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Biomedical subjects

G F Medley

Publications and source records attributed to G F Medley.

90 records · Page 5Linked to original sources
PubMed1992

Evaluating measures to control intestinal parasitic infections.

Intestinal parasitic infections are among the most common infections of humans in developing countries, but the resources available for their control are severely limited. Careful evaluation of control measures is essential to ensure that they are cost-effective. The evaluation of the effects of control on intestinal helminths and intestinal protozoa requires an understanding of the different epidemiological patterns of these two groups of parasites. The transmission dynamics and morbidity associated with the major helminth infections (Ascaris lumbricoides, Trichuris trichiura and the hookworms) are dependent on the size of the worm burdens. Thus the important parameter for evaluating the impact of control on morbidity and transmission is the intensity of infection, which can be assessed by determining the mean density of parasite eggs in faecal specimens. Estimation of intensity is subject to systematic errors, however, due to the complex pattern of worm burden distributions. The frequency distribution of burdens is highly overdispersed, and individuals exhibit predisposition to particular levels of infection. Furthermore, mean intensity is age-dependent, in a species-specific manner, and is clustered spatially and within families. These complex patterns imply that the estimation of intensity is exceptionally sensitive to the size and demographic structure of the population sample selected for assessment. They also have the effect that prevalence estimates, the most commonly used measures of infection in communities, can seriously mislead. Paradoxically, prevalence is least useful where infection is most common because the relationship between prevalence and intensity is most markedly non-linear when the prevalence is high. Thus in areas where control is most needed, evaluation using prevalence might suggest that control had failed while evaluation by intensity would, correctly, show the measure of success. With the major protozoan infections (Entamoeba histolytica, Giardia duodenalis and Cryptosporidium parvum) an estimate of intensity is of little value and the central parameter for evaluation is prevalence. Prevalence does exhibit age and spatial heterogeneity, which may be species-specific, so there remains a need to ensure a consistent sample structure, although this is less critical than for the helminths. The major constraint on evaluating the control of protozoan infections is the need to identify pathogenic species and, in some cases, pathogenic strains. Harmless commensal protozoans are ubiquitous and often morphologically very similar to pathogens, but their control is both unnecessary and impracticable. Species such a E. histolytica appear to exist as strains with differing pathogenicity, thus control will be cost-effective only if the focus is on pathogenic strains. Effective diagnosis is therefore central to the evaluation of the control of protozoan infections.(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗
PubMed1992

Short-term predictions of HIV prevalence and AIDS incidence.

Reports of AIDS cases in Amsterdam up to February 1990 were used to make predictions of future cases up to 1993. Two published methods were applied, which make extrapolations from current cases and simultaneously estimate the extent of delay in reporting. The choice of the exact model greatly influenced the predictions, as did predictions for distinct transmission groups. We present results for the homo/bisexual male group, and the total population of Amsterdam. The AIDS case predictions are used to predict the HIV prevalence using the ratio of HIV prevalence to AIDS incidence and through 'back calculation'. We suggest that the ratio is a simple technique that may be used to estimate HIV prevalence. The estimated number of cumulative HIV infected homo/bisexual males in Amsterdam in January 1990 was between 2100 and 4100 in a total of 2200-4600 infected people.

Acquired Immunodeficiency Syndrome↗
PubMed1991

The use of AIDS surveillance data for short-term prediction of AIDS cases in Madrid, Spain.

The paper presents a preliminary attempt to predict the numbers of AIDS cases in the Community of Madrid (CAM) up to 1992. Using AIDS case surveillance data gathered by CAM, and a statistical procedure that includes a distribution for reporting delays, the numbers of new diagnoses, reports, AIDS deaths and numbers of patients alive is predicted. Approximate confidence limits for the numbers of new diagnoses are given. We emphasise that these predictions are tentative given the nature of the reporting delays, and discuss the use of such predictions and the requirements for their improvement.

Acquired Immunodeficiency Syndrome↗
PubMed1990

The regulation of gastrointestinal helminth populations.

One quarter of the world's human population suffers infection with helminth parasites. The population dynamics of the ten or so species, which cause disease of clinical significance have been well characterized by epidemiological field survey. The parasites are in general highly aggregated between hosts, and their populations seem to be temporally stable and to recover rapidly from perturbation, including interventions designed to alleviate disease. This paper reviews current understanding of the population regulation of helminth species of medical significance. Both empirical (field and laboratory) and theoretical results are included, and we attempt to interpret the findings in the broader context of the population ecology of free-living species. We begin by considering the evidence for regulation from field data concerning the temporal stability of helminth populations within communities and from the results of perturbation experiments. The detection of regulatory processes is then discussed (with regard to statistical and logistical considerations), and the evidence from both the field and laboratory studies reviewed. Deterministic models are described to investigate the possible consequences of regulation imposed at different points in the parasite life-cycle. The causes and consequences of parasite aggregation are considered, and a stochastic model used to investigate the impact of different combination of regulatory processes and heterogeneity generating mechanisms.

Animals↗
PubMed1990

The relationship between the frequency distribution of Ascaris lumbricoides and the prevalence and intensity of infection in human communities.

Observed field data from a range of geographically distinct human communities suggest a consistent non-linear relationship between prevalence and mean intensity of Ascaris lumbricoides infection. Utilizing the negative binomial distribution as a description of observed aggregation, maximum-likelihood analysis reveals that the degree of aggregation is a negative linear function of mean worm burden. The factors responsible for this relationship in human populations require further study but may involve some combination of (i) density-dependent reduction in worm numbers within individuals, (ii) density-dependent parasite-induced host mortality or (iii) self-treatment by heavily infected hosts. Variability in the degree of aggregation appears dependent on the level of infection in a community and independent of geographical differences in the host or parasite populations.

Animals↗
PubMed1989

A process of events with notification delay and the forecasting of AIDS.

Analysis and prediction of a point process are studied when there is delay in notifying the occurrence of events. A primarily parametric approach is taken to studying the rate of the underlying process and to predicting future properties of the system. Data on AIDS are used in illustration.

Acquired Immunodeficiency Syndrome↗
PubMed1988

The distribution of the incubation period for the acquired immunodeficiency syndrome (AIDS).

This paper contains details of methods and full results of estimates of the incubation period of acquired immunodeficiency syndrome (AIDS) that were outlined in a previous paper by Medley et al. (Nature, Lond. 328, 719 (1987)). The original model is modified to assess the influence of age and sex on the incubation period: age, but not sex, is statistically significant. The difficulties associated with interpretation of these data, and the additional information that would be required to resolve these difficulties are discussed.

Acquired Immunodeficiency Syndrome↗
PubMed1987

Is it possible to predict the minimum size of the acquired immunodeficiency syndrome (AIDS) epidemic in the United Kingdom?

A mathematical model of the dynamics of transmission of human immunodeficiency virus within the male homosexual population in the United Kingdom demonstrates that even the minimum size of the acquired immunodeficiency syndrome (AIDS) epidemic in the United Kingdom (based upon the assumption that all transmission ceased at the end of 1986) is difficult to predict. Model predictions are particularly sensitive to assumptions about the distributed incubation period of the disease, differences in frequency and patterns of sexual activity, and the proportion of infected people in whom AIDS later develops. More accurate predictions will depend on the collection of data on the incubation period of the disease, the infectiousness of infected persons, and on the numbers of new sexual partners of each sex per person and the duration of each partnership.

Acquired Immunodeficiency Syndrome↗
PubMed1986

A preliminary study of the transmission dynamics of the human immunodeficiency virus (HIV), the causative agent of AIDS.

The paper describes some preliminary attempts to formulate simple mathematical models of the transmission dynamics of HIV infection in homosexual communities. In conjunction with a survey of the available epidemiological data on HIV infection and the incidence of AIDS, the models are used to assess how various processes influence the course of the initial epidemic following the introduction of the virus. Models of the early stages of viral spread provide crude methods for estimating the basic reproductive rate of the virus, given a knowledge of the incubation period of the disease (AIDS) and the initial doubling time of the epidemic. More complex models are formulated to assess the influence of variation in the incubation period and heterogeneity in sexual activity. The latter factor is shown to have a major effect on the predicted pattern of the epidemic; high levels of heterogeneity decrease its magnitude. Areas of biological uncertainty, future research needs, and public health implications are discussed.

Acquired Immunodeficiency Syndrome↗
PubMed1985

Community control of helminth infections of man by mass and selective chemotherapy.

The design of mass and targetted community-based chemotherapy programmes for the control of the major helminth infections of man is discussed in relation to the population and transmission dynamics of the parasites. Rapid reinfection following a single mass or targetted anthelmintic application is shown to be a universal feature of helminth transmission, as a consequence of the regulatory or feed-back mechanisms controlling population abundance within both individuals and the community as a whole. Control of reinfection requires repeated community treatment where the intensity of application and the interval between treatments are dependent on the reproductive life-expectancy of the adult worm, the net force of transmission prior to control (the basic reproductive rate) and the factors which create aggregation in the distribution of parasite loads within the population. Selective or targetted treatment is shown to be most effective for the control of morbidity as opposed to the control of transmission. The impact of targetted treatment depends critically on the factors that generate heterogeneity in parasite burdens and on whether or not selectivity is based on a single or repeated identification of the 'wormy' fraction of the community. Monte Carlo simulation studies are employed to assess the likely impacts of different control strategies on average parasite abundance/person and the distributions of parasite loads within populations. Future epidemiological research needs are discussed in relation to theoretical work and recent field studies of predisposition to heavy infection.

Age Factors↗
PubMedDate not supplied

The epidemiology and population biology of Necator americanus infection in a rural community in Zimbabwe.

Baseline data from an epidemiological study of hookworm infection in a rural community in Zimbabwe are presented. The infection status of an age-stratified sample of the community was assessed using anthelmintic expulsion techniques. Necator americanus was the only helminth parasite found to be present. The age-prevalence and intensity profiles rose asymptotically to an adult prevalence of about 80% and adult mean burden of 7.7 worms per host. The overall mean burden was 4.8 worms per host. The frequency distribution of N. americanus was overdispersed and well described by the negative binomial distribution with a value for the aggregation parameter, k, of 0.346. Separate estimates of k were lower in males and older hosts. The distribution patterns were difficult to reconcile with any simple process of age-dependent acquisition of an effective immune response. A significant negative correlation was recorded between per caput fecundity and worm burden, providing evidence for a density-dependent regulation of female worm fecundity. The basic reproductive rate (R0 congruent to 2) was found to be similar to estimates from other geographical areas.

Adolescent↗
PubMedDate not supplied

Dynamics of malaria parasitaemia associated with febrile illness in children from a rural area of Madang, Papua New Guinea.

Active community and self-reporting surveillance techniques have been used to describe the dynamics of febrile illness and associated malaria infection in children aged 2 to 15 years from a rural area of Madang Province, Papua New Guinea (PNG). Both history of fever and fever in association with parasitaemia appeared to be reliable indicators of malaria morbidity in this endemic area. Parasite density was observed to be a major determinant of mild malarial disease at both the population level and within an individual. Age-specific prevalence of febrile illness correlated with age-specific patterns of parasite density but not of parasite prevalence. Seasonal changes in fever incidence correlated with parasite density. The transition from afebrile to febrile state within an individual was generally associated with an increase in parasite density. Surveillance and self-reported febrile cases (which differ in severity on the basis of the perceived need for treatment) could be distinguished on the basis of parasite density. Thus surveillance techniques divide clinical malaria in rural PNG into 'mild' and 'very mild' forms. The age-specific pattern of decline of prevalence of malaria-associated febrile illness and parasite density is best explained by induction of strain-specific anti-disease immunity upon infection with a given strain of Plasmodium falciparum. The fever threshold in self-reporting febrile cases was seen to decrease with age and can be explained by an age-specific decline in anti-toxic immunity.

Adolescent↗
PubMedDate not supplied

Control of Ascaris infection by chemotherapy: which is the most cost-effective option?

Cost-effectiveness analysis is used to predict the optimal design of mass chemotherapy strategies in controlling Ascaris lumbricoides infection. The question of who to treat, how many to treat, and how often to treat are addressed using a population dynamic model of helminth transmission that assesses effectiveness in terms of disease reduction, combined with cost data from an actual control programme. Child-targeted treatment can be more cost-effective than population treatment in reducing the number of disease cases. The model also implies that, in the circumstances described here, enhancing coverage is a more cost-effective approach than increasing frequency of treatment.

Adult↗
PubMedDate not supplied

The structural identifiability and parameter estimation of a multispecies model for the transmission of mastitis in dairy cows with postmilking teat disinfection.

A mathematical model for the transmission of two interacting classes of mastitis causing bacterial pathogens in a herd of dairy cows is presented and applied to a specific data set. The data were derived from a field trial of a specific measure used in the control of these pathogens, where half the individuals were subjected to the control and in the others the treatment was discontinued. The resultant mathematical model (eight non-linear simultaneous ordinary differential equations) therefore incorporates heterogeneity in the host as well as the infectious agent and consequently the effects of control are intrinsic in the model structure. A structural identifiability analysis of the model is presented demonstrating that the scope of the novel method used allows application to high order non-linear systems. The results of a simultaneous estimation of six unknown system parameters are presented. Previous work has only estimated a subset of these either simultaneously or individually. Therefore not only are new estimates provided for the parameters relating to the transmission and control of the classes of pathogens under study, but also information about the relationships between them. We exploit the close link between mathematical modelling, structural identifiability analysis, and parameter estimation to obtain biological insights into the system modelled.

Animals↗
PubMedDate not supplied

Vaccination against hepatitis B virus in highly endemic areas: waning vaccine-induced immunity and the need for booster doses.

A dynamic model of the transmission of hepatitis B virus (HBV) was used to investigate some of the possible implications of the loss of vaccine-induced immunity on the effectiveness of mass HBV vaccination in high endemicity countries. The model, which describes the processes of horizontal and vertical transmission of HBV in a rapidly growing population, was based on one previously published, modified to allow for waning vaccine-induced immunity. All model parameters were empirically based. Summaries of published data provided estimates of the mean duration of immunity in vaccine responders and age-dependence in the risk of developing acute HBV disease. Simulations of the temporal and long-term impact of mass infant immunization on the prevalence of HBV chronic carriage and acute disease suggested that booster doses are of little benefit in coping with waning vaccine protection. The key factor is that vaccine-induced protection should be long enough to prevent infection in the first few years of life, when the risk of developing chronic carriage is highest.

Acute Disease↗
PubMedDate not supplied

Incubation period of AIDS in patients infected via blood transfusion.

Temporal trends in the prevalence of HIV (human immunodeficiency virus) infection are uncertain because of the reluctance of most governments to embark on large-scale programmes of serological surveillance. In the absence of such data, attempts have been made to relate the number of reported cases of AIDS (acquired immune deficiency syndrome) in a defined population to the proportion of that population infected with the virus as a specified time point. One crucial determinant of this relationship is the probability distribution of the incubation period of the disease, with the period defined as the time interval from infection to diagnosis. Recent statistical analysis suggests a mean incubation period of 4.5 years with wide confidence limits, whereas a more heuristic study reports a mean of 15 years. Here we report on a new analysis which reveals age-related differences in the mean (and median) incubation period: 1.97 (1.90) years for children (0-4 yrs old at infection), 8.23 (7.97) years for adults (5-59 yrs old), 5.50 (5.44) years for elderly patients (60 yrs and older).

Acquired Immunodeficiency Syndrome↗