Handbook of Medical Entomology Part 17

dd. The last segment of vein M with a broad, gentle curve (fig.

102).

e. Eyes microscopically hairy; each abdominal segment with two spots. Larvae in dung. _Myiospila meditabunda_

ee. Eyes bare; abdomen gray and brown marbled. _Muscina_

f. With black legs and palpi. _M. a.s.similis_

ff. With legs more or less yellowish; palpi yellow. Larvae in decaying vegetable substances, dung, etc. _M. stabulans_

It is almost universally believed that the adults of _Musca domestica_ hibernate, remaining dormant throughout the winter in attics, around chimneys, and in sheltered but cold situations. This belief has been challenged by Skinner (1913), who maintains that all the adult flies die off during the fall and early winter and that the species is carried over in the pupal stage, and in no other way. The cl.u.s.ter-fly, _Pollenia rudis_, undoubtedly does hibernate in attics and similar situations and is often mistaken for the house-fly. In so far as concerns _Musca domestica_, the important question as to hibernation in the adult stage is an open one. Many observations by one of the writers (Johannsen) tend to confirm Dr. Skinner's conclusion, in so far as it applies to conditions in the lat.i.tude of New York State. Opposed, is the fact that various experimenters, notably Hewitt (1910) and Jepson (1909) wholly failed to carry pupae through the winter.

[Ill.u.s.tration: 108. The house or typhoid fly (Musca domestica (4)).

After Howard.]

The house-fly breeds by preference in horse manure. Indeed, Dr. Howard, whose extensive studies of the species especially qualify him for expressing an opinion on the subject, has estimated that under ordinary city and town conditions, more than ninety per cent of the flies present in houses have come from horse stables or their vicinity. They are not limited to such localities, by any means, for it has been found that they would develop in almost any fermenting organic substance. Thus, they have been bred from pig, chicken, and cow manure, dirty waste paper, decaying vegetation, decaying meat, slaughter-house refuse, sawdust-sweepings, and many other sources. A fact which makes them especially dangerous as disease-carriers is that they breed readily in human excrement.

The eggs are pure white, elongate ovoid, somewhat broader at the anterior end. They measure about one millimeter (1-25 inch) in length.

They are deposited in small, irregular cl.u.s.ters, one hundred and twenty to one hundred and fifty from a single fly. A female may deposit as many as four batches in her life time. The eggs hatch in from eight to twenty-four hours.

The newly hatched larva, or maggot (fig. 108), measures about two millimeters (1-12 inch) in length. It is pointed at the head end and blunt at the opposite end, where the spiracular openings are borne. It grows rapidly, molts three times and reaches maturity in from six to seven days, under favorable conditions.

The pupal stage, like that of related flies, is pa.s.sed in the old larval skin which, instead of being molted, becomes contracted and heavily chitinized, forming the so-called _puparium_ (fig. 108). The pupal stage may be completed in from three to six days.

Thus during the warm summer months a generation of flies may be produced in ten to twelve days. Hewitt at Manchester, England, found the minimum to be eight days but states that larvae bred in the open air in horse manure which had an average daily temperature of 22.5 C., occupied fourteen to twenty days in their development, according to the air temperature.

After emergence, a period of time must elapse before the fly is capable of depositing eggs. This period has been tuned the _preoviposition_ period. Unfortunately we have few exact data regarding this period.

Hewitt found that the flies became s.e.xually mature in ten to fourteen days after their emergence from the pupal state and four days after copulation they began to deposit their eggs; in other words the preoviposition stage was fourteen days or longer. Griffith (1908) found this period to be ten days. Dr. Howard believes that the time "must surely be shorter, and perhaps much shorter, under midsummer conditions, and in the freedom of the open air." He emphasizes that the point is of great practical importance, since it is during this period that the trapping and other methods of destroying the adult flies, will prove most useful.

Howard estimates that there may be nine generations of flies a year under outdoor conditions in places comparable in climate to Washington.

The number may be considerably increased in warmer climates.

The rate at which flies may increase under favorable conditions is astounding. Various writers have given estimates of the numbers of flies which may develop as the progeny of a single individual, providing all the eggs and all the individual flies survived. Thus, Howard estimates that from a single female, depositing one hundred and twenty eggs on April 15th, there may be by September 10th, 5,598,720,000,000 adults.

Fortunately, living forms do not produce in any such mathematical manner and the chief value of the figures is to ill.u.s.trate the enormous struggle for existence which is constantly taking place in nature.

Flies may travel for a considerable distance to reach food and shelter, though normally they pa.s.s to dwellings and other sources of food supply in the immediate neighborhood of their breeding places. Copeman, Howlett and Merriman (1911) marked flies by shaking them in a bag containing colored chalk. Such flies were repeatedly recovered at distances of eight to one thousand yards and even at a distance of seventeen hundred yards, nearly a mile.

Hindle and Merriman (1914) continued these experiments on a large scale at Cambridge, England. They "do not think it likely that, as a rule, flies travel more than a quarter of a mile in thickly-housed areas." In one case a single fly was recovered at a distance of 770 yards but a part of this distance was across open fen-land. The surprising fact was brought out that flies tend to travel either _against_ or across the wind. The actual direction followed may be determined either directly by the action of the wind (positive anemotropism), or indirectly owing to the flies being attracted by any odor that it may convey from a source of food. They conclude that it is likely that the chief conditions favoring the disposal of flies are fine weather and a warm temperature.

The nature of the locality is another considerable factor. Hodge (1913) has shown that when aided by the wind they may fly to much greater distances over the water. He reports that at Cleveland, Ohio, the cribs of the water works, situated a mile and a quarter, five miles, and six miles out in Lake Erie are invaded by a regular plague of flies when the wind blows from the city. Investigation showed that there was absolutely nothing of any kind in which flies could breed on the crib.

The omnivorous habits of the house-fly are matters of everyday observation. From our view point, it is sufficient to emphasize that from feeding on excrement, on sputum, on open sores, or on putrifying matter, the flies may pa.s.s to the food or milk upon the table or to healthy mucous membranes, or uncontaminated wounds. There is nothing in its appearance to tell whether the fly that comes blithely to sup with you is merely unclean, or whether it has just finished feeding upon dejecta teeming with typhoid bacilli.

[Ill.u.s.tration: 100. Pulvillus of foot of house-fly, showing glandular hairs.]

The method of feeding of the house-fly has an important bearing on the question of its ability to transmit pathogenic organisms. Graham-Smith (1910) has shown that when feeding, flies frequently moisten soluble substances with "vomit" which is regurgitated from the crop. This is, of course, loaded with bacteria from previous food. When not sucked up again these drops of liquid dry, and produce round marks with an opaque center and rim and an intervening less opaque area. Fly-specks, then, consist of both vomit spots and feces. Graham-Smith shows a photograph of a cupboard window where, on an area six inches square, there were counted eleven hundred and two vomit marks and nine fecal deposits.

From a bacteriologist's viewpoint a discussion of the possibility of a fly's carrying bacteria would seem superfluous. Any exposed object, animate or inanimate, is contaminated by bacteria and will transfer them if brought into contact with suitable culture media, whether such substance be food, or drink, open wounds, or the sterile culture media of the laboratory. A needle point may convey enough germs to produce disease. Much more readily may the house-fly with its covering of hairs and its sponge-like pulvilli (fig. 109) pick up and transfer bits of filth and other contaminated material.

For popular instruction this inevitable transfer of germs by the house-fly is strikingly demonstrated by the oft copied ill.u.s.tration of the tracks of a fly on a sterile culture plate. Two plates of gelatine or, better, agar medium are prepared. Over one of these a fly (with wings clipped) is allowed to walk, the other is kept as a check. Both are put aside at room temperature, to be examined after twenty-four to forty-eight hours. At the end of that time, the check plate is as clear as ever, the one which the fly has walked is dotted with colonies of bacteria and fungi. The value in the experiment consists in emphasizing that by this method we merely render visible what is constantly occurring in nature.

A comparable experiment which we use in our elementary laboratory work is to take three samples of _clean_ (preferably, sterile) fresh milk in sterile bottles. One of them is plugged with a pledget of cotton, into the second is dropped a fly from the laboratory and into the third is dropped a fly which has been caught feeding upon garbage or other filth.

After a minute or two the flies are removed and the vials plugged as was number one. The three are then set aside at room temperature. When examined after twenty-four hours the milk in the first vial is either still sweet or has a "clean" sour odor; that of the remaining two is very different, for it has a putrid odor, which is usually more p.r.o.nounced in the case of sample number three.

Several workers have carried out experiments to determine the number of bacteria carried by flies under natural conditions. One of the most extended and best known of these is the series by Esten and Mason (1908). These workers caught flies from various sources in a sterilized net, placed them in a sterile bottle and poured over them a known quant.i.ty of sterilized water, in which they were shaken so as to wash the bacteria from their bodies. They found the number of bacteria on a single fly to range from 550 to 6,600,000. Early in the fly season the numbers of bacteria on flies are comparatively small, while later the numbers are comparatively very large. The place where flies live also determines largely the numbers that they carry. The lowest number, 550, was from a fly caught in the bacteriological laboratory, the highest number, 6,600,000 was the average from eighteen swill-barrel flies.

Torrey (1912) made examination of "wild" flies from a tenement house district of New York City. He found "that the surface contamination of these 'wild' flies may vary from 570 to 4,400,000 bacteria per insect, and the intestinal bacterial content from 16,000 to 28,000,000."

Less well known in this country is the work of c.o.x, Lewis, and Glynn (1912). They examined over four hundred and fifty naturally infected house-flies in Liverpool during September and early October. Instead of washing the flies they were allowed to swim on the surface of sterile water for five, fifteen, or thirty minutes, thus giving natural conditions, where infection occurs from vomit and dejecta of the flies, as well as from their bodies. They found, as might be expected, that flies from either insanitary or congested areas of the city contain far more bacteria than those from the more sanitary, less congested, or suburban areas. The number of aerobic bacteria from the former varied from 800,000 to 500,000,000 per fly and from the latter from 21,000 to 100,000. The number of intestinal forms conveyed by flies from insanitary or congested areas was from 10,000 to 333,000,000 as compared with from 100 to 10,000 carried by flies from the more sanitary areas.

Pathogenic bacteria and those allied to the food poisoning group were only obtained from the congested or moderately congested areas and not from the suburban areas, where the chances of infestation were less.

The interesting fact was brought out that flies caught in milk shops apparently carry and obtain more bacteria than those from other shops with exposed food in a similar neighborhood. The writers explained this as probably due to the fact that milk when accessible, especially during the summer months, is suitable culture medium for bacteria, and the flies first inoculate the milk and later reinoculate themselves, and then more of the milk, so establishing a vicious circle.

They conclude that in cities where food is plentiful flies rarely migrate from the locality in which they are bred, and consequently the number of bacteria which they carry depends upon the general standard of cleanliness in that locality. Flies caught in a street of modern, fairly high cla.s.s, workmen's dwellings forming a sanitary oasis in the midst of a slum area, carried far less bacteria than those caught in the adjacent neighborhood.

Thus, as the amount of dirt carried by flies in any particular locality, measured in the terms of bacteria, bears a definite relation to the habits of the people and to the state of the streets, it demonstrates the necessity of efficient munic.i.p.al and domestic cleanliness, if the food of the inhabitants is to escape pollution, not only with harmless but also with occasional pathogenic bacteria.

The above cited work is of a general nature, but, especially in recent years, many attempts have been made to determine more specifically the ability of flies to transmit pathogenic organisms. The critical reviews of Nuttall and Jepson (1909), Howard (1911), and Graham-Smith (1913) should be consulted by the student of the subject. We can only cite here a few of the more striking experiments.

Celli (1888) fed flies on pure cultures of _Bacillus typhosus_ and declared that he was able to recover these organisms from the intestinal contents and excrement.

Firth and Horrocks (1902), cited by Nuttall and Jepson, "kept _Musca domestica_ (also bluebottles) in a large box measuring 4 3 3 feet, with one side made of gla.s.s. They were fed on material contaminated with cultures of _B. typhosus_. Agar plates, litmus, glucose broth and a sheet of clean paper were at the same time exposed in the box. After a few days the plates and broth were removed and incubated with a positive result." Graham-Smith (1910) "carried out experiments with large numbers of flies kept in gauze cages and fed for eight hours on emulsions of _B.

typhosus_ in syrup. After that time the infested syrup was removed and the flies were fed on plain syrup. _B. typhosus_ was isolated up to 48 hours (but not later) from emulsions of their feces and from plates over which they walked."

Several other workers, notably Hamilton (1903), Ficker (1903), Bertarelli (1910) Faichnie (1909), and Cochrane (1912), have isolated _B. typhosus_ from "wild" flies, naturally infected. The papers of Faichnie and of Cochrane we have not seen, but they are quoted in _extenso_ by Graham-Smith (1913).

On the whole, the evidence is conclusive that typhoid germs not only may be accidentally carried on the bodies of house-flies but may pa.s.s through their bodies and be scattered in a viable condition in the feces of the fly for at least two days after feeding. Similar, results have been reached in experiments with cholera, tuberculosis and yaws, the last-mentioned being a spirochaete disease. Darling (1913) has shown that murrina, a trypanosome disease of horses and mules in the Ca.n.a.l zone is transmitted by house-flies which feed upon excoriated patches of diseased animals and then pa.s.s to cuts and galls of healthy animals.

Since it is clear that flies are abundantly able to disseminate viable pathogenic bacteria, it is important to consider whether they have access to such organisms in nature. A consideration of the method of spread of typhoid will serve to ill.u.s.trate the way in which flies may play an important role.

Typhoid fever is a specific disease caused by _Bacillus typhosus_, and by it alone. The causative organism is to be found in the excrement and urine of patients suffering from the disease. More than that, it is often present in the dejecta for days, weeks, or even months and years, after the individual has recovered from the disease. Individuals so infested are known as "typhoid carriers" and they, together with those suffering from mild cases, or "walking typhoid," are a constant menace to the health of the community in which they are found.

Human excrement is greedily visited by flies, both for feeding and for ovipositing. The discharges of typhoid patients, or of chronic "carriers," when pa.s.sed in the open, in box privies, or camp latrines, or the like, serve to contaminate myriads of the insects which may then spread the germ to human food and drink. Other intestinal diseases may be similarly spread. There is abundant epidaemiological evidence that infantile diarrha, dysentery, and cholera may be so spread.

Stiles and Keister (1913) have shown that spores of _Lamblia intestinalis_, a flagellate protozoan living in the human intestine, may be carried by house-flies. Though this species is not normally pathogenic, one or more species of _Entamba_ are the cause of a type of a highly fatal tropical dysentery. Concerning it, and another protozoan parasite of man, they say, "If flies can carry _Lamblia_ spores measuring 10 to 7, and bacteria that are much smaller, and particles of lime that are much larger, there is no ground to a.s.sume that flies may not carry _Entamba_ and _Trich.o.m.onas_ spores."

Tuberculosis is one of the diseases which it is quite conceivable may be carried occasionally. The sputum of tubercular patients is very attractive to flies, and various workers, notably Graham-Smith, have found that _Musca domestica_ may distribute the bacillus for several days after feeding on infected material.

A type of purulent opthalmia which is very prevalent in Egypt is often said to be carried by flies. Nuttall and Jepson (1909) consider that the evidence regarding the spread of this disease by flies is conclusive and that the possibility of gonorrhal secretions being likewise conveyed cannot be denied.

Many studies have been published, showing a marked agreement between the occurrence of typhoid and other intestinal diseases and the prevalence of house-flies. The most clear-cut of these are the studies of the Army Commission appointed to investigate the cause of epidemics of enteric fever in the volunteer camps in the Southern United States during the Spanish-American War. Though their findings as presented by Vaughan (1909), have been quoted very many times, they are so germane to our discussion that they will bear repet.i.tion:

"Flies swarmed over infected fecal matter in the pits and fed upon the food prepared for the soldiers in the mess tents. In some instances where lime had recently been sprinkled over the contents of the pits, flies with their feet whitened with lime were seen walking over the food." Under such conditions it is no wonder that "These pests had inflicted greater loss upon American soldiers than the arms of Spain."

Similar conditions prevailed in South Africa during the Boer War. Seamon believes that very much of the success of the j.a.panese in their fight against Russia was due to the rigid precautions taken to prevent the spread of disease by these insects and other means.

Veeder has pointed out that the characteristics of a typical fly-borne epidemic of typhoid are that it occurs in little neighborhood epidemics, extending by short leaps from house to house, without regard to water supply or anything else in common. It tends to follow the direction of prevailing winds (cf. the conclusions of Hindle and Merriman). It occurs during warm weather. Of course, when the epidemic is once well under way, other factors enter into its spread.

In general, flies may be said to be the chief agency in the spread of typhoid in villages and camps. In cities with modern sewer systems they are less important, though even under the best of such conditions, they are important factors. Howard has emphasized that in such cities there are still many uncared-for box privies and that, in addition, the deposition of feces overnight in uncared-for waste lots and alleys is common.