An advanced form of the disease known as chronic obstructive pulmonary disease (COPD). The disease is an allergic bronchitis which leads to a cough, usually in association with feeding and exercise, and to increased respiratory effort. In an advanced form of the disease, the lung tissue becomes thickened and loses its elasticity, making inspiration and expiration more strenuous. Whereas inspiration always involves muscular effort, expiration usually occurs through muscle relaxation and elastic recoil of the lung tissue. In horses with the heaves, muscular effort is required to expell air from the lungs; contraction of the abdominal muscles leads to a "heave line" along the ventral rib cage.