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Prion protein (PrP) alleles associated with scrapie susceptibility persist in many sheep populations even with high frequencies despite centuries of selection against them. This suggests that scrapie susceptibility alleles have a pleiotropic effect or are associated with fitness or other traits that have been subject to selection. We genotyped all lambs in two scrapie-free Scottish Blackface sheep flocks for polymorphisms at codons 136, 154 and 171 of the PrP gene. We tested potential associations of the PrP genotype with lamb viability at birth and postnatal survival using a complementary log-log link function and a Weibull proportional hazard model, respectively. Here we show there is an association between PrP genotype, as defined by polymorphisms at codons 154 ad 171, and postnatal lamb survival in the absence of scrapie. Sheep carrying the wild-type ARQ allele have higher postnatal survival rates than sheep carrying the more scrapie-resistant alleles (ARR or AHQ). The PrP genotypes associated with higher susceptibility to scrapie are associated with improved postnatal survival in the absence of the disease. This association helps to explain the existence, and in many instances the high frequency, of the ARQ allele in sheep populations.

Background Here, we jointly summarise scientific evidence for genetic variation in resistance to infection with Mycobacterium bovis , the primary agent of bovine tuberculosis ( TB ), provided by two recent and separate studies of Holstein-Friesian dairy cow populations in Great Britain ( GB ) and Ireland. Methods The studies quantified genetic variation within archived data from field and abattoir surveillance control programmes within each country. These data included results from the single intradermal comparative tuberculin test ( SICTT ), abattoir inspection for TB lesions and laboratory confirmation of disease status. Threshold animal models were used to estimate variance components for responsiveness to the SICTT and abattoir confirmed M. bovis infection. The link functions between the observed 0/1 scale and the liability scale were the complementary log-log in the GB, and logit link function in the Irish population. Results and discussion The estimated heritability of susceptibility to TB, as judged by responsiveness to the SICTT, was 0.16 (0.012) and 0.14 (0.025) in the GB and Irish populations, respectively. For abattoir or laboratory confirmation of infection, estimates were 0.18 (0.044) and 0.18 (0.041) from the GB and the Irish populations, respectively. Conclusions Estimates were all significantly different from zero and indicate that exploitable variation exists among GB and Irish Holstein Friesian dairy cows for resistance to TB. Epidemiological analysis suggests that factors such as variation in exposure or imperfect sensitivity and specificity would have resulted in underestimation of the true values.

Background Bovine tuberculosis (bTB) is a disease of significant economic importance and is a persistent animal health problem with implications for public health worldwide. Control of bTB in the UK has relied on diagnosis through the single intradermal comparative cervical test (SICCT). However, limitations in the sensitivity of this test hinder successful eradication and the control of bTB remains a major challenge. Genetic selection for cattle that are more resistant to bTB infection can assist in bTB control. The aim of this study was to conduct a quantitative genetic analysis of SICCT measurements collected during bTB herd testing. Genetic selection for bTB resistance will be partially informed by SICCT-based diagnosis; therefore it is important to know whether, in addition to increasing bTB resistance, this might also alter genetically the epidemiological characteristics of SICCT. Results Our main findings are that: (1) the SICCT test is robust at the genetic level, since its hierarchy and comparative nature provide substantial protection against random genetic changes that arise from genetic drift and from correlated responses among its components due to either natural or artificial selection; (2) the comparative nature of SICCT provides effective control for initial skin thickness and age-dependent differences; and (3) continuous variation in SICCT is only lowly heritable and has a weak correlation with SICCT positivity among healthy animals which was not significantly different from zero ( P  > 0.05). These emerging results demonstrate that genetic selection for bTB resistance is unlikely to change the probability of correctly identifying non-infected animals, i.e. the test’s specificity, while reducing the overall number of cases. Conclusions This study cannot exclude all theoretical risks from selection on resistance to bTB infection but the role of SICCT in disease control is unlikely to be rapidly undermined, with any adverse correlated responses expected to be weak and slow, which allow them to be monitored and managed.

The main objective of this work was to undertake analyses which would lead to an improvement in the accuracy of production and type breeding value predictions for Holstein-Friesian dairy cattle and to investigate the relationship between production and type traits and herd life. Production records were obtained from the National Milk Records files and linear type records from the Holstein Friesian Society. An animal model estimation of heterosis and recombination loss between North American Holstein and European cattle for first lactation milk, fat and protein yield showed that the omission of these effects in breeding value prediction would bias the results. Including both these effects in the breeding value estimation increased the predicated sire proof for fat plus protein yield of a typical F1 Holstein X Friessian sire by 3kg. An analysis of approximately 275 000 first lactation records and the 2nd to 5th lactation records of survivors found the regression coefficients of yield in lactations 1 to 5 on heterosis and recombination loss to be fairly consistent over lactations. After fitting first lactation yield in the model and compared to a pure Friesian, there is an increased chance of survival to third lactation of 3.6% for a pure Holstein, 3.8% for an F1 and an F2, and 2.4% for a Friesian backcross. In the investigation of herd life, defined as the completion of a specified number of lactations, in relation to linear type traits and production, results were not completely consistent either over lactations or over different subsets of the population. In general, there was a significant positive association between survival and angularity, fore udder attachment and udder depth and a negative association with chest width, rump width and teat length. Regressions on yield were positive, while those on fat and protein content were usually negative.