Since the late 19th century, respiratory disease has been identified as a leading cause of population declines in bighorn sheep in the western United States. This disease in bighorn sheep is characterized by sporadic outbreaks of bronchopneumonia in all ages of bighorn sheep. Disease is commonly followed by multiple years of poor lamb survival. The cause of disease is most likely multifactorial, including bacterial, viral, parasitic, and environmental (stress, nutrition) factors. The interactions between these causes aren't completely understood nor are any predisposing conditions.

In the winter of 1990/1991 the Whiskey Mountain herd suffered an all-age die-off caused by pneumonia, resulting in an estimated 30% decline in the number of sheep. The herd continues to stay below the desired population size primarily because lamb survival is very low likely due to the persistence of lamb pneumonia. At one time, there were an estimated 2,500 sheep in this population; today there are about 750.

Since 2011, the Wyoming Game and Fish Department and its Wildlife Health Laboratory have been conducting statewide bighorn sheep herd health surveillance. As of February 2022, we have sampled 1,506 bighorn sheep and 107 mountain goats, both alive and post mortem. The purpose of this surveillance is the determine 'herd health' status/baseline for each herd (e.g. trace minerals, parasites, pregnancy rate, etc.). In addition we document all respiratory pathogens and potentially correlate pathogen presence to heard health. Respiratory disease in bighorn sheep is polymicrobial, with the common pathogens found in most Wyoming herds being Mannheimia haemolytica (Lkt+), Bibersteinia trehalosi (Lkt+), Pasteurella multocida, and Mycoplasma ovipneumoniae.

Signs exhibited by animals suffering from respiratory disease include, but are not limited to: coughing, head shaking, nasal discharge, respiratory distress, exercise intolerance, and general depression. Animals are often just found dead.