Among other causes must be named fermented food, the microbian ferments and their products, serving to render the organ torpid, but also to produce fever, lessened secretion and an arrest or retardation of liquid supplies from the mouth or rumen.
Chronic heart disease, causing blood stasis in the omasum, appears to induce torpor and favor impaction.
The ingestion of lead has a very direct action in producing paralysis and consequent impaction.
Finally, finely divided dry food like meal or bran, swallowed hastily, tends to pass in large amount directly into the omasum, and, before the animal has become accustomed to the ration, is liable to clog the viscus and induce impaction.
In nearly all cases, the commencing impaction entails a certain rise of temperature and suppression of secretions, so that the malady tends to move in a vicious circle, each new step tending to aggravate the already existing condition. In chronic cases, which are very common, a careful record of bodily temperature shows oscillations, above and to the normal, at irregular intervals, each rise tending to add to the impaction.
The most acute and fatal forms of the affection occur in connection with a sudden change from dry to rich, luscious, green food in spring, the unwonted stimulus giving rise to general irritation of the whole gastric mucosa, with disordered and impaired function of all four stomachs, but especially of the third. Such cases are usually congestive and inflammatory and the suspension of the gastric movements is a grand cause of impaction. In such cases too the brain or spinal cord, or both, are seriously involved, and the early death is preceded by torpor, paralysis, violent delirium or convulsions, following largely the type of acute lead poisoning.
Symptoms. These vary according to the degree of impaction or gastric torpor, from simple, irregular, or suspended rumination (loss of cud) to the most severe gastric and nervous disorder.
The slighter or less acute cases are marked by a failure to re-establish regular rumination on partial convalescence from a fever or inflammation. The hyperthermia subsides, but the appetite remains poor and capricious, the muzzle dry, the eyes dull, the spirits low, breathing quickened and occasionally accompanied by a moan, especially when moving down hill, slight tympanies of the rumen may appear and the contents of that organ seem consolidated and may be felt as solid masses when pressure is made by the hand. The mouth is hot, clammy and fœtid, and the bowels costive, the fæces being passed in small amount and in the form of hard, black pellets, covered by a film of mucus, or streaks of blood, and containing particles of undigested food. This not unfrequently merges into a transient diarrhœa to be followed in turn by renewed constipation, and such alternations may repeat themselves again and again. The omasum is so deeply seated under the ribs on the right side that exploration is unsatisfactory, especially in the milder cases, yet pressure of the closed fist upward and forward below the middle of the chest will give the impression of a specially solid resistance and the patient may indicate suffering by a moan. Percussion with the closed fist has the same effect. There may be slight tremors of the body, the horns, ears and limbs are cold, and the hair erect in patches, dry and lustreless.
In cases occurring independently of previous disease, diarrhœa may be the first symptom noted, the malady being preceded by local irritation and congestion, but this soon gives place to constipation with alternating diarrhœa and the general train of symptoms above mentioned. The animal leaves the herd and is found lying apart on its left side with the nose in the right flank, the pulse and breathing quickened, the eyes congested, and a moan emitted occasionally in expiration. This is increased if the patient is raised and driven, especially down hill. He walks with stiff, arched back, unsteady gait and dragging limbs. Appetite may not be entirely lost at first, but only impaired and irregular, and as rumination ceases, grinding of the teeth becomes common. The secretion of milk is diminished or altogether arrested, and emaciation advances day by day. Fœtor of the eructations, the result of prolonged and septic fermentation in the rumen, is often a marked symptom.
This form may last from ten to fourteen days and merge finally into paralysis of the hind limbs, drowsiness and stupor, or delirium and convulsions.