Natural history of a bighorn sheep pneumonia epizootic: Source of infection, course of disease, and pathogen clearance

Abstract

A respiratory disease epizootic at the National Bison Range (NBR) in Montana in 2016–2017 caused an 85% decline in the bighorn sheep population, documented by observations of its unmarked but individually identifiable members, the subjects of an ongoing long‐term study. The index case was likely one of a small group of young bighorn sheep on a short‐term exploratory foray in early summer of 2016. Disease subsequently spread through the population, with peak mortality in September and October and continuing signs of respiratory disease and sporadic mortality of all age classes through early July 2017. Body condition scores and clinical signs suggested that the disease affected ewe groups before rams, although by the end of the epizootic, ram mortality (90% of 71) exceeded ewe mortality (79% of 84). Microbiological sampling 10 years to 3 months prior to the epizootic had documented no evidence of infection or exposure to Mycoplasma ovipneumoniae at NBR, but during the epizootic, a single genetic strain of M. ovipneumoniae was detected in affected animals. Retrospective screening of domestic sheep flocks near the NBR identified the same genetic strain in one flock, presumptively the source of the epizootic infection. Evidence of fatal lamb pneumonia was observed during the first two lambing seasons following the epizootic but was absent during the third season following the death of the last identified M. ovipneumoniae carrier ewe. Monitoring of life‐history traits prior to the epizootic provided no evidence that environmentally and/or demographically induced nutritional or other stress contributed to the epizootic. Furthermore, the epizootic occurred despite proactive management actions undertaken to reduce risk of disease and increase resilience in this population. This closely observed bighorn sheep epizootic uniquely illustrates the natural history of the disease including the (presumptive) source of spillover, course, severity, and eventual pathogen clearance.

Bighorn sheep show similar in‐host responses to the same pathogen strain in two contrasting environments

“…Body weight and loin thickness declined during infection among the captive animals (similar to patterns reported in Besser et al, 2021 ). We cannot determine whether those declines were due to environment or disease in this study.…”

Disease Ecology of a Low-Virulence Mycoplasma ovipneumoniae Strain in a Free-Ranging Desert Bighorn Sheep Population

“…We tracked the timing and severity of symptoms in lambs through the spring and summer of 2019 and 2020 using methods adapted from published protocols for documenting symptom emergence in bighorn sheep [16,21,22]. We located marked ewes using very high frequency (VHF) telemetry and observed animals through binoculars or a spotting scope.…”

Abortion and Neonatal Mortality Due to Toxoplasma Gondii in Bighorn Sheep (Ovis Canadensis)

“…Interestingly, within the Bison Range population that produced one abortion and one neonatal death attributed to T. gondii infection, several animals experienced progressive and extreme loss of body mass (wasting) of undetermined etiology during the 4-yr after a severe epizootic of fatal pneumonia in 2016–2017 (Besser et al 2021). Although T. gondii has yet to be detected in the tissues of any affected individuals, and it is currently unclear whether wasting was directly or indirectly related to T. gondii exposure, chronic toxoplasmosis was confirmed in one culled adult, and a protozoal cyst consistent with T. gondii was identified histologically in the brain of the ewe that produced a T. gondii– positive neonate.…”