Several other species occur in man and are thought to be transferred by various insects, among which have been mentioned Tabanidae and tsetse-flies, but there is no experimental proof in support of such conjectures.
_Filaria immitis_ is a dangerous parasite of the dog, the adult worm living in the heart and veins of this animal. It is one of the species which has been clearly shown to undergo its development in the mosquito, particularly in _Anopheles maculipennis_ and _Aedes calopus_ (= Stegomyia). The larval form occurs in the peripheral blood, especially at night. When taken up by mosquitoes they differ from _Filaria bancrofti_ in that they undergo their development in the Malpighian tubules rather than in the thoracic muscles. In about twelve days they have completed their growth in the tubules, pierce the distal end, and pa.s.s to the labium. This species occurs primarily in China and j.a.pan, but is also found in Europe and in the United States. It is an especially favorable species for studying the transformations in the mosquito.
[Ill.u.s.tration: 121. Dracunculus medinensis; female; mouth; embryo. After Bastian and Leuckart.]
_Filariae_ are also commonly found in birds, and in this country this is the most available source of laboratory material. We have found them locally (Ithaca, N. Y.) in the blood of over sixty per cent of all the crows examined, at any season of the year, and have also found them in English sparrows.
In the crows, they often occur in enormous numbers, as many as two thousand having been found in a single drop of the blood of the most heavily infested specimen examined. For study, a small drop of blood should be mounted on a clean slide and the covergla.s.s rung with vaseline or oil to prevent evaporation. In this way they can be kept for hours.
Permanent preparations may be made by spreading out the blood in a film on a perfectly clean slide and staining. This is easiest done by touching the fresh drop of blood with the end of a second slide which is then held at an angle of about 45 to the first slide and drawn over it without pressure. Allow the smear to dry in the air and stain in the usual way with haematoxylin.
OTHER NEMATODE PARASITES OF MAN AND ANIMALS DEVELOPING IN ARTHROPODS
_Dracunculus medinensis_ (fig. 121), the so-called guinea-worm, is a nematode parasite of man which is widely distributed in tropical Africa, Asia, certain parts of Brazil and is occasionally imported into North America.
The female worm is excessively long and slender, measuring nearly three feet in length and not more than one-fifteenth of an inch in diameter.
It is found in the subcutaneous connective tissue and when mature usually migrates to some part of the leg. Here it pierces the skin and there is formed a small superficial ulcer through which the larvae reach the exterior after bursting the body of the mother.
[Ill.u.s.tration: 122. Cyclops, the intermediate host of Dracunculus.]
Fedtschenko (1879) found that when these larvae reach the water they penetrate the carapace of the little crustacean, _Cyclops_ (fig. 122).
Here they molt several times and undergo a metamorphosis. Fedtschenko, in Turkestan, found that these stages required about five weeks, while Manson who confirmed these general results, found that eight or nine weeks were required in the cooler climate of England.
Infection of the vertebrate host probably occurs through swallowing infested cyclops in drinking water. Fedtschenko was unable to demonstrate this experimentally and objection has been raised against the theory, but Leiper (1907), and Stra.s.sen (1907) succeeded in infesting monkeys by feeding them on cyclops containing the larvae.
_Habronema muscae_ is a worm which has long been known in its larval stage, as a parasite of the house-fly. Carter found them in 33 per cent of the house-flies examined in Bombay during July, 1860, and since that time they have been shown to be very widely distributed. Italian workers reported them in 12 per cent to 30 per cent of the flies examined.
Hewitt reported finding it rarely in England. In this country it was first reported by Leidy who found it in about 20 per cent of the flies examined at Philadelphia, Pa. Since then it has been reported by several American workers. We have found it at Ithaca, N. Y., but have not made sufficient examinations to justify stating percentage. Ransom (1913) reports it in thirty-nine out of one hundred and thirty-seven flies, or 28 per cent.
[Ill.u.s.tration: 123. An Echinorhynchid, showing the spinose retractile proboscis.]
[Ill.u.s.tration: 124. June beetle (Lachnosterna).]
[Ill.u.s.tration: Larva]
Until very recently the life-history of this parasite was unknown but the thorough work of Ransom (1911, 1913) has shown clearly that the adult stage occurs in the stomach of horses. The embryos, produced by the parent worms in the stomach of the horse, pa.s.s out with the feces and enter the bodies of fly larvae which are developing in the manure. In these they reach their final stage of larval development at about the time the adult flies emerge from the pupal stage. In the adult fly they are commonly found in the head, frequently in the proboscis, but they occur also in the thorax and abdomen. Infested flies are accidentally swallowed by horses and the parasite completes its development to maturity in the stomach of its definitive host.
_Gigantorhynchus hirudinaceus_ (= _Echinorhynchus gigas_) is a common parasite of the pig and has been reported as occurring in man. The adult female is 20-35 cm. long and 4-9 mm. in diameter. It lacks an alimentary ca.n.a.l and is provided with a strongly spined protractile rostrum, by means of which it attaches to the intestinal mucosa of its host.
The eggs are scattered with the feces of the host and are taken up by certain beetle larvae. In Europe the usual intermediate hosts are the larvae of the c.o.c.kchafer, _Melolontha vulgaris_, or of the flower beetle, _Cetonia aurata_. Stiles has shown that in the United States the intermediate host is the larva of the June bug, _Lachnosterna_ (fig.
124). It is probable that several of the native species serve in this capacity.
A number of other nematode parasites of birds and mammals have been reported as developing in arthropods but here, as in the case of the cestodes, experimental proof is scant. The cases above cited are the better established and will serve as ill.u.s.trations.
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 princ.i.p.al 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 as.e.xual 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 malariae_, which has an as.e.xual 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 "aestivo-autumnal," or "pernicious" fever is caused by _Plasmodium falc.i.p.arum_. 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 r.e.t.a.r.ding 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, _Mbu_. The opinion was not lacking support from medical men. Celli quotes pa.s.sages 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 circ.u.mstantial 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 s.p.a.ce 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 acc.u.mulate 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 ca.n.a.ls, 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.