Tables 4.2 and 4.3 show the results of sampling withininfected flocks. The Poppe et al. (1992) study was a follow-up to the surveylisted in Table 4.1. The original survey identified eight infected Canadianflocks, but was able to measure within-flock prevalence in only seven flocks. Avariable number of hens were cultured in each of these flocks and, in fourflocks, no infected hens were detected, despite previous positive hen orenvironmental test results, or both. The mean of the Beta distribution based onthese results provided a non-zero point estimate for within-flock prevalence(Table 4.2). Table 4.3 summarizes the findings of the studies analysed by Hogueet al. (1997). In two different surveys, 247 positive flocks were detected. Foreach flock, 60 pooled caecal samples comprising five hens each were collected(i.e. caecae were collected from 300 hens per flock). Apparent within-flockprevalence was estimated by assuming that only one infected hen contributed toeach positive pool. Such an assumption is reasonably unbiased (i.e.
The data in Table 4.5 on the frequency of SalmonellaEnteritidis-positive eggs produced by positive flocks were from flocks that weretypically detected via environmental sampling. Estimating egg contaminationfrequencies directly from these data can result in biased estimates, possiblyintroduced because environmental testing is more likely to detect infectedflocks with high within-flock prevalence levels, compared with flocks with lowwithin-flock prevalence levels. Therefore, egg-culturing evidence isdisproportionately influenced by higher prevalence flocks relative to the actualegg contamination frequency in the total population of infected flocks. In US SERA, the proportion of infected flocks classed as high prevalence was adjustedfor the sensitivity of environmental testing to account for thisphenomenon.
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