Popular Science Monthly/Volume 23/October 1883/The Liver-Fluke of Sheep
|THE LIVER-FLUKE OF SHEEP.|
By BYRON D. HALSTED, Sc. D.
THE object of this paper is to briefly summarize the present knowledge of the liver-fluke, causing the much-dreaded and fatal "rot" in sheep. Professor A. P. Thomas, of Balliol College, Oxford, has completed his long and extended researches on this parasite, which have been carried on under the direction of the Royal Agricultural Society of England. Professor Thomas's concluding report appeared in the last issue of the "Royal Agricultural Journal," from which the leading facts here given are drawn and the engravings borrowed.
The liver-fluke, shown twice the natural size in Fig. 1, is a sucking-worm related to the common leech, and known to zoölogists as Fasciola hepatica. It has the shape of a privet-leaf, is pale brown or flesh-colored, and from an inch to an inch and a third in length. There is a short projection at one end, and at its tip, y, is a sucking-mouth by which the fluke can attach itself to the surface of the bile-ducts of the sheep. A second sucker, y', is situated at the place where the head joins the body. These flukes are found in abundance in the livers of sheep and other animals infested with the "rot," and produce vast numbers of eggs. Each of these eggs under proper conditions gives rise to an animal "which is never like its parent, never does become like it, and never lives where its parent lives." It will be seen that in the liver-fluke we have an example of what is known among naturalists as an alternation of generations.
The eggs, one of which is shown in Fig. 2 very highly magnified, are about 200 of an inch in length, but may be made visible to the naked eye by shaking some of the dark-brown contents of an infested bile-duct in a glass of water and holding the vessel up to the light. The line marking off the lid of the egg may be seen near the right end,
and a little to the left is the embryo in its early state of development. It is surrounded by large granular masses, which serve as food. Only three of these masses, at the left hand, have been fully drawn. In one case Professor Thomas found seven million eggs in the gall-bladder of
|Fig. 3.||Fig. 4.|
a single sheep, while the liver contained about two hundred flukes. The number of eggs produced by a single fluke may be safely estimated as half a million. No further changes take place in the egg while it remains within the infested sheep. The eggs are, however, naturally washed away by the bile into the intestines, and finally pass from the sheep and are distributed with the droppings. If the eggs fall upon wet land, further changes take place during warm weather, and an embryo is formed. Fig. 3 shows a fluke-egg with the embryo fully formed within the shell. The body of the embryo is covered with cilia, by the motion of which the young trematode is propelled through the water. Both of the engravings (Figs. 3 and 4) are highly magnified. In swimming, the broader end is directed forward, and in its center is a projection, used as a boring-tool. The embryo has very simple eye-spots, which render it sensitive to light, and aid it in finding its future home. When the swimming embryo comes in contact with any object, it feels about, and, if not suited to its wants, starts off again. If the object met with is the snail (Limnæus truncatulus), shown in Fig. 5, it at once bores into it. In boring through the shell of the snail, the peg-like projection is extended, and the embryo spins around rapidly by means of the cilia. The natural place for the further growth of the embryo is in or near the lung of the snail, and when once lodged there its eye-spots and cilia disappear, and the body becomes oval in shape. Figure 6 shows the embryo while the changes are taking place. When the changes are completed the animal is called a sporocyst, meaning a sac of germs. The sporocysts live at the expense of the snail, and will, in July weather, reach their full growth, 1⁄40 of an inch, in a fortnight. Fig. 7 shows a full-grown
|Fig. 5.||Fig. 6.||Fig. 7.|
sporocyst, or first generation of the liver-fluke. It contains a number of germs, the lower one of which is ready to hatch out. This is the second generation, and is named redia, after Redi, the celebrated anatomist. The young redia, when ready, breaks through the wall of the parent, and the wound thus formed closes up, and the remaining germ redia? continue to grow. The sporocysts also multiply by simple division, thus causing still greater increase in the number of parasites in the snail. A young redia is shown in Fig. 8, with the contents forming into the third generation of the liver-fluke. A full-grown redia is seen in Fig. 9, much magnified. Some of the germs of the cercaria, as they are termed, may be seen within the redia. Each of these germs develops into a tadpole-like animal, with a slender tail. A redia may produce a score of the cercariæ, which escape from the parent through a special opening and then wriggle their way out of the snail. The free-swimming life of the "tadpole" does not last long, and after coming to rest it draws its body into a small sphere, and exudes a gummy substance, which protects it from injury. These encysted cercariæ are destined to find entrance to the liver of the sheep and then develop into the full-grown fluke. Fig. 10 shows a free cercaria as seen swimming in water, and in Fig. 11 is seen a portion of grass upon-which three cysts are fastened. The cysts remain attached to the herbage of the pasture, and are swallowed by the sheep in feeding upon the ground. If the cysts are not picked up by the sheep within a few weeks, the young flukes which they contain will perish. It has been determined that two hundred or more cercariæ may descend from a single fluke-egg, and, if the rediæ give rise to a generation of daughter-rediæ, a single egg may produce more than a thousand cercariæ. "Not only does the race of the liver-fluke multiply and increase abundantly in the sheep by producing myriads of eggs, but there is a further and great increase of the forms within the snail. If only the greatest degree of ordinary increase were reached, a single fluke might give rise to more than a hundred million descendants in the next generation of liver-flukes proper inhabiting the sheep. But, fortunately for farmers, the chances are enormously against any such disastrous increase."
Professor Thomas has determined that at least six weeks elapse, from the time the encysted cercaria is swallowed by the sheep, before the fluke is fully grown and begins to lay eggs. The flukes do not always pass from the sheep in summer-time, as was once thought by leading veterinarians. There is no time of year when sheep-livers
|Fig. 10.||Fig. 11.|
containing flukes can not be obtained. Cases are known where the flukes have been known to live for more than a year.
The summary of the life-history of the liver-fluke is given as follows: "The adult fluke in the liver of the sheep produces enormous numbers of eggs, which are distributed with the droppings of the sheep. If these eggs have moisture and a suitable degree of warmth, they continue to live, and in each is formed an embryo. The embryo leaves the egg and swims in search of the particular snail (Limnæus truncatulus) within which its future life and growth take place. The embryo bores into the snail, and then grows into the form which is called a sporocyst. The sporocyst gives rise to the second generation. This generation is known as the redia. The rediæ in turn produce the third generation, which has the form of a tadpole, and is called cercaria. The cercaria? quit the snail and inclose themselves in envelopes or cysts, which are attached to the grass. When the grass to which the cysts adhere is eaten by sheep or other suitable hosts, the young liver-fluke comes out of the cyst and takes up its abode in the liver of its host, and the fatal circle is thus completed."
The fluke-disease alternates between a kind of snail and the sheep. One sheep can not contract the "rot" directly from another member of the flock, and one snail can not take it from its neighbor. The sheep drop the eggs, and hatching, the embryos, find the snails, and the snails after nursing the young flukes through three generations distribute the tadpole cercariæ, which convey the infection back to the sheep, and it there inhabits the liver and causes the disease.
The Prevention of the Rot.—Now that the life-history of the fluke is known, it is not difficult to comprehend the conditions necessary for its existence. There must be: (1) Fluke-eggs on the ground; (2) wet ground or water during warm weather—(3) the snail Limnæus truncatulus (?); and (4) sheep allowed to feed upon the infested ground.
Under the first condition it may be said that wherever fluked sheep are kept we shall have fluke-eggs. In some districts flukes are always to be found, and where the conditions are the most favorable a sudden outbreak may be expected. The disease sometimes appears in quarters where it was previously unknown, and may have been introduced in manure containing fluke-eggs, or adhering to the feet of cattle, dogs, or men. The eggs and young flukes (embryos) may be conveyed by running streams, floods, etc. Other animals than sheep are infested with the parasite, and rabbits and hares may be the means of introducing the plague. The production of the fluke-eggs may be prevented by killing the sheep so soon as they are found suffering from the "rot." If there is a suspicion that a flock is attacked, one of the members exhibiting the strongest signs may be killed and its liver examined. If rotted sheep are kept, they should be on dry ground, where the fluke-eggs can not fall on wet land, or be swept into brooks by the descending rains. The manure of infested sheep should not be placed on wet land, and the livers of rotted sheep ought to be destroyed or deeply buried.
The remedy for the second condition is a simple one, but not always easy of application. Wet land should be thoroughly drained, and, besides preventing the rot, it will greatly improve the pasturage. When draining can not be done, lime or salt may be scattered over the surface. These substances will destroy the embryos, the more developed encysted form of the fluke, and the snails which serve as hosts. The salt or lime should be applied in early summer, when the young flukes are present in the greatest numbers.
There seems to be only one snail in England which is host to the young flukes, and the accounts of Professor Thomas in exposing other species of Limnæus to the embryos of the fluke are most interesting. The Limnæus truncatulus (Fig. 5) is said not to exist in the United States, though several other kinds of snails belonging to the same large genus are found here. It is probable that with us some other species than Limnæus truncatulus serves as the host of the intermediate forms of our liver-flukes. Draining the wet land will reduce the number of snails; and dressings of lime or salt, as above mentioned, also destroy them. The lime should be scattered especially on or near marshy places and along ditches. If a pasture has been flooded, it is well to use the lime to destroy snails which may have been brought down from infested areas.
If there is suspicion that a lowland is likely to give the "rot" to sheep, the best plan is to not allow them to feed upon it. In seasons of excessive moisture it may be impossible to keep the flocks on dry pastures. Salt, as a preventive, may be used in a second way. It acts injuriously upon the germs when fed to the sheep, and it also improves the general health of the animal. In addition to the salt, use dry feed as much as possible. The sheep should not be allowed to graze too closely, for the snails, as a rule, keep near the ground.
If all farmers would unite in carrying out the above preventive measures given by Professor Thomas, the losses from a fatal disease would be greatly reduced. Rabbits and hares are an obstacle to the total extermination of the "rot" in sheep.
The literature of the liver-rot in sheep is specially interesting in the light of our present knowledge. Jennings, in his work on "The Horse and other Live-Stock" (1866), says: "The malady is unquestionably inflammation of the liver. These fluke-worms undoubtedly aggravate the disease and perpetuate a state of irritability and disorganization, which must necessarily undermine the strength of any animal. . . . The sheep, having a little recovered from the disease, should still continue on the best and driest pasture on the farm, and should always have salt within their reach." Youatt, in his work on sheep, in 1848—back of which date it is not worth while to go—wrote at length upon the "rot." He located the disease in the liver, and states that it has existed from the earliest period of agricultural history. His description of a diseased sheep was full and quite accurate, but the cause was unknown to him. He says, without qualification, that it is inflammation of the liver. The full-grown fluke is too large an object to escape notice in the examination of an infested liver, and therefore was known to Youatt and the veterinarians of his day, but was considered the consequence instead of the cause of the "rot." They believed that the "rot" was connected in some way with the soil, it being confined to wet seasons and to sheep which fed on marshy lands. "It has reference to the evaporation of water, and to the presence and decomposition of moist vegetable matter." In other words, the gases arising from decomposing organic substances cause the "rot." Randall, in his "Sheep Husbandry," states that this view at that date (1860) was universally received by scientific veterinarians. H. Clok, V. S., in his "Diseases of Sheep" (1869), says the "rot" is analogous to "fluke," and is produced by many causes, among which "grazing on marshy or sour meadows" is a leading one. In speaking of the fluke, he says, "The worms are found spontaneously, like all other worms of the viscera, and the opinion that they are introduced into the body with the food, drink, etc., must be considered erroneous."
With the first symptoms of the liver-fluke in sheep there is a tendency to fatten, and animals intended for the shambles have been purposely rotted in order to increase their fattening properties. A celebrated stock-man in England used to overflow his pastures, and, after the water was run off, turn on his sheep which he was preparing for the market. These animals became infested, accumulated flesh rapidly, and by this manœuvre a gain of some weeks was obtained. The practice is certainly questionable, if not positively vicious.
The writer claims no originality in the present paper, and only acts the part—and that imperfectly—of a middle-man in science.