We have here an order of flat-worms of a more or less oval shape, and many of them somewhat in the form of a leaf. They possess, on the ventral surface, one or more sucking discs by which they attach themselves. They have only one opening of the alimentary canal, which is generally forked. These worms are commonly called Flukes from the resemblance in shape of the commonest of them to the fish of that name. The various forms inhabit the bile ducts, except the Distoma haematobium, which is found in the veins of the portal system.

Distoma Hepaticum

This is the commonest worm of this order. As the name implies, it is met with in the liver, where it inhabits the bile ducts. The liver fluke is generally about an inch in length (Fig. 166) and rather more than half an inch in greatest breadth. The body is very flat, and anteriorly it ends in an elongated process, forming a kind of head. This head bears the mouth, and a short distance behind it comes the sucking disc. Between these lies the opening of the sexual apparatus, both male and female organs existing in each individual. The uterus forms a convoluted tube behind the sexual opening, and the seminal tubules lie still further back. This parasite is very common in certain of the lower animals, especially sheep. It occurs in enormous numbers in the bile ducts, which are dilated by it. As many as 1,000 have been obtained from a single sheep. It produces in sheep the disease called commonly the rot, which in some years is very fatal. It is said that in 1830-31 between one and two million sheep perished from it. It occurs also in oxen, where it produces more considerable alterations of the ducts. These become greatly dilated, thickened by inflammation, and encrusted with lime. It sometimes happens that masses of inspissated bile and lime salts form in the liver where the flukes are present. It is probable that in these cases many of the parasites have died and become themselves the seat of incrustation. This fluke has also been met with in horses and asses, and in some rare case's in man. In man. it has not been observed in large numbers, but it may produce serious obstruction of the bile ducts.

The eggs of this parasite are small oval bodies, which, in water, develop into embryoes which swim about by the aid of cilia. The intermediate host of the distoma hepaticum, which long eluded observation, is now known to be a small gasteropod possessed of a thin spiral shell, the Limncea truncatula. This mollusc, whose shell only measures about 1 cm. in length, is most cosmopolitan in its distribution.

Distoma Lanceolatum

This form of fluke is less than three-eighths of an inch in length, and about the fifteenth of an inch in breadth. It is seldom seen in man, and occurs in comparatively small numbers in sheep and cattle, producing little disturbance.

Fig. Hitf. - Distoma hepaticum. (Leuckart).

Distoma Sinense Or Spathulatum

This parasite has been met with in the East, and described by M'Connell in the Lancet in 1875, and independently by Macgregor in the Glasgow Medical Journal in 1877. Although a much smaller worm it is of a more elongated shape than the distoma hepaticum, as will be seen from Fig. 167. It is rather more than half an inch in length, and about an eighth of an inch in greatest breadth. When seen in the bile in the fresh state the edges show a beautiful delicate green colour, tinged with yellow, while the centre is of a deep brown. In the accompanying figure the position and appearances of the various organs are indicated. The eggs are very small, and each animal possesses thousands. This parasite is of very frequent occurrence in China and Japan, especially the latter, where it constitutes a serious malady. It is found in very large numbers in the bile ducts and gall-bladder, and produces symptoms like those produced in animals by the distoma hepaticum. The liver enlarges and becomes painful, and there is a gradual loss of strength and nutrition. Latterly diarrhoea, ascites, oedema of the feet, and general cachexia result.

Distoma Pulmonale Or Ringeri

This is also a parasite prevalent in Japan, Formosa, etc.

According to Manson it is present in 15 per cent, of the inhabitants of Formosa. It is a short, plump worm about a cm. in length; the transverse section is nearly circular. It is found in the lungs enclosed in little capsules formed of connective tissue. It gives rise to cough and haemoptysis. In the sputum the eggs of the worm are present sometimes to the number of several thousands daily. A case has been recorded by Yamagiwa in which the ova of this parasite produced embolism of the brain with symptoms of Jacksonian epilepsy.

Distoma Haematobium

This parasite, also called the Bilharzia haematobia, has the male and female organs in different individuals. The male is about half an inch in length, and flat, but rolled up at the edges, especiallv behind, so as to form a kind of gutter in which the female lodges. The female is about a half longer, but filiform. The eggs are small and furnished with a spine at the end or at the side (see Fig. 168).

Fig. 167. - Distoma sinense: a, oesophagus and stomach tubes; b, uterus; c, yolk glands; d, ovary; e, testes; f, termination of water vascular system, x 5. (After Macgregor).

Fig. 168. - Egg of Bilharzia (distoma haematobium) from the urine, x 230.

The parasite inhabits the blood-vessels of its host, chiefly the portal vein, the splenic and mesenteric veins, and those of the rectum and bladder. The ova penetrate the walls of the vessels as shown in Fig. 169, which is a photograph from the portal vein. The penetration of the eggs in large numbers into the mucous membrane of the rectum and urinary bladder produces great irritation and frequently haemorrhage. Similar irritation may be produced in the pelvis of the kidney and ureters. The eggs are found in the bloody discharges from the intestine and in the urine. This parasite is met with almost solely in Egypt and Abyssinia, and it is said that in Egypt about half the natives are victims of it.

Fig. 169. - Ova of Bilhaizia in wall of portal vein, x 60.

Literature

Leuckhart, in general. Distoma hepaticum - Weinland, Arch. f. Naturgeschichte, ii., 1874; Thomas, History and full account, Quar. Jour, of Mic. Sc, 1883. Human Distomata - M'Connell, Lancet, 1875; Macgregor, Glas. Med. Jour., 1877; Baelz, Berl. klin. Wochenschr., 1883, Die Krankheit. d. Athemorgane, Tokio, 1890; Manson, Med. Times and Gaz., 1881 and 1882; Yamagiwa, Virch. Arch., cxix., 1890.