Popular Science Monthly/Volume 81/July 1912/The Role of the House Fly and Certain Other Insects in the Spread of Human Diseases

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THE RÔLE OF THE HOUSE FLY AND CERTAIN OTHER INSECTS IN THE SPREAD OF HUMAN DISEASES[1]
By W. E. BRITTON, Ph.D.

STATE ENTOMOLOGIST, NEW HAVEN, CONN.

THE rapid progress during recent years in the knowledge and treatment of human diseases has been marked by a number of discoveries so important and fundamental in their nature that intelligent people everywhere are paying homage to the discoverers, some of whom have given the best part of their lives for the benefit of others. In no line of scientific activity are the results of recent discovery more far reaching or have they a more important bearing on the daily lives of men, women and children than in medical entomology—or the relation of insects to the transmission of human diseases.

The diseases that may be spread by insects are of course those that are commonly known as germ diseases, some of which are regarded as contagious or infectious. They are in some cases blood diseases, and affect the entire system, while in others perhaps only one part of the body, or certain organs, are involved. Some, like typhoid fever and tuberculosis, are caused by bacteria, the lowest forms of plant life, and others, like scarlet fever and malaria, by protozoa, which are animals low in the scale of classification. It is manifestly impossible in this paper to place before you all of the evidence, or even brief descriptions of the various studies and experiments which enabled the investigators finally to obtain the facts that make up our present knowledge of the subject. I shall therefore mention only a few of the strategic points and striking illustrations, hoping that these may be sufficient to show the importance of the subject and the necessity for action, and to enlist your interest in it.

The agency of insects in the spread of human diseases is of two sorts—(1) mechanical carriers, (2) essential hosts.

To the first group belongs the common house fly, and whatever germs adhere to the body, feet, tongue or wings of the fly in its perambulations in and over filth, or those that are swallowed by it, may be deposited on food or in other places in such manner as to cause infection.

The mosquito is a good example of the second group. One species, Anopheles maculipennis, is one of the necessary hosts in the development of the malarial parasite. Man is the other host, and in the blood of each host the parasite undergoes a certain development which is essential to complete the life cycle and insure the perpetuation of the malarial organism. Another kind of mosquito, in the tropics, is responsible in a similar manner for infecting human beings with yellow fever.

With these brief introductory remarks, we will now take up a discussion of the house fly and such other insects as are known to carry disease germs.

 

The House Fly, Musca domestica

The common house fly has been associated with typhoid epidemics so frequently during the past few years that Dr. Howard suggests that it be called the "typhoid fly."[2] Even before the rôle of this insect was definitely understood, it was suspected to have a connection with the disease, because typhoid fever is generally most prevalent in late summer, at the time when flies are the most abundant. I do not wish to convey the impression that typhoid is spread only by means of flies, for such is not the case. There are plenty of other agencies, such as a polluted water or milk supply, but flies play a much more important part in this connection than was supposed a few years ago, and the house fly is more important than other species 'because of its great abundance and its habit of occupying the dwellings of man and crawling over his food.

It was an old idea that flies were not only innocuous but were a benefit to mankind. It is said that Sir John Lubbock provoked a laugh in the House of Commons in 1873 by quoting as follows from one of the books used in the elementary schools: "The fly keeps the warm air pure and wholesome by its swift and zigzag flight."[3]

On the other hand, Kircher,[4] writing in 1658, makes the following statement: "There can be no doubt that flies feed on the internal secretions of the diseased and dying, then flying away, they deposit their excretions on the food in the neighboring dwellings, and persons who eat it are thus infected."

As it took 240 years to demonstrate the truth of this theory, and as twelve years have now passed, and the lesson of the Spanish war has not yet come into general practise, it is a mooted question if, after all, our progress is not an idle boast. At the present day there are many persons who regard flies as a necessary nuisance, but who are not awake to the dangers of their abundance.

As a rule flies do not go far from their breeding places, and if they are very abundant in any locality it is reasonably certain that their breeding place is close at hand.

The house fly does not bite, but its mouth parts are fitted for lapping and sucking up liquids. Another fly (Stomoxys calcitrans), called the stable fly, pierces the skin, and as this fly resembles the house fly and sometimes enters houses, many persons are mistaken in thinking that the house fly actually bites.

Breeding in filth and visiting all sorts of foul waste and decaying animal and vegetable matter and crawling over it, flies can not help becoming contaminated. At the first opportunity they will also crawl over food in the kitchen and drink from the milk pitcher. In this way some of the germs are rubbed off and adhere to the food and are swallowed with it by human beings. The diseases most commonly disseminated in this manner are those of the alimentary canal known as enteric diseases, such as typhoid fever, cholera and dysentery, the germs of which are voided in fecal matter, which if left exposed is certain to be visited by hundreds of flies, and some of the causative bacterial germs of these diseases are thus transferred to food, and infection is thus made possible. But it is not these diseases alone that may be and are occasionally carried by flies. There is considerable evidence to show that the house fly and its near relatives may carry the anthrax bacillus an their digestive systems and deposit the germs with their excretions, or may carry these germs exteriorly if the flies have visited foul matter "containing them. They may then infect persons by crawling over wounds or even food. Flies may carry the germs of tuberculosis by "visiting sputum and then crawling over the mouth and nose and food of persons. Kuttall made some interesting experiments in 1897 which proved that house flies not only may carry the germs of bubonic plague, commonly carried by fleas, but that they may actually die of the disease.[5]

It has been shown that the causative germs of some of these diseases may be and are taken into the digestive tract of the house fly and deposited upon food, confections or other substances. Thus the tiny fly specks which are the bane of every good housekeeper may be positively dangerous.

Formerly it was supposed that the house fly bred only in manure from the stables, and that it did breed in such places was pointed out as early as 1831 by Bouche. In 1873 Packard, and in 1880 Taschenberg, published accounts of the house fly showing that it usually breeds in horse manure. Packard records fourteen days as the period required to develop a generation. Dr. Howard in 1895[6] studied the insect, and had some difficulty in rearing it in captivity. The female laid 120 eggs, which hatched in eight hours, and the maggots lived five days before transforming. The pupa or cocoon stage also lasted five days, making only ten days for the complete life cycle. Dr. Howard considered that probably ninety-five per cent, of all house flies in towns and cities breed in the heaps of horse manure about the stables or in the fields. Later investigations of Dr. Howard and others show that the house fly may breed in privies, garbage cans and garbage heaps, street sweepings, waste from slaughter houses, and even between the folds of old paper from ash dumps. In fact, in almost any place where suitable moisture and food conditions exist.

 

Notable Typhoid Epidemics

The thoughtlessness of some persons having charge of the sick is described by Dr. Veeder in the New York Medical Record.[7] He states that he has seen dejecta from a typhoid patient emptied from a commode and the receptacle left standing without disinfection within a few feet of a pitcher of milk, both attracting flies, which fairly swarmed from one to the other. In the summer of 1898, when our armies were in camp in the southern states during the Spanish war, an epidemic of typhoid broke out. It caused much apprehension and cost many lives. Though the water supply was suspected, the authorities were not able to check the disease by the methods usually practised. Dr. Veeder was one of the first to advance the idea that the germs were being carried by flies, and it was not until the camp had been visited by government entomologists from Washington that the matter was properly controlled. I have the statement from a young soldier who at Chickamauga contracted the disease and was carried to a Philadelphia hospital for treatment, that the sinks or latrines had become filled to overflowing, and were not even covered with dirt, but, reeking with filth and disgusting odors, they attracted vast swarms of flies. It was but a short distance to the mess tents, where flies swarmed just as thickly, and during the investigation that followed, flies were taken from the food with their legs whitened by the lime that had been spread over the sinks. Thousands of soldiers were then encamped, hundreds were sick with typhoid, yet the able men had little or nothing to do, and might just as well have kept the camp in a sanitary condition. It appeared afterwards that the sanitary regulations of the surgeon general had not been followed: the privates dared not complain, the officers in charge were indifferent to this phase of the sanitation of the camp, and the surgeons were all busy administering to the sick and wounded. Such a condition is especially dangerous in view of the fact that in typhoid cases the germs are often given off in the dejecta before the disease has been recognized and before the patient takes to the bed, and also for a long time after recovery seems complete.

Permit me to quote from the official report of Messrs. Reed, Vaughan PSM V81 D046 Common house or typhoid fly.pngCommon House or Typhoid Fly, Musca domestica. Enlarged five times. Eggs as they are deposited in horse manure. Natural size. and Shakespeare, who were appointed to investigate this typhoid outbreak:

"About one fifth of the soldiers in the National Encampment in the United States in 1898 developed typhoid fever." "The percentage of deaths among typhoid fever cases was 7.61." "The deaths from typhoid fever were 86.24 per cent, of the total deaths." PSM V81 D046 Puparia and larvae of the house fly.pngPuparia and Maggots or Larvæ of House Fly. Enlarged three times.

"Flies undoubtedly serve as carriers of the infection."[8]

In other words, out of 107,973 officers and men in the camps, 20,738 were sick and 1,578 died from typhoid. From the above figures it is evident that flies are more effective destructive agents than Spanish bullets.

According to Dr. Alice Hamilton,[9] the Chicago typhoid epidemic of 1902 was traced largely to the agency of flies.

One of the most significant publications in recent years was issued in 1908 by the Merchants' Association of New York City on the pollution of New York Harbor.[10] The fecal matter of the sewerage discharge into the water attracted swarms of flies, many of which were caught in traps placed on the piers and afterward examined in the laboratory for bacteria. One individual fly carried more than 100,000 fecal bacteria. The same publication contains a chart sbowing the location where each individual death from intestinal diseases occurred during the season, and they were by far the most prevalent in the downtown districts near the water front, where sewerage and fly conditions are worst. Infantile diarrhœa was the cause of many of these deaths, and the author of this publication, Dr. Jackson, attributes much of the infection to the agency of flies.

Washburn only last year[11] investigated a typhoid outbreak, and found flies chiefly responsible for the spread of this disease on the Iron Range of Minnesota. Many other cases might be cited.

 

Local Conditions to be Avoided

Many persons go from their homes in the cities to spend their vacations at shore resorts or mountain camps, and are soon taken sick with typhoid. The imperfect sanitary conditions at many of these places make it hard to prevent the spread of the disease if a case of it occurs, and where a large number of persons are brought together from different localities there is always danger.

Large gangs of laborers in quarries, lumber camps and on construction work are, on account of carelessness and ignorance, liable to suffer from the spread of typhoid fever by flies, and what has been said of typhoid would doubtless be equally true of cholera and dysentery.

In looking about our cities and towns for breeding places of the house, fly, what do we find? In many stables manure is allowed to accumulate untreated as long as there is room for it, then it is carted away to the suburbs and piled upon the land. Carloads of manure from the large cities are drawn through our towns and allowed to stand on sidings for several days, perhaps, before reaching their destination and being unloaded. Streets are often so filthy as to attract flies, and when cleaned, the sweepings are dumped on vacant lots or drawn into the parks for fertilizer and allowed to remain in heaps several months. In many streets water closets are not installed, and uncared-for dry closets are still in use. All of these conditions are favorable for the breeding of flies, and we should remember that in

PSM V81 D048 Unloading a car of stable manure.png

Unloading a Car of Stable Manure from New York. Four ounces of material from the top of this carload contained between 700 and 800 maggots, by actual count.

warm weather only ten clays are required to develop a generation. Each female may lay 120 eggs, and her possible progeny amounts to more than 3,000,000,000 in a single season. Hence the tremendous increase in the number of flies the latter part of the summer. Then please visit some of our cheap restaurants and meat markets, and note how they are swarming with flies which crawl all over the unprotected food and provisions. If you must eat food from such a place, choose something that can be cooked thoroughly before eating, or if raw, something that has a natural covering to be removed before eating, like an orange or banana. Remember that typhoid fever does not always come through the water or milk supply or by eating oysters.

Of course flies do not originate typhoid fever, but if a case occurs in a locality where conditions are unsanitary they are sure to spread it.

The stable manure which is shipped in carloads from the cities to suburban or country districts is an excellent breeding place for flies. Two years ago one of my assistants examined such a loaded car standing upon the siding near this city (New Haven). The contents had come from New York and was waiting to be unloaded for use upon the land for growing fruit or vegetable crops. The upper two inches of the manure was literally swarming with maggots. Some of the material was taken to the laboratory, and four ounces of it contained between 700 and 800 maggots, by actual count!

What of ordinances and health board regulations? It is true that anti-spitting rules are in force, as are also regulations about the covering of foodstuffs when carted through the streets or exhibited for sale, and garbage cans and wagons must be covered. Is it unreasonable to require that exposed surfaces of manure on platform cars or in stables, yards and fields be either screened or treated once a week to prevent the breeding of flies?

 

Remedies for Flies

Screen buildings: and prevent flies from breeding.

All living rooms in houses, and especially the kitchen and dining room, sick-rooms, and all hotels, restaurants, markets and stores where food supplies are sold or stored should be fitted with screens to. keep out flies.

Breeding places of flies should be abolished where possible by not allowing manure, garbage or filth to accumulate or by screening it to keep flies away, or by treating it to kill the maggots. Manure treated with chloride of lime each day will not produce flies. Kerosene or one of the so-called soluble or miscible oils sold everywhere for spraying orchards will probably kill the maggots if the outer two inches is saturated with the liquid.

Flies in houses may be killed by the use of insect powder, fly-paper, or by five-per-cent. sweetened formalin placed about the rooms in saucers. A recent circular[12] from the North Carolina Agricultural Experiment Station recommends one tablespoonful of commercial formalin (40 per cent.) to a half pint teacup of half milk and half water. The liquid is exposed in a shallow plate with a slice of bread in it to give more space for the flies to alight while drinking. The author of this circular, Professor R. I. Smith, states that in this way

PSM V81 D049 Horse manure piles in the suburbs.png

Horse Manure piled in the Suburbs, in a suitable condition to breed many flies.

he has killed over 40,000 flies (about four quarts) inside of twenty-four hours in a barn where flies were very numerous.

 

Other Flies that Carry Diseases

The terrible scourge of Africa known as sleeping sickness is caused by a protozoan parasite known as Trypanosoma gambiense, and is transmitted by the bite of a species of fly, Glossina palpalis. In our own southern states the disease known as "pink eye" is disseminated by a minute fly of the genus Hippelates.

PSM V81 D050 View of an ill kept city stable yard.png

View of an Ill-kept City Stable Yard, showing a manure heap where thousands of house flies breed.

 

Mosquitoes and Malaria

Malaria has been known ever since 400 B.C. in southern Europe,[13] and from the records it must have been present in Connecticut for about 250 years, though rot generally distributed here, nor did it appear in the form of an epidemic until about 1860, when it broke out in the southwestern corner of the state and spread gradually during the next twenty years until the entire area of the state was involved. It is thought that soldiers returning from the Civil War brought it from the south.

The cause of malaria was formerly thought to be gases or foul emanations from swamps, and it was in 1881 that Laveran, a French army surgeon, discovered in the red corpuscles of human blood a protozoan parasite, which he named Plasmodium malariæ. Though the mosquito was suggested by King in 1882 as a possible agent in transmitting this disease, it remained for Dr. Patrick Manson, of London, to point out in 1895 that the malarial parasite had an alternate mode of generation, and he considered some blood-sucking insect (probably a mosquito) as the most probable host. His pupil, Dr. Ronald Ross, an English military surgeon, soon went to India, and after patiently dissecting the bodies of hundreds of mosquitoes, finally discovered one having pigmented bodies in the stomach after biting a malarial patient. In 1900, Sambon and Low, and Grassi, conducted in the most malarious sections of Italy careful experiments which proved to the world that malaria is transmitted to man through the bites of the malarial mosquito, Anopheles maeulipennis. These experiments have since been duplicated and the results confirmed by many others in different parts of the world.

Mosquitoes breed only in water,-and the malarial mosquito will breed in almost any pool where other kinds flourish, but is never so abundant as the rain-barrel mosquito, Culex pipiens, or the salt-marsh mosquito, Culex sollicitans. It lays its eggs singly on the surface of the water. They hatch in a few hours, and the young larvæ or wigglers feed in the water on minute particles of vegetable matter. Each larva goes to the surface every few minutes to inhale air through the tube or siphon near the tail. In a few days the wiggler changes to a peculiar hunchback pupa, and the adult mosquito emerges two or three days later. Only a week is required in warm weather to complete the life cycle.

As a rule, mosquitoes do not fly far, and usually breed in the vicinity where they occur. The salt marsh mosquito is an exception to this rule, and often flies inland for twenty-five or thirty miles, though it breeds only near the coast.

From the records of the State Board of Health it appears that for the decade ending with 1903, 1,073 deaths, or more than 100 each year, occurred in Connecticut from malarial diseases alone. Dr. Howard obtained similar figures from those states where statistics are kept (less than one half of the states keeping them, and these being in the north), which show that more than 12,000 deaths occurred in eight years from malaria. From the records of a number of cities it appears that two deaths occur from malaria in the south to one in the north, and on this basis and including the non-registration states, he concludes that the annual death rate from malaria in the United States must amount to 12,000, and that it would be 96,000 for the eight-year period.[14]

But with malaria perhaps more than with any other disease the death rate is a small indication of the economic loss suffered. Many are ill for years from malaria, and their capacity for work greatly reduced, and they may finally die from some other trouble.

 

Yellow Fever and Mosquitoes

As early as 1881, Dr. Charles Finlay, of Habaria, noticed a correspondence between the abundance of mosquitoes and the prevalence of yellow fever, but it was not until 1900, when our soldiers occupied the island of Cuba at the time of the Spanish War, that experiments were conducted proving that the disease is transmitted chiefly if not wholly by a mosquito, Stegomyia calopus. The investigating committee was appointed by Surgeon General Sternberg, and consisted of Messrs. Reed, Lazear, Carroll and Agramonte.

A small house was built and effectually screened against the entrance of all mosquitoes. A circulation of air was also prevented and all sunlight excluded. A temperature of 76.20° F., with a moist air, was maintained for sixty-three days—just the conditions favorable to the spread of bacterial diseases. Moreover, clothes, blankets and bedding which had been used by yellow fever patients and not cleaned were put into the building and used by the inmates. Seven non-immunes were kept in this house, two or three sleeping in one room with the contaminated bed-clothing, for about twenty nights, then shifts were made and other subjects placed under the same conditions. All seven were released from quarantine in excellent health at the end of sixty-three days, not a single case of yellow fever appearing. Formerly, contaminated clothing, bedding, etc., were regarded as a dangerous source of infection and were usually burned.

Another similar building was erected by these investigators and was divided into two large rooms, one admitting air and sunlight freely and containing the mosquitoes which had previously bitten yellow fever patients. In this room six out of seven persons bitten came down with yellow fever. From the other room mosquitoes were excluded, and the occupants remained in perfect health. These tests still more strongly confirmed previous experiments implicating the mosquito in transmitting yellow fever, and acting on this knowledge General Wood issued orders requiring the use of mosquito bars at the barracks and for the destruction of mosquito lame in the breeding pools by the use of petroleum. This work, in charge of Colonel Gorgas, was carried out thoroughly, and continued until Habana was a comparatively healthy city. Mosquito extermination has everywhere been practised-—fumigating buildings with tobacco or sulphur to kill the adults, and draining and filling the pools or applying oil to kill the larvæ or wigglers. Of 26,000 of these mosquito breeding places within the city limits in March, 1901, only 300 remained in January, 1902.[15] Moreover, the number of deaths from malaria in Habana was greatly reduced from 325 in 1900 to 151 in 1901, 77 in 1902, and 45 up to the first of November, 1903.

In 1905, yellow fever broke out in New Orleans. The situation was critical, and on August 12 was placed under the control of the Public Health and Marine-Hospital Service under Dr. White. A warfare against the yellow-fever mosquito was at once commenced. This mosquito was found breeding in the rain-water cisterns which abounded in the city. These cisterns were screened, and various pools treated. The epidemic abated at once, and the total number of deaths was 460 as against 4,046 in the epidemic of 1878, 4,858 in 1858, and 7,848 in 1853.

Similar control measures have been inaugurated in the Panama Canal zone, with the result that the canal is soon to be completed and the region is now considered fairly salubrious, though the French had to abandon their work there on account of the unhealthy climate.

 

Measures for Controlling Mosquitoes

Mosquito prophylaxis is usually an engineering problem pure and simple—abolish breeding places. This can be done in nine out of ten cases by filling and draining at small expense. In the tenth case it may be advisable, on account of expense, to make a permanent pool and stock it with carnivorous fishes. The edges should be deep and abrupt, and kept clean and free from vegetation. In the salt marshes, ditches should be opened so that the tide may ebb and flow through them, and mosquitoes will not breed there. Fill all small depressions.

Screen all houses, and also screen all cisterns and rain-water barrels to keep mosquitoes out of them. Treat the surface of all breeding places once each ten days with oil to prevent the breeding of mosquitoes therein until these pools can be made permanently safe.

 

Fleas and Plague

Bubonic plague, or "black death," has always been one of the most dreaded diseases of mankind, and from the scourge of Egypt, beginning about A.D. 542 and lasting sixty years, down to the San Francisco epidemic of 1907-08, communities and government authorities have been powerless to cope with it. In India even at present, according to the newspapers, the mortality from this disease was 43,508 for the month of February and 95,884 for March, 1911. As it has always been serious in India, various commissions there and in other countries have each investigated and made their own contributions toward a knowledge of the disease. From 1896 to 1903, during these investigations, it was learned that the bacterial germ Bacillus pestis, causing the disease, entered through some wound, puncture or abrasion of the skin, and that all fleas and bugs sucking the blood of dying plague-diseased animals contained plague microbes. Fleas attacking rats were then suspected, and experiments proved that they were capable of transmitting the disease to human beings. Rats and ground squirrels die in large numbers from the plague,[16] which is said to be primarily a disease of rats.

So, working in the light of the knowledge previously gained, Dr. Blue, of the Public Health and Marine Hospital Service, in charge of the outbreak of plague in San Francisco in 1907, directed the warfare against rats, killing more than a million in the city, and disinfecting them as soon as they could be caught. At first the work was difficult. Cases of plague were kept secret. The prejudices of ignorance and superstition had to be overcome. City officials as well as the people had to be educated, and laws made and enforced. But the work was finally performed so thoroughly that San Francisco made a new record in sanitation, and only about 140 cases developed. Except for this effective campaign not only San Francisco and California were endangered, but the whole country imperiled.[17]

 

Remedial Measures

Destroy all rats by the use of baited traps and poisons, dipping them immediately in a solution of corrosive sublimate to disinfect them. This immediately kills the rats and the fleas, with the plague germs on them.

Fumigate buildings to kill fleas and disease germs.

Build solid concrete foundations, floors and walls where possible in buildings and wharves to keep out rats.

 

Other Diseases that are Known or Thought to be Transmitted by Insects

There are certain other diseases, mostly tropical, that are known to be carried by insects, and still others that are believed to be similarly transmitted, though the connecting evidence forming the proof is not yet complete. Elephantiasis is caused by worm-like parasites transmitted through the bites of certain mosquitoes of the genus Culex in the East and West Indies. Dengue fever and malta fever are probably disseminated by mosquitoes.

It is thought that the germs of leprosy are transmitted by the bites of mosquitoes, flies, fleas, lice, mites or bedbugs. Dr. Patton, of the Indian Medical Service, has demonstrated that the fatal and infectious tropical disease called "kala-azar" is transmitted by the bites of the Indian bedbug, Cimex rotundatus.[18] Our bedbug, Cimex lectularius, which has always been in disrepute, though constantly appearing in the best society, is now under suspicion. It is not only possible, but quite probable, that in the near future further discoveries will show that some other infectious diseases, like infantile paralysis, and possibly smallpox and scarlet fever, may be transmitted through the bites of some of the common insect parasites of man.

Pestilences were formerly considered as visitations of Providence to punish man for his wickedness. People were powerless to save their own lives or the lives of their friends. Ignorance and superstition are hard to overcome, but in the light of our present knowledge these scourges which I have mentioned are all preventable by controlling the little insects which carry the germs. Is it not therefore a crime to allow them to be repeated?

If the exact history of the world could be written and the truth revealed, it would be interesting to learn whether the decline of Greece was due largely to malaria, as Dr. Ross has suggested,[19] and also to find out how important a part these seemingly insignificant insects have played in shaping the destinies of the nations.

  1. This paper in substance was given, with lantern slides, before the Consumers' League, New Haven, Conn., May 4, 1911. A few paragraphs are taken verbatim from previous papers by the author.
  2. L. O. Howard, Bureau of Entomology, Bull. 78, p. 23.
  3. G. H. F. Nuttall, Johns Hopkins Hospital Reports, Vol. VIII., p. 37.
  4. W. A. Riley, Science, February 18, 1910.
  5. G. H. F. Nuttall, Johns Hopkins Hospital Reports, Vol. VIII., p. 16.
  6. L. O. Howard, Bureau of Entomology, Bull. 4, p. 46, 1896.
  7. H. A. Veeder, New York Medical Record, Vol. 54, September 17, 1898.
  8. Walter Reed, V. C. Vaughn, E. O. Shakespeare, "Report on the Origin and Spread of Typhoid Fever in U. S. Military Camps during the Spanish War of 1898," p. 666.
  9. Hamilton, Journal of American Medical Association, 40, 576.
  10. D. D. Jackson, "Pollution of New York Harbor," Merchants' Association of New York, July, 1908.
  11. F. L. Washburn, Eeport of State Entomologist of Minnesota, 1909-1910, p. 135. See also Popular Science Monthly, August, 1911, p. 137.
  12. R. I. Smith, Press Bulletin No. 23. North Carolina Agricultural Experiment Station.
  13. L. O. Howard, Bureau of Entomology, Bull. 78, p. 37.
  14. L. O. Howard, "Economic Loss to the People of the United States through Insects that Carry Disease." Bureau Entomology, Bull. 78, p. 10.
  15. W. C. Gorgas, "A Few General Directions with Regard to Destroying Mosquitoes." Government Printing Office, Washington, D. C, 1904.
  16. E. W. Doane, "Insects and Disease," p. 155.
  17. Report of Citizens' Health Committee, "Eradicating Plague in San Francisco."
  18. R. W. Doane, "Insects and Disease," p. 173.
  19. L. O. Howard, Bureau of Entomology, Bull. 78, p. 36.