Towards March and April the parasites leave their position, and are conveyed by the current of bile towards the intestine, to be rejected with the fæces. This is the period of convalescence and recovery; but recovery is only relative, for the parasites are never entirely evacuated. The distomata then recommence their life cycle outside the animal body.
Fig. 161.—Macerated specimen of large American fluke, showing the digestive system and acetabulum. X 2. (After Stiles, 1894, p. 226, Fig. 2.)
Unfortunately the mortality caused by distomata is accidentally aggravated by other diseases, and the scourge then becomes an absolute disaster for the districts where such complications occur. Thus Besnoit and Cuillé, of Toulouse have shown that distomatosis may become complicated with a form of very rapidly fatal hæmorrhagic septicæmia, produced by an ovoid bacterium.
Distomatosis, already sufficiently grave, then becomes infinitely more serious, if only from the fact that it may prove the point of origin of an absolutely fatal complication.
In bovine animals the symptoms develop exactly as in sheep, though the cachectic period is uncommon and the injury done is often less important than in sheep. The patients exhibit irregular appetite, wasting without appreciable cause, anæmia, and even diarrhœa. In spite of excellent winter feeding they do not regain condition, and relative recovery only sets in with the approach of spring. Death from simple distomatosis is exceptional, but in animals so predisposed enteritis develops more easily, as do all forms of infection of intestinal origin.
The disease is, however, also grave for bovines because successive reinfection occurs, and the disease may be prolonged for years.
Causation. Distomatosis is due to one cause, viz., the entrance of embryo flukes into the digestive apparatus of herbivora.
The adult distomata in the biliary ducts continually discharge large quantities of eggs, though the process is most active between February and June or July. The eggs are carried away with the bile and fæces and pass on to the pastures, where they continue their life cycle, thanks to moisture and the presence of stagnant water. The embryos, having escaped from the egg, enter the bodies of the snails found in or near stagnant water (Limnæa truncatula), become converted into sporocysts, and afterwards into rediæ and cercariæ. The cercariæ become encysted on the lower surface of blades of grass in damp pastures, whence they are transferred to the animals’ stomachs along with the grass itself.
