Diseases of the Respiratory System Diseases of the Respiratory System Sheep pneumonia is especially common in newborn lambs and in feedlot lambs, it can also occur in the mature ewe flock with milder clinical signs. Pneumonia is caused by a complex interaction between the environment which produces stress, microorganisms, and the sheep's immune response. Pneumonia occurs in all ages of sheep, in all breeds, in every country of the world. As the management practices become more intensive the level and risk of pneumonia become much greater.
Close contact allows for rapid spread of infectious organisms from one lamb to the next.
Intensive management also leads to a buildup of pathogens in the environment. Manure in sheds also leads to the production of ammonia which irritates and damages the respiratory mucosa and reduces the sheep's ability to fight the infection. In shed or semi-confinement lambing operations the pneumonia will peak towards the end of the lambing season in many operations. In acute outbreaks of pneumonia, Pasteurella haemolytica is the organism most frequently isolated from the affected lung tissue, but only rarely is it the primary organism. P. haemolytica is a common inhabitant of upper respiratory and tonsils of normal healthy sheep.