Abstract:
Infectious pneumonia associated with the bacterial pathogen Mycoplasma ovipneumoniae is
an impediment to bighorn sheep (Ovis canadensis) population recovery throughout western North
America, yet the full range of M. ovipneumoniae virulence in bighorn sheep is not well-understood.
Here, we present data from an M. ovipneumoniae introduction event in the Zion desert bighorn
sheep (Ovis canadensis nelsoni) 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). Individuallevel immune responses were lower than expected, and M. ovipneumoniae appeared to fade out
approximately 1.5 to 2 years after introduction. Several mechanisms could explain the limited burden
of this M. ovipneumoniae event. First, most work on M. ovipneumoniae has centered on Rocky Mountain
bighorn sheep (O. c. candensis), but the Zion bighorns are members of the desert subspecies (O.
c. nelsoni). Second, the particular M. ovipneumoniae 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.
