Popular Science Monthly/Volume 23/September 1883/Insects and Disease - Mosquitoes and Malaria
|INSECTS AND DISEASE—MOSQUITOES AND MALARIA.|
By Professor A. F. A. KING, M.D.
THE animalcular, or insect, origin of disease is not a new idea. It was suggested by Linnæus, by Kircher, and by Nyander, but gained little ground. It received a new impetus after the publications of Ehrenberg on the Infusoria. Later, it received attention in Bradley's work on "The Plague of Marseilles," in Dr. Drake's books on "Epidemic Cholera" and on "The Topography and Diseases of the Mississippi Valley," as well as in Sir Henry Holland's "Medical Notes," and other works.
More recently the researches of Dr. Patrick Manson in China, Dr. Bancroft in Australia, Dr. J. R. Lewis in India, and Dr. Sonsino in Egypt, have tended to show that the mosquito "acts as the intermediary host of Filaria sanguinis hominis" and is thus indirectly instrumental in the production of chyluria, elephantiasis, lymph-scrotum, etc. (London "Medical Times and Gazette," January 12, 1878, p. 69; September 7, 1878, p. 275; December 28, 1878, p. 731; and June 4, 1881, p. 615).
Still later, M. le Dr. Ch. Finlay has hypothetically considered the mosquito an agent of transmission of yellow fever ("El mosquito hipoteticamente considerado como agente de transmission de la fiebre amarilla," Havana, 1881; and "Pathogonia de la fiebre amarilla," 1882). These papers were communicated to l'Academie royale des sciences médicales, physiques et naturelles at the dates mentioned. A review of them, by Dr. A. Corre, appears in the "Archives de méd. Navale," tome xxxix, pp. 67-70, 1883, Paris. (See also "Lancet," 1878, i, p. 69.)
Viewed in the light of our modern "germ theory" of disease, the punctures of proboscidian insects, like those of Pasteur's needles, deserve consideration, as probable means by which bacteria and other germs may be inoculated into human bodies, so as to infect the blood and give rise to specific fevers. It has long ago been demonstrated that "malignant pustule" is produced in man by the bite of a fly ("British Medical Journal," January 24, 1863, p. 239). Dr. Budd, in the article just quoted, refers to the greater frequency of this disease in hot, dry summers where insect life is active and teeming; and this, he thinks, would go far to explain the greater frequency of the malignant pustule in Burgundy than in England and the north of France, as also its greater frequency in Siberia and Lapland, where insects of the mosquito tribe are the great pest of the traveler. In Lapland the popular belief was long ago universal that the disease was caused by a peculiar insect, which suddenly descended from the air, and as suddenly disappeared. In the London "Times" (1860) it is reported that four hundred persons lost their lives in the south of Russia and in the province of Kiev from the sting of a "venomous fly" imported from Asia, the same fly having made its appearance there on another occasion, sixty or seventy years before. Virchow, who has made malignant pustule a special study, says: "Most probably, insects with piercing probosces effect the inoculation, such as gadflies (Bremse); but flies which make no wound may also implant the poison on the skin by their soiled wings and feet." The bites of these same flies may be generally harmless; they have no venomous power of their own, but only convey poison from sources of infection to man and animals.
Furthermore, when it is remembered that disease-producing bacteric germs are so minute that a million may rest on the head of a pin, and that the smallest puncture of the finest needle-point (as in Pasteur's experiments with chicken-cholera), when charged with an atom of infecting matter, may be sufficient to infect the body with the septic matter, it scarcely seems possible to ignore any longer the punctures of mosquitoes and other proboscidian insects as possible sources of both infection and contagion. With our present knowledge of the "germ theory" one would hardly dare, even once, to plunge an inoculating needle into the blood of a yellow-fever or typhus-fever patient, whether living and comatose or recently dead, and then withdraw it and plunge it into his own blood or the blood of other persons, yet this is exactly what the mosquito is doing in nearly every yellow-fever epidemic, and what, perhaps, the flea is doing in the filthy jails and ships infested with typhus. In the yellow-fever instance, it is to be noted, also, that the spread of the disease ceases with frost; so also do the peregrinations of the mosquito.
In this paper, however, my chief design is to present what facts I may be able in support of the mosquital origin of malarial disease—in fact, of ague. And, while the data to be presented can not be held to prove the theory, they may go so far as to initiate and encourage experiments and observations by which the truth or fallacy of the views held may be demonstrated, which, either way, will be a step in the line of progress. It is scarcely necessary to premise that other—nay, all—insects that infest and wound the human body may share in the guilt that will here be charged, in particular, to the culex; and so, of course, other diseases than ague, yellow fever, etc., may possibly have a similar history. Be it noted en passant that, so far back as 1848, Dr. Josiah Nott, of Mobile, Alabama, published a lengthy essay on yellow fever, in which he maintained the insect origin of that disease, and also suggested the "mosquito of the lowlands" as a probable cause of malarial fever, in place of the marsh-vapors of Lanscisci ("New Orleans Medical and Surgical Journal," vol. iv, pp. 563-601, 1848). And, even before his time, I find that a paper on the "Mosquital Origin of Malarial Disease" was published by Dr. John Crawford in a periodical known as the "Baltimore Observer," 1807, no copy of which I have yet been able to get hold of.
I now propose to present a series of facts—some of the best known and most generally established facts—with regard to the so-called "malarial poison," and to show how they may be explicable by the supposition that the mosquito is the real source of disease, rather than the inhalation or cutaneous absorption of a marsh-vapor. These facts are, briefly, as follows:
1. "Malaria affects, by preference, low and moist localities"—in fact, swamps, fens, jungles, marshes, etc. This statement no one will dispute. Conformably with it we find the mosquito does the same. The female lays her eggs, to the number of two hundred and fifty or three hundred, in a boat-shaped mass, on the surface of any natural or artificial receptacle for fresh water. Early in spring the larvæ are found in the bottoms of pools and ditches, feeding upon decaying matter (hence the works on entomology state that they are of great benefit in clearing swamps of miasms(?)). These larvæ are the so-called "wrigglers," or "wigglers," to be found in great numbers in any stagnant pools of water during summer. They change into pupæ, and, in a few days more, the pupa-skin is cast, and floating on this latter, like a raft, the insect finally takes flight, a full-developed gnat. Many thousands perish by drowning, or are devoured by fish while extricating themselves from their pupa-cases. As the eggs develop into perfect insects in three or four weeks, many broods are hatched during the warm season, which accounts for their increasing numbers during the later summer and autumnal months. Some species deposit their eggs in soft mud or in dry sand, but all require moisture in the larval state.
2. "Malaria is hardly ever developed at a lower temperature than 60° Fahr." A temperature of 60° F. is necessary for the development of the mosquito.
3. "The evolution or active agency of malaria is checked by a temperature of 32° F." The mosquito is killed or paralyzed, so that its active agency is checked, by a temperature of 32°.
4. Malaria "is most abundant and most virulent as we approach the equator and the sea-coast." The swarms of mosquitoes (as well as of sand-flies, ants, and other insect-plagues) that infest many equatorial regions are well known; and, with regard to sea-coasts, the accumulation of mosquitoes is both a fact and easily susceptible of explanation. Under the influence of gentle land-breezes the mosquito is wafted toward the ocean, but, in the absence of strong winds that would carry it out to sea, the water will form a barrier to its farther progress seaward, for it is not a marine insect. Mosquitoes, therefore, accumulate on sea-coasts notably at some of our familiar summer resorts, Cape May, Atlantic City, etc.
5. Malaria "has an affinity for dense foliage, which has the power of accumulating it when lying in the course of winds blowing from malarious localities."
6. "Forests or even woods have the power of obstructing or preventing its transmission under these circumstances."
These last two propositions, embodying, first, the "accumulation," and, second, the "obstruction," of malaria by forests and trees, may be considered together. That a wind coming from a marsh (from, in fact, a mosquito nursery), and bearing a colony of mosquitoes, should be screened or sifted of its insect burden by passing through the foliage of a forest, or a belt of trees, is certainly far more comprehensible than the conception of a malarial vapor being so screened by virtue of its "affinity for foliage." And though, in the case of a single belt of trees, even the mosquital filter may appear imperfect, the insect, should it have been carried far, is probably anxious to settle, and may so vary its course by steering as to take the first opportunity of clinging to anything that may come in its way; and, having settled, we may readily conceive its shifting round to the leeward side of a leaf or branch, and there holding on until the wind sufficiently subsides to allow of safer flight. Thus mosquitoes, "like malaria, may both accumulate in, and be obstructed by, forests and trees.
The conduct, or rather the mechanical properties, of the mosquito, when carried by the wind, can hardly be better described than in the following verbatim quotation from Sir Francis Day, in his description of malaria. He says: "Malaria may be carried by the winds to places where it was not generated; it is obstructed by and hangs in the foliage of trees, or in mosquito-curtains; it subsides into low places, and may be blown over a hill, and, may be very virulent on the side opposite to that on which it was formed. In like manner it may be taken up the side of a hill, and, as a lull takes place in the atmosphere, consequent upon its weight it rolls down, and may thus envelop its base with a deadly belt of fever, for there, hanging in the leaves of the trees, it gradually sinks through them to the earth beneath, in which situations it is most dangerous to pass the night" (Sir Francis Day's work, p. 87).
7. "By atmospheric currents it" (malaria) "is capable of being transported to considerable distances, probably as far as five miles." So, certainly, is the mosquito.
8. "It" (malaria) "may be developed in previously healthy places by turning up of the soil, as in making excavations for the foundations of houses, tracks for railroads, and beds for canals." Here two things are confounded, viz.: 1. Turning up of the soil, as by plowing or digging; and, 2. Making excavations. Which of these two is the more fruitful in producing malaria is not stated, nor is the modus operandi of either suggested. In Hong-Kong, an island consisting of little more than bare and barren rocks of weather-beaten granite, and whose soil contains but two per cent of organic matter, malarial disease was formerly unknown, and only became prevalent, as it is at present, after excavations had been made in digging granite for building purposes. So, again, tanks and pools of water—cess-pools, mill-ponds, reservoirs, and bilge-water on shipboard—appear to be specially productive of malaria. In Ceylon, the tanks of Candelay and Minery—the one twenty miles in circumference, the other twelve—have been considered the cause of malaria in that region (see Davy on "Diseases of the Army," p. 51, etc.). It is easy to comprehend how such pools, tanks, and excavations containing water may constitute mosquito nurseries, where the female may deposit her eggs and propagate, which would probably have been prevented in the absence of such water accumulations. How simply digging up the soil may contribute to the formation of malaria, or to the development of mosquitoes, without excavations, I am not able to explain.
9. "In certain countries it" (malaria) "seems to be attracted and absorbed by bodies of water lying in the course of such winds as waft it from the miasmatic source." That the malaria itself is absorbed by water is pure hypothesis. The known fact embodied in this ninth statement is really this: A body of water intervening between a healthy locality and a fever district will, provided it be sufficiently wide (three fourths of a mile or more) prevent the transmission of fever from the infected to the healthy locality, also provided, of course, that the prevailing wind does not blow the fever-generating element from one side to the other, as we have already seen it may do over a much wider space—probably five miles. This, again, is not difficult of possible explanation by the mosquito theory. All depends upon the answer to this question: Over how wide a sheet of water will the mosquito, in the absence of irresistible winds, attempt to fly? I am unable to answer this question positively. It may depend upon the degree to which the insect possesses far-sightedness, for, if it can not see land across a body of water three fourths of a mile wide, such a width would appear to its vision boundless as an ocean, and under those circumstances it might not voluntarily attempt to cross. Furthermore, the flight of the insect being mostly nocturnal, long vision would be all the more difficult. These suggestions need confirmation: they are tentative, but still sufficient to suggest the possibility of the protective influence of wide bodies of water being explicable on the mosquital basis.
10. "In proportion as countries, previously malarious, are cleared up and thickly settled, periodical fevers disappear." Here, too, we may remark that in such countries the land is cultivated, and' its swamps and pools drained, so that the mosquito can not so readily find a place suitable to deposit her eggs. And, as the forests and underbrush disappear before the implements of the agriculturist, colonies of mosquitoes, wafted from a distance by winds, are not "obstructed" and "accumulated" by foliage, nor can the insect so readily escape, as before, the numerous fly-catching birds that feed upon it. Even here, however, artificial pools, tanks, and excavations containing water, may constitute mosquito-nurseries from which many millions may be developed in a single summer.
11. Malaria usually keeps near the surface of the earth; it is said to "hug the ground" or "love the ground." When blown by the wind, however, or drifting up ravines, it has been known to rise several thousand feet. Dr. Russell, in his address before the New York Public Health Association, April 13, 1876, stated that, "under ordinary circumstances, a certain altitude affords immunity from malaria, although low elevations of 200 or 300 feet above a miasmatic tract are often more dangerous than the flat lands, the poison seeming to float upward and become intensified." This, he says, has long been noticeable on the heights of Bergen Hill, West Hoboken, and Weehawken, which overlook the Jersey flats. In accordance with the malarial vapor theory, these facts are completely mysterious. The mosquito, on the other hand, is known to hover near the ground (or water) from which it springs, and, being wafted by winds, can readily be understood to be "obstructed" and "accumulated" by forests on the brows of hills, etc.
12. Malaria is most dangerous when the sun is down, and it seems almost inert during the day. Of this there is no doubt, and the various hypotheses on the marsh-vapor theory, that have been alleged in explanation of it, are almost as numerous as they are unsatisfactory. With regard to the mosquito, however, it is well known that it remains, for the most part, during the day, harbored in woods, weeds, or low underbrush, and comes out after sunset and at night to indulge its blood-sucking proclivities.
13. The danger of exposure after sunset is greatly increased by the person exposed sleeping in the night air. Again have the hypotheses based on the marsh-vapor theory been altogether insusceptible of explaining this circumstance satisfactorily. With regard to mosquital inoculation, however, it is undoubtedly true that, while awake, the person exposed will move about, or brush away the insect, while he will submit to be bitten during sleep.
14. In malarial districts, the use of fire, both in-doors, and to those who sleep out, affords a comparative security against malarial disease. Explanations on the marsh-vapor theory are numerous, various, and unsatisfactory. With regard to the mosquito, however, it is well known to be attracted by lamps, lights, and fires, into which it heedlessly flies at the cost of life. In countries where these insects are extremely numerous, lamps are extinguished by the accumulation of their dead bodies. Every fire, therefore, whether in-doors or out, is a sort of mosquito hades. In some tropical countries, despite heat of climate, fires are kept up all night in every apartment as a preventive against fevers; and experience has demonstrated that they are more effective when placed between the open window (or door) and the body of the person to be protected. In this way it is easy to comprehend how every mosquito will fly directly into the light and the fire before reaching the thus protected sleeper.
15. "The air of cities in some way renders the poison innocuous, for, though a malarial disease may be raging outside, it does not penetrate far into the interior."
In conformity with this statement, we may easily conceive that mosquitoes, while invading cities during their nocturnal pilgrimages, will be so far arrested by walls and houses, as well as attracted by the lights in the windows and streets of the suburbs, as that many of them will in this way be prevented from penetrating far "into their interior." Even a single row of houses, on one side of a road, with its contiguous fences, lamps, and closely-knit hedge-rows, may so far completely obstruct the onward flight of mosquitoes coming from some neighboring swamp as to prevent their crossing the street. The curious instances in which people living on one side of a road are attacked with ague, while those living on the other side escape, as on the high-road between Chatham and Feversham (see Macculloch on "Malaria," p. 121), and in Cività Vecchia (see Johnson on "Tropical Climates," p. 315), are quite as susceptible of possible explanation by the mosquito theory as by the marsh-vapor conception, for that the infected air from the marshes does not cross the street is inconceivable.
16. "Malarial diseases are most prevalent toward the latter part of summer and in the autumn."
It has been already explained in what manner—and the fact is a common observation—mosquitoes are more numerous also during the later summer and autumn months.
17. Malaria is arrested, not only by trees, walls, etc., but also by canvas curtains, gauze veils, and mosquito-nets.
Sir Francis Day (p. 87) tells us that travelers, besides being warned against night and morning temperature, should be instructed at night to employ mosquito-curtains "through which malaria can seldom or never pass."
Dr. Macculloch (pp. 137, 138) says that, by surrounding the head with a gauze veil or conopeum, the action of malaria is prevented, and that thus it is possible even to sleep in the most pernicious parts of Italy without hazard of fever. The prophylactic efficacy of fine cloth or gauze at night is further attested by Dr. Johnson ("Tropical Climates," pp. 316, 317), as quoted on p. 318 of La Roche's well-known work. (See also p. 416 of Dr. Johnson's work, and p. 15 of Dr. W. J. Evans on "Endemic Fevers of West Indies," 1837.)
Dr. Oldham ("What is Malaria?" p. 172) tells us that the Jeevas of the Punjaub, employed in fishing and catching wild-fowl, spend the whole night in their boats, under the reeds of the marshes, "unharmed in the midst of malaria"; but they are wrapped from "head to foot" in a peculiar costume that completely envelops them, and which they always put on at sunset; and, moreover, a smoldering fire is kept up in the boat.
It is almost needless to add that, while these nets, curtains, etc., can hardly be conceived to intercept marsh-air, they certainly can and do intercept and protect from mosquitoes.
18. Malaria spares no age, but it affects infants much less frequently than adults.
The child in utero has even been alleged to suffer from ague when its mother was affected: she has felt it, as she supposed, shivering or executing spasmodic movements during the paroxysm, and such infants have been born with enlarged spleens. Nevertheless, it is a matter of daily observation that the sucking infant is less liable to malarial disease than older children and adults.
Young infants, however, be it remembered, are usually carefully housed, and in summer their beds or cradles are generally provided with mosquito-curtains to keep off house-flies, and they may thus be protected from mosquital inoculation. Furthermore, since the human infant, in savage life, and without the artificial protection of gnat-curtains, would be presumably helplessly exposed to mosquito-bites, it would not surprise us if Nature had given the infant some inbred eccentricity by which the inoculations would be rendered harmless; just as the bite of the African tsetse-fly, which will destroy cows and oxen, is perfectly harmless when inflicted upon the sucking calf, as attested by Burton, Livingstone, and Stanley.
19. "Of all human races the white is most susceptible to marsh fevers, the black least so."
The black man, however, is not entirely exempt, and is probably more secure in his native clime than in the United States and other civilized countries to which he has been imported. Acclimatization is alleged to be the proper explanation of this exemption. The negro, it is said, is born in a country where he is obliged almost incessantly and universally to breathe malarial emanations; he is descended from ancestors who, from prehistoric times, have lived in such poisoned air; he has thus become acclimatized to it more than any other race, and on this account is able to prosper in places where the white man would suffer for a long time (see Quatrefages's "Human Species," p. 223, Appletons' "International Scientific Series"). But we are not told in what this acclimatization consists. Will the mosquito theory furnish any probable explanation?
Crude, simple, and at first sight ludicrous as it may appear, it is nevertheless worthy of consideration that the negro is black, and, in the absence of moonlight or artificial illumination, can not at night be so readily seen as a white man; he may therefore on this account escape the sight, and consequently the bite, of the mosquito. The deep tint of his skin possibly constitutes a "protective coloring," such as in many other species affords a defense against natural enemies. Moreover, the habit, common with many negro races in their native heath, of daily anointing the body with grease, affords additional protection against mosquitoes, as do also, probably, the offensive odor and greasiness of his cutaneous secretions. In this connection the work of Dr. Balfour on "Sol-Lunar Influence" in malarial fevers, hitherto ignored, deserves reconsideration. The light of a full moon would render the negro more visible to his culicidal enemy.
When the negro is imported into this country, and subjected to the conditions of civilized life—which usually comprise, among the rest, his remaining during a part of the evening in rooms artificially illuminated, or sleeping with his black body portrayed in bold relief upon white bedclothing—the mosquital blood-suckers will then have little difficulty in finding their victim, notwithstanding his protective melanotic mantle.
The Conservative Function of Ague.—In my original paper, read before the Philosophical Society of Washington, on the 10th of February last (and of which, as I have said, the present production is an abstract), I suggested that the natural conservative design of the processes embodied in the term ague was to develop malarial melanosis—in fact, to change the skin from white to black, thus securing adaptation to (i. e., protection against) the environment of inoculating gnats, by clothing the individual with a cutaneous mantle of "protective coloring." The spleen I therefore regarded as the organ, one of whose offices is, as it were, to preside over and determine the cutaneous coloring of the individual—a function not hitherto ascribed to it; though its pigment-forming function, by destruction of blood-corpuscles in its substance, has been long known. The absolute transformation from white to black under the influence of malarial disease has been frequently observed (see a case by Dr. William II. Falls, in the "Cincinnati Lancet and Observer," November 18, 1882, pp. 479-488). Surgeon-General C. A. Gordon, in his account of the fever among the British troops in Cyprus ("London Medical Press and Circular," new series, vol. xxx, p. 303), in 1878, says: "The Forty-second Regiment suffered terribly at Cyprus; the men looked worse than they did on the Gold Coast—pale and sallow, or black (italics his), the pure malarial melanosis."
Be it noted, further, that the causes or conditions that lead to different tints of color in different races of men—since it is now known not to depend, as once supposed, upon heat of climate—is a complete and inexplicable mystery; a mystery, however, that may be now possibly explained by man's geographical relations with his zoölogical tormentors, the proboscidean diptera.
That malarial diseases occur frequently without the development of malarial melanosis is not difficult of explanation. The disease is not permitted to pursue its natural course; it is interrupted by quinine. The individual, moreover, is clothed, or protected from the sun—an artificial appendage and addition to the organism which the ancestrally inherited powers of adaptation could scarcely anticipate would occur. Possibly, if every ague-patient were exposed to a broiling sun, naked, during the chill, and were then suffered to follow the bent of his successive inclinations during the remaining stages of the disease, the accumulated pigment enlarging the spleen would find its natural and more salutary destination in an even distribution over the cutaneous surface—a phase in the natural career of the disease which seems to be further indicated by the circumstances that the chills of ague only occur, in typical cases, between sunrise and sunset, the paroxysms getting of later and later occurrence, as they are wont to do, until reaching sunset, when the night is "skipped," and the attacks begin early next morning. It seems as if Nature required the sun during the chill, in order that her beneficent purpose of protective cutaneous pigmentation should be carried out.
Although the ethnological bearings of this subject hardly belong to a medical paper, I can not refrain from expressing the suggestion that it is not at all impossible that future study and observation may demonstrate that much of the difference in type between the lowest grade of negro and the most perfect Caucasian white may find its true explanation in the changes produced by an environment of inoculating gnats. Even the characteristic type of the negro skeleton and the capacity of his brain, it is not impossible, may be susceptible of explanation in this manner; for, when we remember that the spleen and its allies are not the only pigment-forming organs, this function being also performed by the marrow of bones, and when we recall, further, the aching of bones that so often attends an ague-chill, in some cases so severe as to have originated the term "break-bone fever," it is not difficult to conceive that the bone-marrow, like the spleen, may become congested during the chill, and in this way, in the course of time, so far lead to modified nutrition of the osseous structures as to set up, finally, a change of type in the embryological formation of the skeleton.
In certain tropical regions it has already been observed by ethnologists that tribes inhabiting elevated regions in the interior are superior to those dwelling on low tracts on the sea-coast, the superiority being manifest both in mental and bodily qualities. The lowlands and sea-coasts, however, are favorite habitats of the fever-producing and pigment-producing mosquito.
In further support of the mosquital origin of malarial disease, we may remark the general admission on the part of medical authorities that sickly seasons and localities are usually accompanied with an extraordinary number of winged insects, many of them being bloodsucking diptera. Lind, in his "Essay on the Health of Semen" (p. 58), referring to an army of Christians, half of whom were lost by fever while passing through Hungary, tells us, "The air swarmed with insects—a sure sign of its malignancy" (p. 60); and, in referring to the climate of Guinea and of the East and West Indies as being fatal to Europeans, he adds, "More especially when molested with heat within-doors, and the plague of mosquitoes, they have ventured to sleep in the open night-air" (p. 71). And again, describing a journey from the interior of Guinea to Senegal, he says (p. 94): "The earth had its white ants, the air its wild bees, its sand-flies, and its mosquitoes. These insects, though not the most tremendous, were perhaps their (the travelers') most distressing enemies." On page 77 he remarks: "The greatest plague was the mosquitoes and sand-flies, whose incessant buzzing and painful stings were more insupportable than any symptom of the fever." After landing on the Canaries the health of the men immediately improved, but they were here no longer "tortured with swarms of blood-sucking gnats and flies" (pp. 83, 85).
Mosquitoes are not generally troublesome in England, yet in the ague-fens of Lincolnshire and the swamps of Essex the use of mosquito-nets is as necessary and common as in India or any other tropical climate ("Chambers's Cyclopædia," article "Gnat"). The prevalence of the mosquito-plague in the fever-districts of Italy is also well known.
Furthermore, in certain districts where the so-called "malarial poison" is supposed to be lodged in trees and bushy plants near the ground, it has been observed that those persons are particularly prone to fever who cut down or disturb these malaria-laden plants, which is extremely suggestive of the mosquitoes being disturbed from their reposing haunts, just as one might get stung by stirring up a bee-tree or a hornet's nest. La Roche, in his well-known work (p. 282), says: "Malaria is collected by plants, particularly those of a dense and entangling foliage, so as to be disengaged on cutting them down or rooting them up, thus exciting fever in the laborers who might otherwise have escaped, as proved by the circumstance that in all these situations, while the workmen are in the erect posture and engaged at their work, they escape the fever, but are attacked if they sit, and more particularly if they lie down on the ground—and that whether they sleep or not."
Here it may be observed that the circumstances stated to conduce to the production of fever (viz., sitting or lying still) are exactly those which would favor being bitten by mosquitoes, the insect having less chance of inflicting its inoculating wound while the men when in motion are "engaged at their work."
The same thing has been observed by Macculloch in the Roman Campagna, where, he says (p. 124), it is remarked that, "if the laborers cut down certain plants (a bushy thistle chiefly), a fever, that would otherwise not have occurred, is the consequence."
In opposition to the mosquital origin of malarial disease it is known that numerous mosquito wounds may be inflicted without the occurrence of malarial disease; but this is by no means incompatible with the theory. We do not yet know whether the poison be mosquital saliva, or whether the fever-producing element be a bacillus with which the puncturing proboscis of the insect may be loaded at the time of inflicting its wounds. The scratch of a lancet will not produce vaccinia, unless the instrument be charged with vaccine matter; the puncture-needles of Pasteur would be harmless and impotent, did he not load them with infecting bacteria; so with dog-bites and hydrophobia, etc.
Nay, it may even turn out that, under certain circumstances, mosquito-bites shall even be protective against malarial disease, for as Pasteur and others are able to produce, artificially, "attenuated culture-fluids," the inoculation of which, while producing slight symptoms, protects from more serious phases of disease, so may there exist in nature naturally "attenuated" fever-poison fluids, the inoculation of which, by mosquital puncture, may produce trivial symptoms, and thus protect from more decided attacks of veritable fever. What product of man's art has not been anticipated by Nature? Hardly any.
In the absence of direct experiments with the mosquito as a fever producing agent, I have endeavored to ascertain if the geographical distribution of the insect had any relation with that of malarial disease. But the insect and the disease are both so wide-spread that it is difficult to find any locality entirely exempt from either. Tasmania, Singapore, New Zealand, Ceylon, and the Dismal Swamp of North Carolina, as well as the bog-country of Ireland, are said by some medical writers to be entirely exempt from fevers. Others discredit this statement. I do not know who is right; but I have endeavored to ascertain whether insects of the mosquito tribe were or were not rife in these localities. On this point there seems to be little scientific knowledge available. Of the insects of Singapore—a locality in which the absence of fever seems to be generally admitted—I have been able to find no account. With relation to Tasmania (Van Diemen's Land), where exemption from fever is again generally admitted, I have only been able to find, in the Capitol Library, one reference to its insects. This is from the work of James Bischoff (London, 1832, p. 33), who, quoting from Widowson's "Present State of Van Diemen's Land," says: "The insects are not so numerous or so annoying as in most other tropical countries. The ant, the mosquito, and a common green fly are chiefly seen. The mosquito does not sting so severely as in hotter climates."
With regard to Tasmania, it may also be observed that the native inhabitants are universally of one color—absolutely black—which would suggest a previous history of malarial disease to which a prolonged succession of generations has finally secured complete adaptation and consequent exemption from symptoms. How far the eucalyptus-tree, which here abounds, Las been instrumental in correcting malarial disease, also deserves consideration. Possibly the sticky, pendent leaves, and camphoraceous odor of the plant are not conducive to the prosperity of the mosquito genus.
The Dismal Swamp, so far from being exempt from mosquitoes, is said to abound with them during the autumn. Alexander Hunter, writing in "Potter's American Monthly," July, 1881, page 15, says: "The mosquitoes were in uncounted millions; they came armies on armies, waves upon waves, clouds upon clouds, and charged in platoons and single file, and threw themselves with bloodthirsty voracity upon every living thing in reach." On the same page, however, his negro guide, "Bob," is made to say that he reckons he would be quite fat "but for the 'skeeters and chills." Another writer ("Harper's Magazine," vol. xiii, 1856, page 450) refers to an hotel having been erected for a summer resort in the "Dismals," but "before the month of August visitors, servants, and proprietors had all cleared out and left the place in full possession of the mosquitoes and yellow flies. These insects are said to be savage enough to worry the life out of a mule. . . . The hotel was taken down."
In so far, therefore, as regards the geographical relation between mosquitoes and malarial disease, it may be said: 1. The two often coexist; 2. There is no decided proof that localities alleged to be exempt from ague are also exempt from mosquitoes; 3. There is no locality noted for malarial disease where mosquitoes or other bloodsucking insects do not exist.
In those isolated cases of ague occurring during the winter or early spring, before inoculating insects have made their appearance, there may of course be other modes of inoculation. We have only to admit the production of the bacillus malariæ, its transmission in the air and its deposit upon the skin, to see how easily it may be inoculated into the body by accidental wounds, such as pin-scratches, the cut from a pocket-knife, or of a razor in shaving, etc. Furthermore, it is generally admitted by medical authorities that the period of inoculation after the poison is introduced into the system may, exceptionally, extend weeks and even months before symptoms are developed. In these, or in some other ways, the isolated winter cases referred to may therefore be explained without necessarily conflicting with the mosquito theory. Finally, it seems incredible that a function so necessary to life as respiration—a function that can not be suspended in any atmosphere—should be the means of infecting the body with a fatal disease. It was surely never designed that breathing the indigenous air of any natural environment should be, and without warning of danger, a means of death. Such a supposition is inconsistent with that general beneficence of Nature as exhibited in the signals of danger, instinctively recognized by all living things, when contemplating their natural enemies, and when in the presence of conditions that are destructive to life.
Man naturally loves beautiful things—woman and the flowers. But the serpent also is beautiful—superficially smooth, tapering in form, elegantly elastic, absolute in symmetry, undulating in motion, every element of beauty in woman finds its counterpart in the snake; yet we love the one, loathe the other—loathe it because we have inherited the instinct that tells us it is one of our natural enemies, whose touch is destruction; and, when sight is not sufficient, the rattle is added, so that even in darkness we may hear the warning note of danger.
When man is prompted by Nature to invade a swamp in pursuit of fowl and fish, his natural foods, it can not be that the silent air he breathes shall, like a subtle enemy, and without any admonition, destroy his life. But when night comes, should he there lie down to rest, the annoying puncture of the mosquito and its siren-song, like the warning note of the serpent's rattle, would emphatically and persistently tell him, "This is no place to sleep!"
The mosquital origin of malarial disease is in this respect, therefore, more in accord with the beneficence of Nature's arrangements than is the conception of malarial fevers being produced by the respiration of a marsh-vapor.
I have before referred to "color-protection" as a means of defense' from natural enemies. Acuteness of audition is another well-known, means. It curiously happens that some forms of fever are followed in those who convalesce by a remarkable acuteness of hearing, which lasts for weeks and months, thus indicating another phase of adaptation to environment, an additional means of recognizing the warning note of the inoculating mosquito, or other insects inaudible to ordinary ears. Dr. J. B. Allan refers to this symptom in his description of a remittent fever prevailing on the African sea-coast ("Monthly Journal of the Medical Sciences," August, 1841, page 545). He says: "The acuteness of hearing sometimes came on during the second day in those who recovered. It was very distressing for the first six weeks or two months of convalescence; and every wave that burst on the distant reef was counted with pain and even dread."
If the mosquital origin of malarial fevers be correct—if protection from mosquital inoculation protects from ague the means of prophylaxis from malarial disease will not be difficult. It comprises the following items:
1. Personal protection from all winged insects, especially during evening and night, by gauze curtains, window-screens, or clothing impenetrable to their probosces—a further protection from these, as well as from the bites of creeping insects, especially during epidemics and endemics in jails, ships, etc., by a daily inunction of the whole body with some terebinthinate, camphorated, or eucalyptalized ointment or liniment.
2. Domiciliary protection, exteriorly, by screens of trees, walls, fences, etc., interposed at some distance between dwellings and the sources of malaria or mosquito nurseries, together with fires, lamps, or electric lights, to act as traps for the attraction and destruction of such winged insects as may approach nearer; a further protection in the interior of dwellings being secured by the use of smoke (such as that of tobacco or pyrethrum), or of some volatile aromatic oil, as of camphor, etc., which may be offensive to proboscidian intruders.
3. Municipal protection, by the destruction or draining of swamps and pools which produce mosquitoes; and by the planting of forests to obstruct the latter in their flight, or cordons of electric lights for the same purpose, as well as for the destruction of insects that may be attracted by the flame or incandescence.
- Abstract of a paper on "The Prevention of Malarial Disease, illustrating, inter alia, the Conservative Function of Ague," read before the Philosophical Society of Washington, February 10, 1882. For another paper, on "The Conservative Design of Organic Disease," see this journal for June, 1875.
- Most of them are quoted from a paper read by Dr. John T. Metcalfe, United States Sanitary Commission, 1862; see, also, Flint's "Practice," p. 826, edition of 1867.