INSECTS AND DISEASE It had been noted that ‘‘malaria is freely generated at the bases of mountain ranges in tropical climates,’ that “temperature exercises Sreat influence over its development and activity,’ ““many places can be visited with impunity in winter which are dangerous in summer and autumn”; that “malaria drifts along plains to a considerable distance from its source, when aided by winds sufficiently strong to propel, but not to dispel it’’; that ‘“‘under the influence of currents of heated air it can ascend, in dangerous concentration, far above its source’; that ‘‘a belt of forest interposed between any malarial place and human habitations affords considerable protection.”’ Could anything be more mysterious than this picture? Yet only three years after this article appeared in Quain’s Encyclopedia the mystery was solved.
The germ of malaria, a Protozoan parasite which destroys the red cells of the blood, had been first seen by a French surgeon, Laveran, in 1880, and an individual observer here and there had suggested a possible connection between malaria and mosquitoes. In 1883 an American, A. F. A. King, had urged with special force the hypothesis that ‘“‘the mosquito is the real source of the disease rather than the inhalation or cutaneous absorption of a marsh vapor.”
It was only in the early nineties, however, that the proof of this assumption was at last furnished. Patrick Manson and Ronald Ross, two English physicians, at that time began serious work on the mosquito theory and in 1897 Ross discovered the germ of bird malaria in the stomach of the mosquito. The Italians, Grassi and Bignami, first demonstrated the germ of human malaria in the body of the mosquito in 1898 and in 1900 Sambon and Low showed that it was possible to remain immune from disease in the most malarious region of the Roman Campagna if protected against mosquitoes while Dr. Manson’s son and another voiunteer were inoculated with malaria in England by the bites of infected mosquitoes shipped alive from Italy.
The malaria germ is an example of a parasite which requires two different hosts in which to complete its life history. Asexual cycles are completed in the blood of man at regular intervals of 48 or 72 hours, the recurrent period of chill and fever corresponding to the time at which new generations are set free in the blood stream. The complete development of the parasite, however, must be accomplished in the stomach of a mosquito belonging to the genus A nopheles. 37
AMERICAN MUSEUM GUIDE LEAFLETS When one of these mosquitoes sucks the blood of a malaria patient it draws in the germ which passes through its sexual stage in the body of the mosquito; and after a period of ten or twelve days a new generation of germs find their way into the salivary glands of the mosquito, where they lie ready to infect any new victim who may be bitten by the insect.
The control of mosquito breeding, then, at last offered definite hope of checking this disease which had laid so heavy a burden upon many populous countries and some of the most fertile regions of the earth.
Celli estimated that malaria caused two million cases of disease and 15,000 deaths a year in Italy. We have no adequate statistics of its ravages in the southern United States, although the importance of the problem has recently been forcibly set forth in an admirable monograph by Mr. F. L. Hoffman.
A careful study made in Alabama in 1911 revealed 70,000 cases and 770 deaths in that one state and that one year. Certain counties in southern Missouri have experienced death rates from malaria of over 100, and in one instance nearly 300 per 100,000 population. Howard estimates the money loss due to this disease in the United States at $100,000,000 a year.
During the past summer (1917) malaria was the second largest cause of sickness among our American troops in mobilization camps.
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INSECTS AND DISEASE CONTROL OF MOSQUITO-BORNE DISEASE The control of mosquito-borne disease involves five principal measures: (1) the elimination of mosquito-breeding places, (2) the destruction of mosquito larvz in accumulations of water which can- Fig. 17. MOSQUITO-BREEDING NEW JERSEY MARSHLAND BEFORE DRAINAGE Fig. 18. SAME AREA SHOWN IN Fig. 17 AFTER DRAINAGE not be removed, (3) the destruction of adults, (4) the protection of human beings from access of mosquitoes, and (5) immunization and treatment by the use of quinine.
AMERICAN MUSEUM GUIDE LEAFLETS Those who would control injurious insects should use a combination of common sense and knowledge of the insects’ habits. It should not be necessary to mention the second factor, for common sense would point out the necessity of such knowledge, except that so many otherwise sensible people pick up a smattering of entomology and straightway feel competent to handle the complicated problems presented in nature. No important piece of work should be undertaken without expert help. On the other hand, mere knowledge of the insect’s habits is not sufficient, as has been shown by some utterly impractical methods of control which have been suggested. The notes which have been given here on the habits of mosquitoes are the merest outlines and, furthermore, refer chiefly to those species which seriously trouble man.
The most fundamental of all measures of protection is the removal of the original source of all mosquito breeding, accumulations of stagnant or sluggishly moving water. The particular measures adopted must vary widely with the type of mosquito to be dealt with. Along the shores of the eastern United States, for example, the salt-marsh mosquito, Culex sollicitans, furnishes one of the most important problems from the standpoint of nuisance, although this species is not a carrier of disease. The salt-marsh areas and the neighboring country may be kept reasonably free from mosquitoes by the construction of comprehensive systems of drainage ditches which keep down the water level of the land adjacent and are themselves flushed out by the tide. Fresh water swamps and pools where species of Anopheles breed may be treated in a similar way or in some cases may more economically be filled in.
In dealing with the common malarial mosquito of the central United States, Anopheles quadrimaculatus, special attention must be devoted to sluggish streams clogged with vegetable growth. If such streams are straightened and cleared of weeds their rapid flow will no longer be suitable for mosquito breeding. On the other hand the Culex pipiens and Aédes calopus are house mosquitoes, and the most important measure in checking these insects is the removal of small accumulations of stagnant water in the immediate neighborhood of habitations. Such inconspicuous breeding places may produce in the aggregate great numbers of mosquitoes; and every anti-mosquito campaign must include provision for systematic houseto-house inspection if it is to be successful.
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INSECTS AND DISEAS£ Watch, then, for unused barrels, tubs, and buckets; roadside puddles; hoof prints, ruts, and depressions of all sorts; barn-yard pools; and the like. In cities, special attention must be paid to the catch basins of street sewers which are often prolific breeding places. In attempting to locate wrigglers in such places water should be skimmed (not scooped) from the margins of the water in a whitelined cup or dipper. Fountain-basins should contain goldfish since these fish are voracious feeders on mosquito eggs and larve, or the basins should be emptied and thoroughly dried at least once a week Fig. 19. OILING A MOSQUITO-BREEDING SWAMP during the mosquito season. If for any reason a pool or pond cannot be drained there are two principal methods of preventing it from furnishing the neighborhood with mosquitoes: the surface may be covered once a week with a film of oil; or surface-vegetation may be removed, the sides of the pool made sharp and steep, and the pool well stocked with goldfish. The first method is not entirely satisfactory since the oil must be renewed so often, and also because an oiled pool has an unpleasant look and odor. Kerosene spreads well but evaporates quickly: crude oil lasts longer but does not 41
AMERICAN MUSEUM GUIDE LEAFLETS spread so well and larve survive between the patches of oil. A mixture of about equal parts of kerosene and crude oil is rather satisfactory. It should be applied with a spray-pump. Very few, if any, of the numerous patented oils are worth the money. “‘Larvicide”’ is made from crude carbolic acid; it was used in the Panama Canal work, but water treated with it is poisonous to stock. Of all the natural enemies of mosquitoes, fish are the most important, and of these goldfish are the most dependable and easily secured, but small ones are much better than large ones. Top minnows, such as species of Gambusia and Heterandria (Fundulus) are the most important natural enemies of mosquitoes. It is true that numerous aquatic insects feed, to some extent, on mosquito larve and pup, but they are rarely numerous enough to be of much value, and their numbers cannot easily be increased in any given pool. The usefulness of dragon-fly larve and adults has been sreatly exaggerated. Tadpoles are vegetarians and do not make a practice of eating larve.
Since Anopheles larve frequently live in water which flows rapidly enough to wash away oil, and since they keep close to the surface of the water, getting above leaves of aquatic plants where fish . do not see them, they are rather difficult to control. To make matters worse, they are the mosquitoes which we are particularly anxious to get rid of since they are the only ones known to carry disease in the North. Cleaning the edges of streams or ponds, clearing away aquatic vegetation, and keeping a good stock of fish seem to be the only feasible methods of control, although in small brooks a constant supply of oil may be furnished from a specially designed automatic drip can.
In regions where malaria or yellow fever or any other mosquitoborne disease prevails, such measures should be supplemented by careful exclusion of mosquitoes by the use of window screens and bed canopies, and special precautions should be taken to prevent the biting of those suffering from disease by mosquitoes which may carry the infection to others. | The application of these various methods of mosquito control, with thoroughness and over sufficient areas, has yielded results of the most definite and tangible kind. In the Italian Campagna the proportion of the population infected with malaria was reduced from 65% to 12% by measures directed toward the eradication of the Anopheles mosquito and was then brought down to 4% by the free distribution and vigorous advertisement of the value of prophylactic doses of quinine. Between 1902 and 1908 the deaths from malaria in the whole kingdom of Italy fell from 16,000 to 4,000. 42 e ne te.
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AMERICAN MUSEUM GUIDE LEAFLETS THE CONQUEST OF YELLOW FEVER The discovery of the mosquito transmission of yellow fever is one of the most striking and dramatic chapters of sanitary science and one of the brightest episodes in the history of our country. Between the years 1702 and 1800 this terrible disease had appeared in the United States thirty-five times and between 1800 and 1879 it visited the country every year with two exceptions. In 1793 a tenth of the population of Philadelphia are said to have perished from its ravages.
Mathew Carey writes of this epidemic, “‘the consternation of the people of Philadelphia at this period was carried beyond all bounds...:..°<.- People hastily shifted their course at the sight of a hearse coming towards them. Many never walked on the foot-path but went into the middle of the street, to avoid being infected in passing by houses wherein people had died. | Acquaintances and friends avoided each other in the streets and only signified their regard by a cold nod. The old custom of shaking hands fell into such disuse that many shrunk back with affright at even the offer of the hand. A person with a crépe, or any appearance of mourning, was shunned like a viper. And many valued themselves highly on the skill and address with which they got to the windward of every person they met. Indeed, it is not probable that London, at the last stage of the plague, exhibited stronger marks of terror than were to be seen in Philadelphia from the 24th or 25th of August till pretty late in September.”’ The Philadelphia epidemic was the occasion of a vigorous discussion as to the contagiousness or non-contagiousness of the disease in which the eminent Dr. Benjamin Rush was finally converted from the latter to the former view. For over a century arguments were advanced pro and con, without conclusive result, and as late as 1898 the United States Marine Hospital Service summed up the matter as follows: ‘“While yellow fever is a communicable disease, it is not contagious in the ordinary acceptation of the term, but is spread by the infection of places and articles of bedding, clothing and furniture. .... One has not to contend with an organism or germ which may be taken into the body with food or drink, but with an almost inexplicable poison so insidious in its approach and entrance that no trace is left behind.” 44