Man usually acquires trichinosis by eating uncooked or improperly-cooked pork, and the disease is so widely spread and of such a serious nature that most civilised countries have adopted rigorous methods for the detection of trichinised meat. The pigs either acquire the disease by eating uncooked swine's flesh, which is frequently given them in the form of offal, or by devouring rats, which are very susceptible to the disease.
IV. Family Filariidae.
Mouth with two lips, or without lips. Six oral papillae often present, and sometimes a horny oral capsule. Four pre-anal pairs of papillae, and sometimes an unpaired one as well. Two unequal spicula or a single one.
Genera: Filaria, Ichthyonema, Hystrichis, Spiroptera, Dispharagus, and others.
The genus Filaria is a very large one. Like Ascaris, it is confined to Vertebrates, but usually lives in the tissues of the body and not in the intestines. F. (Dracunculus) medinensis Gmel., the guinea-worm, is well known as a human parasite in hot countries; it also occurs in the horse and dog. The female has an average length of 50 to 80 cm., but gigantic forms with a length of 4 metres have been described. The alimentary canal is degenerate. In adult females the body is completely occupied by a uterus crowded with eggs and embryos, which can only escape by the rupture of the mother's body, as the genital ducts have disappeared. Its original home is tropical Asia and Africa, but it has been introduced into South America with the negroes.
The female lives coiled up in the subcutaneous tissues, usually in those of the legs. Its presence gives rise to painful tumours. When these break the female protrudes, and may be withdrawn from the body by very carefully rolling it round a stick or pencil. This must be done very slowly, a few inches a day, as the rupture of the body sets free the contained embryos, and may result in the death of the host. The embryos normally bore their way into the body of the fresh-water Cyclops, and are re-introduced into their Vertebrate hosts with the drinking-water. It is usually stated that the female alone is known, and that it is uncertain whether it is hermaphrodite or whether both sexes are present in the Cyclops. Recently Dr. Charles[[184]] has described a specimen found in the mesentery of a human subject, from an orifice in the middle of whose body he was able to draw a much smaller specimen, and he thinks this may be the long-sought-for male.
Fig. 73.—A, View of the heart of a dog infested with Filaria immitis[[185]] Leidy; the right ventricle and base of the pulmonary artery have been opened. a, Aorta; b, pulmonary artery; c, vena cava; d, right ventricle; e, appendix of left auricle; f, appendix of right auricle. B, A female F. immitis removed from the heart to show its length. Natural size.
Filaria immitis Leidy, the cruel worm, is common in dogs in China and the East generally. It is not unknown in America and Europe. It occurs in such large clusters in the right ventricle that it is difficult to see how the circulation can proceed. The intermediate host is unknown, but from the prevalence of the disease in marshy country it is probably some aquatic animal. The larvae are said by Manson to disappear from the peripheral circulation of the dog during the day, but not to such a marked extent as do the F. sanguinis hominis Lew., var. nocturna Man. They were found by Galeb and Pourquier in the foetus of an infested bitch, a fact which establishes the transmission of such parasites through the placenta.
Filaria sanguinis hominis nocturna.—The female of this parasite has been described as living in the lymphatic glands of man. The embryos escape from it into the lymph, and thus reach the blood. According to Manson the intermediate host is the mosquito, in whose stomach the embryos undergo their larval changes. When the mosquito dies the larvae escape into the water, and then make their way into the alimentary canal of man, where they are believed to pair, and whence the female makes its way to the lymphatics. The presence of this Filaria causes great functional disturbance. One of the most remarkable features of it is that the larvae, which are very numerous in the blood during the night, disappear during the day, and are not to be found. Recently Manson[[186]] has described two new varieties: F. san. hom. diurna, in which the conditions of things are reversed, the larvae being found by day and not by night; and F. san. hom. perstans, in which the larvae occur both by day and by night. The larvae are long-lived, and were found by Manson in the blood of a negro who had not been in Africa, where it is endemic, for six years. The same observer is inclined to associate the presence of F. san. hom. perstans with the fatal disease known as "sleeping sickness." He also suggests that the mature form of the variety diurna is the F. loa, which is not uncommon in the eyes of negroes, and that its intermediate host may be one of the blood-sucking flies so common on the west coast of Africa.