Anopheles

“ Anopheles is often conveyed great distances by the wind, or in railway trains or ships; but of itself it does not fly far; about five or six hundred yards—some authorities place it much lower—is its limit. Beyond this distance they do not voluntarily stray from their breeding-places. Both Anopheles and Culex lay their eggs, as is well known, in standing water, and here three out of the four stages in their life-history—the egg, the larva, and the pupa—are passed through. ”

“ Knowing the cause, we can now guard against malaria; mosquito-nets and wire-protected windows and doors are a sufficient check on the access of Anopheles to man. If the mosquito and man could only be kept permanently apart, we might hope for the disappearance of the parasite from our fauna. In relieving man from this world-wide pest, all genuine lovers of animals will rejoice that we are also relieving the far more serious lesions of one of the most delicate and beautiful insects that we know. ”

“ Some observers maintain that Anopheles does not “sing,” like the common mosquito, and its bite is much less irritating. Only the females suck blood; the act is believed to be necessary for fertilization and reproduction. Anopheles rarely bites by day, and then only in dark places. ”

“ So it is ten or twelve days or sometimes as much as eighteen or twenty days from the time an Anopheles bites a malarial patient before it is dangerous or can spread the disease. On the other hand, the sporozoites may lie in the salivary gland alive and virulent for several weeks. It does not give up all the parasites at one time, so that three or four or more people may be affected by a single mosquito. ”

“ The anopheles is especially a northern insect. The ordinary culex mosquito, when it alights upon a wall, stands with its body parallel to the wall, as a house-fly stands; the anopheles mosquito stands with its tail raised from the wall at an angle. A mosquito lays its eggs in any pool of still water, and the "wrigglers" seen in an open rain-barrel are the larvae from these eggs. The larvae come to the surface of the water to get air, and they may be smothered with petroleum; but the only effective way to get rid of malaria and yellow fever is to drain or fill pools of water and marshes. ”

“ Other mosquitoes may be there in abundance, but if no Anopheles, there is no malaria. In certain regions this is well demonstrated. The west coast of Africa is one of the worst pest-holes of malaria and Anopheles. The east coast has no malaria and no Anopheles. In many islands the same condition exists. On the other hand, the Fiji Islands have Anopheles but no malaria. No malaria has ever been introduced there to infect the mosquitoes. In the same way [Pg 118] Stegomyia occurs in some of the South Sea islands and yet there is no yellow fever there. ”

“ There seems small wonder that the carnivorous feeder and spirituous drinker from a chilly latitude should fall a victim in the East to malarial and other fevers: and this without any assistance from Lady Anopheles or her sister mosquitoes. To her a meed of praise would seem due, for where the mosquito exists there is proof of a need of drainage, clearance, and general sanitary attention. But man, who has stoned the prophets throughout the ages, equally execrates the insects that come as warnings. ”

“ Amongst them were numbers of anophele, the carriers of the malarial fever microbe. Only the female anophele stings, and she has got to be herself previously infected by the fever germ before she can convey infection to the person bitten. Consequently, anopheles inhabiting densely populated regions are far more dangerous than those found in comparatively deserted ones, such as we were now in. In and around the big villages practically every anophele is a germ carrier, and capable of breeding infection, while those breeding out in the bush are comparatively innocuous. ”

“ In the same proportion as the population becomes less malarious, so the mosquitoes become less dangerous, and theoretically a millennium is possible in which man and anopheles, mutually purged of the malarial organism, may live happily together. Unfortunately, a malarious man it is believed remains infectious to anopheles for no less than three years, instead of the three days' limit of yellow fever, and this greatly increases the difficulty of exterminating malaria. ”

“ The destruction of the mosquito, at any rate in neighbourhoods inhabited by man, is a matter of difficulty, but is worth attempting. To expect to destroy the mature insect seems a vain thing, but the larva can be more easily dealt with. Anopheles—unlike the common gnat, which breeds close to houses, in cisterns, garden fountains, old tubs, drains, etc.—prefers rain-water puddles, natural hollows by the roadside, small ponds, and rice-fields. We have occasionally found the larvæ of Anopheles and Culex in the same water in England, but this is probably{153} exceptional. ”

“ These spores, or merozoites, enter new blood cells and repeat the process (auto-infection) . When taken into the stomach of the mosquito Anopheles, the parasites become much larger, some develop male gametes (o, p, q) on the Polymitus form {p) ; others, female gametes (r—u) . These conjugate (v) in the stomach of Anopheles, and the copula (w) penetrates the epithelial lining of the stomach and ultimately lies suspended in the body cavity (x—zz) . When mature, the parasite forms spores (A) , which form, in turn, naked sporozoites (B) . ”

“ 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. ”

“ Grassi has shown that several species belonging to the genus Anopheles, more particularly—at all events as regards Italy—Anopheles maculipennis, are the special mosquito-hosts of the malaria parasites of man. Step by step he traced the crescent-forming and the non-crescent-forming malaria parasites through their mosquito hosts, finding that in their evolution they closely resembled that which Ross had so successfully demonstrated for the avian plasmodium, and had so clearly foreshadowed and partly demonstrated for the malaria parasite of man. ”

“ If one is obliged to be in a region where malaria is common, the disease can be avoided absolutely by protecting [Pg 385] one's self from mosquitoes, and since the anopheles prefer the early morning and evening hours, it is at those times of the day particularly when precautions must be taken. It was once thought that the night air caused malaria, and this had some foundation in fact, because it is in the early evening that the anopheles is on the wing. ”

“ It is owing to this fact that Anopheles is able to convey the germs of malarial fever from person to person. When biting any one who is afflicted with malaria, the insect drawls in with the blood the germs of the disease, which it afterwards carries on into the blood of another victim. The vast majority of mosquitoes never get human blood for food. ”

“ Anopheles minimus, the malaria vector in this area, has a flight pattern of about three feet above ground and, thanks to the stilts, contents itself with feeding on the livestock kept below the homes. When mosquitoes do stray above the usual three feet, the smoke in the house serves as a further deterrent to keep them away from its inhabitants [49] .
The example of the highland Vietnamese scenario demonstrates how culture can act as a buffer to disease transmission, shielding the population from insults that originate in the environment. ”

“ In regard to the differentiation of the species, I must refer you to the special literature, and content myself by observing that about 150 species of Culex and about fifty species of Anopheles have been described, of which fifty about four are found in Europe. [The number of known Anophelines now is more—100 species—of other Culicidæ over 700.—F. V. T.] According to our present knowledge it appears that the entire genus Anopheles can transmit malaria to man; this observation has been confirmed in Anopheles claviger, Fabr.; A. maculipennis, Meig. ”

“ Generally, in any one locality, there are only three or four species of mosquito occurring in any abundance, so that when the inspector has learned to differentiate these, he is pretty well educated for work in that particular locality. Each species usually has some prominent trait; the anopheles, for instance, has the hind legs very much longer than the fore legs, which gives her when at rest the appearance of standing on her head. ”

“ If a yellow fever patient is put into a room in which the mosquitoes have been killed and the doors and windows are screened, he is as harmless, as far as contagion is concerned, as a man with a broken leg. The disease is not spread by fomites.
Malaria is caused by plasmodia, which are protozoa, not bacteria, and it is carried from case to case by mosquitoes of the genus Anopheles. So certain are we that this is the mode of infection that the expression "no anopheles, no malaria" has almost become a medical axiom. ”

“ The evidence shows that they belong to a family of mosquitoes, known by the infamous name of Anopheles, and they have used malaria, yellow fever and several other diseases with which to slay man; that the female is the entertainer of a very small creature, a kind of Jonah, whom she swallows, and after a time he gets down from her stomach into her proboscis, and when she bites her human victim he leaps into the wound where he remains and eats red-blood corpuscles, and when grown he breaks up into from six to ten pieces, each piece making a new animal life like the original ”

“ Anopheles, on the other hand, carries its body at a sharp angle with the surface upon which it rests, and its larva lies flat below the surface-film and parallel with it. The malarial parasite lives in the blood-cells of man, but at a certain period it breaks up into spores, which escape into the fluid of the blood, and it is at this moment that the sufferer feels the access of fever. ”

“ No one ever slept in the tent, and I never found Anopheles bite in the day-time.
The best way of ‘downing’ mosquitos is to prevent the imago hatching, and this, as has been indicated, can be done by killing the larvae and the pupae, which is effected by brushing oil on the water in which they live. The petrol or crude mineral oil should be renewed from time to time as it evaporates. When once the mosquitos are hatched, every effort should be made to keep them outside dwelling-houses by means of wire screens, but if that be impracticable mosquito-nets should be used at nights. ”

“ The wings in Culex have not the same dappled appearance. Anopheles is also a more slender insect, with a smaller head, narrower body and thinner legs. There are further differences in the other stages of life. Mosquitoes go through four phases: (1) ovum, (2) larva, (3) nympha, (4) complete insect. The ova of Anopheles are tiny black rod-shaped objects, which are deposited on the water of natural puddles, ponds, or slowly moving streams, by preference those which are well supplied with vegetation; they float, singly or attached to other objects or clustered together in patterns. ”

“ So that, while a region might be simply swarming with ordinary mosquitoes, it would frequently be found that the only places which fulfilled all the requirements for breeding-homes for the Anopheles, that is, isolation from running water or larger streams, absence of fish, and persistence for at least three months continuously, would not exceed five or six to the square mile. Drain, fill up, or kerosene these puddles,—for they are often little more than that,—and you put a stop to the malarial infection of that particular region. ”

“ Some mosquitoes that breed in salt marshes may be carried for miles, so that a land breeze will bring millions of the pests to seashore cottages which, with a sea breeze, are quite free from them. The anopheles has a habit of clinging to weeds, shrubs, and bushes when the wind blows, so that it is seldom carried more than about two hundred yards from the place where it is hatched. If all pools of water, therefore, within this radius are disposed of, the elimination of malaria will logically follow. ”

“ Fig. 13 Last Segment of Larva Profile. It does not turn its head upside down when feeding as does Anopheles. Its mandibles are strikingly large and powerful and are prominently toothed. It lacks the frontal tufts or brushes which are conspicuously present in Anopheles, and its antennæ, which extend directly forward parallel with the sides of the head, are much longer and more slender, and are tipped each with three hairs of equal size. The thorax is broadly elliptical and is much wider in comparison with its abdominal segments than is the thorax of Anopheles. ”

“ The genus with which Anopheles is most likely to be confounded is Culex, which is the commonest of all mosquitoes, has a world-wide distribution, and is generally a greedy blood-sucker. A distinctive feature is the position assumed in resting; Culex has a humpbacked attitude, while in Anopheles the proboscis, head and body are in a straight line, and in many species inclined at an angle to the wall, the tail sticking outwards. ”