The Aqency of Insects in Disease

COLONEL G. M. GILES June 1 1913

The Aqency of Insects in Disease

COLONEL G. M. GILES June 1 1913

The Aqency of Insects in Disease


It must be astonishing to everyone reflect just how quickly the news scientific world become the property as surprising how soon people acquai and theories of the laboratory. The microbes, bacilli, cultures, and so forth years ago would not understand at all. the attention of the world with his supp article tells about some of the researches of both hemispheres, and how some fearful ledge gained there. Insects have played a Giles, who here tells about them, is a resident

who takes time to think and of the new discoveries in the of the common people. It is equally nt themselves with the phraseology ordinary person talks about germs, so fluently that people of twenty-five At present Dr. Friedmann is claiming osed cure for tuberculosis. The present that have been made in various hospitals diseases have been checked by the knowlarge part in spreading of disease. Col. of England.

OF ALL the plagues conjured by Moses upon Pharoah and his much tried subjects, perhaps the most vexatious and disgusting must have been that of flies. It is noteworthy too that two others of the ten, the plagues of lice and locusts, employed insect agency, while the boils and murrain, in the light of modern science, were probably spread in the same way. From the Egyptian point of view, indeed, it was as well that Moses lacked our present knowledge of the ways of insects, or he might have made himself even more disagreeable to them than he did, without going outside the insect world for his agents. Those who have sojourned in Egypt know that though its plague of flies may have abated, it is there to-day; and large tracts of the African continent are useless for stock-raising, through the ravages of flies that, to the untrained eye, are indistinguishable from the common house-fly.

Up to well on in the last century, the public mind, and even that of the scientific world, was paralysed by the oldfashioned idea that all created beings are designed for some wise and munificent purpose, and even to-day, the conservative scientist is fond of pointing out the inconvenient results of the acclimatization of rabbits in Australia, forgetful of the fact that, if his argument be logical, some parallel evil should have resulted from the introduction jof our common domestic animals,

of songbirds, and of trout. As if forsooth, our entire civilization were not one huge object lesson of man’s success in “flying in the face of providence” as they were pleased to conceive it. With singular perversity, these good folks ignored some of the most valuable teachings of Holy writ, for a literal obedience to the sanitary code of Leviticus would have placed Europe in a position, from the sanitary point of view, in many respects better than was reached till well into the Victorian era, and which, in the matter of meat inspection is only exceptionally attained to-day.

Indeed the escape of the Hebrews, while the Egyptians suffered, might be fairly explained by their adherence to such a code. ' Fortunately for mankind, such fatalistic folly is now well nigh a thing of the past, and, when the child of the day obeys the wholesome sanitary instinct of destroying insects, he is no longer chidden, but encouraged to “swat that fly.”


The possible connection between im sects and disease is so obvious that it is hardly surprising that speculative guesses on the point have been made by shrewd observers in most parts of the world, but perhaps the most argued suggestion in that direction was that made by Inspector General Maclean, Professor of Military Medicine at Netley, in the late seventies. Maclean was certain-

ly the greatest authority on malaria of his time: indeed the subject seemed to obsess him : and in discussing the causation of the disease, he was wont to lay great stress on the fact he had observed, that a mosquito net afforded great protection. In a long experience of being lectured at, the writer never “sat under” so attractive an orator, and the very words of his racy “Doric” still linger in his memory. “That those who make a rule of sleeping under a mosquito-net rarely contract malaria, is a fact of which I have no doubt. We are taught that malaria is due to a “miasm,” a something impalpable. I do not see how a net can keep out anything much smaller than a mosquito, but the fact remains.” Maclean was too sound a scientist, and too cautious a Scot to indulge in futile conjecture, but his manner leaves little doubt in my mind that he shrewdly suspected that the mosquito, and not the miasm was the true culprit.

Guesses of this sort have, however, no scientific value, and it is rather pitiable to find the parochial conceit of certain scientific men of to-day leading them to claim priority for some forgotten compatriot, on the score of surmises of this description.

One of the earliest proven cases of insect-transmission of disease was that of the spread of certain tape-worms, among domestic animals, such as dogs, cats, sheep and cattle, by the agency of the lice that infest their pelts. These lice are known as Trichodectes, and each animal is infested by its own special species of the genus, and an equally special tape-worm, which passes one stage of its existence in the tissues of the louse, and the other in the intestine of the mammal.

As regards man, however, the first crime to be brought home to our insect tormentors was the conveyance of filariasis, through the agency of mosquitoes, by Sir Patrick Manson, in 1879.


In this disease, which is widely spread throughout the tropical world, the blood of the unfortunate patient

literally swarms with worms, every minute, but of the same class in the animal world, as that well-known parasite, the common round worm.

By a laborious and carefully planned series of investigations, Sir Patrick, then a hard-working practitioner in Southern China, demonstrated that a part of the life-history of these worms must necessarily be passed within the body of a mosquito, into which they gain admission, along with the blood the insect has sucked from the capillaries of a human being affected with the disease. After passing through certain necessary stages of their development within the mosquito, they find their way into its proboscis, or piercer, and are so inoculated into the tissues of the next human being on which the insect chances to feed. The astonishing story of the after history of the worm, within its human host, is rather beside the subject of our present thesis, and so must be passed over for the present.

Now, as already mentioned, the problem of the causation of malaria, was still an unsolved mystery, and the best suggestions as to how it was carried, depended on mere guesswork. We had not even the remotest suspicion as to the character of the physical agency concerned in the production of the malady, the fashionable plan of hiding our ignorance being to ascribe it to a mysterious emanation from the soil which we were pleased to dub a “miasm.” The use of crack-jaw terms of this sort affords a soothing splint to a certain class of mind, and to many, the comfort of a sense of explanation, but is a poor crutch to men of the type of Maclean, and of the names that, follow. The writer once served beneath the harrow of a chief who spoiled some 600 pages of Blue-book paper, to show that cholera was not somehow communicated from man to man, but was due to a “pandemic wave.” The worst of it was that he was apt to order the excision from official papers, of facts that accorded with common experience, but “that is another story.”

About the same time that Dr. Manson was running to earth the malefac-

tor of filariasis, a French military surgeon, Dr. Laveran, achieved a great step in the elucidaion of the malaria problem, by discovering that the disease was caused by a minute animal parasite, inhabiting the red" corpuscles of the blood. It seems strange now that these bodies should have been till then overlooked, even with the microscopic powers we then possessed, but the fact is that the normal blood had been so little studied, that we knew not the abnormal from the normal. Blood, you see, changes so strangely, after it is drawn, that even now, quite practised observers may be misled by what are really post mortem appearances, and hence some years elapsed before Laveran’s discovery gained general acceptance. The possibility that mosquitoes might play the same part for this disease that they served in filariasis, was mooted by King, of Madras, in 1883, and by Laveran himself in 1884, but the time was not yet ripe, and hence the proposition attracted little notice till 1894, when Manson published the suggestion in terms that led more than one investigator to seriously attack the problem. The principal of these were Major Ronald Ross, of the Indian Medical service, and Professor Grassi, of Rome. The race was rather a close one,

but the Britisher won. Ross commenced his research in 1895, and published, in the British Medical Journal of December 18th, 1897, the proof that human malaria was carried by a species which he termed the “dapple-winged” mosquito, and it was not till November 6th, 1898, that Grassi sent a far less detailed note to the Royal Academy of the Lincei, while his magnificent “Studi di uno Zoologo sulla Malaria” did not appear till 1901.

A regrettable dispute as to priority arose between the rival scientists, but Ross’ claim was at once conceded by the scientific world, and was acknowledged by the fellowship of the Royal Society, and the award of the Nobel prize for the greatest discovery of the year, while comparatively recently, our King has made him “Sir Ronald Ross.”

Although a member of the same service, the writer never met Ross till he chanced to do so in the insect laboratory of the British Museum, in 1899, for our work had lain in widely distant parts of India. On examining Ross’ specimens, I found that while his “grey” and “brindled“ species were well known, at least two of the inculpated “dapplewings” were new to science, and it was obviously urgent that someone accustomed to this branch of work should undertake the humbler but laborious task of monographing the mosquitoes.


I accordingly set to work, and after a year’s labor, brought out the first collected descriptions of the family, including the original descriptions of no less than 242 species, uniformly translated into English, and an account of the little that was known of their anatomy and life history. A second edition was soon called for, but in the meantime collectors had been busy in all parts of the world, and much origim al anatomical and field work had to be undertaken: and it was not till 1902 that the entirely re-written work appeared. The number of species at present known cannot be far short of 600, and is being constantly augmented.

The gain to humanity involved in this “epoch-making” discovery is incalculable, as at a single stroke, it made possible the healthy habitation of the tropics, but it is deplorable how little its advantages have been utilized by the short-sighted governments of tropical lands. It is an undoubted fact that, but for Ross’ discovery, the construction of the Panama Canal would have proved a physical impossibility, for neither laborers nor engineers can work when stricken with malaria ; for to their everlasting credit be it recorded, the great American republic alone has fully availed itself of the potentialities of antimalarial sanitation. At the outset, the usual custom of the sanitarian being there merely to advise, had its customary results, but President Taft, with instincts of a true statesman, grasped the administrative nettle, and made his sanitary chief, Col. Gorgas, the virtual dictator of the canal zone, with the result that it is now one of the healthiest of tropical places, and the great work already approaches completion.


The ball having thus been set rolling, fresh discoveries followed in rapid succession. By a series of investigations commenced in 1898 by the Papanese Ogata, and continued by others till its final demonstration by Capt. Liston, I.M S., bubonic plague was shown to be conveyed, in the vast majority of cases, by rat fleas. As early as 1881, Dr. Charles Finlay, of Havana, had attempted to prove that yellow fever was*caused by the bites of mosquitoes, but he missed the point of their being mere agents, and tried to work with uninfected insects. He chose, however, the right mosquito, Stegomyia calopus, Ross’ “brihdled,” and in 1900, an American Commission, consisting of Drs. Reid, Carroll, Lazear, and Agramonte conclusively proved that yellow fever can only naturally be communicated by the bite of this insect. Poor Lazear died “on the* field of honor” during this hazardous investigation, the first of a lengthening series of casualties in this dangerous branch of

research. The Americans have also shown that the “Texas fever” of cattle is conveyed by the bite of a tick, while a brilliant series of achievements lies to the credit of the Missions of the Liverpool School of Tropical Medicine over which Professor Ross still presides. Britishers from all parts of the empire have shared in these sorties, Professor Todd, of McGill’s, having done yeoman service on some of the earlier, while another Canuck, Macconnell, of the same university, was detailed as my colleague on another of these scientific jaunts to the West coast of Africa.

The list, however, is too long to be recounted in an article of the present scope and character, so I will close with a few words onj the last culprit, who, though long siispected, has only recently been haled before the court of science. '

IfTrin the term common fly, we inc$udb insects commonly confused with it, "there would be a new count to the indictment, for, only the other day, a strong case was made out against the stable fly. Stomoxys, as the probable agent of the terrible poliomyelitis, or infantile paralysis, but we will confine ourselves to the common domestic insect. It is true that it has not as yet been shown to be the chosen vehicle of transmission of any one disease but its life history and habits are such that it must necessarily constantly deposit infective matter on food, and the obstinate way in which it refuses to be driven from wounds and ulcers shows that it cannot fail to frequently infect the surface organs of the body. That the dreaded Egyptian ophthalmia is commonly spread in this way, no one who has watched Egyptian children with their faces literally a crawling mask of flies can doubt.


The common fly is enormously prolific. Howard, of Washington, estimates that, assuming all the progeny of three female flies to survive, the resulting swarm would weigh a ton in 40 days, and even if but one per thousand survive, it is obvious that to keep their

numbers down, our efforts must be constant and unremitting.

Like man, the adult insect is almost omniverous “only more so” and they consequently swarm wherever food is exposed. The larva, or maggot on the other hand, feeds by preference on the excreta of animals, especially that of horses, but can put up with any putrescent matter, and it is rather with the view of selecting a suitable home for its

young, than in search of food, that flies congregate so thickly on offensive matter. With this double habitat, it is obvious that flies cannot fail to carry the infection of such diseases as infantile diarrhoea and typhoid from the dejecta of patients, to food destined for healthy people, and enough, I think, has been said to show that the banishment of flies is one of the most urgent sanitary tasks of the time. f

But how? it will be asked. The full discussion of this question would require a separate article, but I should like to point out that the “Swat that fly” campaign, in summer, is a somewhat futile proceeding and that the larval stage is the most vulnerable period of the insect’s life. If no dung, or other offensive matter be left without removal and destruction, for over four days, flies must die out in the district so protected, and, in other words, scrupulous scavenging of towns is the key of the situation.

There is, however, a season during which fly swatting may be most advantageously pursued. Flies cannot stand cold, and in climates such as that of Canada the hope of the survival of the species through each winter, depends on such insects as succeed in hiding themselves in dwellings. Systematic swatting at this time of the year is, therefore most valuable, and if combined with an unreasonable “Spring cleaning” could not fail to have a marked influence on the prevalence of flies, during the ensuing summer.