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Data from: Bighorn sheep show similar in-host responses to the same pathogen strain in two contrasting environments

Citation

Manlove, Kezia et al. (2022), Data from: Bighorn sheep show similar in-host responses to the same pathogen strain in two contrasting environments, Dryad, Dataset, https://doi.org/10.5061/dryad.0vt4b8h1p

Abstract

Ecological context – the biotic and abiotic environment, along with its influence on population mixing dynamics and individual susceptibility – are thought to have major bearing on epidemic outcomes. However, direct comparisons of disease events in contrasting ecological contexts in wildlife systems are often confounded by concurrent differences in host genetics, exposure histories, or pathogen strains. Here, we compare disease dynamics of a Mycoplasma ovipneumoniae spillover event that affected bighorn sheep populations in two contrasting ecological contexts. One event occurred on the herd’s home range near the Rio Grande Gorge in New Mexico, while the other occurred in a captive facility at Hardware Ranch in Utah. While data collection regimens varied between the two sites, general patterns of antibody signal strength and symptom emergence were consistent. Symptoms appeared in the captive setting an average of 12.9 days post-exposure, average time to seroconversion was 24.9 days, and clinical signs peaked at approximately 36 days post-infection. These patterns were consistent with serological testing and subsequent declines in symptom intensity in the free-ranging herd. At the captive site, older animals exhibited more severe declines in body condition and loin thickness, higher symptom burdens, and slower antibody response to the pathogen than younger animals.  Younger animals were more likely than older animals to clear infection by the time of sampling at both sites. The patterns presented here suggest that environment may not be a major determinant of epidemiological outcomes in the bighorn sheep - M. ovipneumoniae system, elevating the possibility that host- or pathogen-factors may be responsible for observed variation.

Methods

The data were gathered over a series of three bighorn sheep handling events in the late winter/early spring of 2020.  Animals were initially captured for the intention of translocation, but several animals tested positive for Mycoplasma ovipneumoniae during transit from the capture site in NM to the release site in UT.  Animals that were either serologically or PCR-positive for M. ovipneumoniae were sequestered and euthanized, but the remainder were held in captivity in order to see whether they would develop infections.  They did develop infections, and we tracked their clinical signs and sampled them twice prior to a decision to euthanize them in late March.  Thus, the first cpature event in this dataset occurred at the home range Rio Grande Gorge bighorn sheep herd near Taos Pueblo, NM.  The remainder of the handling events occured at Hardware Ranch in northern UT. For the first event, animals were captured via net gun; in the latter events, animals were chemically immobilized.  Animals were live sampled and then euthanized and necropsied on March 26th, 2020.  Sampling was conducted in accordance with standard UDWR animal handling procedures.  Captures at Hardware Ranch relied on protocols approved in Utah State University IACUC protocol # 11117. 

Body condition score is BCS. Rump fat and Loin thickness were both measured by ultrasound using standing Utah Division of Wildlife Resources protocols. Age estimates were based on teeth (usually) and horns (as a back-up). Fetus sex and weight were determined at necropsy. Exposure day was assumed to be the day of the first capture, February 21st, for all animals that did not show evidence of M. ovipneumoniae exposure on the first sampling event. Severity of clinical signs was derived from daily observations of all animals at Hardware Ranch. We did not regather basic allometry measurements (body length, chest girth) on captures after the first capture event. Most M. ovipneumoniae PCR testing was based on samples from nasal swabs (cycle thresholds reported in Movi_Ct_nasal), but post-mortem, we also gathered bronchial junction swabs (Movi_Ct_bronch).

Nasal and oropharyngeal swabs and serum were collected during the original capture and subsequent animal handling events at Hardware Ranch. Nasal swabs were collected by inserting a single DACRONTM swab into each nostril and gently swabbing the nasal mucosa by swirling the swab. One nasal swab was stored in Tryptic Soy Broth (TSB) for whole genome sequencing for a different project on the first and second captive sampling events. Diagnostic testing and strain typing were conducted by WADDL. M. ovipneumoniae DNA from six individuals were strain typed using a four-locus MLST method (Cassirer et al. 2017; Kamath et al. 2019). WADDL provided cycle threshold values corresponding to PCR diagnostic tests. Oropharyngeal swabs were gathered during each event except for the March 12th captive sampling, and were submitted to WADDL for bacterial culturing. Blood was collected by jugular venipuncture into PAXGene and serum separator tubes that were centrifuged within 4 hours of collection (results presented in Bowen et al. 2022). Serum was separated and stored frozen in cryogenic vials until it was shipped frozen to WADDL. The sensitivity of the WADDL cELISA test is 90.7%, and its specificity is 95.8%.

Body conditions at Hardware Ranch were estimated using ultrasound by measuring rump fat and loin thickness of the animals in similar locations as described for deer (Cook et al., 2007) and by palpation. Rump fat, which is the conventional target for ultrasound measurement of ungulate body condition (Stephenson et al. 1998; Cook et al. 2001; Cook et al. 2007), was very low in most animals at the first sampling event (median among positives = 1.25 mm; median among negatives = 1.00 mm). Therefore, we tracked loin thickness as a relative measure of condition changes within an animal in subsequent sampling events.

Fetal age was estimated based on polished developmental benchmarks for domestic sheep (Sivachelvan et al. 1996) and published birth weights for Rocky Mountain bighorn lambs (Hogg et al. 1992).

Clinical signs were scored for the captive animals each day. Observers simultaneously watched all animals in the pen for 45 minutes and recorded signs including inappetance, nasal discharge, coughing (including number, quality, and pacing of coughs), and lethargy. Nasal discharge at Hardware Ranch was given a numeric score between 0 and 5. Shiny noses were given scores of 1, 2 indicated clear discharge from one nostril, 3 indicated clear discharge from both nostrils, 4 indicated purulent discharge from one nostril, and 5 indicated purulent discharge from both nostrils. Coughs were scored as 1 for isolated coughs, 2 for bouts of five or more consecutive coughs, and 2.5-4 for 2 or more bouts of 5 or more consecutive coughs, depending on depth of cough and number of bouts. Nose licking and head shaking were assigned scores of 1 if present and 2 if consistent throughout the observation period. Total daily scores for each animal were determined by summing the nasal, head shaking, and nose licking scores, and twice the individuals coughing score. Coughing scores were cumulative, with each coughing event contributing separately to the symptom score, so scores had no pre-defined upper-bound. Individual daily symptom scores ranged from 0 to 12.

At RGG, observers recorded coughing (including quality) at the individual level, and nasal discharge status whenever viewers were close enough to see discharge clearly during the summer lamb surveys. However, no observer watched animals at both RGG and Hardware Ranch and scoring rubrics differed between the sites.

Antemortem sampling included collection of the full suite of samples described previously; postmortem sampling included bronchial junction swabs, extraction and measurements of the fetus, and full tissue collection, and gross assessment for sinus tumors.

Usage Notes

The .csv files should be easily opened using Microsoft Excel or any other software capable of reading .csvs. 

Funding

Utah Agricultural Experiment Station, Award: UTA01427