Clinical signs of tracheal collapse usually are seen in young calves exhibiting inspiratory distress. The inspiratory dyspnea causes strider in the calves' breathing and a marked increase in the duration of the inspiratory phase of respiration. Calves breathe with their mouth open and head and neck extended, but body temperatures are seldom elevated more than 1 degrees to 1.5 degrees C above normal. Auscultation of the respiratory tract shows harsh, often bubbly, tracheal sounds that are referred into the chest. Calves can be stressed easily to the point of collapse by even the simplest restraint procedures, so extreme care must be taken during examination. Historically, most calves with tracheal collapse have been cases of dystocia at birth. Although evidence of the collapse seldom is seen from birth, it is thought that the initiating injury to the trachea occurs at that time. Compression of the anterior chest wall with fracturing of the first few pairs of ribs may cause injury to the tracheal rings at the thoracic inlet. Because of the elastic nature of the rings, they tend to spring back to a circular shape, but the compressed rings are left less rigid and have the tendency to malformation in response to pressure changes in the chest and trachea. As the calf ages, either of two events may contribute to a gradual worsening of the condition-exuberant callus formation around the rib fractures compresses the trachea at the thoracic inlet or the thoracic segment of the trachea weakens secondarily to the increased negative pressure exerted by the calf during inspiration (dynamic collapse).
