CHAPTER VIII ARTHROPODS AS ESSENTIAL HOSTS OF PATHOGENIC PROTOZOA Mosquitoes and Malaria

Under the name of malaria is included a group of morbid symptoms formerly supposed to be due to a miasm or bad air, but now known to be caused by protozoan parasites of the genus Plasmodium, which attack the red blood corpuscles. It occurs in paroxysms, each marked by a chill, followed by high fever and sweating. The fever is either intermittent or remittent.

There are three principal types of the disease, due to different species of the parasite. They are:

1. The benign-tertian, caused by Plasmodium vivax, which undergoes its schizogony or asexual cycle in the blood in forty-eight hours or even less. This type of the disease,—characterized by fever every two days, is the most wide-spread and common.

2. The quartan fever is due to the presence of Plasmodium malariÆ, which has an asexual cycle of seventy-two hours, and therefore the fever recurs every three days. This type is more prevalent in temperate and sub-tropical regions, but appears to be rare everywhere.

3. The sub-tertian "Æstivo-autumnal," or "pernicious" fever is caused by Plasmodium falciparum. Schizogony usually occurs in the internal organs, particularly in the spleen, instead of in the peripheral circulation, as is the case of the tertian and quartan forms. The fever produced is of an irregular type and the period of schizogony has not been definitely determined. It is claimed by some that the variations are due to different species of malignant parasites.

It is one of the most wide-spread of human diseases, occurring in almost all parts of the world, except in the polar regions and in waterless deserts. It is most prevalent in marshy regions.

So commonplace is malaria that it causes little of the dread inspired by most of the epidemic diseases, and yet, as Ross says, it is perhaps the most important of human diseases. Figures regarding its ravages are astounding. Celli estimated that in Italy it caused an average annual mortality of fifteen thousand, representing about two million cases. In India alone, according to Ross (1910) "it has been officially estimated to cause a mean annual death-rate of five per thousand; that is, to kill every year, on the average, one million one hundred and thirty thousand." In the United States it is widespread and though being restricted as the country develops, it still causes enormous losses. During the year 1911, "in Alabama alone there were seventy thousand cases and seven hundred and seventy deaths." The weakening effects of the disease, the invasion of other diseases due to the attacks of malaria, are among the very serious results, but they cannot be estimated.

Not only is there direct effect on man, but the disease has been one of the greatest factors in retarding the development of certain regions. Everywhere pioneers have had to face it, and the most fertile regions have, in many instances been those most fully dominated by it. Herrick (1903) has presented an interesting study of its effects on the development of the southern United States and has shown that some parts, which are among the most fertile in the world, are rendered practically uninhabitable by the ravages of malaria. Howard (1909) estimates that the annual money loss from the disease in the United States is not less than $100,000,000.

It was formerly supposed that the disease was due to a miasm, to a noxious effluvia, or infectious matter rising in the air from swamps. In other words its cause was, as the name indicated "mal aria," and the deep seated fear of night air is based largely on the belief that this miasm was given off at night. Its production was thought to be favored by stirring of the soil, dredging operations and the like.

The idea of some intimate connection between malaria and mosquitoes is not a new one. According to Manson, Lancisi noted that in some parts of Italy the peasants for centuries have believed that malaria is produced by the bite of mosquitoes. Celli states that one not rarely hears from such peasants the statement that "In such a place, there is much fever, because it is full of mosquitoes." Koch points out that in German East Africa the natives call malaria and the mosquito by the same name, MbÙ. The opinion was not lacking support from medical men. Celli quotes passages from the writings of the Italian physician, Lancisi, which indicate that he favored the view in 1717.

Dr. Josiah Nott is almost universally credited with having supported the theory, in 1848, but as we have already pointed out his work has been misinterpreted. The statements of Beauperthuy, (1853) were more explicit.

The clearest early presentation of the circumstantial evidence in favor of the theory of mosquito transmission was that of A. F. A. King, an American physician, in 1883. He presented a series of epidemiological data and showed "how they may be explicable by the supposition that the mosquito is the real source of the disease, rather than the inhalation or cutaneous absorption of a marsh vapor." We may well give the space to summarizing his argument here for it has been so remarkably substantiated by subsequent work:

1. Malaria, like mosquitoes, affects by preference low and moist localities, such as swamps, fens, jungles, marshes, etc.

2. Malaria is hardly ever developed at a lower temperature than 60° Fahr., and such a temperature is necessary for the development of the mosquito.

3. Mosquitoes, like malaria, may both accumulate in and be obstructed by forests lying in the course of winds blowing from malarious localities.

4. By atmospheric currents malaria and mosquitoes are alike capable of being transported for considerable distances.

5. Malaria may be developed in previously healthy places by turning up the soil, as in making excavations for the foundation of houses, tracks for railroads, and beds for canals, because these operations afford breeding places for mosquitoes.

6. In proportion as countries, previously malarious, are cleared up and thickly settled, periodical fevers disappear, because swamps and pools are drained so that the mosquito cannot readily find a place suitable to deposit her eggs.

7. Malaria is most dangerous when the sun is down and the danger of exposure after sunset is greatly increased by the person exposed sleeping in the night air. Both facts are readily explicable by the mosquito malaria theory.

8. In malarial districts the use of fire, both indoors and to those who sleep out, affords a comparative security against malaria, because of the destruction of mosquitoes.

9. It is claimed that 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. 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 lights in the suburbs, that many of them will in this way be prevented from penetrating "far into the interior."

10. Malarial diseases and likewise mosquitoes are most prevalent toward the latter part of summer and in the autumn.

11. Various writers have maintained that malaria is arrested by canvas curtains, gauze veils and mosquito nets and have recommended the rise of mosquito curtains, "through which malaria can seldom or never pass." It can hardly be conceived that these intercept marsh-air but they certainly do protect from mosquitoes.

12. Malaria spares no age, but it affects infants much less frequently than adults, because young infants are usually carefully housed and protected from mosquito inoculation.

Correlated with the miasmatic theory was the belief that some animal or vegetable organism which lived in marshes, produced malaria, and frequent searches were made for it. Salisbury (1862) thought this causative organism to be an alga, of the genus Palmella; others attributed it to certain fungi or bacteria.

In 1880, the French physician, Laveran, working in Algeria, discovered an amoeboid organism in the blood of malarial patients and definitely established the parasitic nature of this disease. Pigmented granules had been noted by Meckel as long ago as 1847, in the spleen and blood of a patient who had died of malaria, and his observations had been repeatedly verified, but the granules had been regarded as degeneration products, and the fact that they occurred in the body of a foreign organism had been overlooked.

Soon after the discovery of the parasites in the blood, Gerhardt (1884) succeeded in transferring the disease to healthy individuals by inoculation of malarious blood, and thus proved that it is a true infection. This was verified by numerous experimenters and it was found that inoculation with a very minute quantity of the diseased blood would not only produce malaria but the particular type of disease.

Laveran traced out the life cycle of the malarial parasite as it occurs in man. The details as we now know them and as they are illustrated by the accompanying figure125, are as follows:

The infecting organism or sporozoite, is introduced into the circulation, penetrates a red blood corpuscle, and forms the amoeboid schizont. This lives at the expense of the corpuscle and as it develops there are deposited in its body scattered black or reddish black particles. These are generally called melanin granules, but are much better referred to as hÆmozoin, as they are not related to melanin. The hÆmozoin is the most conspicuous part of the parasite, a feature of advantage in diagnosing from unstained preparations.

As the schizont matures, its nucleus breaks up into a number of daughter nuclei, each with a rounded mass of protoplasm about it, and finally the corpuscles are broken down and these rounded bodies are liberated in the plasma as merozoites. These merozoites infect new corpuscles and thus the asexual cycle is continued. The malarial paroxysm is coincident with sporulation.

As early as Laveran's time it was known that under conditions not yet determined there are to be found in the blood of malarious patients another phase of the parasite, differing in form according to the type of the disease. In the pernicious type these appear as large, crescent-shaped organisms which have commonly been called "crescents." We now know that these are sexual forms.

When the parasite became known there immediately arose speculations as to the way in which it was transferred from man to man. It was thought by some that in nature it occurred as a free-living amoeba, and that it gained access to man through being taken up with impure water. However, numerous attempts to infect healthy persons by having them drink or inhale marsh water, or by injecting it into their circulation resulted in failure, and influenced by Leuckart's and Melnikoff's work on Dipylidium, that of Fedtschenko on Dracunculus, and more especially by that of Manson on Filaria, search was made for some insect which might transfer the parasite.

Laveran had early suggested that the rÔle of carrier might be played by the mosquito, but Manson first clearly formulated the hypothesis, and it was largely due to his suggestions that Ross in India, undertook to solve the problem. With no knowledge of the form or of the appearance in this stage, or of the species of mosquito concerned, Ross spent almost two and a half years of the most arduous work in the search and finally in August, 1897, seventeen years after the discovery of the parasite in man, he obtained his first definite clue. In dissecting a "dappled-winged mosquito," "every cell was searched and to my intense disappointment nothing whatever was found, until I came to the insect's stomach. Here, however, just as I was about to abandon the examination, I saw a very delicate circular cell, apparently lying amongst the ordinary cells of the organ and scarcely distinguishable from them. On looking further, another and another similar object presented itself. I now focused the lens carefully on one of these, and found that it contained a few minute granules of some black substance, exactly like the pigment of the parasite of malaria. I counted altogether twelve of these cells in the insect."

Further search showed that "the contents of the mature pigment cells did not consist of clear fluid but of a multitude of delicate, thread-like bodies which on the rupture of the parent cell, were poured into the body cavity of the insect. They were evidently spores."

With these facts established, confirmation and extension of Ross's results quickly followed, from many different sources. We cannot trace this work in detail but will only point out that much of the credit is due to the Italian workers, Grassi, Bignami, and Bastianelli, and to Koch and Daniels.

It had already been found that when fresh blood was mounted and properly protected against evaporation, a peculiar change occurred in these crescents after about half an hour's time. From certain of them there were pushed out long whip-like processes which moved with a very active, lashing movement. The parasite at this stage is known as the "flagellated body." Others, differing somewhat in details of structure, become rounded but do not give off "flagella."

The American worker, MacCallum (1897), in studying bird malaria as found in crows, first recognized the true nature of these bodies. He regarded them as sexual forms and believed that the so-called flagella played the part of spermatozoa. Thus, the "flagellated body" is in reality a microgametoblast, producing microgametes, or the male sexual element, while the others constitute the macrogametes, or female elements.

It was found that when blood containing these sexual forms was sucked up by an Anopheline mosquito and taken into its stomach, a microgamete penetrated and fertilized a macrogamete in a way analogous to what takes place in the fertilization of the egg in higher forms. The resultant, mobile organism is known as the migratory ookinete. In this stage the parasite bores through the epithelial lining of the "stomach" (mid-intestine) of the mosquito and becomes encysted under the muscle layers. Here the oocyst, as it is now known, matures and breaks up into the body cavity and finally its products come to lie in the salivary glands of the mosquito. Ten to twelve days are required for these changes, after which the mosquito is infective, capable of introducing the parasite with its saliva, when feeding upon a healthy person.

Thus the malarial parasite is known to have a double cycle, an alternation of generations, of which the asexual stage is undergone in man, the sexual in certain species of mosquitoes. The mosquito is therefore the definitive host rather than the intermediate, as usually stated.

The complicated cycle may be made clearer by the diagram of Miss Stryke (1912) which, by means of a double-headed mosquito (fig.126) endeavors to show how infection takes place through the biting of the human victim, (at A), in whom asexual multiplication then takes place, and how the sexual stages, taken up at B in the diagram, are passed in the body of the mosquito.

The experimental proof that mosquitoes of the Anopheline group are necessary agents in the transmission of malaria was afforded in 1900 when two English physicians, Drs. Sambon and Low lived for the three most malarial months in the midst of the Roman Campagna, a region famous for centuries as a hot-bed of malaria. The two experimenters moved about freely throughout the day, exposed themselves to rains and all kinds of weather, drank marsh water, slept exposed to the marsh air, and, in short, did everything which was supposed to cause malaria, except that they protected themselves thoroughly from mosquito bites, retiring at sunset to a mosquito-proof hut. Though they took no quinine and all of their neighbors suffered from malaria, they were absolutely free from the disease.

To complete the proof, mosquitoes which had fed in Rome on malarious patients were sent to England and allowed to bite two volunteers, one of them Dr. Manson's own son, who had not been otherwise exposed to the disease. Both of these gentlemen contracted typical cases of malaria and the parasites were to be found in abundance in their blood.

Since that time there have been many practical demonstrations of the fact that malaria is transmitted exclusively by the bite of mosquitoes and that the destruction of the mosquitoes means the elimination of the disease.

We have said that the malarial parasite is able to undergo its development only in certain species of mosquitoes belonging to the Anopheline group. It is by no means certain that all of this group even, are capable of acting as the definitive host of the parasites, and much careful experiment work is still needed along this line. In the United States, several species have been found to be implicated, Anopheles quadrimaculatus and Anopheles crucians being the most common. The characteristics of these species and the distinctions between them and other mosquitoes will be discussed in Chapter XII.

In antimalarial work it is desirable to distinguish the anopheline mosquitoes from the culicine species in all stages. The following tabulation presents the more striking distinctions between the groups as represented in the United States.

These malarial-bearing species are essentially domesticated mosquitoes. They develop in any accumulation of water which stands for a week or more. Ponds, puddles, rain barrels, horse troughs, cess-pools, cans, even the foot-prints of animals in marshy ground may afford them breeding places.

It is clear from what has been said regarding the life cycle of the malarial parasite that the mosquito is harmless if not itself diseased. Hence malarial-bearing species may abound in the neighborhood where there is no malaria, the disease being absent simply because the mosquitoes are uninfected. Such a locality is potentially malarious and needs only the introduction of a malarial patient who is exposed to the mosquitoes. It is found that such patients may harbor the parasites in their blood long after they are apparently well and thus may serve as a menace, just as do the so-called typhoid carriers. In some malarious regions as high as 80-90 per cent of the natives are such malaria-carriers and must be reckoned with in antimalaria measures.

Based upon our present day knowledge of the life cycle of the malarial parasite the fight against the disease becomes primarily a problem in economic entomology,—it is a question of insect control, in its broadest interpretation.

The lines of defence and offence against the disease as outlined by Boyce (1909) are:

Mosquitoes and Yellow Fever

Yellow fever was until recently one of the most dreaded of epidemic diseases. It is an acute, specific and infectious disease, non-contagious in character but occurring in epidemics, or endemics, within a peculiarly limited geographical area. It is highly fatal, but those who recover are generally immune from subsequent attacks.

It is generally regarded as an American disease, having been found by Cortez, in Mexico, and being confined principally to the American continents and islands. It also occurs in Africa and attempts have been made to show that it was originally an African disease but there is not sufficient evidence to establish this view.

There have been many noted outbreaks in the United States. Boston suffered from it in 1691 and again in 1693; New York in 1668 and as late as 1856; Baltimore in 1819. In 1793 occurred the great epidemic in Philadelphia, with a death rate of one in ten of the population. In the past century it was present almost every year in some locality of our Southern States, New Orleans being the greatest sufferer. In the latter city there were 7848 deaths from the disease in 1853, 4854 in 1858, and 4046 in 1878. The last notable outbreak was in 1905. Reed and Carroll (1901) estimated that during the period from 1793 to 1900 there had not been less than 500,000 cases in the United States.

As in the case of the plague, the most stringent methods of control proved ineffective and helplessness, almost hopelessness marked the great epidemics. A vivid picture of conditions is that given by Mathew Cary, 1793 (quoted by Kelly, 1906) in "A Short Account of the Malignant Fever Lately Prevalent in Philadelphia."

"The consternation of the people of Philadelphia at this period was carried beyond all bounds. Dismay and affright were visible in the countenance of almost every person. Of those who remained, many shut themselves in their houses and were afraid to walk the streets. * * * The corpses of the most respectable citizens, even those who did not die of the epidemic, were carried to the grave on the shafts of a chair (chaise), the horse driven by a negro, unattended by friends or relative, and without any sort of ceremony. People hastily shifted their course at the sight of a hearse coming toward them. Many never walked on the footpath, but went into the middle of the streets to avoid being infected by 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 crape, 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 until pretty late in September."

Such was the condition in Philadelphia in 1793 and, as far as methods of control of the disease were concerned, there was practically no advance during the last century. The dominant theory was that yellow fever was spread by fomites, that is, exposed bedding, clothing, baggage, and the like. As late as 1898 a bulletin of the United States Marine Hospital Service stated:

"While yellow fever is a communicable disease, it is not contagious in the ordinary acceptance of the term, but is spread by the infection of places and articles of bedding, clothing, and furniture."

Based upon this theory, houses, baggage, freight, even mail, were disinfected, and the most rigid quarantine regulations were enforced. The hardships to which people of the stricken regions were subjected and the financial losses are incalculable. And withal, the only efficient check upon the disease seemed to be the heavy frosts. It was found that for some reason, the epidemic abated with cold weather,—a measure beyond human control.

It is not strange that among the multitude of theories advanced to explain the cause and method of dissemination of the disease there should be suggestions that yellow fever was transmitted by the mosquito. We have seen that Beauperthuy (1855) clearly urged this theory.

More detailed, and of the greatest influence in the final solution of the problem were the arguments of Dr. CÁrlos Finlay, of Havana. In 1881, in a paper presented before the "Real Academia de Ciencias MÉdicas, FÍsicas y Naturales de la Habana," he said:

"I feel convinced that any theory which attributes the origin and the propagation of yellow fever to atmospheric influences, to miasmatic or meteorological conditions, to filth, or to the neglect of general hygienic precautions, must be considered as utterly indefensible."

He postulated the existence of a material transportable substance causing yellow fever,—"something tangible which requires to be conveyed from the sick to the healthy before the disease can be propagated" and after discussing the peculiarities of the spread of the disease and the influence of meteorological conditions, he decides that the carriers of the disease must be sought among insects. He continues:

"On the other hand, the fact of yellow fever being characterized both clinically and (according to recent findings) histologically, by lesions of the blood vessels and by alterations of the physical and chemical conditions of the blood, suggested that the insect which should convey the infectious particles from the patient to the healthy should be looked for among those which drive their sting into blood vessels in order to suck human blood. Finally, by reason of other considerations which need not be stated here, I came to think that the mosquito might be the transmitter of yellow fever."

"Assimilating the disease to small-pox and to vaccination, it occurred to me that in order to inoculate yellow fever it would be necessary to pick out the inoculable material from within the blood vessels of a yellow fever patient and to carry it likewise into the interior of a blood vessel of a person who was to be inoculated. All of which conditions the mosquito satisfies most admirably through its bite."

In the course of his study of the problem, Finlay made detailed studies of the life history and habits of the common mosquitoes at Havana, and arrived at the conclusion that the carrier of the yellow fever was the Culex mosquito or AËdes calopus, as it is now known. With this species he undertook direct experimental tests, and believed that he succeeded in transmitting the disease by the bite of infected mosquitoes in three cases. Unfortunately, possibility of other exposure was not absolutely excluded, and the experiments attracted little attention.

Throughout the next twenty years Finlay continued his work on yellow fever, modifying his original theory somewhat as time went on. Among his later suggestions was that in the light of Smith's work on Texas fever, his theory must be "somewhat modified so as to include the important circumstance that the faculty of transmitting the yellow fever germ need not be limited to the parent insect, directly contaminated by stinging a yellow fever patient (or perhaps by contact with or feeding from his discharges), but may be likewise inherited by the next generation of mosquitoes issued from the contaminated parent." He believed that the bite of a single mosquito produced a light attack of the disease and was thus effective in immunizing the patient. Throughout the period, many apparently successful attempts to transmit the disease by mosquitoes were made. In the light of present day knowledge we must regard these as defective not only because possibility of other infection was not absolutely excluded but because no account was taken of the incubation period within the body of the mosquito.

In 1900, while the American army was stationed in Cuba there occurred an epidemic of yellow fever and an army medical board was appointed for "the purpose of pursuing scientific investigations with reference to the acute infectious diseases prevalent on the island." This was headed by Walter Reed and associated with him were James Carroll, Jesse W. Lazear and Aristides Agramonte, the latter a Cuban immune. For a detailed summary of this work the lay reader cannot do better than read Dr. Kelly's fascinating biography "Walter Reed and Yellow Fever."

Arriving at the army barracks near Havana the Commission first took up the study of Bacillus icteroides, the organism which Sanarelli, an Italian physician, had declared the causative agent in yellow fever. They were unable to isolate this bacillus either from the blood during life or from the blood and organs of cadavers and therefore turned their attention to Finlay's theory of the propagation of yellow fever by means of the mosquito. In this work they had the unselfish and enthusiastic support of Dr. Finlay himself, who not only consulted with them and placed his publications at their disposal, but furnished eggs from which their experimental mosquitoes were obtained. Inoculations of eleven non-immunes through the bite of infected mosquitoes were made, and of these, two gave positive results. The first of the two was Dr. Carroll who allowed himself to be bitten by a mosquito which had been caused to feed upon four cases of yellow fever, two of them severe and two mild. The first patient had been bitten twelve days before.

Three days after being bitten, Dr. Carroll came down with a typical case of yellow fever. So severe was the attack that for three days his life hung in the balance. During his convalescence an incident occurred which showed how the theory of mosquito transmission of the disease was generally regarded. We quote from Dr. Kelly: "One of his nurses who came from Tennessee had had considerable experience with yellow fever, having indeed, lost her husband and several children from it. One day early in his illness Dr. Carroll mentioned to her that he had contracted the disease through the bite of a mosquito, and noticed that she looked surprised. Some time later, when well enough to look over the daily records of his condition, he found this entry: 'Says he got his illness through the bite of a mosquito,—delirious'."

The second case was that of an American who was bitten by four mosquitoes, two of which had bitten severe (fatal) cases of yellow fever twelve days previously, one of which had bitten a severe case (second day) sixteen days before and one which had bitten a severe case eight days before. Five days later, the subject developed a well pronounced but mild case of the disease.

In the meantime, another member of the Commission, Dr. Lazear, was accidentally bitten by a mosquito while collecting blood from yellow fever patients. Five days later he contracted a typical case which resulted fatally.

So clear was the evidence from these preliminary experiments that the commission felt warranted in announcing, October 27, 1900, that, "The mosquito serves as the intermediate host for the parasite of yellow fever, and it is highly probable that the disease is only propagated through the bite of this insect."

In order to extend the experimental evidence under conditions which could leave no possibility of infection from other sources, a special experimental sanitary station, named in honor of the deceased member of the Commission, was established in an open field near the town of Quemados, Cuba. Here there were constructed two small buildings known respectively as the "infected clothing building" and the "infected mosquito building."

The infected clothing building, 14 × 20 feet in size, was purposely so constructed as to exclude anything like efficient ventilation, but was thoroughly screened to prevent the entrance of mosquitoes. Into this building were brought sheets, pillow-slips, blankets, etc., contaminated by contact with cases of yellow fever and their discharges,—many of them purposely soiled with a liberal quantity of black vomit, urine, and fecal matter from patients sick with yellow fever. Nothing could better serve as the fomites which were supposed to convey the dread disease.

Three non-immunes unpacked these articles, giving each a thorough handling and shaking in order to disseminate through the air of the room the specific agent of the disease. They were then used in making up the beds which the volunteers occupied each night for a period of twenty days. The experiment was repeated three times, volunteers even sleeping in the soiled garments of yellow fever victims but in not a single case was there the slightest symptom of disease. The theory of the spread of yellow fever by fomites was completely demolished.

The infected mosquito building, equal in size to its companion, was the antithesis as far as other features were concerned. It was so constructed as to give the best possible ventilation, and bedding which was brought into it was thoroughly sterilized. Like the infected clothing building it was carefully screened, but in this case it was in order to keep mosquitoes in it as well as to prevent entrance of others. Through the middle of the room ran a mosquito-proof screen.

On December 5, 1900, a non-immune volunteer who had been in the quarantine camp for fifteen days and had had no other possible exposure, allowed himself to be bitten by five mosquitoes which had fed on yellow fever patients fifteen or more days previously. The results were fully confirmatory of the earlier experiments of the Commission—at the end of three days, nine and a half hours, the subject came down with a well marked case of yellow fever.

In all, ten cases of experimental yellow fever, caused by the bite of infected mosquitoes were developed in Camp Lazear. Throughout the period of the disease, other non-immunes slept in the little building, separated from the patient only by the mosquito-proof screen, but in no circumstances did they suffer any ill effects.

It was found that a yellow fever patient was capable of infecting mosquitoes only during the first three or four days after coming down with the disease. Moreover, after the mosquito has bitten such a patient, a period of at least twelve days must elapse before the insect is capable of transmitting the disease.

Once the organism has undergone its twelve day development, the mosquito may remain infective for weeks. In experiments of the Commission, two of the mosquitoes transmitted the disease to a volunteer fifty-seven days after their contamination. No other volunteers presenting themselves, one of these mosquitoes died the sixty-ninth and one the seventy-first day after their original contamination, without it being determined whether they were still capable of transmitting the disease.

So carefully carried out was this work and so conclusive were the results that Dr. Reed was justified in writing:

"Six months ago, when we landed on this island, absolutely nothing was known concerning the propagation and spread of yellow fever—it was all an unfathomable mystery—but today the curtain has been drawn—its mode of propagation is established and we know that a case minus mosquitoes is no more dangerous than one of chills and fever."

The conclusions of the Commission were fully substantiated by numerous workers, notably Dr. Guiteras of the Havana Board of Health, who had taken a lively interest in the work and whose results were made known in 1901, and by the Brazilian and French Commission at Sao Paulo, Brazil, in 1903.

Throughout the work of the Army Commission and down to the present time many fruitless efforts have been made to discover the specific organism of yellow fever. It was clearly established that the claims of Sanarelli for Bacillus icteroides were without foundation. It was found, too, that whatever the infective agent might be it was capable of passing through a Berkefeld filter and thus belongs to the puzzling group of "filterable viruses." It was further found that the virus was destroyed by heating up to 55° C for ten minutes. It is generally believed that the organism is a Protozoan.

The question of the hereditary transmission of the yellow fever organism within the mosquito was left unsettled by the Army Commission, though, as we have seen, it was raised by Finlay. Marchoux and Simond, of the French Commission devoted much attention to this phase of the problem and basing their conclusions on one apparently positive case, they decided that the disease could be transmitted through the egg of an infected AËdes calopus to the second generation and thence to man. The conclusion, which is of very great importance in the control of yellow fever, has not been verified by other workers.

Once clearly established that yellow fever was transmitted solely by mosquitoes, the question of the characteristics, habits, and geographical distribution of the insect carrier became of vital importance.

AËdes calopus, more commonly known as Stegomyia fasciata or Stegomyia calopus (fig.134) is a moderate sized, rather strikingly marked mosquito. The general color is dark-brown or reddish-brown, but the thorax has a conspicuous broad, silvery-white curved line on each side, with two parallel median silvery lines. Between the latter there is a slender, broken line. The whole gives a lyre-shaped pattern to the thorax. The abdomen is dark with silvery-white basal bands and silvery white spots on each side of the abdominal segments. Legs black with rings of pure white at the base of the segments.

Size of the female 3.3 to 5 mm.; male 3 to 4.5 mm.

It is preeminently a domesticated species, being found almost exclusively about the habitation of man. "Its long association with man is shown by many of its habits. It approaches stealthily from behind. It retreats upon the slightest alarm. The ankles and, when one is sitting at a table or desk, the underside of the hands and wrists are favorable points of attack. It attacks silently, whereas other mosquitoes have a piping or humming note. The warning sound has doubtless been suppressed in the evolutionary process of its adaptation to man. It is extremely wary. It hides whenever it can, concealing itself in garments, working into the pockets, and under the lapels of coats, and crawling up under the clothes to bite the legs. In houses, it will hide in dark corners, under picture moldings and behind the heads of old-fashioned bedsteads. It will enter closets and hide in the folds of garments."—Howard.

It was claimed by the French Commission, and subsequently often stated in discussions of the relation of the mosquito to yellow fever that the mature AËdes calopus will bite only at night. If this were true it would be of the greatest importance in measures to avoid the disease. Unfortunately, the claim was illy founded and numerous workers have clearly established that the exact converse is more nearly true, this mosquito being pre-eminently a day species, feeding most actively in early morning, about sunrise, and late in the afternoon. On cloudy days it attacks at any time during the day. Thus there is peril in the doctrine that infected regions may be visited with perfect safety during the daytime and that measures to avoid the mosquito attack need be taken only at night.

Dr. Finlay maintained that the adult, even when starved, would not bite when the temperature was below 23°C, but subsequent studies have shown that this statement needs modification. The French Commission, working at Rio de Janeiro, found that AËdes calopus would bite regularly at temperatures between 22° and 25° and that the optimum temperature was between 27° and 30° C, but their experiments led them to believe that it would bite in nature at a temperature as low as 17° C.

The yellow fever mosquito breeds in cisterns, water barrels, pitchers and in the various water receptacles about the house. In our own Southern States it very commonly breeds in the above-ground cisterns which are in general use. Often the larvÆ (fig.135b) are found in flower vases, or even in the little cups of water which are placed under the legs of tables to prevent their being overrun by ants. They have been repeatedly found breeding in the holy water font in churches. In short, they breed in any collection of water in close proximity to the dwellings or gathering places of man.

The life cycle under favorable conditions is completed in from twelve to fifteen days. These figures are of course very dependent upon the temperature. The Army Commission in Cuba found that the cycle might be completed in as brief a period as nine and a half days. Under less favorable conditions it may be greatly lengthened.

The adults are long lived. We have seen that during the experimental work in Cuba specimens were kept in captivity for sixty-nine and seventy-one days, respectively, and that they were proved to retain their infectivity for at least fifty-seven days. Dr. Guiteras subsequently kept an infected adult for one hundred and fifty-four days.

Low temperatures have a very great effect not only on development, but on the activity and even life of the adults. Long before the method of transmission of yellow fever was discovered it was well known that the epidemics were brought to a close by heavy frosts, and it is now known that this is due to the killing of the mosquitoes which alone could spread the disease.

AËdes calopus has a very wide distribution since, as Howard says, being a domestic mosquito, having a fairly long life in the adult stage, and having the custom of hiding itself in the most ingenious ways, it is particularly subject to carriage for long distances on board vessels, in railway trains, even packed in baggage. In general, its permanent distribution is from 40 degrees north latitude to 40 degrees south latitude (Brumpt), in a belt extending around the world. In the United States it breeds in most of our Southern States.

Thus, as in the case of malaria, there are many places where the insect carrier is abundant but where yellow fever does not occur. Such, for instance, are Hawaii, Australia and Asia. An outbreak may occur at any time that a patient suffering from the disease is allowed to enter and become a source of infection for the mosquitoes. In this connection various writers have called attention to the menace from the Panama Canal. When it is completed, it will allow of direct passage from regions where yellow fever is endemic and this will greatly increase the possibility of its introduction into these places where it is now unknown. The result, with a wholly non-immune population, would be appalling.

On the other hand, there are places wholly outside of the normal range of AËdes calopus where the disease has raged. Such are New York, Boston, and even Philadelphia, which have suffered notable epidemics. These outbreaks have been due to the introduction of infected mosquitoes during the heat of summer, when they have not only conveyed the disease but have found conditions favorable for their multiplication. Or, uninfected mosquitoes have been thus accidentally brought in and developed in large numbers, needing then only the accidental introduction of cases of the disease to start an epidemic.

Methods of control of various diseases have been revolutionized by the discovery that they were insect-borne, but in no other case has the change been as radical or the results as spectacular as in the case of yellow fever. The "shot-gun quarantine," the sufferings and horrors, the hopelessness of fighting absolutely blindly have given way to an efficient, clear-cut method of control, based upon the knowledge that the disease is carried from man to man solely by the mosquito, AËdes calopus. The lines of defense and offense are essentially as follows:

In the first place, when a case of yellow fever occurs, stringent precautions must be adopted to prevent the infection of mosquitoes and the escape of any already infected. This means that the patient must be removed to a mosquito-proof room, or ward beyond reach of the insects, and that the infected room must be thoroughly fumigated at once, to kill the mosquitoes hiding within it. All cracks and openings should be closed with strips of paper and fumigation with burning sulphur or pyrethrum carefully carried out.

It should be remembered that if the first case noted is that of a resident rather than imported, it means that the mosquito carriers became infected more than two weeks before the case was diagnosed, for as we have seen, the germ must undergo a twelve-day period of development within its insect host. Therefore a careful search must be made for mild cases which, though unrecognized, may serve as foci for the spread of the disease.

In face of a threatened epidemic one of the most essential measures is to educate the citizens and to gain their complete coÖperation in the fight along modern lines. This may be done through the schools, the pulpit, places of amusement, newspapers and even bulletin boards.

Emphasis should be placed on the necessity of both non-immunes and immunes using mosquito curtains, and in all possible ways avoiding exposure to the mosquitoes.

Then the backbone of the fight must be the anti-mosquito measures. In general, these involve screening and fumigating against adults, and control of water supply, oiling, and drainage against the larvÆ. The region involved must be districted and a thorough survey undertaken to locate breeding places, which must, if possible, be eradicated. If they are necessary for water supplies, such as casks, or cisterns, they should be carefully screened to prevent access of egg-laying adults.

The practical results of anti-mosquito measures in the fight against yellow fever are well illustrated by the classic examples of the work in Havana, immediately following the discoveries of the Army Commission and by the stamping out of the New Orleans epidemic in 1905.

The opportunities for an immediate practical application of the theories of the Army Commission in Havana were ideal. The city had always been a hotbed of yellow fever and was the principal source from which the disease was introduced year after year into our Southern States. It was under martial law and with a military governor who was himself a physician and thoroughly in sympathy with the views of the Commission, the rigid enforcement of the necessary regulations was possible. The story of the first campaign has been often told, but nowhere more clearly than in Dr. Reed's own account, published in the Journal of Hygiene for 1902.

Closer home was the demonstration of the efficacy of these measures in controlling the yellow fever outbreak in New Orleans in 1905. During the spring and early summer of the year the disease had, unperceived, gained a firm foothold in that city and when, in early July the local Board of Health took cognizance of its existence, it was estimated that there had been in the neighborhood of one hundred cases.

Conditions were not as favorable as they had been under martial law in Havana for carrying on a rigid fight along anti-mosquito lines. The densely populated city was unprepared, the public had to be educated, and an efficient organization built up. The local authorities actively began a general fight against the mosquito but in spite of their best efforts the disease continued to spread. It was recognized that more rigid organization was needed and on August 12th the United States Public Health and Marine Hospital Service was put in absolute charge of the fight. Up to this time there had been one hundred and forty-two deaths from a total of nine hundred and thirteen cases and all of the conditions seemed to threaten an outbreak to exceed the memorable one of 1878 when, as we have seen there were four thousand and forty-six deaths.

With the hearty coÖperation of the citizens,—physicians and laymen alike,—the fight was waged and long before frost or any near approach thereto the disease was stamped out,—a thing unheard of in previous epidemics. The total loss of life was four hundred and sixty—about 11 per cent as great as that from the comparable epidemic of 1878. If the disease had been promptly recognized and combated with the energy which marked the fight later in the summer, the outbreak would have made little headway and the great proportion of these lives would have been saved.

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