| Summary: | Infectious pneumonia associated with the bacterial pathogen <i>Mycoplasma ovipneumoniae</i> is an impediment to bighorn sheep (<i>Ovis canadensis</i>) population recovery throughout western North America, yet the full range of <i>M. ovipneumoniae</i> virulence in bighorn sheep is not well-understood. Here, we present data from an <i>M. ovipneumoniae</i> introduction event in the Zion desert bighorn sheep (<i>Ovis canadensis nelsoni</i>) population in southern Utah. The ensuing disease event exhibited epidemiology distinct from what has been reported elsewhere, with virtually no mortality (0 adult mortalities among 70 animals tracked over 118 animal-years; 1 lamb mortality among 40 lambs tracked through weaning in the two summers following introduction; and lamb:ewe ratios of 34.9:100 in the year immediately after introduction and 49.4:100 in the second year after introduction). Individual-level immune responses were lower than expected, and <i>M. ovipneumoniae</i> appeared to fade out approximately 1.5 to 2 years after introduction. Several mechanisms could explain the limited burden of this <i>M. ovipneumoniae</i> event. First, most work on <i>M. ovipneumoniae</i> has centered on Rocky Mountain bighorn sheep (<i>O. c. candensis</i>), but the Zion bighorns are members of the desert subspecies (<i>O. c. nelsoni</i>). Second, the particular <i>M. ovipneumoniae</i> strain involved comes from a clade of strains associated with weaker demographic responses in other settings. Third, the substructuring of the Zion population may have made this population more resilient to disease invasion and persistence. The limited burden of the disease event on the Zion bighorn population underscores a broader point in wildlife disease ecology: that one size may not fit all events.
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