To this group belong many symptoms in common, and it is therefore convenient to consider them together. The following list comprises all of the class of substances whose compounds are likely to be the cause of poisoning in horses: -
Caustic Potash or Soda.
Lead. Silver. Zinc.
Spanish-fly. Oxalic Acid.
Antimony. Carbolic Acid. Creosote.
Caustic Ammonia. Nitric Acid.
Symptoms common to this class of poisons are the result of their irritating or caustic action upon the membranes of the digestive tract, in some cases beginning with the lips, and in the majority affecting the stomach and intestines. The mouth and gullet, although first (after the lips) to have contact with the irritant, are frequently less affected, as it will be remembered that some parts of the digestive tract are provided with thick and comparatively resistant coverings. The tongue on its upper surface is protected with a dense membrane of epithelium, and the oesophagus or gullet is likewise lined with a thick and tough membrane capable of resisting puncture from such prickly or spinous plants as gorse and comfrey. The fauces, being less guarded, more frequently suffer irritation, and the bowels, with thinner and more highly vascular walls, are more readily acted upon by poisons of the class we are considering.
While the symptoms are the result of pain, the tissues are damaged or destroyed by chemical action.
Though our equine patient is not able to state in words the burning anguish he feels, there is a language of pain which it is the veterinarian's business to learn. It has a large vocabulary, with many shades of meaning to him who understands it by intimate association with the objects of his solicitude.
The common symptom - loss of appetite - will hardly be observed, as poisons of this class are so rapid in their effects that the last meal may have been partaken of in perfect health.
Amongst the earliest symptoms are rigors and fidgetiness, staring coat, colicky pains, evinced by stamping and scraping the feet, crouching and looking round at the flank, trembling of the body and shaking of the tail, sweating profusely over scattered patches, staggering, and either falling or going down recklessly. The lips are sometimes swollen, and from between them may issue great quantities of frothy saliva, which hangs in ropes from the corners of the mouth. Inside, the tongue may also be swollen, the lining membrane discoloured, the gums and mouth generally being of a purple hue, with, in certain cases, patches of sloughing tissue and a most offensive odour. The cavity seems filled with sticky mucus, and the animal can with difficulty close his mouth owing to the swelling of the tongue.
The blood-shot eyes and anxious countenance bespeak intense pain and fear. There is an agonized look of such intensity as is seldom observed as the result of ordinary illness. Respiration is hurried, nostrils dilated, the pulse small and almost indistinct, the extremities having an icy coldness. Vomition, so rare in the horse, is sometimes effected, the stomach contents passing through the nostrils. The bowels may be either purged or constipated, urine very high-coloured, and evacuated with groans.
Such are the symptoms common to this group of poisons, but they vary with the particular agent as well as in individuals.
Some poisons declare themselves by their odour, as in carbolic acid, or the lesions of the mouth may point to corrosive sublimate or a mineral acid as the particular poison.
Irritant poisons are not all corrosive, in the sense of quickly destroying and perforating the tissues. Ammonia is an example of an intense irritant, and corrosive sublimate the most immediately destructive of living tissues. The mineral acids show some difference in their local action. A yellow staining results from nitric acid, sulphuric acid whitens the membranes, and hydrochloric acid imparts a whity-brown appearance to the injured parts. Carbolic acid, we have said, leaves a tell-tale odour, but it and the caustic alkalies also make a white, drawn, or puckered surface of the membrane with which they come in contact. Patches thus injured become presently detached, or slough, leaving a raw surface and the expectoration of bloodstained mucus. In dogs and other animals the vomit indicates to some extent the amount of mischief in the stomach, but with the horse vomition is uncertain and accomplished with so much difficulty that we are unable to place any great reliance upon the appearance of ingesta ejected via the nostrils, and probably stained in the effort to get rid of it.
If death does not follow irritant poisoning in a few hours, the animal may pass into a condition of fever with variable results. If the toxic agent was of the corrosive class the patient may die of perforated stomach or intestine when a general improvement in his condition has raised false hopes of recovery in those interested. Gradual recovery is probable when a week has passed, in the case of merely irritant poison, but not in those of a corrosive nature.
Permanent constrictions in the oesophagus, stomach, or bowels may result from the healing scars where large areas of tissue are destroyed.