Ligula

LIGULA (strapworm), Ligula intestinalis (syn.: Ligula avium Blanch., Ligula simplicissima), a tapeworm that lives in its sexually mature form in the intestines of water birds (ducks, gulls, terns), and in its larval form in the body cavity of teleost fish, primarily cyprinids, causing a specific disease in the latter known as ligulosis or so-called fish rabies. Ligula belongs to the family Diphyllobothriidae, to the subfamily Ligulinae. The genus Ligula is characterized by the following features. In the young, i.e., larval state (in the body cavity of fish), the strobila is not externally segmented at all; in the adult, it is weakly segmented only in its anterior part, with the external segmentation not coinciding with the internal one. The head and suction pits are very poorly developed; in the larva, the latter are completely absent. The reproductive apparatus in some is simple (one longitudinal row of ovaries, etc.), in others it is double (two longitudinal rows of reproductive organs); the former are separated into the subgenus Monogramma, the latter into the subgenus Digramma; the larvae of both are indistinguishable in form, but individuals of the subgenus Monogramma reach larger sizes. Each of these subgenera includes one species: Monogramma uniserialis Rud.—length usually 20–40 cm, sometimes up to 1 m, and Digramma alternans—length usually 10–20 cm. The developmental cycle of the strapworm as a representative of Diphyllobothriidae proceeds with the participation of 3 hosts: 1) an intermediate—a cyclops, 2) an intermediate—a fish, and 3) a definitive—a fish-eating bird. Water birds disperse the eggs of the strapworm with their feces, from which a coracidium hatches, which is eaten by a cyclops, in the body cavity of which a procercoid is formed. The infested cyclops is swallowed by a fish, in the abdominal cavity of which a plerocercoid develops, which usually attracts the attention of both the consumer of the fish and the sanitary physician. In the body of a large fish, the strapworm can reach up to 1 m; the abdominal cavity of the fish swells, the swimming mechanism is disrupted, as a result of which the fish loses the ability to descend to the bottom and stays all the time near the surface of the water basin, making restless movements, thanks to which ligulosis received the name 'fish rabies.' This circumstance, in turn, makes it easier for fish-eating birds to catch fish swimming on the surface, swallow them, and become infected. In the intestines of birds, the parasite reaches full development quickly—in 1-2 days. There are cases when, under the influence of the parasitism of several strapworms, a rupture of the abdominal wall of the fish occurs, as a result of which the fish dies, and the parasite swims freely in the water for some time. If such live parasites are swallowed by a corresponding bird, it again becomes infected with ligulosis. The geographical distribution of Ligula is very wide; in the USSR, it is found in fish of all our water basins. There are lakes where the fish are infected with ligulosis in such a huge percentage that the local population refuses to consume fish from the corresponding lakes (e.g., Lake Kinon near Chita, some lakes of the Ural region, the Baraba Steppe, etc.). Sanitary and veterinary physicians have to perform examinations and evaluate to what extent fish infected with the strapworm are dangerous for human consumption. Man is not the definitive host of this parasite, therefore the strapworm is harmless to him. The figure [see separate table (cols. 175–176), fig. 6] shows a fragment of a strapworm found in canned fish.