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Biomedical subjects

G Marion

Publications and source records attributed to G Marion.

10 recordsLinked to original sources
PubMed2000

Stochastic modelling of environmental variation for biological populations.

We examine stochastic effects, in particular environmental variability, in population models of biological systems. Some simple models of environmental stochasticity are suggested, and we demonstrate a number of analytic approximations and simulation-based approaches that can usefully be applied to them. Initially, these techniques, including moment-closure approximations and local linearization, are explored in the context of a simple and relatively tractable process. Our presentation seeks to introduce these techniques to a broad-based audience of applied modellers. Therefore, as a test case, we study a natural stochastic formulation of a non-linear deterministic model for nematode infections in ruminants, proposed by Roberts and Grenfell (1991). This system is particularly suitable for our purposes, since it captures the essence of more complicated formulations of parasite demography and herd immunity found in the literature. We explore two modes of behaviour. In the endemic regime the stochastic dynamic fluctuates widely around the non-zero fixed points of the deterministic model. Enhancement of these fluctuations in the presence of environmental stochasticity can lead to extinction events. Using a simple model of environmental fluctuations we show that the magnitude of this system response reflects not only the variance of environmental noise, but also its autocorrelation structure. In the managed regime host-replacement is modelled via periodic perturbation of the population variables. In the absence of environmental variation stochastic effects are negligible, and we examine the system response to a realistic environmental perturbation based on the effect of micro-climatic fluctuations on the contact rate. The resultant stochastic effects and the relevance of analytic approximations based on simple models of environmental stochasticity are discussed.

Animals↗
PubMed1998

Stochastic effects in a model of nematode infection in ruminants.

We illustrate the importance of stochastic effects in population models of biological systems and demonstrate a number of analytic and simulation-based approaches that can usefully be applied to such models. In so doing, we compare the stochastic approach to the more usual deterministic one. The model studied represents the gastrointestinal infection of ruminants by nematodes when the hosts maintain a fixed density. The incorporation of a feedback mechanism, which accounts for the immune response of the infected animals, results in a highly nonlinear model; similar forms of nonlinearity are a feature of many plausible models in population biology. In the absence of an analytic solution to the full stochastic model we explore a number of approximations and compare them to simulations of the full stochastic process. We explore three modes of behaviour of the system. In the endemic regime the stochastic system fluctuates widely around the non-zero fixed points of the deterministic model. In the managed regime, where the system is subject to external periodic perturbation, stochastic effects are negligible. Finally, we find that in a regime in which the deterministic model predicts the long-term persistence of oscillations the stochastic model shows that extinction can occur. Of the approximation procedures we consider, the Normal approximation to the full stochastic process is the most generally applicable, and it is also the most accurate in the light of simulation results. Local linearization provides reasonably accurate prediction of the variance-covariance structure, and a transfer function approach allows calculation of the time-lagged auto- and cross-correlations in the endemic regime. Linearization of the stochastic updates themselves results in poor prediction of the population variances.

Animals↗
PubMed1987

Prevalence of islet cell antibodies, insulin antibodies and hyperglycaemia in 2291 schoolchildren.

Islet cell antibodies, insulin antibodies and hyperglycaemia were investigated in 2291 healthy schoolchildren. Eight of the subjects had islet cell antibodies and eight had insulin antibodies. However, no schoolchild who was positive for islet cell antibodies also had insulin antibodies present. Hyperglycaemia was observed in five children but neither islet cell antibodies nor insulin antibodies could be detected in the sera from these particular subjects.

Adolescent↗
PubMed1985

The detection of abscesses with diffusible tracers.

In six patients with suspected infection, scintigraphy with diffusible tracers outlined the margins and central portion of an abscess. An animal model was developed to study this process quantitatively. Small inflammatory lesions yielding volumes of pus of 0.1-0.2 mL were shown to have increased blood volume of 0.7 +/- 0.5 mL, increased blood flow by a factor of three and increased extracellular fluid volume of 10 +/- 6 mL. This supports the patient data and indicates that the pathophysiologic features characterising the clinical studies are hyperemia and oedema associated with an abscess. The slow, central area of tracer diffusion corresponded to the presence of pus. Scintigraphy with 99mTc-DTPA is a convenient way of detecting suspected inflammatory lesions and localising collections of pus.

Abscess↗