Debility doubtless renders an animal more susceptible, yet the disease usually kills nearly all horses attacked, excepting such as have been immunized. This is the same for casual and inoculated cases. When, however, the virulence has been lessened by culture or by passing through the body of an unsuitable animal the results are very uncertain; some horses it will kill, while in others it produces a slight and harmless fever.
Forms. Two leading forms of the disease are known: 1st, the lung sickness (Dunpaardziekte), and 2nd, the head sickness (Dikkopziekte). A variety of this last is blue tongue (Blautong), which has been confounded with gloss-anthrax.
Symptoms. The lung sickness may appear as a fulminant affection following on the usual incubation of about eight days. Suddenly, in the midst of apparently full health and vigor, the breathing becomes accelerated and dyspnœic; this encreases for about an hour, then the patient staggers, falls, ejects a mass of white froth from mouth and nostrils and dies. When death is more delayed, there may be noted a rigor, and in the evening a rise of temperature to 103° F., lowering a little next morning and rising again toward night, yet making an encrease day by day until near the end, when it becomes subnormal. Death in such cases occurs on the third or fourth day, preceded by great prostration, hurried, labored breathing, dark red or cyanosed mucosæ, loud rattling over the large bronchia or lower end of the trachea, coughing and dropping of a serous fluid from the nose, or accumulation of white froth around nostrils and mouth. The froth soon condenses in part into a straw-colored liquid, which collects in considerable quantity. The abundance of froth blocking the air passages produces death by suffocation.
In the head œdema, the muzzle, lips, head and neck become the seat of excessive exudation, the swelling of the face drawing back the lips so as to expose the teeth of the lower jaw. The skin is rendered tense and exudes the straw-colored serum, as do also the buccal and pituitary mucosæ.
In blue tongue the exudate is concentrated in the lingual organ, which swells to an enormous size, forcing the jaws and lips apart, and hanging out as a dark blue, cold mass. A foul, liquid mixture of saliva and exudate drivels from the mouth. The pressure on the larynx may cause marked stertor, advancing to asphyxia.
While these three types may seem to be distinct and uncomplicated in some cases, more commonly the exudation appears to a slight extent in all three situations, and it is only the predominance of the symptoms in one particular part that assigns the attack to one type rather than another. All are very fatal, but the lung sickness is preëminently so, very few such surviving.
Lesions. There is usually a mass of white froth around the mouth and nostrils. The serous exudate coagulates readily in the presence of minute traces of blood, and forms a solid mass of clot when heated. A yellow gelatinoid exudate is found in streaks or patches, subcutaneously and between the muscles, but especially along the jugular furrow. In the head sickness the whole subcutaneous and intermuscular tissue in the head and neck are infiltrated, and the straw-colored liquid escapes abundantly when the part is scarified. The same is true of the tongue, which is stained throughout with blood that has gravitated into it.
The pleuræ contain an abundant exudate more or less deeply stained with blood. The same is true of the pericardium. In the latter Edington has found 140 fluid ounces. On the surface of the lungs and pericardium are extensive yellowish exudates. They are covered with petechiæ mostly small or punctiform. If the horse has stood at rest throughout the illness the lungs seem pale, yellowish, yet swollen and indisposed to collapse. The interlobular tissue especially is infiltrated with serum so that toward the free margin the lobules may be separated by intervals of half an inch in breadth as in lung plague of cattle. In horses that have been worked during the illness the whole organ is congested and firm, resembling the condition of croupous pneumonia. The trachea and bronchia show dark congestion of the mucosæ and a mixture of froth and serous exudate. The large blood vessels contain diffluent blood clots of an intensely dark color. In the vascular furrows of the heart and along the large vessels are yellow exudates or blood extravasations. The muscular tissue of the heart appears normal or rather pale, and under the microscope the striæ are found to be obscured by cloudy swelling, and minute blood extravasations and hæmatin are met with. The endocardium is cloudy, with blood extravasations, and exceptionally ante-mortem clots are found. The spleen is usually swollen, very dark, blood gorged and covered with petechiæ. On section there are found extensive extravasations, with masses of blood pigment and crystals. The kidneys are enlarged, the capsule easily detached, the epithelium of the glomeruli and convoluted tubes swollen and their nuclei multiplied. A gelatinoid exudate is usually present in the renal pelvis. Congestions have been found in the right gastric cul-de-sac and less frequently in the intestines. Exudations have also been found in the cerebro-spinal nervous system, the laryngeal mucosa and the conjunctiva. The latter is usually cyanosed.
Prevention. The first consideration is to keep work horses indoors or in a kraal during the summer or sickly season. Here they must be fed on dry hay and grain only, grass being strictly withheld. If it becomes absolutely necessary to feed green fodder of any kind it should not be cut until all dew or rain has completely dried off in the heat of the sun, and if kept over night should be kept under cover and again dried before feeding. When taken out to work the animal should wear a check rein or muzzle so that he cannot by any chance reach the green vegetation. This rule must be most strictly adhered to at night or during damp weather.
For horses turned to pasture a fair amount of protection may be secured by shutting them in a stable or kraal before sundown, and until the vegetation has been thoroughly dried by the sun the next morning.