From a consideration of all these data, the conclusion must at once be drawn, that as living flukes cannot reach the liver from without, they must of a necessity be produced only in particular states of the animal they inhabit. How they originate we cannot of course determine, and this is not the place to hazard physiological conjectures; but it will be found that their appearance in the bile is always preceded by tuberculous deposits in the lungs or liver. This I have proved by numerous dissections, in which I have occasionally found tubercles without flukes, but never met with flukes where I did not at the same time discover tubercles. Fluke worms, therefore, can never be regarded as a cause of rot, they must be looked upon merely as a symptom. We cannot, however, say that tubercles give rise to the liver-fluke, for tubercles are often present in cases where flukes are absent; and if the latter were the effect of the former, their presence under such circumstances would in all probability be constant.
III. Particular plants have been said to cause rot, but the proofs of their evil tendencies being in every instance about as logically supported as the fluke theories already mentioned, I need not trouble myself or the reader by proceeding to details.
Real Causes of Rot. Everything that has a tendency to weaken the animal, will be more or less liable to lead to rot. Exposure to cold and wet, mishaps at lambing time, food bad in quality or deficient in quantity, and over-driving, will all predispose the constitution to the deposition of tubercles. It is from the causes being in this way common to the whole flock, that contagious properties have been ascribed to rot, it having been observed, from the time of Virgil, to break out in many animals at once.[ [38]
The reason of so many different things having, from first to last, been reckoned capable of producing this disease, appears to lie in the known fact, that if a sheep be exposed to any of the above depressing agents, rot, if the animal be as yet untainted, will not, at the moment, shew itself; but a chain of morbid actions will in all probability then commence, and, being beyond the ken of ordinary observers, will pass unheeded, till some slight mismanagement in food or shelter, hastens their progress, and renders them apparent to the plainest understanding. The final symptoms of rot may thus occur on any kind of pasture, and the scene of the catastrophe will incur a stigma which ought to be attached to herbage which the sheep have consumed at some distant place or date.
Bad food is justly regarded as one of the most common causes of rot, and ranks, in my estimation, next to cold and wet, in its power of producing it. I shall only remark, on this point, that of all the food on which sheep can possibly be kept, none is known to act so deleteriously as grass which has sprouted quickly. Rot is well known to occur most frequently on land which has been irrigated during summer, for at this season any excess of moisture is peculiarly injurious to the economy of a plant.
When plants by heat and moisture are stimulated to increased exertion on a poor soil, they acquire bulk without having it in their power to obtain at the same time those saline matters which constitute a healthy plant, becoming in fact, to the eye of an inexperienced person, thriving vegetables, while to the palate they prove wersh and watery.
The same result may follow from a different process. The saline matter may not be taken up, even when the soil is rich in such ingredients, from the functional derangement into which the roots or digestive organs have been thrown by the unnatural circumstances in which it has been placed. A plant is composed, like all organized bodies, of a certain number of proximate principles, which are more or less numerous in different kinds. These are combined with varying quantities of potass, soda, lime, magnesia, and iron, which, though formerly supposed to be too trifling in quantity materially to affect the quality of the plant, have yet been recently and satisfactorily proved completely to change the character of the compound, even when the excess or deficiency amounts only to a 1/10000th part so that, supposing an animal to thrive on plants which contain salts of any or all of the above bodies, it will soon fall off if these plants are in any way deprived of a single adjunct; for by the removal of that one salt, their nature has been entirely altered.
The certainty and rapidity with which Bakewell could rot his sheep, by pasturing them, in Autumn, on land over which water had been allowed to flow during the previous summer, may seem to controvert what I have above stated, as to time and frequent change of pasture often intervening between the origin of the disease, and its termination; but when it is recollected that he pursued the destructive system of breeding in and in, of itself sufficient to induce a tuberculous predisposition, the reader will perceive that his sheep were, in all likelihood, more or less tainted, and therefore, sure to fall victims to the disease the moment they were subjected to the deleterious influence of an unwholesome pasture.[ [39]
Over-driving and hurrying of every kind, is, in my opinion, a fruitful source of rot, not only from the fatigue it causes, or the risk it leads to of taking cold, but also from the injury, which in many cases results, to the delicate texture of the lungs. As shown in the note to paragraph (157), no animal is more easily put out of breath by running, than the sheep. Whenever the breathing is hurried, the circulation through the lungs is quickened also. If the tissue of the lungs be in any way delicate, the force with which the blood is propelled is sure to make it yield, and in this manner the animal is often suffocated by the large quantity of blood, which issues into the air tubes at once from many points. Fig. 1, Plate VI. exhibits a good illustration of this taken from a sheep. Numerous red points are seen sprinkled over the surface of the section, indicating that blood has been effused from many minute torn vessels. Now, if this animal had survived, each speck of blood would have formed a centre, round which tuberculous matter, as in Fig. 2, Plate VI. would have been secreted, and death from rot, at some ulterior period, would, in all probability, have been the result.[ [40]
(165.) Treatment of Rot. As reason and experience have taught us that tathy herbage is a common cause of this complaint, we should, when it shows itself, at once remove the animals to a better pasture, where they should be exempted from teazing of every kind.