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Among the other diseases which may be disseminated in this way, we may mention in particular Egyptian ophthalmia, Florida "sore-eye,” and Asiatic cholera. For the first two maladies, the evidence is based on observation, but the facts at hand are so complete that they can scarcely be doubted. In the case of Asiatic cholera, however, the bacillus which causes the disease has actually been found in flies caught in a room where post-mortem examinations on cholera cadavers were being made.
What, now, are the practical lessons which we should draw from these facts-practical lessons not only for the medical fraternity, but also for the laity, and especially for the housewife? A moment's thought will show that they are: First, prevent flies from breeding; second, keep them away from diseased material ; third, keep them away from our food—three propositions which, at first thought, may appear purely theoretical, but which, nevertheless, are thoroughly feasible.
You have all heard of anthrax or malignant pustule, which a fiects both man and live stock. It is quite generally supposed that this malady is transmitted by the bite of flies and mosquitoes. There is not, however, very much evidence in support of this viewv, and all the experimental evidence as yet available is contrary to it. It would, in fact, appear that the danger involved has probably been greatly exaggerated. If, indeed, anthrax were normally transmitted in this manner, we should expect to find it much more general than the malady really is, for biting insects are common around live stock, and they do not confine themselves to one animal, but fly from one to another. If, therefore, a case of anthrax appeared in a herd, the probabilities are that within a few hours, or a few days at most, practically the entire herd, as well as the people in the vicinity, would be suffering from this disease. At present, we must admit that it is possible that anthrax may be disseminated in this manner, but at the same time it would appear that such transmission is probably the exception rather than the rule. It seems probable that one of the common methods of keeping this disease alive is the leaving of carcasses of anthrax animals on the fields. Cattle later chew the bones, and thus expose themselves to infection with anthrax spores. That biting insects would undoubtedly swallow the germs of this disease while sucking the blood of a sick animal, and thus disseminate the bacilli through their droppings, is, of course, possible; but ali experimental evidence thus far ac
cessible indicates that the germs rapidly lose their virulence in the insects' intestines.
Similarly it has been maintained that bubonic plague, erysipelas, and leprosy are transmitted by the bite of insects, but this theory cannot at present be admitted as of general application. Much more probable does it seem that insects may transmit these diseases in another manner. Suppose a flea, infected with bubonic plague, succeeds in reaching a healthy person, and the latter, in an attempt to allay the irritation caused by his unwelcome visitor, scratches himself and at the same time crushes the fea. The possibility is present that some of the germs from the intestine of the insect will be rubbed into the fresh scratch, and thus bring about the disease; or, since fleas deposit their excreta on the skin of their victims, the possibility is present that the germs enter the system by way of the wound caused by the flea bites, instead of by the act of biting on the part of the flea. From experimental evidence before us, these two possibilities must undoubtedly he admitted, not only in connection with fleas, but also with the itch mite. We must also admit the very high probability that flies transnuit hubonic plague accidentally to the food, after lighting upon plague discharges. Finally, we cannot deny that even ants, which have fed on rats dying from plague, may perhaps spread the disease.
None of these facts or possibilities should frighten us or lead us to ridicule science for taking them into consideration, but all of them should induce us to take every precaution possible, especially in cases of bacterial diseases, to have the sick-room scrupulously clean and free from insects of all kinds.
Let us now turn from bacterial diseases, the spread of which by insects is possible, to some of the diseases caused by animal parasites, and for the dissemination of which insects and closely allied arthropods are necessary.
Mosquitoes are positively proved to be necessary for the spread of at least two diseases in man, known as malaria and filaria disease, or Arabian elephant foot. Thanks to the efforts of two of our fellow-townsmen, Drs. Reed and Carroll, and their army colleagues, the present indications are that mosquitoes are necessary for the spread of vellow fever also. Now a curious, but from a biologic standpoint not an unnatural, fact is that each of these diseases requires a certain kind of mosquito.
Malaria is caused by a microscopic protozoan which lives in the blood. In the common type of the malady it takes this para
site just forty-eight hours to complete its development, and every forty-eighth hour, corresponding to the time when your chill begins, each germ breaks up into about fifteen young germs. Assuining that you have only one of these minute parasites in your blood to-day, you may at this rate be the happy possessor of about 576,000,000,000 wriggling parasites after twenty days, cach germ destroying a blood cell. This is compound interest with a vengeance, and leaves in the shade any Wall Street accumulation as yet recorded.
If this rapid increase were to continue, every person who is once attacked with malaria would soon become skin, bones and germs, and every case of malaria would inevitably be fatal. Fortunately, however, it is a more or less common biologic law that protozoa which thus increase in geometrical ratio, by asexual reproduction, soon reach a point where further multiplication is inipossible until the organisins pass through asexual stage. It is this law (the explanation of which is not entirely clear) more than all the doctors in the world combined which accounts for the comparatively low death rate of malaria.
After a number of generations of asexual forms, male and female parasites appear in the blood, and these cannot reproduce further until they enter the body of a mosquito belonging to the genus Anopheles. The next generation forms in the mosquito, and the young parasites then reach the insect's salivary glands and are introduced into the blood of the next person bitten.
The insect which disseminates malaria in this way is a small dapple-winged mosquito, with long antennæ ; in a resting position its body is straight, and is likely to assume a posture at nearly right angles to the wall; and while serenading it sings about four tones lower than does the ordinary mosquito glee club. The female lays from forty to one hundred boat-shaped eggs, which float around on the water, and in three days' time hatch out into larvæ. The latter swim around in the water, coming to the surface to breathe and feed; they assume a position nearly parallel to the surface, and being possessed of a “rubber neck," they can turn their head half way around so that the mouth appears to be on the back; after about sixteen days they turn into peculiar pupæ which have very large heads and bear a more or less close resemblance to a lobster salad nightmare. Five days of fasting in this stage, and the adult winged mosquito issues and is ready to spread malaria, if conditions are favorable.
Another disease transmitted by mosquitoes, and more interesting to the biologist than to the patient, is known as filaria disease. This malady is caused by a worm. The adult parasite lives in the lymphatic system, while the embryonic worms swim around in the blood. Curious to relate, these embryos are found in the peripheral circulation only at night, or more properly speaking, during the hours of sleep. If certain mosquitoes, while sucking blood, swallow these embryos, the young parasites undergo a series of changes in the insect, and finally reach the salivary glands; from there they pass down the proboscis, and are inoculated into the body of another person.
In this instance the guilty mosquito is different from the one which spreads malaria. It possesses short antennæ, and belongs to the genus Culex. I have no illustration of this species (Culex cilaris) here this evening, but can present to you its American first cousin, which sings tenor, and is rather likely to hold its hump-backed body more or less parallel to the wall when in a resting position. Instead of laying boat-shaped eggs, it deposits two hundred to four hundred ova which are fastened together. Having become very proficient in the trick so long practiced by Columbus, the eggs are all placed on end and remain perpendicular. The larvæ hang downward in the water, and the pupa have smaller heads than we saw in Anopheles.
You have all heard of the important work which General Sternberg's yellow fever commission is accomplishing in connection with this inuch-dreaded disease. I cannot show you the germ which causes this malady, for it is not definitely determined as yet. The mosquito in question is known as Stegomyia fasciata; it was formerly called Culer fasciatus, and is closely related to the form which transmits filaria disease. It possesses banded legs, and has conspicuous silver stripes on the thorax and abdomen. It breeds in standing water, and its larvæ and pupa are much like the same stages of Culex.
It has thus far been shown that this insect, if it has fed on the blood of a yellow fever patient, may, after about twelve days, transmit the disease by means of its bite to a healthy and nonimmune person. This discovery is of extreme value, and we may all well be proud that we can count the discoverers among our personai friends. It is, however, to be expected that until the germ which causes the disease is recognized, scientific men in general will be somewhat conservative in giving to this discovery its full
apparent value from a standpoint of public hygiene. The fact that yellow fever is transmitted by the bite of the mosquito not until a considerable time has elapsed-namely, twelve days after the insect has sucked blood-indicates very strongly that yellow fever is caused, not by a bacterium, but by a sporozoan. If this latter supposition can be demonstrated, then the conclusion will, from analogy, be warranted that mosquitoes, and probably only mosquitoes, can transmit this disease. If, on the other hand, the disease should be proved to be bacterial in nature, analogy would not at present warrant us in concluding that mosquitoes are either necessary to its transmission or that they are the only transmitters. As already stated, however, analogy at present points very strongly to a sporozoan as causal agent; and if this can only be proved, the importance of the discoveries already made by our friends will be iar greater than is at present recognized.
The facts brought forward in connection with the last three diseases mentioned should convince us of the necessity of a public campaign against the mosquito. Practical zoologists and sanitarians see the possibility of killing off these insects and thus eradicating the diseases in question. This cannot, of course, be done all at once, but it can be done with comparatively little expense in all well-settled communities. Viewed purely from the standpoint of dollars and cents, it would be an excellent business investment for malaria districts to eradicate mosquitoes.
In the Southern States there exists among the cattle a disease known as Texas fever. Its economic importance may be partially appreciated when we recall that a quarantine line runs across this country from the Atlantic to the Pacific, and that except during a few weeks in winter it is permitted only under stringent regulations to ship cattle from places south of that line to the Northern markets, while driving the animals to Northern pastures is prohibited. This disease is quite similar to malaria, being caused by a minute parasite in the blood. So-called cattle ticks act as transmitters of the malady. When ticks have filled themselves with blood, they drop to the ground and lay their eggs. The young of the next generation, although they have not been in contact with an infected steer, seem to have taken the disease from their mother, for they can transmit it to healthy cattle.
Another disease which may be mentioned in this connection is found among the live stock in certain parts of Africa, and is one