# Popular Science Monthly/Volume 81/December 1912/Insects as Agents in the Spread of Disease

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* INSECTS AS AGENTS IN THE SPREAD OF DISEASE
By CHARLES T. BRUES
BUSSEY INSTITUTION, HARVARD UNIVERSITY
LESS than fifteen years have elapsed since the scientific world entertained its first definite suspicion that certain human diseases might be spread through the agency of insects. Twelve have gone by since that suspicion became an established fact, and in this short space of time so much has been learned concerning the pernicious activities of these small animals in disseminating disease-causing organisms among man and the higher animals, that the science of preventive medicine can now be applied to many important diseases which were before utterly beyond its reach. Every year brings forth fresh evidence that insects are important factors in relation to public health, and adds to the list of diseases that are partially or entirely dependent upon certain insects for their spread.
A brief statement of the nature of communicable diseases and of the general habits of the kinds of insects that are implicated in carrying disease will serve to define roughly the field of medical investigation which is open to the entomologist. Communicable diseases are invariably due to parasitic organisms in the body which are capable of inducing similar symptoms in other persons or animals if transferred to healthy individuals from diseased ones. Many conditions modify the transfer of communicable diseases; some individuals are more easily infected than others; some may be immune as the result of a previous attack; and, on the other hand, the virulence of pathogenic organisms often varies greatly in accordance with conditions to which they have been subjected previously. A simple method of spread occurs with many diseases, for example typhoid fever and pulmonary tuberculosis With the former, the Bacillus typhosus which is the disease-producing organism, is present in the dejecta of an infected person and may find its way from these to food, carried by flies or otherwise; ingested by a healthy person, it may quite likely multiply and induce a second case of typhoid. With tuberculosis, the tubercle bacillus from desiccated sputum may enter the lungs of a healthy person with dust and there reproduce the disease. As we shall see later, certain insects are commonly associated with the spread of diseases of this type, although from the very nature of such diseases, insects are not exclusive factors, and may be referred to as contaminative carriers.
A second type of communicable diseases differs from the one just ​mentioned in that the organism which causes the disease must live for a time in the body of some other animal to undergo certain definite changes before it can again induce the disease in another individual. The most important insect-borne diseases belong to this type, for in the case of man and domestic animals, certain insects and ticks act as the secondary host animals for the organisms of many diseases. Thus, yellow fever is spread only through the agency of a certain mosquito, for in its body alone can the yellow fever organism live and undergo the changes that are necessary before it can be introduced into another patient by the bite of an infected mosquito. Malaria belongs to the same category, for it spreads only through the bite of certain mosquitoes that obtain the organisms with their meal of blood, and then afterwards inject into the blood of another person, a later stage of the malarial parasite which has developed meanwhile within the mosquito.
Diseases in which certain insects act as specific carriers are most numerous and prevalent in the warmer parts of the world, although temperate regions are by no means free from diseases of this kind, which may be referred to as the inoculative type.
Among insects which disseminate pathogenic organisms without any specific association, the common housefly is without doubt the most important. The rank of the fly in this unenviable profession is due to several facts in its life-history which render it eminently suited to act as a vector for several diseases such as typhoid fever, diarrhœa, dysentery, summer complaint of children, etc. The eggs of the house fly are laid preferably in horse manure, upon which the larval stages or maggots feed, but human excrement serves equally well, and when exposed is very likely to provide food for a brood of fly maggots. Less than two weeks are required for the larvæ to mature, and after a short resting stage of two or three days the adult flies emerge from their bed of filth. At this time they may quite possibly bear in their alimentary tract bacteria derived from their larval food. Usually, however, they are quite clean when hatched, in spite of the surroundings whence they have come. They do not long remain so, however, for they feed upon animal and human dejecta of all sorts, garbage and other fermenting material, and if still hungry invade markets or houses, where they may leave upon food any bacteria brought upon their bodies, legs, mouth parts or wings. In addition, their excrement deposited as "fly specks" may contain virulent pathogenic bacteria, if they have had access to matter from which these could be obtained.
This, in brief, is the status of the house fly as a disease carrier and it is readily seen why filth diseases are the ones naturally spread by this insect. Attracted to the nursing bottle or to the baby's mouth they may infect him with bacteria of any of the numerous enteric troubles known as summer complaint. If they have had access to the dejecta of a person ​suffering from typhoid fever, or to those of a chronic carrier of this disease, they may deposit the virulent bacteria upon food that later finds its way into our own bodies. Bacillary dysentery may be spread in the same fashion as well as many other gastro-enteric infections.
The house fly occurs practically throughout the entire civilized world and under all conditions is a continual menace to public health. In rural communities, however, where the proper disposal of waste matter of all kinds is most difficult, the importance of these insects is correspondingly increased.
The house fly is not the only insect which may act as a carrier of typhoid, for Dutton has shown experimentally that this may be spread by the bed-bug. These insects become infected through feeding on the blood of a person in the acute stage of the disease and for at least twenty-four hours retain the bacteria in a sufficiently virulent condition to inoculate a second person whom they may bite. That other biting insects such as fleas and mosquitoes may act in the same way is as yet unproved, but is by no means improbable.
Several other species of flies appear regularly in houses, but in far lesser numbers, and none exhibit to such a marked degree the peculiar tastes of the house fly, which wanders back and forth from filth to food, feeding on each in turn. In this method of feeding lies the danger of infection by house flies; they are equally fond of clean and filthy materials, and their frequent migrations from one to the other multiply their opportunities to pick up pathogenic organisms that may be later deposited upon foods.
The flea is another domestic insect which was looked upon only as a nuisance until it was shown that certain kinds of fleas are agents in spreading bubonic plague. The most terrible epidemics of which we have any historical record have been those of plague, or "black death." One swept from Egypt in the sixth century before the Christian era and invaded Europe and Asia, where it raged for sixty years. A similar one spread through the whole known world in the fourteenth century and is thought to have caused over twenty-five million deaths before it subsided.
In 1898 Simond suspected fleas as agents in the spread of plague and his suspicions have since been fully justified by Verjbitski and others. Plague is common to rats, certain other rodents and man, and is usually carried to man by the bites of fleas which have become infected from plague-stricken rats. The flea most commonly concerned is the rat flea, Læmopsylla cheopis. The transfer of plague bacteria is mechanical in nature, and other fleas, bed-bugs, etc., may also act as carriers, although far less commonly.
The plague bacilli (Bacillus pestis) appear only in fleas and bugs which have bitten affected persons or rats twelve to twenty-six hours ​previous to death, for after this time the bacilli do not occur in the blood. The vitality and virulence of the bacilli are preserved for nearly a week at least and sometimes fully a month; and there is actually an increase in their number during the first few days. Infection from these insects may then occur through their bites, if they contain extremely virulent bacilli, but probably occurs more commonly by the insects being crushed in situ after they have punctured the skin. Plague is confined more generally to the tropics and in recent years has threatened to become epidemic in the United States only in the region bordering San Francisco, Cal. Prompt measures of repression, based on a knowledge of the manner in which the disease spreads, have, however, been very successful and future wide-spread epidemics are not to be expected.
Plague is primarily a disease of rats, and its occurrence as a human disease is rather secondary, so nearly so that it can almost be said "no rats, no plague." On this account the destruction of rats is the first prophylactic measure to be undertaken for the suppression of plague, since this is much more readily accomplished than the destruction of the fleas directly. In parts of California the wild ground squirrel has become infected with plague from rats and presents a menace to the human population, although apparently not so great a one as the rat.
Another disease that has very recently been demonstrated to be insect-borne is typhus fever. This should not be confused with typhoid; it is a very different disease, occurring in the tropics and colder regions alike, and usually associated with dirty, unsanitary surroundings. On this account, it is becoming less prevalent in civilized countries every year, but has at times in the past claimed many victims. During our own civil war, the inmates of army prisons suffered greatly from the ravages of typhus fever, and similar conditions of crowding many people together in filthy surroundings have long been known to be favorable for the development of typhus fever epidemics. We now know through the researches of Ricketts and others that typhus is carried by vermin, the body louse, Pediculus vestimenti, acting as the vector. Thus the etiology of typhus has suddenly been made clear and we are in a position to formulate measures for prophylaxis and quarantine which will prevent the development of the disease in epidemic form. There is much yet to be learned; perhaps other insects also may act as carriers, but there can be no doubt that typhus is almost exclusively insect-borne.
Another disease which has puzzled the medical profession for centuries is a peculiar malady known as pellagra. Pellagra develops very slowly, and the origin of individual cases is correspondingly difficult to trace. It is also usually rather erratic or sporadic in occurrence, but appears to be rapidly increasing in prevalence in many parts of the United States. At one time it was thought that pellagra was contracted ​through eating moldy corn or corn products, and investigators went so far as to describe a certain fungus as the specific cause of the disease. This hypothesis was never satisfactorily in accord with the facts, and has been abandoned very generally in favor of a belief that pellagra is insect-borne. This has not yet been sustained by actual proof, and is far from being generally accepted, but Sambon and others have adduced much evidence to show that the "black-fly" (Simulium) may be the carrier for the virus of pellagra. These flies are widely distributed throughout the world, always occurring in proximity to rapidly flowing streams of water in which the larva? live. The adults, though small, are vicious biters. They appear mainly in the spring, more rarely in the fall, and agree in seasonal distribution with the incidence of pellagra. The causal organism has never been found and is evidently an ultramicroscopical or filterable virus.
One of the best known insect-borne diseases, and one which is of great importance in many parts of our own country is malarial fever, variously termed ague, chills and fever, etc. This was the first human disease traced directly to insect carriers and gave the impetus which has led to the unraveling of the facts connected with other insect-borne diseases. There are many types of malarial fevers, due to a number of similar but different blood parasites and the disease is most common in tropical regions, although in our own country it extends well into the northern states, even quite commonly into Massachusetts. The protozoan blood parasites that cause malaria were first demonstrated many years ago, in 1880, by a French surgeon, Laveran, who discovered them in the blood of persons suffering from malaria. Five years later an Italian, Golgi, distinguished three kinds, each associated with one of the more familiar types of malaria. They were found to go through a regular life cycle in the red blood corpuscles and, from analogy with other known Protozoa, it was suspected that in addition to their non-sexual generations in the human blood there must be a sexual development in some cold-blooded animal. Manson was led to suspect that some insect might be the secondary host and, working on this hypothesis, Eoss in India first found the malarial parasites in a certain kind of mosquito in 1898. He had worked for nearly three years on a common mosquito belonging to the genus Culex without result, but finally in a mosquito of the genus Anopheles was able to trace the development of the parasite. His epoch-making discovery has been since amply confirmed and extended by experimental proof till we now know that the various types of malarial blood parasites complete their life-cycles in anopheline mosquitoes, the latter acting as the sole carriers of the disease.
The details of growth and development of these parasites, which belong to the Protozoan genus Plasmodium, are extremely interesting, but far too complicated to discuss briefly. In general it may be said ​that the blood of persons suffering from malaria contains the parasitic organisms, and that these, on being taken into the stomach of the proper kind of mosquito, undergo certain changes and later penetrate the wall of the stomach to form vesicular swellings. Within these they multiply, and finally on the bursting of the nodule are set free in the body cavity and find their way to the salivary glands. After becoming infected, a period of twelve to twenty days are required for these changes in the mosquito. Then for a period of several weeks the virulent organisms remain in the salivary glands and if the mosquito bites a second person the parasites are introduced with the salivary secretion, through the puncture into the circulation. Here they multiply and produce another case of human malaria, which develops from ten days to three weeks after inoculation.
As previously stated, only certain mosquitoes can transmit malaria, for when the parasites are ingested by other species of mosquitoes they do not continue their development, but die without passing through the stomach into the salivary glands. In the United States only one form, Anopheles maculipennis, is capable of harboring malaria, but in other parts of the world, especially in the tropics, other species of Anopheles and related genera act as hosts for the several kinds of Plasmodium.
Although in temperate regions the number of deaths from malaria is rather small, in spite of the wide-spread occurrence of the disease, the economic loss is very great, due to the debilitated condition which invariably occurs in the population of malarial districts. In the tropics, however, malaria in its various forms causes an enormous number of deaths and predisposes its victims to so many other dread diseases that it ranks as perhaps the most important human disease.
Fortunately prophylactic measures against malaria are not difficult, although they have been shamefully neglected in our own country. They consist in the elimination of anopheline mosquitoes, which is best accomplished by the destruction of mosquitoes in general. The larval or preparatory stages of anopheline, and of practically all other mosquitoes, are passed in the water of small quiet ponds, puddles, exposed vessels containing water, rain-barrels, etc., and it is during this period that they are most easily controlled. This is accomplished by oiling the water with either crude or refined petroleum or with some miscible oil. The petroleum forms a film over the surface of the water through which the larvæ can not extrude their breathing tubes and they are thus suffocated. The application of miscible oils is efficacious, but attended with some danger, since it destroys fish and predatory insects which are themselves some of the most important natural enemies of mosquitoes. Very frequently even oiling is not necessary, as much swamp land may be permanently freed from mosquitoes by very simple systems of drainage ditches which prevent the accumulation of the stagnant water in which the larvæ occur.
​Anti-malarial work of this sort has been undertaken and successfully prosecuted in many parts of the world; in the Federated Malay States, in the Suez Canal region, at Havana, Cuba, and in the Panama Canal Zone. Many other regions might be mentioned, but we must look in vain for such concerted and sustained work in the United States. Attempts have been made by many entomologists and public-spirited citizens to inaugurate measures against mosquitoes on account of their relation to malaria, but with the excepion of the most successful work accomplished by Smith in New Jersey and by others on Long Island little has been done to aid the efforts of the energetic few. We have, however, reason to believe that such apathetic contemplation on the part of the American public will some day develop into an active interest, and that the population of our extensive malaria-ridden areas will gradually see the possibilities of improvement in public health through the destruction of the malarial mosquito.
Another mosquito-borne disease which has aroused more interest in America on account of its spectacular appearance and higher mortality is yellow fever. This is due to a filterable virus, concerning the nature of which we can only speculate at the present time, although enough has been ascertained through experimental work to demonstrate that the virus is a living organism which undergoes a development of definite periodicity in mosquitoes of a single species known as Stegomyia calopus. This mosquito enjoys a very wide distribution in many parts of the world, mainly in the tropics, but also extends into the warmer temperate regions. Yellow fever is not so extensively distributed, being absent in many places where Stegomyia occurs, but it is nevertheless present in many parts of the tropics in both hemispheres and all that is necessary for the development of a possible epidemic in a region where Stegomyia occurs is the introduction of a human case in the early stages of the fever.
The larval habits of Stegomyia are in quite marked contrast to those of Anopheles. The adults are strictly domestic mosquitoes and occur almost entirely in the neighborhood of human habitations. Their larvae occur in the same places, breeding preferably in vessels containing small amounts of water, rain barrels, cisterns, stray tin cans filled with rain water, etc. On this account, extermination work against the yellow-fever mosquito resolves itself mainly into the examination and treatment of cities, towns and the immediate environment of smaller settlements.
A Stegomyia feeding upon the blood of a person suffering from yellow fever becomes infected only during the first three or four days after the onset of the fever; later than this mosquitoes do not obtain the virus. An incubation period of at least twelve or fourteen days in the mosquito is now necessary before the mosquito can infect a second ​person, after which the Stegomyia remains infectious for a long period and may be responsible for a series of new cases. These facts were first discovered during the summer of 1900 by a Yellow Fever Commission consisting of Drs. Reed, Carroll, Lazear and Agramonte, of the U. S. Army. Two of these men, Carroll and Lazear, allowed themselves to be bitten by infected mosquitoes, and Lazear died from a severe case of fever thus contracted.
Little further has since been learned of the etiology of yellow fever, but wonderful strides have been made in the application of these simple facts for its eradication. In Cuba, where the commission conducted their investigations, the first attempts were made, and in 1902 yellow fever had been entirely eliminated in Havana. Other West Indian islands were formerly badly infested with yellow fever, but at the present time there is little more danger from this disease there than in the United States. Rio de Janeiro was once a hot bed for yellow fever, but it too has yielded to the destruction of mosquitoes and the screening of patients, till after a six years' fight, the fever has vanished. Still more remarkable are the results accomplished in the Panama Canal Zone under the direction of Dr. Gorgas. Here the warfare against yellow fever has gone hand in hand with anti-malarial work and the isthmus has been transformed from a veritable death-trap to a condition which compares favorably with that of any region on earth.
Our own country has suffered from yellow fever in the past, mainly in the south, but extending to southern Illinois in 1878, to Philadelphia in the terrible epidemic of 1793 and even to Boston and into interior New England towns in the earlier days. The last epidemic occurred during the summer of 1905 in New Orleans, where the application of rational methods rapidly checked the spread of the disease and resulted in its complete eradication long before cold weather. The success of this campaign has undoubtedly sounded the death knell of the yellow fever epidemic and panic in the United States, for New Orleans has amply demonstrated what may be accomplished in the control of an epidemic by an efficient group of workers, backed by a sympathetic public and supplied with reasonable funds.
Rocky Mountain spotted fever, an important human disease which occurs in certain parts of the Rocky Mountain region in the northern United States, has been shown to be insect-borne. In this case the vector is a tick, not a true insect, but a member of the arthropod group Acarina, whose members are so much like insects in many ways that it is hardly necessary to make any distinction in the present discussion. In 1902 Wilson and Chowning suggested that ticks might carry this disease, and four years later Ricketts definitely proved such to be the case. Spotted fever occurs in its most severe form with 70-80 per cent, mortality in western Montana, but extends into several other near-by ​states in a much milder form with only 5 per cent, mortality. A common wood-tick of that region, Dermacentor venustus, seems to be the only carrier under natural conditions, but recently Maver has shown experimentally that other ticks can transfer the' virus. One of these is Amblyomma americanum, a common form in the eastern states, and two others are members of the genus Dermacentor which occur also in the east. Whether spotted fever will eventually become established beyond its present range must remain a matter of conjecture, although there appears to be nothing that precludes such a grave possibility.
One of the most important insect-borne human diseases which does not exist in the new world is African sleeping sickness. In recent years this malady has decimated the native population in certain parts of eastern equatorial Africa and any extension of its range would be most serious. It seems very unlikely that America will ever have to face an epidemic, for the introduction of sleeping sickness together with its carrier is not at all probable, and the possibility of its becoming established, even after introduction, is still more remote. As is well known, sleeping sickness depends for its spread entirely upon certain biting flies known as tsetse-flies belonging to the same family as our common house fly and stable fly. The genus Glossina in which these flies are included is restricted to the African continent, but is there represented by a number of species, several of which have been shown to act as carriers for trypanosome diseases in animals. One only, Glossina palpalis, is known to carry the trypanosome of human sleeping sickness, Trypanosoma gambiense. The disease appears to have been originally endemic only in West Africa, but was found in eastern equatorial Africa something over ten years ago, and it is in this latter region that its ravages have been so pronounced. Owing to certain peculiarities in the habits of the tsetse-flies, the distribution of sleeping sickness is limited to very definite areas in the region where it occurs. The fly, which has a sharp needle-like beak for sucking blood, resembles our own, stable-fly (Stomoxys calcitrans) in general appearance but is considerably larger, measuring about half an inch in length. It is found only in the dense brush which grows along the edges of streams, ponds and lakes. In such places persons and animals may be bitten by the flies and it is exclusively through such bites that these insects may obtain virus of sleeping sickness from the blood of a person or animal suffering with the disease. Should the fly obtain a meal of blood containing trypanosomes, these may multiply in the body of the fly, although not always, for only about one in twenty of such flies becomes infectious. A considerable period must now elapse before the infected fly is in condition to inoculate a new patient, usually thirty or forty days, but after this for at least seventy-five days it remains infectious, and may introduce the trypanosomes into the blood of any animal upon which it feeds during this period.
​The tsetse-flies develop in a very different way from most insects. The female does not deposit her eggs, but a single one develops to the fully grown larval condition before being deposited. This larva soon pupates in the shade beneath the brush bordering the water where it has been dropped by the parent fly, and later emerges in the winged adult condition. The pupa? require such moist shade, and it is apparently for this reason alone that the flies never occur away from the immediate vicinity of the water. As a result of its method of development, the tsetse-flies do not multiply rapidly, and under favorable conditions only one larva is produced in a ten-day period.
The trypanosome of sleeping sickness was discovered by Bruce in 1902 and a year later the role of Glossina palpalis in its transmission was proved. Since then much energy has been expended in attempting to stamp out the disease by every possible means. It was thought at first, that by moving all the natives back from the edges of the water the flies thus left without opportunities for re-infection, would become free from trypanosomes, and that by isolating and treating cases of the disease in fly-free areas it would be possible to eliminate it entirely. In conjunction with this, the cutting of brush, especially about boat landings and watering-places, has been practised as far as possible. Contrary to expectations, it has been found that even after three or four years, infected flies still occur along the uninhabited shores. This led to experimentation upon animals and it is now known that various wild antelopes as well as certain domestic animals may act as reservoirs for the virus of sleeping sickness which may thus persist in the complete absence of any human subjects. As a result of this discovery the great difficulties of combatting the disease among the ignorant African natives have been vastly increased.
The regions surrounding the Mediterranean Sea are the centers of distribution for a very interesting, but far less dangerous insect-borne disease known as phlebotomus fever. In this case the carriers are minute gnat-like flies of the family Psychodidæ known as Phlebotomus papatasii. These insects are semi-aquatic in the larval condition, occurring in damp situations, drains, cellars, etc., where they feed on plant matter. The adult is a vicious biter in spite of the fact that it is scarcely over one millimeter in length. It rarely bites except at night, following the habits of certain mosquitoes in this respect. The specific cause of phlebotomus fever is not known, but it has been shown to be an invisible virus. At the present time it is impossible to state whether other insects may play a part in its transmission, although such does not seem probable. We have at least one species of Phlebotomus in the United States and it is possible that it might act as a vector should the disease be introduced into our country, although it would seem that such a ​possibility would have been realized already if it were likely to occur, for cases of this common European fever must undoubtedly have been imported.
We have already referred to ticks as carriers of spotted fever in this country. Another important disease, or group of closely similar diseases, known as relapsing fever, is known to be tick-borne. This malady is due to a very small spirochete, a protozoan organism known as Spirochæta recurrentis. In acute human cases of this disease these are present in the peripheral blood from whence they may be withdrawn by ticks. Within the alimentary tract of at least some species of ticks the parasites undergo a sort of development which is not well understood, entering the Malpighian tubules or other parts of the body and later assuming a somewhat different spirochete form. Infection of another person may then occur from a subsequent bite by the infected tick, the virus not passing into the body from the salivary glands or mouth, but entering the wound after having been excreted by the attached tick. As occurs with the tsetse-fly carriers of sleeping sickness, only a part, in this case about one third, of the ticks feeding upon a person with relapsing fever become infected themselves. Of those which do, however, some may transmit the infection to their offspring, which are then capable of infecting man with the virus thus received. Relapsing fever is very widely distributed, mainly in warm countries, although in Europe it has occurred in epidemic form as far north as St. Petersburg. The etiology and method of dissemination of the African type appear to be best known. A common tick, Ornithodoros moubata, was first found to act as vector and was until very recently believed to be the only carrier. Now it has been demonstrated that other ticks may act in the same way, and there is a possibility that other carriers may exist, probably in the form of blood-sucking insects.
Relapsing fever has been occasionally reported from the United States, but has never become established.
Among the less important insect-borne diseases is a very widespread tropical fever known as dengue which occasionally spreads into temperate regions in epidemic form. This is due to the presence in the red blood corpuscles of a protozoan smaller than the malarial parasite, probably a spirochete of some sort. However this may be, we know from experimental tests that dengue may be spread through the agency of certain mosquitoes. The widespread Culex fatigans is capable of transmitting the infection and there is good evidence to incriminate the yellow fever mosquito, Stegomyia calopus. Dengue is quite common in the southern United States, where in the minds of many people it is confused with malaria. It is less serious, however, although an even more unpleasant ailment to endure.
The same Culex fatigans has been shown to be at least partly ​responsible for the transmission of a parasitic disease of the tropics known as filariasis. The direct cause is a nematode worm belonging to the genus Filaria which is present in the circulation and lymphatics of the infected person. In the late stages of the disease the microscopic larval worms occur abundantly in the blood. For some unexplained reason they remain in the deep-seated blood vessels during the day, but usually appear more abundantly in the peripheral circulation during the night. Here they are readily obtained by mosquitoes with their meal of blood. In the alimentary canal of the mosquito the larval Filaria discards a sheath-like envelope which has previously invested it, and works its way through the wall of the stomach into the thoracic muscles where it increases greatly in size and finally migrates to the base of the proboscis. From two to three weeks are necessary for this metamorphosis, and for some time longer the Filaria may remain in the proboscis awaiting its opportunity to enter another person through the wound occasioned by the mosquito's beak. Once they have been transferred to their human host, the parasites enter the lymphatics where they attain sexual maturity and give rise to the abundant microscopic larval Filarias that reenter the circulation to await ingestion by another mosquito.
Filariasis is most common in equatorial regions, but extends less commonly into the subtropics. The parasites themselves do not ordinarily cause great inconvenience, but their presence in the lymphatics may clog these vessels to such an extent that secondary swellings may be developed in the limbs or other parts of the body.
Several insects have been associated with a peculiar tropical disease of the old world, variously known as kala-azar, dum-dum fever and leishmaniosis. In this case the organism is a flagellate protozoan, Leishmania donovani, of which there are possibly two forms, one producing a children's disease termed infantile kala-azar and the other the true leishmaniosis of adults. It has been shown that among domestic animals the dog at least is susceptible, and other animals may be also. In 1907 Patton discovered various stages of the Leishmania parasites in bed-bugs that had fed on persons suffering from kala-azar and this insect has been considered to be one, if not the exclusive, carrier. Very recently, however, some doubt has been expressed regarding the role of the bed-bug and a certain reduviid bug has been suspected. There seems to be no doubt, however, that kala-azar is insect-borne.
The diseases which we have enumerated are the more important insect-borne ones that affect man. A number of others of greater rarity or of minor nature are known to be carried regularly or occasionally by various insects, and many others are now being investigated in the light of present knowledge to ascertain if they, too, may not be spread by insects. It seems probable that flies may take some part in the dissemination of the bacilli of leprosy, although to how great an extent ​can not be said, and the same is true to a greater or less degree of cholera, pink-eye, yaws, syphilis, and many other diseases which can not be considered as typically insect-borne.
One other disease which has been increasing at an alarming rate in our own country during the past several decades is infantile paralysis. This malady occurs in certain parts of Europe, whence it is probable that it was brought to America. As a rule it affects children during the first few years of life and, although the mortality is not so very great, a majority of the children affected are left permanently lame after recovery. The virus of this disease is an ultramicroscopic organism which causes lesions of the spinal cord that sometimes lead to paralysis. At the present time it appears probable that infantile paralysis is insect-borne, and it has been suggested by Brues and Sheppard that the stable fly, Stomoxys calcitrans, acts as a carrier of the virus, although it is quite possible that some other insect also may be concerned.
No account of insect-borne diseases, however brief, could be complete without some reference to animal diseases. A few of these have already been referred to incidentally as affecting both man and animals, and it is quite likely that other human diseases whose etiology is at present obscure, will in the future be shown to bear some relation to those of animals. Apart from this, the economic loss occasioned by such affections of domestic animals is enormous, although it is in great part preventable.
A wide-spread disease of cattle in the southern part of the United States, known as splenetic fever, or "Texas fever," is the most important insect-borne animal disease that occurs in this country, and is particularly interesting since it was the first disease of any kind shown to be carried exclusively by insects or ticks. It occurs very generally throughout the gulf states as far north as the thirty-sixth parallel of latitude and is the cause of immense pecuniary loss to this region, not only on account of the cattle lost, but as a result of the greatly weakened condition of the animals in general. Southern cattle are usually immunized by an attack at an early age, but northern animals die in large numbers when exposed to the disease.
Smith and Kilborne showed, in 1893, that the protozoan blood-parasite, Piroplasma bigeminum, which Smith had discovered several years earlier to be the cause of the disease, is carried by ticks. The common cattle-tick of the southern United States, Margaropus annulatus, acts as the exclusive vector, becoming infected during its period of engorgement when feeding on the blood of a diseased animal and then ​transmitting the Piroplasma through its eggs to the young ticks of the next generation. These may feed on healthy animals the next season, conveying to them the parasites that have been handed down from the mother tick.
Several similar diseases of cattle occur in other parts of the world. In Africa, related forms of Piroplasma carried by ticks are the cause of redwater, East Coast fever, Rhodesian fever, and in various parts of the world other piroplasmoses have been observed in many animals.
Spirochætosis in animals, due to organisms similar to those producing relapsing fever, is well known. The most familiar example is probably a disease of fowls which is carried by Argas miniatus, a common tick which infests these birds.
Trypanosomiasis is a general term for diseases like sleeping sickness due to trypanosomes and there are many diseases of this type, among which may be mentioned an old-world affection of horses known as Surra; an African one, Nagana, that attacks other domestic animals as well; and a South American type termed Mai de Caderas. Flies are the insects implicated in the transmission of these diseases, mainly the large tabanid horseflies and the smaller stable flies of the genus Stomoxys. Surra was recently introduced into the United States, but was successfully stamped out before it had become established.
Among bacterial diseases of animals, anthrax may be mentioned as one which is sometimes transmitted by biting flies, the insects acting as mechanical or contaminative carriers only.
The foregoing enumeration of insect-borne diseases is by no means complete. Indeed, it would be well-nigh impossible to make it so, in view of the rapid strides which are being made at the present time toward a knowledge of these many problems which bear on the question of public health. New discoveries are being rapidly announced in all parts of the world, and while it is difficult to see how the fundamentally important revelations of the past fifteen years can be equalled in the near future, we should be very unwise to predict that they will not be exceeded.