was about to abandon the examination, I saw a very delicate circular cell, apparently lying amongst the ordinary cells of the organ and scarcely distinguishable from them. On looking further, another and another similar object presented itself. I now focused the lens carefully on one of these, and found that it contained a few minute granules of some black substance, exactly like the pigment of the parasite of malaria. I counted altogether twelve of these cells in the insect."

Further search showed that "the contents of the mature pigment cells did not consist of clear fluid but of a mult.i.tude of delicate, thread-like bodies which on the rupture of the parent cell, were poured into the body cavity of the insect. They were evidently spores."

With these facts established, confirmation and extension of Ross's results quickly followed, from many different sources. We cannot trace this work in detail but will only point out that much of the credit is due to the Italian workers, Gra.s.si, Bignami, and Bastianelli, and to Koch and Daniels.

It had already been found that when fresh blood was mounted and properly protected against evaporation, a peculiar change occurred in these crescents after about half an hour's time. From certain of them there were pushed out long whip-like processes which moved with a very active, lashing movement. The parasite at this stage is known as the "flagellated body." Others, differing somewhat in details of structure, become rounded but do not give off "flagella."

The American worker, MacCallum (1897), in studying bird malaria as found in crows, first recognized the true nature of these bodies. He regarded them as s.e.xual forms and believed that the so-called flagella played the part of spermatozoa. Thus, the "flagellated body" is in reality a _microgametoblast_, producing _microgametes_, or the male s.e.xual element, while the others const.i.tute the _macrogametes_, or female elements.

It was found that when blood containing these s.e.xual forms was sucked up by an Anopheline mosquito and taken into its stomach, a microgamete penetrated and fertilized a macrogamete in a way a.n.a.logous to what takes place in the fertilization of the egg in higher forms. The resultant, mobile organism is known as the _migratory ookinete_. In this stage the parasite bores through the epithelial lining of the "stomach"

(mid-intestine) of the mosquito and becomes encysted under the muscle layers. Here the _oocyst_, as it is now known, matures and breaks up into the body cavity and finally its products come to lie in the salivary glands of the mosquito. Ten to twelve days are required for these changes, after which the mosquito is infective, capable of introducing the parasite with its saliva, when feeding upon a healthy person.

Thus the malarial parasite is known to have a double cycle, an alternation of generations, of which the as.e.xual stage is undergone in man, the s.e.xual in certain species of mosquitoes. The mosquito is therefore the definitive host rather than the _intermediate_, as usually stated.

The complicated cycle may be made clearer by the diagram of Miss Stryke (1912) which, by means of a double-headed mosquito (fig. 126) endeavors to show how infection takes place through the biting of the human victim, (at A), in whom as.e.xual multiplication then takes place, and how the s.e.xual stages, taken up at B in the diagram, are pa.s.sed in the body of the mosquito.

[Ill.u.s.tration: 126. Life cycle of the malarial parasite. After Miss Anna Stryke.]

The experimental proof that mosquitoes of the Anopheline group are necessary agents in the transmission of malaria was afforded in 1900 when two English physicians, Drs. Sambon and Low lived for the three most malarial months in the midst of the Roman Campagna, a region famous for centuries as a hot-bed of malaria. The two experimenters moved about freely throughout the day, exposed themselves to rains and all kinds of weather, drank marsh water, slept exposed to the marsh air, and, in short, did everything which was supposed to cause malaria, except that they protected themselves thoroughly from mosquito bites, retiring at sunset to a mosquito-proof hut. Though they took no quinine and all of their neighbors suffered from malaria, they were absolutely free from the disease.

To complete the proof, mosquitoes which had fed in Rome on malarious patients were sent to England and allowed to bite two volunteers, one of them Dr. Manson's own son, who had not been otherwise exposed to the disease. Both of these gentlemen contracted typical cases of malaria and the parasites were to be found in abundance in their blood.

[Ill.u.s.tration: 127. Eggs of Anopheles. After Howard.]

Since that time there have been many practical demonstrations of the fact that malaria is transmitted exclusively by the bite of mosquitoes and that the destruction of the mosquitoes means the elimination of the disease.

We have said that the malarial parasite is able to undergo its development only in certain species of mosquitoes belonging to the Anopheline group. It is by no means certain that all of this group even, are capable of acting as the definitive host of the parasites, and much careful experiment work is still needed along this line. In the United States, several species have been found to be implicated, _Anopheles quadrimaculatus_ and _Anopheles crucians_ being the most common. The characteristics of these species and the distinctions between them and other mosquitoes will be discussed in Chapter XII.

In antimalarial work it is desirable to distinguish the anopheline mosquitoes from the culicine species in all stages. The following tabulation presents the more striking distinctions between the groups as represented in the United States.

_Anopheles_ _Culex, Aedes, etc_.

_Eggs_: Laid singly in small Deposited in clumps in the numbers upon the surface of the form of a raft (Culex group) or water. Eggs lie upon their sides deposited singly in the water or and float by means of lateral on the ground in places which expansions (fig. 127). may later be submerged.

_Larva_: When at rest floats in When at rest (with few exceptions) a horizontal position beneath the floats suspended in an surface film. No respiratory oblique or vertical position, or tube but instead a flattened more rarely nearly horizontal, area on the eighth abdominal with the respiratory tube in segment into which the two contact with the surface film spiracles open (fig. 128). (fig. 128).

_Adults_: Palpi in both s.e.xes Palpi short in the female, in nearly or quite as long as the the male usually elongate.

proboscis. Proboscis projecting Proboscis projects forward at an forward nearly on line with the angle with the axis of the body.

axis of the body. When at rest When at rest on a vertical wall on a vertical wall the body is the body is usually held parallel usually held at an angle with the or the tip of the abdomen inclined vertical (fig. 128). Wings frequently towards the wall (fig. 128).

spotted (fig. 130). Wings usually not spotted.

[Ill.u.s.tration: 128. (_a_) Normal position of the larvae of Culex and Anopheles in the water. Culex, left; Anopheles, middle; Culex pupa, right hand figure.]

These malarial-bearing species are essentially domesticated mosquitoes.

They develop in any acc.u.mulation of water which stands for a week or more. Ponds, puddles, rain barrels, horse troughs, cess-pools, cans, even the foot-prints of animals in marshy ground may afford them breeding places.

[Ill.u.s.tration: 128. (_b_) Normal position of Culex and Anopheles on the wall.]

It is clear from what has been said regarding the life cycle of the malarial parasite that the mosquito is harmless if not itself diseased.

Hence malarial-bearing species may abound in the neighborhood where there is no malaria, the disease being absent simply because the mosquitoes are uninfected. Such a locality is potentially malarious and needs only the introduction of a malarial patient who is exposed to the mosquitoes. It is found that such patients may harbor the parasites in their blood long after they are apparently well and thus may serve as a menace, just as do the so-called typhoid carriers. In some malarious regions as high as 80-90 per cent of the natives are such malaria-carriers and must be reckoned with in antimalaria measures.

Based upon our present day knowledge of the life cycle of the malarial parasite the fight against the disease becomes primarily a problem in economic entomology,--it is a question of insect control, in its broadest interpretation.

[Ill.u.s.tration: 129. Larva of Anopheles. After Howard.]

The lines of defence and offence against the disease as outlined by Boyce (1909) are:

1. Measures to avoid the reservoir (man):

Segregation.

Screening of patients.

2. Measures to avoid Anopheles:

Choice of suitable locality, when possible.

Screening of houses and porches.

Sleeping under mosquito nets.

3. Measures to exterminate the Anopheles:

Use of natural enemies.