Investigation of a Simple Model for Within-Flock Transmission of Scrapie.
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In order to satisfactorily analyze the effectivity of control programs at the population level, insight is needed at the flock level, i.e., how the grouping of sheep in flocks affects the population-level transmission risk.We show that the data are consistent with a relatively simple transmission model assuming horizontal transmission and homogeneous mixing between animals.Here we provide an estimate of its mean value and variation for Dutch flocks.
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Affiliation: Central Veterinary Institute, part of Wageningen UR, P.O. Box 65, 8200 AB Lelystad, The Netherlands.
ABSTRACT
Genetic control programs for scrapie in sheep build on solid knowledge of how susceptibility to scrapie is modulated by the prion protein genotype at the level of an individual sheep. In order to satisfactorily analyze the effectivity of control programs at the population level, insight is needed at the flock level, i.e., how the grouping of sheep in flocks affects the population-level transmission risk. In particular, one would like to understand how this risk is affected by between-flock differences in genotype frequency distribution. A first step is to model the scrapie transmission risk within a flock as a function of the flock genotype profile. Here we do so by estimating parameters for a model of within-flock transmission using genotyping data on Dutch flocks affected by scrapie. We show that the data are consistent with a relatively simple transmission model assuming horizontal transmission and homogeneous mixing between animals. The model expresses the basic reproduction number for within-flock scrapie as a weighted average of genotype-specific susceptibilities, multiplied by a single overall transmission parameter. The value of the overall transmission parameter may vary between flocks to account for random between-flock variation in non-genetic determinants such as management practice. Here we provide an estimate of its mean value and variation for Dutch flocks. No MeSH data available. Related in: MedlinePlus |
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Mentions: The subset of flocks with at least two positive cases and at least eight tested animals of genotype ARQ*/VRQ contained 22 flocks, out of which 17 had a negative FIS value. In Fig 2 we plot a histogram of the estimated values for the transmission scale parameter ρ0 for these 22 flocks, based on a value for the sensitivity parameter of Se = 0.75. The corresponding histogram of values is presented as Fig 3. In Fig 2 we also plot a Weibull frequency distribution with mean and variance equal to the mean and variance of the ρ0 estimates. In Fig 4 we show how mean and variance of the distribution of ρ0 values change with the value assumed for Se. We observe that both the mean and the variance of the estimated ρ0 values are only weakly sensitive to the assumed Se, in other words that our results are robust against the uncertainty in Se. |
View Article: PubMed Central - PubMed
Affiliation: Central Veterinary Institute, part of Wageningen UR, P.O. Box 65, 8200 AB Lelystad, The Netherlands.
No MeSH data available.