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Biomedical subjects

J Swinton

Publications and source records attributed to J Swinton.

At least 19 recordsLinked to original sources
PubMed2002

A status-based approach to multiple strain dynamics.

We present and investigate a new model for cross-immunity. Past models classify hosts according to their infection history. Here we represent hosts through their status: their current ability to respond to strains. This framework allows a different, a wider, and a more biologically interpretable range of forms of cross-immunity to be studied. Using this new form of cross-immunity we then consider a previously studied case of four strains, each of which confers partial immunity to two of the others. In this interesting special case, with applications to the genetic maintenance of strain diversity, we can make substantial analytical progress. We present methods for exploiting the symmetries of the system to show that only a particular invariant subspace need be considered for characterizing the dynamics of the whole system. A complete bifurcation structure is given for this subspace. In contrast to systems previously studied, this system does not exhibit sustained oscillations for any set of parameter values.

Disease Susceptibility↗
PubMed2001

Spirituality. Come all ye faithful.

A growing body of research suggests that religion and spirituality can have a positive effect on mental and physical health. Like any other powerful belief system, they also have potential for harm. Further research is needed if they are to be understood and therapeutically incorporated into healthcare.

Health Status Indicators↗
PubMed2000

Community, culture and character: the place of the virtues in psychiatric nursing practice.

In this paper, the authors address the serious neglect of the 'classical' virtues in ethical reflection upon mental health nursing. The virtues are offered as a possible alternative paradigm for mental health nursing in its search for new models and approaches. Embodied in the notion of a moral community, the virtues have important implications in addressing problems adherent in various 'dualisms' so described. By invoking the concepts of practical wisdom and praxis, the interrelationship between theory and practice, action and reflection and self and community, is shown. The contextual nature of the virtues is shown in a number of examples and their potential for the transformation of practice is shown. The opportunity is now present for mental health nursing to strengthen its basis for practice by distancing itself from scientific models. In so doing, it can embrace an approach which is consensual and contextual and which places ethical reflection at the centre of practice.

Character↗
PubMed1998

Extinction times and phase transitions for spatially structured closed epidemics.

This paper considers the time to extinction for a stochastic epidemic model of SEIR form without replacement of susceptibles. It first shows how previous rigorous results can be heuristically explained in terms of the more transparent dynamics of an approximating deterministic system. The model is then extended to include a host population structured into patches, with weak nearest-neighbour mixing of infection. It is shown, by considering the approximating deterministic system, that the expected time to extinction in a population of n + 1 patches each of size N is of the form a log N + bn, provided that N > Nc where Nc is a critical patch size below which transits are unlikely to occur. This corresponds to the simple decomposition of the time of an epidemic into the time it takes to spread through one patch plus the time it takes to transit to each of n successive patches. Expressions for this threshold and the coefficients of the time to extinction are given in terms of the transmission parameters of infection and the coupling strength between patches. These expressions are compared with numerical results using parameters relevant to a study of phocine distemper virus in North Sea seals, and the agreement is found to be good for large and small N. In the region when N approximately Nc, where transits may or may not occur, interesting transitional behaviour is seen, leading to a non-monotonicity of the extinction time as a function of N.

Animals↗
PubMed1997

A comparison of fertility control and lethal control of bovine tuberculosis in badgers: the impact of perturbation induced transmission.

In this paper we use mathematical modelling to consider the broad advantages and disadvantages of fertility control over lethal control for bovine tuberculosis in badger populations. We use a deliberately simple model, attempting to capture only the key transmission processes. The model is parametrized with reference to the long-term Woodchester Park study. Estimates of mortality rate from this study suggest no significant extra mortality risk for animals with evidence of infection as indicated by the presence of anti-Mycobacterium bovis antibodies or M. bovis isolation. We find that large reductions in prevalence are sometimes the consequence of only moderate reductions in population numbers. If we assume that the act of control does not in itself affect transmission rates, then as far as eradication is concerned, both fertility control and mortality control operate through the same epidemiological mechanism, the removal of susceptibles: if one is in principle capable of keeping a population low enough to be infection free then so is the other. It is necessary to continue either form of control at regular intervals to maintain a constant level of infection in the long term. If control were to be stopped, return to precontrol levels of badger population and infection prevalence would be expected within a few years. Fertility control is less effective in reducing population density than lethal control since it can only act, at maximum, to remove one age cohort per year. It is also less effective in reducing transmission as it can only ever remove susceptibles, while lethal control also removes infectious badgers. However, if the social disturbance caused by lethal control does in fact increase contact rates for the remaining infectious badgers, the relative efficacies of the two strategies become a great deal less clear. While we have no quantitative data on the extent to which social perturbation does act to promote transmission, model simulations show that it is possible to develop plausible scenarios in which the lethal control may actually act to increase the absolute numbers of animals infected, while reducing the number of uninfected animals to very low numbers.

Animals↗
PubMed1997

Vaccination in pulses: a strategy for global eradication of measles and polio?

Recent American successes against poliomyelitis and measles have been attributed to repeated 'pulse' vaccination campaigns. Whilst logistic and economic constraints will be crucial, a deeper epidemiological understanding of the mechanism, strengths and weaknesses of pulse vaccination will optimize the chances of success elsewhere in the world.

Age Factors↗
PubMed1997

The role of sexual partnership networks in the epidemiology of gonorrhea.

BACKGROUND: Empirical studies have the potential to collect data on patterns of sexual mixing and network structures. GOAL: To explore the contribution of different network measures in sexually transmitted disease epidemiology. STUDY DESIGN: Individual-based stochastic simulations of a network of sexual partnerships and sexually transmitted disease transmission are analyzed using logistic regression. RESULTS AND CONCLUSIONS: Measures accumulated over times similar to the duration of infection are more informative than are static cross sections. The patterns of sexual mixing and network structure influence patterns of infection. In particular, the establishment of infection is most sensitive to the proportion of nonmonogamous pairs, the component distribution and cohesion among those with high activity. The subsequent prevalence is most sensitive to the assortativeness of mixing in the high-activity class and a measure of cohesion, both of which reflect the decrease in prevalence brought about by less widespread connections. A person's risk for infection is determined by local rather than global network structures.

Contact Tracing↗
PubMed1995

Potential impact of low efficacy HIV-1 vaccines in populations with high rates of infection.

A safe and effective HIV vaccine to prevent infection and/or to moderate disease is urgently needed. Research progress has been slower than anticipated for a variety of reasons including uncertainty over which immunogen to use (i.e. recombinant subunit envelope proteins or whole HIV-1 products), confusion on which immunological markers best correlate with protection, the relevance of the HIV-1 chimpanzee model to infection in humans and the significance of the rapid evolution of HIV-1, with different clades of the virus emerging in different parts of the world. However, what some would interpret as encouraging results, from Phase I and II trials of recombinant envelope glycoprotein vaccines, have raised the question of whether the time is right to start Phase III trials in humans with immunogens that may have low to moderate efficacy. By using mathematical models and data from epidemiological studies, we examine the potential impact of such vaccines within heterosexual communities with high rates of infection. Analyses suggest that it will be difficult to block HIV-1 transmission even with very high levels of mass vaccination. The cost of sustaining high levels of herd immunity with a vaccine of short protection duration is likely to be high. However, assessments of impact over the long duration of an HIV-1 epidemic indicate that many cases of HIV infection and associated mortality can be prevented by immunogens with efficacy of 50% or less and a five year protection duration.(ABSTRACT TRUNCATED AT 250 WORDS)

AIDS Vaccines↗
PubMed1995

The control of childhood viral infections by pulse vaccination.

Pulse vaccination, the repeated application of vaccine over a defined age range, is gaining prominence as a strategy for the elimination of childhood viral infections such as measles and polio. However, unlike routine or continuous mass infant immunization, epidemiological understanding of this control method is in its infancy. This paper develops initial work by Agur et al. (1993) using simple steady-state and age-structured dynamic models to extend the theory of the mechanism of action of pulse vaccination, and to explore the relationship between the maximum permitted interval between pulses and key epidemiological, demographic and vaccination variables. Initially, a conceptual model is presented to illustrate the principles of pulse vaccination and to make comparison with routine immunization procedures. An ordinary differential equation model, which assumes homogeneous mixing, is then used to derive equilibrium expressions for the pulse interval in relation to (i) different demographic profiles, (ii) population growth characteristics (stationary or exponentially increasing), (iii) combined routine and pulse immunization, and (iv) the age range vaccinated. Finally, simulations using age-structured compartmental deterministic models illustrate complex epidemiological dynamics associated with pulse vaccination, particularly where there is age heterogeneity in contact rates in the population. The resultant uncertainty in defining an optimal pulse interval raises concerns of a practical nature.

Age Factors↗
PubMed1994

Two signal activation as an explanation of high zone tolerance: a mathematical exploration of the nature of the second signal.

Much work on the induction of non-responsiveness in lymphocytes suggests that it takes two signals to activate a cell correctly, and that receipt of the first signal alone not only causes suboptimal response, but also subsequent non-responsiveness or anergy. This paper discusses not anergy, but the original suboptimal response in the context of the well known but poorly understood phenomenon of high zone tolerance in experiments studying immunological responses to a range of antigen concentrations (low to high "zones"). We proceed to ask specific questions about the nature and timing of the signals involved. We construct three different models describing how the second signal might be received and use them to predict the shapes of dose-response curves. Comparison with data enables us to select, from a dynamical perspective, the most likely mechanism and to tentatively exclude a number of proposed mechanisms for high zone tolerance. We suggest that the stage at which a cell becomes susceptible to anergy is crucial and that it is over-contact, rather than lack of contact, which is important in the induction of tolerance.

Antigens↗
PubMed1994

Theoretical studies of the effects of heterogeneity in the parasite population on the transmission dynamics of malaria.

Periodicity in malaria transmission has generally been ascribed to seasonal fluctuations in mosquito population density or spatial heterogeneity with periodic migration. In this paper we demonstrate that simple models of strain heterogeneity can generate periodic behaviour as a consequence of the interaction between parasite strains and host immunological defences. As the degree of cross-immunity between strains increases, the system moves from a régime of independent strain transmission and coexistence, through increasingly coupled behaviour, to the displacement of the strain of lower transmissibility by the strain with a higher basic reproductive rate (R0). Cross-immunity thus serves both to bring the strains into competition, and also to couple the dynamics. We find analytical and numerical results on strain coexistence to show how the range of possible outcomes may be read as an effect of the tension between these two effects of cross-protection.

Animals↗
PubMed1993

Dynamic interaction between Leishmania infection in mice and Th1-type CD4+ T-cells: complexity in outcome without a requirement for Th2-type responses.

The interaction between Leishmania parasites and Th1 cells is investigated using a simple mathematical model of immunological responses and parasite population growth within the host. The model generates patterns of resistance and susceptibility to infection that mirror observed trends in experimental infections of mice and of humans exposed to infection in areas of endemic transmission. The heterogeneity in outcome predicted by the model can arise either through differences in the values of the parameters that characterize the genetic background of the host or as a consequence of differences in the size of the infecting inoculum of the parasite. Detailed analyses of equilibrium states and of the time course of infection within a host suggest that a limitation in the availability of precursor T-cells, as a consequence of high levels of recruitment into the activated pool, may play a significant role in the progression of infection in susceptible hosts. A brief discussion is presented of the implications of model prediction for therapeutic intervention.

Animals↗
PubMed1992

Gonococcal infection, infertility, and population growth: I. Endemic states in behaviourally homogeneous growing populations.

Sexually transmitted diseases such as gonorrhoea are a significant cause of infertility in women when the infection is untreated. They have the potential to alter human population growth rates in many developing countries where sexually transmitted diseases are prevalent due to limited public health facilities for diagnosis and treatment. The authors develop a simple model describing the conditions under which such a disease can exist in a growing population in which sexual partners are chosen at random by proportionate mixing. Using the model, the impact that such a disease could have for a range of possible parameters is examined. Then a full parameter set for gonorrhoea in a developing country is estimated. Analysis demonstrates the significant influence gonococcal infection may have in reducing population growth rate in some communities. For example, the simple model predicts that a prevalence of 20% in sexually active adults results in a 50% reduction in the population growth rate. Finally, the authors discuss how potential control initiatives may change the parameter values that determine transmission and alter the demographic impact of gonorrhoea.

Africa↗
PubMed1992

Gonococcal infection, infertility, and population growth: II. The influence of heterogeneity in sexual behaviour.

Heterogeneity in sexual behaviour has an important influence on the transmission dynamics of sexually transmitted diseases (STDs). The authors describe the development of a simple mathematical model, incorporating such heterogeneity, to investigate the demographic impact of gonorrhoea on human population growth in developing countries where the disease is endemic. Earlier predictions, based on a model with homogeneous mixing, are shown to be in good qualitative agreement with the predictions of a more complex mathematical framework in which the population is stratified both by sex and into two subgroups representing low and high sexual activity, defined on the basis of rates of sexual partner change. Analyses also demonstrate that the pattern of mixing (assortative to random) between the sexual activity classes has an important influence on the predicted prevalence of gonococcal infection in a defined community. The more complex model supports earlier conclusions that gonorrhoea, via its impact on fertility, can significantly reduce net population growth rates.

Adolescent↗
PubMed1991

Gonococcal infection and human fertility in sub-Saharan Africa.

An analysis is presented of the influence of Neisseria gonorrhoeae on human population growth in regions of sub-Saharan Africa where gonococcal infections are prevalent in sexually active adults. Combining epidemiological and demographic data within the framework of a mathematical model, we show that gonorrhoea has a major impact on fertility and, concomitantly, on net population growth in areas with a high prevalence of untreated infections. Specifically, a 20% prevalence in sexually active adults is predicted to induce a 50% reduction in net population growth. Model predictions are in good agreement with observed data from Uganda, and the sensitivity of the prediction to various complications, such as heterogeneity in sexual behaviour, is assessed. The analysis suggests that the predicted increase in fertility arising from expanded sexually transmitted disease (STD) control programmes in Africa to help combat the spread of human immunodeficiency viruses (HIV-1 and HIV-2) will help to offset the predicted demographic impact of AIDS in the worst afflicted areas. In other areas the rise in fertility associated with effective STD control will need to be countered by the linkage of STD control programmes with family planning initiatives.

Adolescent↗
PubMed1985

The suitability of immunosuppressed mice kept in a standard animal unit as recipients of human tumour xenografts.

CBA/Lac mice were immunosuppressed by thymectomy and whole body irradiation with 250 kVp X-rays following pretreatment with cytosine arabinoside. The optimum radiation dose for immunosuppression with prolonged survival was 7.35 Gy. The animals were kept in a standard animal unit with an overall survival rate of 83%. They were found to be suitable for large scale, long-term, xenotransplantation experiments at 20% of the cost of nude mice.

Animals↗