Handbook of Medical Entomology Part 18

Not only unicellular organisms, such as bacteria and protozoa, but also the eggs, embryos and larvae of parasitic worms have been found to be transported by house-flies. Ransom (1911) has found that _Habronema muscae_, a nematode worm often found in adult flies, is the immature stage of a parasite occurring in the stomach of the horse. The eggs or embryos pa.s.sing out with the feces of the horse, are taken up by fly larvae and carried over to the imago stage.

Gra.s.si (1883), Stiles (1889), Calandruccio (1906), and especially Nicoll (1911), have been the chief investigators of the ability of house-flies to carry the ova and embryos of human intestinal parasites. Graham-Smith (1913) summarizes the work along this line as follows:

"It is evident from the investigations that have been quoted that house-flies and other species are greatly attracted to the ova of parasitic worms contained in feces and other materials, and make great efforts to ingest them. Unless the ova are too large they often succeed, and the eggs are deposited uninjured in their feces, in some cases up to the third day at least. The eggs may also be carried on their legs or bodies. Under suitable conditions, food and fluids may be contaminated with the eggs of various parasitic worms by flies, and in one case infection of the human subject has been observed. Feces containing tape-worm segments may continue to be a source of infection for as long as a fortnight. Up to the present, however, there is no evidence to show what part flies play in the dissemination of parasitic worms under natural conditions."

Enough has been said to show that the house-fly must be dealt with as a direct menace to public health. Control measures are not merely matters of convenience but are of vital importance.

Under present conditions the speedy elimination of the house-fly is impossible and the first thing to be considered is methods of protecting food and drink from contamination. The first of these methods is the thorough screening of doors and windows to prevent the entrance of flies. In the case of kitchen doors, the flies, attracted by odors, are likely to swarm onto the screen and improve the first opportunity for gaining an entrance. This difficulty can be largely avoided by screening-in the back porch and placing the screen door at one end rather than directly before the door.

The use of sticky fly paper to catch the pests that gain entrance to the house is preferable to the various poisons often used. Of the latter, formalin (40 per cent formaldehyde) in the proportion of two tablespoonfuls to a pint of water is very efficient, if all other liquids are removed or covered, so that the flies must depend on the formalin for drink. The mixture is said to be made more attractive by the addition of sugar or milk, though we have found the plain solution wholly satisfactory, under proper conditions. It should be emphasized that this formalin mixture is not perfectly harmless, as so often stated. There are on record cases of severe and even fatal poisoning from the accidental drinking of solutions.

When flies are very abundant in a room they can be most readily gotten rid of by fumigation with sulphur, or by the use of pure pyrethrum powder either burned or puffed into the air. Herrick (1913) recommends the following method: "At night all the doors and windows of the kitchen should be closed; fresh powder should be sprinkled over the stove, on the window ledges, tables, and in the air. In the morning flies will be found lying around dead or stupified. They may then be swept up and burned." This method has proved very efficaceous in some of the large dining halls in Ithaca.

The writers have had little success in fumigating with the vapors of carbolic acid, or carbolic acid and gum camphor, although these methods will aid in driving flies from a darkened room.

All of these methods are but makeshifts. As Howard has so well put it, "the truest and simplest way of attacking the fly problem is to prevent them from breeding, by the treatment or abolition of all places in which they can breed. To permit them to breed undisturbed and in countless numbers, and to devote all our energy to the problem of keeping them out of our dwellings, or to destroy them after they have once entered in spite of all obstacles, seems the wrong way to go about it."

We have already seen that _Musca domestica_ breeds in almost any fermenting organic material. While it prefers horse manure, it breeds also in human feces, cow dung and that of other animals, and in refuse of many kinds. To efficiently combat the insect, these breeding places must be removed or must be treated in some such way as to render them unsuitable for the development of the larvae. Under some conditions individual work may prove effective, but to be truly efficient there must be extensive and thorough cooperative efforts.

Manure, garbage, and the like should be stored in tight receptacles and carted away at least once a week. The manure may be carted to the fields and spread. Even in spread manure the larvae may continue their development. Howard points out that "it often happens that after a lawn has been heavily manured in early summer the occupants of the house will be pestered with flies for a time, but finding no available breeding place these disappear sooner or later. Another generation will not breed in the spread manure."

Hutchinson (1914) has emphasized that the larvae of houseflies have deeply engrained the habit of migrating in the prepupal stage and has shown that this offers an important point of attack in attempts to control the pest. He has suggested that maggot traps might be developed into an efficient weapon in the warfare against the house-fly. Certain it is that the habit greatly simplifies the problem of treating the manure for the purpose of killing the larvae.

There have been many attempts to find some cheap chemical which would destroy fly larvae in horse manure without injuring the bacteria or reducing the fertilizing values of the manure. The literature abounds in recommendations of kerosene, lime, chloride of lime, iron sulphate, and other substances, but none of them has met the situation. The whole question has been gone into thoroughly by Cook, Hutchinson and Scales (1914), who tested practically all of the substances which have been recommended. They find that by far the most effective, economical, and practical of the substances is borax in the commercial form in which it is available throughout the country.

"Borax increases the water-soluble nitrogen, ammonia and alkalinity of manure and apparently does not permanently injure the bacterial flora.

The application of manure treated with borax at the rate of 0.62 pound per eight bushels (10 cubic feet) to soil does not injure the plants thus far tested, although its c.u.mulative effect, if any, has not been determined."

As their results clearly show that the substances so often recommended are inferior to borax, we shall quote in detail their directions for treating manure so as to kill fly eggs and maggots.

"Apply 0.62 pound borax or 0.75 pound calcined colemanite to every 10 cubic feet (8 bushels) of manure immediately on its removal from the barn. Apply the borax particularly around the outer edges of the pile with a flour sifter or any fine sieve, and sprinkle two or three gallons of water over the borax-treated manure.

"The reason for applying the borax to the fresh manure immediately after its removal from the stable is that the flies lay their eggs on the fresh manure, and borax, when it comes in contact with the eggs, prevents their hatching. As the maggots congregate at the outer edge of the pile, most of the borax should be applied there. The treatment should be repeated with each addition of fresh manure, but when the manure is kept in closed boxes, less frequent applications will be sufficient. When the calcined colemanite is available, it may be used at the rate of 0.75 pound per 10 cubic feet of manure, and is a cheaper means of killing the maggots. In addition to the application of borax to horse manure to kill fly larvae, it may be applied in the same proportion to other manures, as well as to refuse and garbage. Borax may also be applied to the floors and crevices in barns, stables, markets, etc., as well as to street sweepings, and water should be added as in the treatment of horse manure. After estimating the amount of material to be treated and weighing the necessary amount of borax, a measure may be used which will hold the proper amount, thus avoiding the subsequent weighings.

"While it can be safely stated that no injurious action will follow the application of manure treated with borax at the rate of 0.62 pound for eight bushels, or even larger amounts in the case of some plants, nevertheless the borax-treated manure has not been studied in connection with the growth of all crops, nor has its c.u.mulative effect been determined. It is therefore recommended that not more than 15 tons per acre of the borax-treated manure should be applied to the field. As truckmen use considerably more than this amount, it is suggested that all cars containing borax-treated manure be so marked, and that public-health officials stipulate in their directions for this treatment that not over 0.62 pound for eight bushels of manure be used, as it has been shown that larger amounts of borax will injure most plants. It is also recommended that all public-health officials and others, in recommending the borax treatment for killing fly eggs and maggots in manure, warn the public against the injurious effects of large amounts of borax on the growth of plants."

"The amount of manure from a horse varies with the straw or other bedding used, but 12 or 15 bushels per week represent the approximate amount obtained. As borax costs from five to six cents per pound in 100-pound lots in Washington, it will make the cost of the borax practically one cent per horse, per day. And if calcined colemanite is purchased in large shipments the cost should be considerably less."

Hodge (1910) has approached the problem of fly extermination from another viewpoint. He believes that it is practical to trap flies out of doors during the preoviposition period, when they are s.e.xually immature, and to destroy such numbers of them that the comparatively few which survive will not be able to lay eggs in sufficient numbers to make the next generation a nuisance. To the end of capturing them in enormous numbers he has devised traps to be fitted over garbage cans, into stable windows, and connected with the kitchen window screens. Under some conditions this method of attack has proved very satisfactory.

One of the most important measures for preventing the spread of disease by flies is the abolition of the common box privy. In villages and rural districts this is today almost the only type to be found. It is the chief factor in the spread of typhoid and other intestinal diseases, as well as intestinal parasites. Open and exposed to myriads of flies which not only breed there but which feed upon the excrement, they furnish ideal conditions for spreading contamination. Even where efforts are made to cover the contents with dust, or ashes, or lime, flies may continue to breed unchecked. Stiles and Gardner have shown that house-flies buried in a screened stand-pipe forty-eight inches under sterile sand came to the surface. Other flies of undetermined species struggled up through seventy-two inches of sand.

So great is the menace of the ordinary box privy that a number of inexpensive and simple sanitary privies have been designed for use where there are not modern sewer systems. Stiles and Lumsden (1911) have given minute directions for the construction of one of the best types, and their bulletin should be obtained by those interested.

Another precaution which is of fundamental importance in preventing the spread of typhoid, is that of disinfecting all discharges from patients suffering with the disease. For this purpose, quick-lime is the cheapest and is wholly satisfactory. In chamber vessels it should be used in a quant.i.ty equal to that of the discharge to be treated. It should be allowed to act for two hours. Air-slaked lime is of no value whatever.

Chloride of lime, carbolic acid, or formalin may be used, but are more expensive. Other intestinal diseases demand similar precautions.

STOMOXYS CALCITRANS, THE STABLE-FLY--It is a popular belief that house-flies bite more viciously just before a rain. As a matter of fact, the true house-flies never bite, for their mouth-parts are not fitted for piercing. The basis of the misconception is the fact that a true biting fly, _Stomoxys calcitrans_ (fig. 110), closely resembling the house-fly, is frequently found in houses and may be driven in in greater numbers by muggy weather. From its usual habitat this fly is known as the "stable-fly" or, sometimes as the "biting house-fly."

_Stomoxys calcitrans_ may be separated from the house-fly by the use of the key on p. 145. It may be more fully characterized as follows:

The eyes of the male are separated by a distance equal to one-fourth of the diameter of the head, in the female by one-third. The frontal stripe is black, the cheeks and margins of the orbits silvery-white. The antennae are black, the arista feathered on the upper side only. The proboscis is black, slender, fitted for piercing and projects forward in front of the head. The thorax is grayish, marked by four conspicuous, more or less complete black longitudinal stripes; the scutellum is paler; the macrochaetae are black. The abdomen is gray, dorsally with three brown spots on the second and third segments and a median spot on the fourth. These spots are more p.r.o.nounced in the female. The legs are black, the pulvilli distinct. The wings are hyaline, the vein M_{1+2} less sharply curved than in the house-fly, the apical cell being thus more widely open (cf. fig. 110). Length 7 mm.

[Ill.u.s.tration: 110. Stomoxys calcitrans; adult, larva, puparium and details, (5). After Howard.]

This fly is widely distributed, being found the world over. It was probably introduced into the United States, but has spread to all parts of the country. Bishopp (1913) regards it as of much more importance as a pest of domestic animals in the grain belt than elsewhere in the United States. The life-history and habits of this species have a.s.sumed a new significance since it has been suggested that it may transmit the human diseases, infantile paralysis and pellagra. In this country, the most detailed study of the fly is that of Bishopp (1913) whose data regarding the life cycle are as follows:

The eggs like those of the house-fly, are about one mm. in length. Under a magnifying gla.s.s they show a distinct furrow along one side. When placed on any moist substance they hatch in from one to three days after being deposited.

The larva or maggots (fig. 110) have the typical shape and actions of most maggots of the Muscid group. They can be distinguished from those of the house-fly as the stigma-plates are smaller, much further apart, with the slits less sinuous. Development takes place fairly rapidly when the proper food conditions are available and the growth is completed within eleven to thirty or more days.

The pupa (fig. 110), like that of related flies, undergoes its development within the contracted and hardened last larval skin, or puparium. This is elongate oval, slightly thicker towards the head end, and one-sixth to one-fourth of an inch in length. The pupal stage requires six to twenty days, or in cool weather considerably longer.

The life-cycle of the stable-fly is therefore considerably longer than that of _Musca domestica_. Bishopp found that complete development might be undergone in nineteen days, but that the average period was somewhat longer, ranging from twenty-one to twenty-five days, where conditions are very favorable. The longest period which he observed was forty-three days, though his finding of full grown larvae and pupae in straw during the latter part of March, in Northern Texas, showed that development may require about three months, as he considered that these stages almost certainly developed from eggs deposited the previous December.

The favorite breeding place, where available, seems to be straw or manure mixed with straw. It also breeds in great numbers in horse-manure, in company with _Musca domestica_.

Newstead considers that in England the stable-fly hibernates in the pupal stage. Bishopp finds that in the southern part of the United States there is no true hibernation, as the adults have been found to emerge at various times during the winter. He believes that in the northern United States the winter is normally pa.s.sed in the larval and pupal stages, and that the adults which have been observed in heated stables in the dead of winter were bred out in refuse within the warm barns and were not hibernating adults.

Graham-Smith (1913) states that although the stable-fly frequents stable manure, it is probably not an important agent in distributing the organisms of intestinal diseases. Bishopp makes the important observation that "it has never been found breeding in human excrement and does not frequent malodorous places, which are so attractive to the house-fly. Hence it is much less likely to carry typhoid and other germs which may be found in such places."

Questions of the possible agency of _Stomoxys calcitrans_ in the transmission of infantile paralysis and of pellagra, we shall consider later.

OTHER ARTHROPODS WHICH MAY SERVE AS SIMPLE CARRIERS OF PATHOGENIC ORGANISMS--It should be again emphasized that any insect which has access to, and comes in contact with, pathogenic organisms and then pa.s.ses to the food, or drink, or the body of man, may serve as a simple carrier of disease. In addition to the more obvious ill.u.s.trations, an interesting one is the previously cited case of the transfer of _Dermatobia cyaniventris_ by a mosquito (fig. 81-84). Darling (1913) has shown that in the tropics, the omnipresent ants may be important factors in the spread of disease.

CHAPTER VI

ARTHROPODS AS DIRECT INOCULATORS OF DISEASE GERMS

We have seen that any insect which, like the house-fly, has access to disease germs and then comes into contact with the food or drink of man, may serve to disseminate disease. Moreover, it has been clearly established that a contaminated insect, alighting upon wounded or abraded surfaces, may infect them. These are instances of mere accidental, mechanical transfer of pathogenic organisms.

Closely related are the instances of direct inoculation of disease germs by insects and other arthropods. In this type, a blood-sucking species not only takes up the germs but, pa.s.sing to a healthy individual, it inserts its contaminated mouth-parts and thus directly inoculates its victim. In other words, the disease is transferred just as blood poisoning may be induced by the p.r.i.c.k of a contaminated needle, or as the laboratory worker may inoculate an experimental animal.

Formerly, it was supposed that this method of the transfer of disease by arthropods was a very common one and many instances are cited in the earlier literature of the subject. It is, however, difficult to draw a sharp line between such cases and those in which, on the one hand, the arthropod serves as a mere pa.s.sive carrier or, on the other hand, serves as an essential host of the pathogenic organism. More critical study of the subject has led to the belief that the importance of the role of arthropods as direct inoculators has been much overestimated.

The princ.i.p.al reason for regarding this phase of the subject as relatively unimportant, is derived from a study of the habits of the blood-sucking species. It is found that, in general, they are intermittent feeders, visiting their hosts at intervals and then abstaining from feeding for a more or less extended period, while digesting their meal. In the meantime, most species of bacteria or of protozoan parasites with which they might have contaminated their mouth-parts, would have perished, through inability to withstand drying.

In spite of this, it must be recognized that this method of transfer does occur and must be reckoned with in any consideration of the relations of insects to disease. We shall first cite some general ill.u.s.trations and shall then discuss the role of fleas in the spreading of bubonic plague, an ill.u.s.tration which cannot be regarded as typical, since it involves more than mere pa.s.sive carriage.

SOME ILl.u.s.tRATIONS OF DIRECT INOCULATION OF DISEASE GERMS BY ARTHROPODS

In discussing poisonous arthropods, we have already emphasized that species which are of themselves innocuous to man, may occasionally introduce bacteria by their bite or sting and thus cause more or less severe secondary symptoms. That such cases should occur, is no more than is to be expected. The mouth-parts or the sting of the insect are not sterile and the chances of their carrying pyogenic organisms are always present.