“And these horrid worms in the bladder,” queried Jules, “no doubt destroy the brain matter, little by little.”
“They grow at the expense of the brain.”
“I can well believe then, that the sheep is unable to stand.”
“Each of these little bladders is a tænia in its first stage of development, and comes from the germ sown by the severed link or joint that the dog ejects with its excrement. As indisputable proof of this, if lambs are made to swallow some of the tænia links ejected by the dog, these lambs soon show themselves to be seized with the staggers, and in their brains are found the bladder-like organisms that cause the disease. The germs contained in the severed pieces of the tænia must therefore hatch in the lamb’s intestines, and the worms thus brought into being must make their way, through a thousand obstacles, to the animal’s brain, the only part of its body adapted to the development of the parasite.”
“Then it is in the brain that the little worms grow and become bladders as large as hens’ eggs?”
“It is only there that they can flourish. But these bladder-shaped worms are only incomplete beings, comparable to the larvæ of insects; and as long as they remain in the sheep’s brain their final development will not be attained. To acquire their final form, to become tænias, tape-worms, these larvæ must pass into the dog’s intestines. A conclusive [[350]]experiment shows it. If a dog is made to take with its food some vesicular worms from a sheep’s brain, the animal soon gives unequivocal signs of the presence of the tænia: its excrement contains chaplets more or less long of ripe joints. Furthermore, by sacrificing the dog so as to be able to decide the question more conclusively, one finds in the intestines the vesicular worms converted into veritable tænias or tape-worms. So the dog gives the sheep the germs that develop in the brain into vesicular worms; and the sheep gives the dog back these vesicular worms, which change into tape-worms in the intestines.”
“But how,” asked Louis, “can the dog become infected with vesicular worms when they are not expressly given to it with its food, as an experiment?”
“Nothing easier. The sheep affected with the staggers is slaughtered, and its head, the seat of the disease, is thrown away. The dog that finds it feasts on it.”
“And there we have shepherd dogs attacked by tænia,” said Louis. “Their excrement will spread the staggers among the flock.”
“We must, then,” concluded Uncle Paul, “as is recommended by those who have studied this subject experimentally in veterinary schools, exercise careful supervision over shepherd dogs and exclude from the flock those that are attacked with the tænia; finally, if the infection shows itself in the sheep, we must bury beyond the reach of any dog the heads of the slaughtered animals.” [[351]]