Part 27

The disease caused by the presence of bladder-worms in the brain is called “gid,” or “sturdy,” also “staggers.” Two stages can be distinguished in it. During the first period the symptoms of disease are chiefly due to the wounds which the larvæ make on the surface of the brain, and the consequent inflammation of the investing membranes. Even in this stage the disease may be fatal, but the symptoms cease as soon as the larvæ cease to move about. The bladder-worms are only able to do harm by the pressure they exert, which is small at first, though it gradually becomes greater. The first stage of gid manifests itself in late summer, the second in late autumn, winter, or spring, according to the rate at which the bladder-worms develop. The symptoms of the first stage are caused by the increased flow of blood to the brain and head generally, resulting from the irritation which the larvæ exert. (Head very hot. Eyes bloodshot. The sheep are sluggish but restless, and carry their heads in a strange way, to one side, bent back, or turned up. They often run round in a large circle. There may also be gnashing of the teeth, foaming at the mouth, squinting, convulsive movements, and loss of consciousness.) The symptoms of the disease in its second stage are the result of pressure on the brain and some of the arteries of the brain and skull. (The sheep affected separate themselves from the others, and generally hang their heads. Gaze fixed. Occasional loss of consciousness. Movements strange; run or turn round in a circle. The sheep often staggers and falls to the ground. The bones of the skull become thinner at the place under which the bladder-worm is found, sometimes even as thin as paper.)

Remedies: Trepanning, as the bladders lie just underneath the roof of the skull. Prevention: Reducing the number of sheep-dogs; giving the dogs medicine to rid them of tapeworms in spring and summer; administering a specific to the yearling sheep in July and August, suitable for driving out any tapeworm larvæ which may be present in their intestines; burning (instead of burying) the heads of sheep affected with staggers which have died or been slaughtered. It is but rarely that all the larvæ living on the surface of the brain die between the first and second stages of the disease. A sheep affected by gid is therefore almost sure to die, and should be slaughtered as soon as possible. Compare “false gid,” p. 189.

The =Minute Tapeworm of the Dog= (Tænia eckinococcus)

is at most scarcely one-sixth of an inch long, and consists of only three or four joints, often lives in large numbers in the intestine of the dog without injuring its host, but causes huge cysts (from the size of a pigeon’s egg to that of a child’s head) in the liver, lungs, or other organs of man, pig, and ruminants. These cysts are of the type represented on the right of Fig. 138. They may prove fatal, owing to the pressure they exert in the organs containing them and the blood-vessels of the same.

The =Expanded Tapeworm= (Tænia expansa)

lives in the intestine of the ox, where it may attain the length of two feet, while in the intestine of the lamb it reaches at most the length of about nineteen inches. It is sometimes present in large numbers in lambs, causing difficulty of digestion with resulting emaciation, poverty of blood, and even death. Bladder-worm still unknown.

ORDER: =Trematoda= (FLUKES).

Flattened parasitic worms, tongue-shaped or leaf-shaped, living on or in a host, and provided with one or two ventral suckers. The intestine forks into two branches immediately behind the gullet, and these two divisions in some species (e.g. the large liver-fluke) branch again repeatedly in a tree-like fashion. Anus and blood-vessels absent. Almost all flukes are hermaphrodite; they fertilize themselves. All the forms living within a host (i.e. endoparasitic forms) lay an immense number of small eggs, from which larvæ are hatched that become asexually reproducing animals. These or their progeny produce once more the sexual form. The development is therefore an example of metagenesis (p. 16), but there is a change of host. Only the two liver-flukes need be dealt with here.

The =Large Liver Fluke= (Distoma hepaticum),

(Fig. 141), is flat, but swollen in the middle, when it contains a large number of eggs, under which circumstances the edges only are flat. The last are of a dirty brown colour, while the middle of the body is greyish yellow. There is a triangular projection at the front end of the body with a sucker at the apex, and another at hinder end of the triangle. The outer surface of the body is covered with minute backwardly projecting spines, which present no hindrance to the forward creeping of the body in the bile-ducts, but make backward creeping or gliding impossible.

FIG. 141.—The Liver Fluke (Distoma hepaticum), natural size. ]

Eggs ¹⁄₁₈₀ of an inch long, with a red shell possessing a greenish sheen. They can only develop if they get into water, and in wet years the conditions are very favourable on damp meadows, for under such conditions the sheep-dung, with the contained fluke eggs, is liable to fall into pools of water. But many of the eggs can develop in years that are not so wet and on dryer fields, as they may be carried into a ditch by rain or on the feet of the sheep. The larva hatched from the egg of the liver-fluke is ¹⁄₁₂₅ of an inch long, elongated in shape, and swims about freely in the water by means of a covering of cilia (Fig. 142, a). It soon finds its way into the lung cavity of a small water snail (Lymnæus truncatulus, Fig. 142, i), loses its covering of cilia, and becomes broader, even almost spherical. In the free-swimming larva there are already to be found a collection of germinal cells in the hinder part of the body; later on, these cells repeatedly divide and form little heaps, which ultimately become fresh individuals. The larva has, therefore, passed into a second stage, and this is now known as a sporocyst (Fig. 142, b, c). The stage developed asexually from the heap of germinal cells is distinguished from the sporocyst by a differently shaped body and by the possession of a gut (with mouth, pharynx, and intestine, but no anus), which is absent in the latter. This stage is termed a “redia” (Fig. 142, d). From five to eight of them are usually developed within one sporocyst. Germinal cells are also present in the redia, and they develop into little heaps of cells from which, while the weather is warm, new rediæ are developed, that live in the lung cavity of the snail, like their parents. When the weather becomes colder, “cercariæ” (Fig. 142, f) are developed from the germinal cells, some fifteen to twenty of them within a single redia (Fig. 142, e). The cercaria has a long tail, by means of which it can propel itself in the water, after leaving the mother redia and the snail. It possesses two suckers, and a forked intestine without anus. After the cercariæ have moved about in the water for some time they fix themselves, by means of their suckers, to various plants growing in the water. The tail being now superfluous is lost, the cercaria becomes spherical, and contracts somewhat within an investment which it secretes (Fig. 142, g). When, later on, the pools in which the cercariæ live dry up, the plants which were before submerged are now on dry land, and may be devoured by sheep, together with the encysted cercariæ found upon them. The wall of the cyst is dissolved in the intestine of the sheep, and the contained cercaria gradually becomes a young liver-fluke, which at first is oval and very small (Fig. 142, h), while, later on, its anterior third becomes the triangular projection at the front end of the adult fluke, and its posterior two-thirds grow very considerably. Meanwhile the young liver-fluke has travelled from the intestine of the sheep into its bile-ducts. It appears that the fluke is completely grown within a few weeks after its introduction into the body of the sheep. Infection almost always takes place in summer or autumn. The life history of the liver-fluke exemplifies change of host and metagenesis (p. 16). From the egg of a sexual worm a free-swimming larva is developed, which becomes an asexual sporocyst within the snail. This sporocyst produces asexual rediæ, and from these, also asexually, are developed cercariæ, which leave the snail, pass into water, attach themselves to plants, and reach the intestine of the sheep later on. The cercariæ become flukes by a process of metamorphosis, and are to be regarded as fluke larvæ.

FIG. 142.—Liver Fluke: a, free-swimming ciliated larva; b, young Sporocyst, a few days after migration into the snail; c, sporocyst, containing rediæ in various stages of development; d, redia, containing balls of germinal cells; e, mature redia, containing its brood of cercariæ; f, mature cercaria; g, just-encysted cercaria; h, young fluke, soon after entering sheep; i, the water snail, Lymnæus truncatulus.

a, b, c, d × 200; e × 150; f × 300; g × 150; h × 25; i, natural size and × 3½. ]

It is easily explained why the “fluke disease,” or “liver rot,” caused by the fluke should appear much oftener in some regions than others. Low-lying pastures which are now and then flooded are best suited for the development of the disease, since the fluke eggs have a better chance of developing upon them. This is because at certain times of the year there are in such spots numerous submerged plants on which cercariæ encyst, and which, later on, when the pools dry up, are nibbled by sheep. It seems that the cercariæ mostly attach themselves to certain kinds of plants, hence the local nature of the disease, which of course also depends on the presence of Lymnæus truncatulus (Fig. 142, i).

Fluke disease first makes itself apparent in the sheep one or two months after infection. If the sheep does not die the parasites often remain in its liver till the next spring. The liver of a single sheep may contain from 200 to 250 individuals. They live in the various branches of the bile-duct, and the irritation they set up first causes an unusual quantity of blood to flow to the liver. Inflammation and internal bleeding may result, and gall stones are deposited in the thickened walls of the biliary passages. The flow of bile from the liver is made difficult or impossible. Later on, the parasites draw so much blood from the liver that this organ is very insufficiently nourished, so that no more bile is secreted, and the liver substance shrivels up. In consequence of these changes the digestion is incomplete, and the whole body of the sheep is very insufficiently nourished. The diseased sheep are therefore sluggish and enfeebled; they eat little, but drink a great deal, and suffer from poverty of blood. Their wool gets dry, and does not hold together, and they become emaciated. Dropsy and diarrhœa set in, and jaundice often makes its appearance. If a sheep is infested by a tolerably large number of flukes, death is almost always the final result.

“The best preventative is a trustworthy and careful shepherd,” who will not let the sheep feed in spots where previous experience shows the disease is usually contracted during damp summers. Furthermore, sheep badly infested should be slaughtered as soon as possible, so that the flukes they contain may not get to the egg-laying stage. Sick sheep should be fed with nourishing food (various kinds of hay, oats; with addition of cooking salt), so that young sheep containing but few flukes may get back their strength, while this treatment will make the badly infested sheep die more quickly.

Footnote 5:

Spinola.

Oxen are also frequently affected by fluke disease in many regions, other animals to a less extent.

The =Small= or =Lancet-shaped Liver Fluke= (Distoma lanceolatum)

is about three-eighths of an inch long, one-tenth of an inch broad, thin, and lancet-shaped. Lives, like the preceding kind, in the liver of the sheep. Development unknown.

=Fourth Sub-Kingdom: MOLLUSCA= (MOLLUSCS).

This sub-kingdom, to which oysters, snails, cuttle-fish, etc., belong, includes unsegmented animals devoid of internal skeleton, and with bilaterally symmetrical (p. 16) embryo, while the adult may be much modified, especially in snails, which are enclosed in spiral shells, and also have their bodies partially coiled. The skin of molluscs possesses a peculiar and characteristic covering. From a definite part of the body a larger or smaller fold of skin (“mantle”) grows out, which encloses a space termed the “mantle-cavity,” and also, from its function, the “respiratory cavity.” In most molluscs this mantle secretes an external or internal calcareous mass. In this way a “shell” is developed (Fig. 143, A, S), which is usually external. This is the case, for example, with the edible, or Roman snail, and the common garden snail. In other cases (cuttlefishes), the shell is formed inside the mantle, or it may be replaced by a collection of calcareous granules (various slugs). In most molluscs the chief organ of locomotion is the so-called “foot” (Fig. 143, F). This is a very muscular region of the body, covered by skin, and serving either for crawling (snails), or else for digging in the sand, and even springing (bivalve molluscs). Its structure varies according to its function. I mention three classes, those of Cephalopoda (Cuttlefishes), Gastropoda (Snails and Slugs), and Lamellibranchiata (Bivalve Molluscs).

FIG. 143.—Diagrams to explain the form of the body of a Mollusc. A, a Bivalve Mollusc; B, a Snail (cross sections). In both; K, body; F, foot; S, shell; m, mantle; k, k, gills. ]

CLASS: =CEPHALOPODA= (CUTTLEFISHES).

The head is sharply marked off from the rest of the body, and the mouth is surrounded by a circle of arms bearing suckers or claw-like hooks. These arms are supposed to be a modification of part of the foot, the rest of this organ being constituted by a hollow, funnel-shaped structure, which places the mantle-cavity in communication with the exterior. The water used for respiration is forced suddenly out from the mantle-cavity, through the funnel, so as to propel the animal in the opposite direction. A gland, known as the “ink sac,” secretes a brownish fluid, which may be pressed out of the sac so as to shield the animal from attack. All cephalopods are marine, and, since they are therefore of no agricultural importance, it is unnecessary to enter into details regarding their form and structure, and I will only mention the following: =Pearly Nautilus= (Nautilus pompilius), with external shell and numerous arms; =Paper Nautilus= (Argonauta argo), with a very thin, boat-shaped shell, and eight arms; =Poulpe= (Octopus), without shell, and with eight arms; =Cuttle-fish= (Sepia), and =Squid= (Loligo), also without external shell, and possessing ten arms.

CLASS: =GASTROPODA= (SNAILS AND SLUGS).