133,371 research outputs found
An economic evaluation of preclinical testing strategies compared to the compulsory scrapie flock scheme in the control of classical scrapie
No full textCost-benefit is rarely combined with nonlinear dynamic models when evaluating control options for infectious diseases. The current strategy for scrapie in Great Britain requires that all genetically susceptible livestock in affected flocks be culled (Compulsory Scrapie Flock Scheme or CSFS). However, this results in the removal of many healthy sheep, and a recently developed pre-clinical test for scrapie now offers a strategy based on disease detection. We explore the flock level cost-effectiveness of scrapie control using a deterministic transmission model and industry estimates of costs associated with genotype testing, pre-clinical tests and the value of a sheep culled. Benefit was measured in terms of the reduction in the number of infected sheep sold on, compared to a baseline strategy of doing nothing, using Incremental Cost Effectiveness analysis to compare across strategies. As market data was not available for pre-clinical testing, a threshold analysis was used to set a unit-cost giving equal costs for CSFS and multiple pre-clinical testing (MT, one test each year for three consecutive years). Assuming a 40% within-flock proportion of susceptible genotypes and a test sensitivity of 90%, a single test (ST) was cheaper but less effective than either the CSFS or MT strategies (30 infected-sales-averted over the lifetime of the average epidemic). The MT strategy was slightly less effective than the CSFS and would be a dominated strategy unless preclinical testing was cheaper than the threshold price of (sic)6.28, but may be appropriate for flocks with particularly valuable livestock. Though the ST is not currently recommended, the proportion of susceptible genotypes in the national flock is likely to continue to decrease; this may eventually make it a cost-effective alternative to the MT or CSF
Advanced survival models for risk-factor analysis in scrapie
No full textBecause of the confounding effects of long incubation duration and flock management, accurate epidemiological studies of scrapie outbreaks are difficult to carry out. In this study, 641 Manech red-faced sheep from six scrapie-affected field flocks in Pyrénées Atlantiques, France, were monitored for clinical scrapie over a 6–9 year period. Over this period, 170 scrapie clinical cases were recorded and half of the culled animals were submitted for post-mortem transmissible spongiform encephalopathy diagnosis to assess their infectious status. Collected data were analysed using a ‘mixture cure model’ approach, which allowed for the discriminating effect of PrP genotype and flock origin on incidence and incubation period. Simulations were performed to evaluate the applicability of such a statistical model to the collected data. As expected, ARR heterozygote sheep were less at risk of becoming infected than ARQ/ARQ individuals and had a greater age at clinical onset. Conversely, when compared with ARQ/ARQ, the VRQ haplotype was associated with an increased infection risk, but not a shorter incubation period. Considering the flock effect, we observed that a high incidence rate was not associated with shorter incubation periods and that the incubation period could be significantly different in flocks harbouring similar infection risks. These results strongly support the conclusion that other parameters, such as the nature of the agent or flock management, could interfere with epidemiological dynamics of the infection in scrapie-affected flocks
Studies of the flocking behaviour of Sanderlings
No full textThe behaviour of sanderlings, Calidris alba, was studied at Teesmouth, N. E. England. Flock sizes and within-flock spacing were related to factors such as substrate type, season, and prey distribution. Sightings of individually-identifiable colour-ringed birds showed that turnover was high. Individuals varied in their vagility and in the way in which they allocated their time between feeding sites. There was a high degree of consistency between winters in the distributions of individual birds. When individual sighting frequencies were taken into account, associations between individual sanderlings, in terms of both flock membership and of occurrences at the same sites on the same days, were non-significantly different from those expected by chance. Previously used methods for detecting non-random associations gave inadequate null models. Each individual appeared to make a decision about where to feed independently of the decisions made by any other particular individuals .A number of aspects of the dynamics of flocks were examined, including flock cohesion; how flocks built up and broke down; whether birds tended to move to the largest or smallest, the nearest or furthest flocks; the effects of disturbance on flock dynamics; and the relationship between group size and the distance flown between flocks. The responses of sanderlings to experimental disturbances tended to support the hypothesis that responses should be varied according to the costs and benefits of different courses of action rather than the hypothesis that birds should take flight as soon as a predator is sighted. Birds often break off from performing an activity in order to raise their heads (scan). Preening birds had shorter inter-scan intervals than feeding birds. Flock size and spacing explained only a small proportion of the variance in vigilance. Vigilance was greater in autumn than in winter. There was some evidence for both feeding and preening birds avoiding very short inter-scan intervals but not for the avoidance of long intervals. Sequences of inter-scan interval durations of preening birds were non-random
Diversification of prey capture techniques among the piscivores in Lake Tana's (Ethiopia) Labeobarbus species flock (Cyprinidae)
No full textLake Tana harbours the only known intact species flock of large cyprinid fishes (15 Labeobarbus spp.). One of the most curious aspects of this species flock is the large number (8) of piscivorous species. Cyprinids are not well designed for piscivory (i.e. small slit-shaped pharyngeal cavity, lack of teeth in the oral jaws, lack of a stomach), which raises the question how well adapted these labeobarbs actually are to function as piscivores? In this study we analyse the kinematics of prey capture (by varied combinations of suction, swimming and jaw protrusion) among Lake Tana's piscivorous labeobarbs. Suction feeding kinematics were similar to values reported for other piscivorous fish species. A detailed analysis of several Labeobarbus species displayed distinct types of techniques (overswimming, velocity/volume suction with jaw protrusion) suited to capture elusive prey in different macro-habitats, Lake Tana's Labeobarbus species evolved a wide range of piscivorous predation techniques, a unique scenario for cyprinid fishes
Effect of hen rearing techniques and egg storage duration on albumen functional properties.
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Evaluation of approaches to control of Maedi-Visna disease of sheep using a Markov chain simulation model for a range of typical British Flocks
No full textAn epidemiological model is described that closely mimicked results of a published serological study of natural transmission of Maedi-Visna virus in a low ground flock of sheep. We adjusted parameters in the model from this baseline to explore the possible implications for the control of Maedi-Visna virus in typical British flocks. On closed hill farms, low probability of effective contact was most critical for control. In open low ground flocks, purchasing accredited replacements eliminated disease spread, otherwise flock size was the most important factor governing flock prevalence. Results highlighted the need for more epidemiological information about Maedi-Visna, particularly whether hill farms act as a hidden reservoir of virus or reduce the impact of this disease on the industry by providing a source of clean replacementsLivestock Production/Industries, Maedi-Visna, Model, Markov Chain, Sheep, Control,
Misfolding of the amyloid beta-protein: A molecular dynamics study
No full textAmyloid beta-proteins spontaneously aggregate and build plaques in the
brains of Alzheimer's disease patients. The polypeptide has been the
subject of extensive in vitro and computational research. Still, the
pathway to aggregational forms and their exact conformations remain
largely unclear. Here we present an extensive molecular dynamics
approach simulating the protein in various temperatures, pH conditions,
and with different charge states of the N- and C-termini, thus exploring
the conformational space of the protein at large. Our results show that
the protein is able to sample different conformations, many of which are
rich in beta structure content, and all characterized by a rapid loss of
helix 1 that converts into a pi-helix, while helix 2 samples random and
beta-rich structures. Moreover, a hydrophobic cluster is observed
involving Val18, Phe19, Ala21, and Gly25. The results are carefully
compared with recent NMR and spectroscopic data, and are in global
agreement with the experimental findings
An investigation of pathways for rebuilding Australia’s sheep flock
No full textPublished online: 26 May 2023lContext. A significant opportunity remains to further increase the supply of premium sheepmeat products to Australia’s customers, which requires a more rapid rebuilding of the national sheep flock. To help meet this challenge, developing relevant information for sheep producers to assist them to achieve a profitable flock-rebuilding outcome is viewed as highly desirable. Aims. (1) Develop flock-rebuild scenarios that track inventory, cashflow, profit and loss, and the overall balance sheet over a 5-year projection. (2) Model the impact on flock-rebuilding pathways of exposure to variation in supplementary-feed costs, purchase of stock and price received for sale animals and flock structure. (3) Model variation in benefits and costs for a range of production zones, enterprise types and flock-age profiles. Methods. Fourteen flock-rebuilding pathways were assessed for up to nine flock types, using a spreadsheet model that tracked inventory, cashflow, profit and loss, and the overall balance sheet over 5 years. Key results. The top four pathways for profitability and capacity to rapidly rebuild flock numbers include retention of more older ewes, purchasing young ewes, purchasing older ewes (mostly 5–6-year olds) in Merino and Merino-cross flock types only and joining ewe lambs in Maternal and Cleanskin flock types only. These pathways were not sensitive to variation in the cost of supplementary feed, stock prices or flock structure, although joining Merino ewe lambs became one of the top four pathways when feed costs were lower. Conclusions and implications. The well established practices of retaining ewes for longer and purchasing ewes, especially young ewes, have the most potential to both rapidly and profitably rebuild flock numbers. However, joining ewe lambs, particularly in Maternal and Cleanskin flocks, can also profitably contribute to rapid flock rebuilding. While reducing reproductive wastage or increasing reproductive potential were mostly profitable, they could not rapidly rebuild flock numbers. In contrast, accelerated lambing systems can rapidly rebuild flock numbers, but are only marginally profitable.F. D. Brien, R. L. Pitchford, S. P. Vogt and D. J. Koopma
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