A Bayesian hierarchical analysis to compare classical and atypical scrapie surveillance data; Wales 2002-2006.

We describe the application of Bayesian hierarchical models (BHM) to the analysis of risk of sheep scrapie using data from multiple surveillance sources. More specifically, we analysed data from the test results of three surveillance sources on classical and atypical scrapie in Wales for the period 2002-2006. For each form of scrapie, a BHM was fitted to assess the occurrence of spatial patterns of risk shared by the multiple surveillance sources and the association between covariates and disease. We defined a shared-component model whereby the two types of data sources: exhaustive lists (e.g. reports of clinical cases) and sample-based data sources (e.g. abattoir survey) shared a common spatial pattern of risks at parish level. This shared component was adjusted by a risk-gradient parameter that moderated the individual contribution of the datasets. For both forms of scrapie, the risk-gradient was not significantly different indicating that the sensitivity of the two types of dataset was similar for the two diseases. The spatial patterns of the combinations of data sources appeared similar within disease. However, our results suggest that classical and atypical scrapie differ in their spatial patterns and disease determinants. The joint approach permitted inference from all the available evidence and resulted in robust and less biased estimates of risk, particularly for atypical scrapie where the number of observations was very limited.

The determination of 129I in milk and vegetation using neutron activation analysis.

A new method has been developed to measure 129I in the environment with detection limits below 10 mBq/kg of vegetation and 10 mBq/l of cows' milk. The method is based on extraction of 129I from the milk or vegetation sample, onto an ion exchange resin. An inactive carrier of 127I is added to the sample before separation, to monitor losses throughout the entire procedure. The ion exchange resin is irradiated for 7.5 h in a neutron flux of 10(16) n m-2 s-1 to induce the 129I(n, gamma) 130I reaction with thermal neutrons. The 127I carrier undergoes a (n,2n) reaction with fast neutrons to produce 126I. Iodine is extracted from the ion exchange resin after irradiation with an elution scheme which removes contamination from the radionuclide 82Br, the main interference in the analysis. Finally iodine is precipitated as AgI for gamma ray analysis. The sample is counted for 3 h on a Ge semiconductor detector to measure the radionuclide 130I, which has a half life of 12.4 h and 126I, which has a half life of 13.0 days. The measured 130I activity is compared to a known standard to deduce the amount of 129I in the sample, and the concentrations are corrected for losses during processing using the measured activity of 126I. The detection limits for 129I by this method are below 10 mBq/l for milk samples and 10 mBq/kg for vegetation. In addition to routine monitoring of milk and grass samples the method has been used to measure 129I deposition on grass and soils in a field near the Sellafield plant. Results of these analyses, along with measurements of 129I in air and rainfall using the same methodology, have been used to determine deposition velocity and retention coefficients of 129I to grass.

Stolon growth and branching in Glechoma hederacea L.: an application of a plastochron index.

Stolon growth and branching in Glechoma hederacea L. was analyzed, using a plastochron index based on the number of ramets on each stolon and internode growth. New methods, employing this index, were used to detect fluctuations in the rate of stolon extension, and to analyse the development of secondary stolons. The plastochron index had a strong linear correlation with time, with a new primary ramet appearing every 3.5 d. The overall stolon extension rate fluctuated between 1.9 and 2.9 cm d-1 through each plastochron cycle. The age at which each node produced secondary stolons was related to its position on the plant, but the initial rates of secondary stolon development differed little between nodes. Rate of development of secondary stolons was strongly correlated with the time at which they started to grow. Thus, later developing secondary stolons grew more slowly than those developing earlier from nodes in equivalent positions.