Fatal interactions: pneumonia in bighorn lambs following experimental exposure to carriers of Mycoplasma ovipneumoniae.

We hypothesized that bighorn sheep ewes with chronic nasal Mycoplasma ovipneumoniae carriage are the source of infection that results in fatal lamb pneumonia. We tested this hypothesis in captive bighorn ewes at two study facilities over a 5-year period, by identifying carrier ewes and then comparing lamb fates in groups that did (exposed pens) or did not (non-exposed pens) include one or more carrier ewes. Most (23 of 30) lambs born in exposed pens, but none of 11 lambs born in non-exposed pens, contracted fatal pneumonia. In addition, surviving lambs in exposed pens showed obvious signs of respiratory disease while lambs in non-exposed pens did not. In crossover experiments, individual non-carrier ewes had lambs that experienced fatal pneumonia in years when housed in exposed pens, but not in years when housed in non-exposed pens. The results of these studies clearly associate lamb pneumonia to exposure to M. ovipneumoniae carrier ewes, consistent with a necessary role for this agent in epizootic pneumonia of bighorn sheep. These data specifically highlight the role of chronic M. ovipneumoniae carriage by some bighorn ewes in the epidemiology of this population-limiting wildlife disease.IMPORTANCEBighorn sheep populations, historically important in mountain and canyon ecosystems of western North America, declined precipitously following European settlement of North America and remain depressed today. One factor contributing to these declines and lack of recovery is epizootic pneumonia caused by the bacterium Mycoplasma ovipneumoniae. This pathogen arrived with settlers' domestic sheep and goats and spilled over to infect bighorn sheep, a process that continues to this day. Bighorn losses from this disease include high rates of mortality (median, approaching 50%) of all ages of bighorn sheep on initial exposure, followed in subsequent years to decades by mortality largely limited to young lambs. The source of infection causing persistent lamb losses is the focus of the research described here. Conducting these studies on groups of captive bighorn sheep enabled demonstration of clear linkage between largely asymptomatic nasal carriage of M. ovipneumoniae by ewes and outbreaks of fatal pneumonia in lambs.