The course of the disease is not invariable. Certain animals may appear ill for years without clinically showing the least apparent aggravation; others on the contrary, though living under similar conditions, are rapidly attacked, and in six to twelve months exhibit all the signs of advanced phthisis. Pregnancy, suckling, and prolonged lactation favour the development of the disease by taxing the physical resources of the animal.
Animals suffering from phthisis exhibit a peculiar appearance. They are extremely thin, all their soft tissues are wasted, the limbs are dragged in moving, respiration is rapid and sometimes jerky, the mucous membranes are pale and discoloured, and the skin is tight and adherent to the subjacent tissues.
These general signs, however, would not warrant a diagnosis, for, apart from the cough, certain other diseases present all the external appearances of the last period of tuberculosis (chronic diarrhœa, chronic forms of poisoning—bacterial or otherwise—dyspepsia, etc.).
In cases of doubt it is essential to discover by percussion and auscultation that the external signs are really the result of lesions of the lung, and that the lung disease has developed gradually in accordance with the signs shown by simple external inspection.
The symptoms presented during the development of the pulmonary lesions may be divided into three phases.
In the first phase percussion gives no information, though auscultation reveals rough respiration, inspiration and expiration being also unequal. Expiration, which, in the healthy subject, is silent, becomes clearly perceptible, not over the whole lung, but usually over the anterior lobes, particularly the cardiac lobes. This sign is the result of tuberculous infiltration and of the neighbouring pulmonary tissue having lost its elasticity.
Inspiration is rough and rasping, and sometimes occurs in several stages, the act being interrupted or jerky; expiration lasts longer than inspiration, is rough and prolonged, but never blowing in character. These peculiarities are only found in one other condition of the lung, viz., emphysema.
The patients appear little affected in this, the first, stage of tuberculosis. But for the cough they may seem perfectly healthy.
In the second phase the tuberculous infiltration extends and ends in the massing, by fusion or centrifugal growth, of the tuberculous masses.
Percussion may now indicate localised dulness, but this is not invariable, because the diseased anterior and middle lobes of the lung are concealed beneath the muscles of the shoulder. When dulness is noted, it is usually over the lower part of the posterior lobes, very rarely at any higher point on the side of the chest. Frequently the dulness is only partial.