Biomedical subjects
Publications and source records attributed to M R Evans.
In September 2002, facsimiles were sent to 360 primary-care physicians alerting them to a local outbreak of Q fever. The physicians subsequently submitted serology samples on significantly more patients than in a previously comparable period in 2001. Facsimile cascade assists effective communication with primary-care physicians in an outbreak investigation.
We study real-space condensation in a broad class of stochastic mass transport models. We show that the steady state of such models has a pair-factorized form which generalizes the standard factorized steady states. The condensation in this class of models is driven by interactions which give rise to a spatially extended condensate that differs fundamentally from the previously studied examples. We present numerical results as well as a theoretical analysis of the condensation transition and show that the criterion for condensation is related to the binding-unbinding transition of solid-on-solid interfaces.
AIMS: To estimate the incidence of active tuberculosis (TB) and study the use of chemoprophylaxis for latent TB in children in Wales, and to identify potential areas for improving prevention and management. METHODS: Active surveillance for TB in children aged 0-15 years from July 1996 to December 2003, using the Welsh Paediatric Surveillance Scheme. RESULTS: A total of 232 children, 102 with active TB (2.3 per 100 000) and 130 with latent TB (2.9 per 100 000), were identified. Nearly half (45%) belonged to ethnic minorities (19% were of black African origin), a much higher proportion than the base population. Pulmonary disease was the most common presentation (47%), including six (9%) children who were sputum smear positive. There were 10 cases of disseminated TB, nearly all in white children under 10 years of age. Less than two thirds of eligible children (27/46, 59%) were known to have received BCG immunisation. The source of infection was an adult household contact in most cases, but was not known in 44 cases, particularly among teenagers. Four community outbreaks occurred during the surveillance period, including three in high schools. CONCLUSION: TB incidence in children in Wales remains low, but the epidemiology is changing with an increasing proportion of cases in black African children. The high proportion of patients with disseminated TB is of particular concern. TB in teenagers was often associated with school outbreaks. Many eligible children do not receive BCG immunisation, indicating further scope for prevention.
Dehydration and the associated impairment of cardiovascular and thermoregulatory function comprise major veterinary problems in horses performing prolonged exercise, particularly under hot and humid conditions. For these reasons, there is considerable interest in using pre-exercise hyperhydration to help maintain blood volume in the face of the excessive fluid loss associated with sweat production during prolonged exertion. However, recently it was reported that pre-exercise hyperhydration causes arterial hypoxemia in horses performing moderate intensity exercise simulating the second day of an equestrian 3-day event competition (E3DEC) which may adversely affect performance (Sosa Leon et al. in Equine Vet J Suppl 34:425-429, 2002). These findings are contrary to data from horses performing short-term maximal exertion, wherein hyperhydration did not affect arterial O2 tension/saturation. Thus, our objective in the present study was to examine the impact of pre-exercise hyperhydration on arterial oxygenation of Thoroughbred horses performing an exercise test simulating the second day of an E3DEC. Control and hyperhydration studies were carried out on seven healthy Thoroughbred horses in random order, 7 days apart. In the control study, horses received no medications. In the hyperhydration experiments, nasogastric administration of NaCl (0.425 g/kg) 5 h pre-exercise induced a plasma volume expansion of 10.9% at the initiation of exercise. This methodology for inducing hypervolemia was different from that of Sosa Leon et al. (2002). Blood-gas tensions/pH as well as plasma protein, hemoglobin and blood lactate concentrations were measured pre-exercise and during the exercise test. Our data revealed that pre-exercise hyperhydration neither adversely affected arterial O2 tension nor hemoglobin-O2 saturation at any time during the exercise test simulating the second day of an E3DEC. Further, it was observed that arterial blood CO2 tension, pH, and blood lactate concentrations also were not affected by pre-exercise hyperhydration. However, hemodilution in hyperhydrated horses caused an attenuation of the expansion in the arterial to mixed-venous blood O2 content gradient during phases B and D of the exercise protocol, which was likely offset by an increase in cardiac output. It is concluded that pre-exercise hyperhydration of horses induced in the manner described above is not detrimental to arterial oxygenation of horses performing an exercise test simulating the second day of an E3DEC.
A case-control study was undertaken in an acute district general hospital to identify risk factors for hospital-acquired bacteraemia caused by methicillin-resistant Staphylococcus aureus (MRSA). Cases of hospital-acquired MRSA bacteraemia were defined as consecutive patients from whom MRSA was isolated from a blood sample taken on the third or subsequent day after admission. Controls were randomly selected from patients admitted to the hospital over the same time period with a length of stay of more than 2 days who did not have bacteraemia. Data on 42 of the 46 cases of hospital-acquired bacteraemia and 90 of the 92 controls were available for analysis. There were no significant differences in the age or sex of cases and controls. After adjusting for confounding factors, insertion of a central line [adjusted odds ratio (aOR) 35.3, 95% confidence interval (CI) 3.8-325.5] or urinary catheter (aOR 37.1, 95% CI 7.1-193.2) during the admission, and surgical site infection (aOR 4.3, 95% CI 1.2-14.6) all remained independent risk factors for MRSA bacteraemia. The adjusted population attributable fraction, showed that 51% of hospital-acquired MRSA bacteraemia cases were attributable to a urinary catheter, 39% to a central line, and 16% to a surgical site infection. In the United Kingdom, measures to reduce the incidence of hospital-acquired MRSA bacteraemia in acute general hospitals should focus on improving infection control procedures for the insertion and, most importantly, care of central lines and urinary catheters.
We study a one-dimensional lattice flocking model incorporating all three of the flocking criteria proposed by Reynolds [Computer Graphics 21, 4 (1987)]: alignment, centering, and separation. The model generalizes that introduced by O. J. O'Loan and M. R. Evans [J. Phys. A. 32, L99 (1999)]. We motivate the dynamical rules by microscopic sampling considerations. The model exhibits various flocking regimes: the alternating flock, the homogeneous flock, and dipole structures. We investigate these regimes numerically and within a continuum mean-field theory.
Routine surveillance data underestimate incidence of foodborne gastrointestinal (FGI) illness and provide little information on illness related to travel. We analysed data from the Welsh Health Survey to estimate population incidence, and to examine risk factors for FGI and factors associated with consulting a doctor. Reported frequency of any FGI in the 3 months before interview was 20.0% [95% confidence interval (CI) 19.5-20.4; equivalent to 0.8 episodes per person-year], and for travel-related FGI was 1.6% (95% CI 1.5-1.8). In the final model, sex, age group, marital status, self-reported health, long-term illness, smoking and alcohol consumption were all independent predictors of FGI. People who consulted a doctor were likely to be older, in poorer health, taking regular medication, or to report mental illness. FGI is common but risk factors for illness and consultation differ and impressions of the epidemiology of the disease based on surveillance data are therefore distorted.
An exactly solvable model for the rewiring dynamics of weighted, directed networks is introduced. Simulations indicate that the model exhibits two types of condensation: (i) a phase in which, for each node, a finite fraction of its total out-strength condenses onto a single link; (ii) a phase in which a finite fraction of the total weight in the system is directed into a single node. A virtue of the model is that its dynamics can be mapped onto those of a zero-range process with many species of interacting particles--an exactly solvable model of particles hopping between the sites of a lattice. This mapping, which is described in detail, guides the analysis of the steady state of the network model and leads to theoretical predictions for the conditions under which the different types of condensation may be observed. A further advantage of the mapping is that, by exploiting what is known about exactly solvable generalizations of the zero-range process, one can infer a number of generalizations of the network model and dynamics which remain exactly solvable.
Zero-range processes, in which particles hop between sites on a lattice, are closely related to rewiring networks, in which rewiring of links between nodes takes place. Both systems exhibit a condensation transition for appropriate choices of the dynamical rules. The transition results in a macroscopically occupied site for zero-range processes and a macroscopically connected node for networks. Criticality, characterized by a scale-free distribution, is obtained only at the transition point. This is in contrast with the widespread scale-free complex networks. Here we propose a generalization of these models whereby criticality is obtained throughout an entire phase, and the scale-free distribution does not depend on any fine-tuned parameter.
We study the phenomenon of real space condensation in the steady state of a class of one-dimensional mass transport models. We derive the criterion for the occurrence of a condensation transition and analyze the precise nature of the shape and the size of the condensate in the condensed phase. We find two distinct condensate regimes: one where the condensate is Gaussian distributed and the particle number fluctuations scale normally as L(1/2) where L is the system size, and the second regime where the particle number fluctuations become anomalously large and the condensate peak is non-Gaussian. We interpret these results within the framework of sums of random variables.
Seasonal peaks in both human campylobacter infections and poultry isolates have been observed in several European countries but remain unexplained. We compared weekly data on human campylobacter infections with thermophilic Campylobacter isolation rates from fresh, retail chicken samples (n = 514) purchased weekly in Wales between January and December 2002. Human isolates (n = 2631) peaked between weeks 22 and 25 (early June) and chicken isolates (n = 364) between weeks 24 and 26 (late June). In the absence of a temporal association, we postulate that the seasonal rise in humans is not caused by a rise in isolation rates in poultry but that both are more likely to be associated with a common, but as yet unidentified, environmental source.
We consider a disordered asymmetric exclusion process in which randomly chosen sites do not conserve particle number. The model is motivated by features of many interacting molecular motors such as RNA polymerases. We solve the steady state exactly in the two limits of infinite and vanishing nonconserving rates. The first limit is used as an approximation to large but finite rates and allows the study of Griffiths singularities in a nonequilibrium steady state despite the absence of any transition in the pure model. The disorder is also shown to induce a stretched exponential decay of system density with stretching exponent phi=2/5 .
In birds, elongated tails are one of the most common and most studied ornaments. However, the tail also has an aerodynamic function, playing a role in turning and slow flight. If a tail is to function as an ornament, then there will be an inevitable conflict between the aerodynamic role and the signaling role. Aerodynamic theory has developed to the point where it is possible to predict the aerodynamic and mechanical consequences of ornamental tails of different sizes and shapes. Tail elongation will influence many different mechanical and aerodynamic parameters. For at least some and possibly all of these parameters, there will be limits that are placed by the bird's anatomy, morphology, or physiology on the extent to which the effect of tail elongation could be tolerated. For example, if a particular tail morphology meant that the power required to fly exceeded the power available from the flight muscles, then the bird would obviously be unable to fly with such a tail. To examine whether these considerations could limit the development of ornamental tails, the effect of long tails of different shapes was examined on three parameters: static balance, lift-to-drag ratio, and the cost of flight. All three of these parameters were found to limit the range of possible tail morphologies that could be developed by birds. These limits were most acute for small birds, which may not be able to operate with a long tail of any shape. Slightly larger birds would be able to develop elongated streamers and forked tails but not pintails or wedge-shaped tails. Medium to large birds are less constrained and could develop a much wider range of tails than small birds, but there will always be limits to the sizes of tail ornament that could be developed. Thus the physical consequences of ornamentation on ecology and behavior are likely to be responsible for some of the patterns of ornamentation observed in nature.
We study condensation transitions in the steady state of a zero-range process with two species of particles. The steady state is exactly soluble-it is given by a factorized form provided the dynamics satisfy certain constraints-and we exploit this to derive the phase diagram for a quite general choice of dynamics. This phase diagram contains a variety of mechanisms of condensate formation, and a phase in which the condensate of one of the particle species is sustained by a "weak" condensate of particles of the other species. We also demonstrate how a single particle of one of the species (which plays the role of a defect particle) can induce Bose condensation above a critical density of particles of the other species.
OBJECTIVE: To examine the epidemiology of chickenpox in Wales from 1986 to 2001. DESIGN: Descriptive analysis of chickenpox consultations reported by the Welsh general practice sentinel surveillance scheme for infectious diseases, compared with annual shingles consultation rates from the same scheme to exclude reporting fatigue and data from a general practice morbidity database to validate results. SETTING: A total of 226,884 patients registered with one of 30 volunteer general practices participating in the sentinel surveillance scheme. MAIN OUTCOME MEASURES: Age standardised and age specific incidence of chickenpox. RESULTS: Crude and age standardised consultation rates for chickenpox declined from 1986 to 2001, with loss of epidemic cycling. Rates remained stable in 0-4 year olds but declined in all older age groups, particularly those aged 5-14 years. Shingles consultation rates remained constant over the same period. Data from the morbidity database displayed similar trends. CONCLUSION: General practitioner consultation rates for chickenpox are declining in Wales except in pre-school children. These findings are unlikely to be a reporting artefact but may be explained either by an overall decline in transmission or increased social mixing in those under 5 years old, through formal child care and earlier school entry, and associated increasing rates of mild or subclinical infection in this age group. Further investigation, particularly by serological surveillance, is necessary before universal varicella immunisation can be considered in the UK.
We study the dynamics of the East model, comprising a chain of uncoupled spins in a downward-pointing field. Glassy effects arise at low temperatures T from the kinetic constraint that spins can only flip if their left neighbor is up. We give details of our previous solution of the nonequilibrium coarsening dynamics after a quench to low T [Phys. Rev. Lett. 83, 3238 (1999)], including the anomalous coarsening of down-spin domains with typical size d approximately t(T ln 2), and the pronounced "fragile glass" divergence of equilibration times as t(*)=exp(1/T(2) ln 2). We also link the model to the paste-all coarsening model, defining a family of interpolating models that all have the same scaling distribution of domain sizes. We then proceed to the problem of equilibrium dynamics at low T. Based on a scaling hypothesis for the relation between time scales and length scales, we propose a model for the dynamics of "superdomains" which are bounded by up-spins that are frozen on long time scales. From this we deduce that the equilibrium spin correlation and persistence functions should exhibit identical scaling behavior for low T, decaying as g(t). The scaling variable is t=(t/t(*))(T ln 2), giving strongly stretched behavior for low T. The scaling function g(.) decays faster than exponential, however, and in the limit T-->0 at fixed t reaches zero at a finite value of t.
We investigate shock formation in an asymmetric exclusion process with creation and annihilation of particles in the bulk. We show how the continuum mean-field equations can be studied analytically and hence derive the phase diagrams of the model. In the large system-size limit direct simulations of the model show that the stationary state is correctly described by the mean-field equations, thus the predicted mean-field phase diagrams are expected to be exact. The emergence of shocks and the structure of the phase diagram are discussed. We also analyze the fluctuations of the shock position by using a phenomenological random walk picture of the shock dynamics. The stationary distribution of shock positions is calculated, by virtue of which the numerically determined finite-size scaling behavior of the shock width is explained.
Recent studies have shown that one-dimensional driven systems can exhibit phase separation even if the dynamics is governed by local rules. The ABC model, which comprises three particle species that diffuse asymmetrically around a ring, shows anomalous coarsening into a phase separated steady state. In the limiting case in which the dynamics is symmetric and the parameter q describing the asymmetry tends to one, no phase separation occurs and the steady state of the system is disordered. In the present work, we consider the weak asymmetry regime q=exp(-beta/N), where N is the system size, and study how the disordered state is approached. In the case of equal densities, we find that the system exhibits a second-order phase transition at some nonzero beta(c). The value of beta(c)=2pi square root 3 and the optimal profiles can be obtained by writing the exact large deviation functional. For nonequal densities, we write down mean-field equations and analyze some of their predictions.