Bighorn sheep die-offs have followed contact with domestic sheep and goats since the nineteenth century. The pattern is consistent: an all-age pneumonia event that removes a large fraction of the herd in weeks or months.
Modern work identifies Mycoplasma ovipneumoniae as the key agent, an organism widespread and largely harmless in domestic flocks. Once introduced to bighorn, it damages the airway lining and opens the way for Pasteurellaceae bacteria to cause severe pneumonia. Worse than the initial crash is what follows: surviving ewes can carry the organism for years and infect each successive lamb crop, so recruitment stays near zero and the herd fails to rebuild.
Management therefore centres on physical separation, keeping domestic flocks off shared range and removing wandering bighorn rams that could carry infection home. Whether targeted test-and-remove of chronic carriers reliably restores lamb survival, and how much strain differences matter, are still open questions in the field.