Date of this Version
Disease has contributed significantly to the decline of bighorn sheep (Ovis Canadensis) populations throughout much of western North America, decreasing many native herds to less than 10% of their historical size and imperiling some populations and subspecies (Valdez and Krausman 1999). According to historical accounts (e.g., Grinnell 1928; Honess and Frost 1942; Shillinger 1937; Warren 1910), epidemics in some locations coincided with the advent of domestic livestock grazing in bighorn ranges, suggesting that novel pathogens may have been introduced into some bighorn populations beginning in the 1800s.
Native North American wild sheep species—bighorn sheep and thinhorn (Dall’s and Stone’s) sheep (O. dalli)—are very susceptible to pneumonia and particularly to pasteurellosis (Miller 2001). The generic term “pasteurellosis” is used here for disease (often respiratory) caused by bacteria in the family Pasteurellaceae but now classified in the genera Pasteurella, Mannheimia, or Bibersteinia. In some recent pneumonia epidemics in bighorns, the cause has been attributed to endemic respiratory pathogens or strains of Pasteurellaceae (Rudolph et al. 2007), and in other epidemics the cause has been attributed to Pasteurellaceae strains or other pathogens introduced via interactions with domestic sheep (O. aires; George et al. 2008). This Commentary reviews current knowledge on pneumonic pasteurellosis in domestic and wild sheep, the risks of transmission between these species, and approaches for lowering the overall risk of epidemics in wild sheep.