We investigated the influence of detection probability (i.e., the probability of detecting the disease or organism of interest) on the repeatability of results reported from bacterial culture tests used to demonstrate the presence of species in the Pasteurellaceae family that infect bighorn sheep (Ovis canadensis). We also estimated occupancy probabilities (i.e., the probability an individual bighorn in a herd is infected) for each cultured biovariant and examined the effects of detection probability on the number of samples needed to detect the Pasteurellaceae biovariants from within an individual sheep as well as from within a herd. We collected 5–15 samples from free-ranging bighorns in Colorado, using oropharyngeal swabs or swabs of lungs, and submitted these swabs either immediately or after 2 days for bacterial culture. We saw significant variability in results for repeated samples from each of the sheep, and detection probabilities were ≤0.71 for all Pasteurellaceae biovariants cultured. The delayed (≥2 days) sample submission reduced both the microbial diversity detected and the detection probability for the biovariants characterized when compared to samples submitted immediately. Oropharyngeal sampling had higher detection probabilities of the individual biovariants than did lung swabs, and there was a difference in the biovariants detected between oropharyngeal and lung sampling. Depending on the biovariant of interest, estimates of occupancy probabilities ranged from 0.37–0.89, and we estimated that three to >30 swab samples were necessary to obtain a 95% confidence of detecting the cultured biovariants if they were present in an individual sheep. We estimated that the optimal sample sizes to detect the observed biovariants within a sheep herd with a 95% confidence ranged from sampling two bighorns twice to sampling 40 individuals once. Detection probability impacts the results reported from bacterial cultures for Pasteurellaceae in bighorn sheep, and confounding effects of the detection process should be addressed to improve the rigor of surveillance.
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Vol. 48 • No. 3