The eruption may invade the conjunctiva, the cornea (inducing blindness), the nasal mucosa (causing difficult breathing), the mouth (salivation, difficult mastication), the pharynx (difficult deglutition), the stomach and intestines (diarrhœa), the bronchia (cough), or the vulva.
With the occurrence of desiccation, in discrete cases, the fever subsides, the swelling of the skin and head disappears, the discharges from the nose and eyes cease, and the animal is restored to health.
A secondary eruption will sometimes take place, but only papules form, which disappear without reaching maturity.
The duration of discrete sheep pox is usually about three weeks, but it may reach four in cool weather. It is more rapid and more prone to dangerous complications in hot weather. As the members of a flock are usually attacked in succession, it may take three months to pass through a flock.
In confluent or complicated cases the fever of invasion is very high, the sheep dull, prostrate, tender to touch along the back and loins particularly, with hurried, labored, panting, breathing, and mawkish, fevered breath, the weakness increases rapidly, standing even may become difficult, the wool is loosened and falls off in patches, the exposed skin is red, shading, it may be, into a violet hue. The head droops, the lips and nostrils swell, saliva drivels abundantly, anorexia is complete, though thirst may be ardent, a yellowish and even reddish, fœtid discharge may flow from the nose, respiration is difficult, the eyes are watery and deeply sunken, and the head, limbs, breast and abdomen are extensively infiltrated and œdematous.
Fever does not subside with the occurrence of eruption, which may appear thickly over the whole body, the woolly parts as well as the bare or hairy. The vesicles, which have often a dark, unhealthy look, tend to become confluent, and instead of proceeding regularly to maturity, they may remain hard or indolent papules or nodules or blacken and dry up. The secretion, when it takes place, is liable to be a thick, yellowish, fœtid pus. The eruption has much more tendency than with the discrete form to invade mucosæ, and not infrequently the serosæ are involved, especially those covering the lungs, liver and spleen. In the worst cases death may result from the fever before any eruption has taken place, in other cases the extent of the internal lesions tends to hasten a fatal result. With the amount of care that can be given to a flock, confluent cases are usually fatal.
In connection with the itching and scratching of the nose, abrasions and complex infections ensue, resulting in extensive ulceration and gangrene implicating the nasal cartilages and bones. The eruption around the pasterns may lead to shedding of the hoofs, and sloughing of the whole digital structures. Suppurating and gangrenous swellings form subcutaneously and in the lymph glands with fatal results. Blindness is especially liable to happen from the formation of the eruption on the conjunctiva and cornea. Ulcerations and sloughs are common on the internal mucosæ, and even on the serosæ. In implication of the abdominal organs, a fœtid diarrhœa is usually present, and in pregnant ewes abortion is the usual result.
Lesions. The cutaneous lesions are in their multilocular structure, the same as in cowpox, but differ materially in size and form. The typical vesicle is small (5 to 12 mm.), and neither broad and umbilicated like cowpox, nor conical and pointed as in smallpox, but round with a slight flattening on the summit. On the same animal may often be found all the successive stages at once, red points or hæmorrhagic spots, vesiculation, pustulation and even desiccation. The early changes are seen in the papillary layer and rete mucosum, in the form of swelling and congestion of the papillæ, exudation, active proliferation of the epidermic and tissue cells, and migration of leucocytes, (embryonic cells polynuclear) vacuolation of the papule, the spaces being filled with a straw colored exudate, and finally, the replacing of this by pus. If the local inflammation is very acute, the pustule may be resolved into a small abscess. On section of the skin the affected parts are found to be the seat of congestion and gelatinoid exudation, extending into the subcutaneous and intermuscular connective tissue, and even the muscles themselves are blackish and their capillaries engorged. The exudation is especially abundant in the dependent parts (head, neck, sternal and abdominal regions, legs). In proportion to the severity of the attack there are congestion, exudation, swelling, and blood extravasation in the lymph glands.
The nasal mucosa is congested, thickened or even ulcerated with abundant, tenacious, muco-purulent secretion and the nasal chambers narrowed or obstructed, and similar changes may be present in the larynx, trachea and bronchia. Spots of ecchymosis and even distinct variolous vesicles are to be expected. The pleuræ are often the seat of congestion, petechiæ, exudation, and discoloration (red or pale). The lung may show congestion and hepatisation in circumscribed areas, or there may be on the surface of such hepatised portions red or grayish foci, like a lentil, pea or bean, the seat of degeneration or forming minute abscesses. The pericardium and myocardium may be involved to a limited extent.
The buccal and pharyngeal mucosæ may be the seats of vesicles, or erosions or ulcers the result of their destruction, and the gastro-intestinal mucosa presents ecchymoses, vesicles, abrasions, and open sores in the midst of inflamed catarrhal patches. The mesenteric and other lymph glands are congested, swollen, softened and friable. Congestions and petechiæ are found on the peritoneum, liver, spleen and kidneys, with, at times, nodular foci. In certain cases congestions and effusions have been found on the pia mater and arachnoid, in the cerebral ventricles and the brain substance.