Bighorn sheep show similar in-host responses to the same pathogen strain
in two contrasting environments
Abstract
Ecological context – the particular environment, and how it shapes
mixing dynamics and individual susceptibility surrounding infectious
disease events – can have major bearing on epidemic outcomes, yet
directly comparable disease events with contrasting ecological contexts
are relatively rare in wildlife systems due to concurrent differences in
host genetics or pathogen strain. Here, we present a case study of one
such event: a spillover of a “goat-clade” Mycoplasma ovipneumoniae
strain into one bighorn sheep population that played out against two
very different ecological backdrops. One event occurred on the herd’s
home range near the Rio Grande Gorge in New Mexico, while the other
progressed in a captive facility at Hardware Ranch in Utah. We collected
data on antibody and pathogen load patterns through time at the
individual level, and examined demographic responses to pathogen
invasion to compare the intensity of, and in-host responses to,
infection in both settings. While data collection regimens varied
between the two sites, general patterns of antibody expansion and gross
timing of symptoms were consistent. Symptoms emerged in the captive
setting 12.9 days post-exposure, and we estimated an average time to
seroconversion among the captive animals of 24.9 days. Clinical signs
peaked among the captive animals at approximately 36 days
post-infection, consistent with subsequent declines in symptom intensity
in the free-ranging herd. At the captive site, older animals exhibited
more severe declines in body condition as determined through declines in
loin thickness, higher symptom burdens, and a decelerated antibody
response to the pathogen. Younger animals were more likely than older
animals to clear infection at or before the time of sampling at both
sites. This study presents one of the richest datasets on immune
responses in bighorn sheep over the course of a newly introduced M.
ovipneumoniae strain available to-date.