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Biomedical subjects

R M Anderson

Publications and source records attributed to R M Anderson.

At least 253 records · Page 14Linked to original sources
PubMed1988

Measles in developing countries. Part II. The predicted impact of mass vaccination.

A mathematical model is developed to mimic the transmission dynamics of the measles virus in communities in the developing world with high population growth rates and high case fatality rates. The model is used to compare the impacts of different mass vaccination programmes upon morbidity and mortality arising from infection by measles virus. Analyses identify three conclusions of practical significance to the design of optimal vaccination programmes. First, there is no single optimum age at which to vaccinate children for all urban and rural communities in developing countries. For a given community the best age at which to vaccinate depends critically on the age distribution of cases of infection prior to the introduction of control measures. Second, numerical studies predict that the introduction of mass vaccination will induce a temporary phase of very low incidence of infection before the system settles to a new pattern of recurrent epidemics. Mass vaccination acts to lengthen the inter-epidemic period in the post-vaccination period when compared with that prevailing prior to control. Third, numerical simulations suggest that two-phase and two-stage vaccination programmes are of less benefit than one-stage programmes (achieving comparable coverage) aimed at young children. The paper ends with a discussion of the needs for: improved programmes of data collection; monitoring of the impact of current vaccination programmes; and the development of models that take account of viral transmission dynamics, host demography and economic factors.

Adolescent↗
PubMed1988

Distributed incubation and infectious periods in models of the transmission dynamics of the human immunodeficiency virus (HIV).

Distributions describing variation in the incubation and infectious periods of the human immunodeficiency virus (HIV) are derived from a series of risk or hazard functions. Four possible forms of the probability density function are considered, namely, exponential, Weibull, Erlang/gamma, and rectangular, and the properties and underlying risk functions are compared and contrasted. Models of the transmission dynamics of the virus, encapsulating different assumptions concerning the distributed incubation and infectious periods, are analysed, and their properties compared by steady-state and local-stability analyses and numerical methods.

Acquired Immunodeficiency Syndrome↗
PubMed1988

Variable infectiousness in HIV transmission models.

Two different approaches to the encapsulation of temporal variation in the infectiousness of HIV-infected persons, and variability in the incubation period of the disease AIDS, in simple homogeneous mixing models of viral transmission in male homosexual communities are described. The first approach is based on the division of the infected population into a series of subclasses with differing levels of infectivity and different durations of occupancy. The second approach is more mechanistic in character and is based on an attempt to relate changes in viral abundance within an infected person to the likelihood that the disease AIDS develops. Variable incubation is induced by variation in the rate of change of viral abundance in the infected population. Numerical projections of changes in the incidence of AIDS through time, generated from both types of model, are compared with projections based on the assumption of constant infectivity throughout the incubation period of AIDS. Model formulation highlights areas in which more detailed quantitative epidemiological studies are required. Methods of parameter estimation and future research needs are discussed.

Acquired Immunodeficiency Syndrome↗
PubMed1988

Heterogeneous sexual activity models of HIV transmission in male homosexual populations.

A proportionate mixing one-sex model of sexual transmission of HIV is described, in which sexual activity (new partners per unit time) is defined as a continuous variable in a set of integro-partial-differential equations. A discrete activity-class approximation is developed by matching equilibrium state and rate variables as closely as possible with the continuous-variable model, and consists only of ordinary differential equations. Activity-class boundaries are arbitrary, and each class is characterized by a single level of activity. If there are N classes, the level of activity in N - 1 of them is such that the steady-state susceptible class sub-population is equal to the population in the equivalent section of the continuous model. The activity level for the remaining class is chosen such that the condition for endemicity of the infection in the approximation is equal to that for the equivalent continuous-variable model; this minimizes errors in the steady-state population. The relationship between the discrete and continuous-variable models is explored, via numerical and analytical studies, in order to evaluate the accuracy of the approximation.

Acquired Immunodeficiency Syndrome↗
PubMed1988

The role of mathematical models in the study of HIV transmission and the epidemiology of AIDS.

The role of mathematical models in the study of human immunodeficiency virus (HIV) transmission, the epidemiology and demographic impact of acquired immune deficiency syndrome (AIDS), and the course of infection within an individual is reviewed. Attention is focused on the part models can play in short-term prediction, parameter estimation, the interpretation of observed patterns, and the identification of areas in which knowledge is inadequate. Methods for short-term predictions, over 1-3 years, have proved reliable and can take account of distributed delays in the interval between reporting and diagnosis. Parametric and nonparametric methods have been developed for estimating summary statistics (e.g., means) for the distributed incubation period of AIDS. Current estimates, based on cohort or transfusion-associated AIDS cases, lie in the range of 7-10 years. Transmission models for longer-term prediction are less reliable at present given the many uncertainties concerning key epidemiological parameters. Theoretical studies highlight the need for quantitative data on temporal changes in the distribution of rates of sexual partner change and fluctuations in viral abundance in serum, secretions, and excretions (i.e., infectiousness) throughout the long and variable incubation period of the disease.

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↗
PubMed1987

Neuropathology in cerebral lactic acidosis.

The neuropathology in two young infants with cerebral lactic acidosis is presented. Both cases showed microcephaly, ventricular dilatation, absent corpus callosum, absent or extremely hypoplastic pyramids, heterotopic inferior olives, focal neuroglial overgrowth into meninges, focal cystic change with gliosis and recent degenerative changes in the germinal matrix. The cerebellum, basal ganglia, Ammon's horn, inferior colliculi and layering of the cortex were well preserved. The features suggest a continuous damage with very old, relatively old and recent lesions. This striking combination of abnormalities should raise the possibility of a disturbance of lactate/pyruvate metabolism.

Acidosis, Lactic↗
PubMed1987

Age-related prevalence and intensity of Trichuris trichiura infection in a St. Lucian community.

Age-related changes in the average worm burden and the prevalence of Trichuris trichiura infection, in a village community in St. Lucia, were examined by field studies based on worm expulsion techniques. Horizontal age-intensity profiles were convex in form with peak parasite loads occurring in the 2 to 15-year-old children. Prevalence is shown to be a poor indicator of changes in average worm load with age. Faecal egg counts (epg and epd) provide a qualitative measure of worm burdens since fecundity is shown to be approximately independent of worm load. The parasites were highly aggregated within the study community, with most people harbouring low burdens while a few individuals harboured very heavy burdens. Of the total parasite populations in the study sample, 84% were harboured by the 2 to 15-year-old children. Of those individuals harbouring 100 worms or more, 87% were in the 2 to 10-year-old age range. Crude estimates of population parameters (basic reproductive rate, 4-5; rate of reinfection, 90 year-1) suggest that the rate of reinfection is higher than for other helminth parasites of man. The control of morbidity and parasite transmission is discussed in the context of targeting drug treatment at the child segment of the study population.

Adolescent↗
PubMed1987

Evidence for predisposition of individual patients to reinfection with Schistosoma mansoni after treatment.

Statistical analysis of the relationship between intensities of infection before treatment and during reinfection after treatment in a sample of 119 Kenyan schoolchildren demonstrated a positive association, indicating that the individuals differed consistently in their tendency to become infected. This association was stronger in young children but the trend was detectable in older individuals. Possible reasons for this variation and for its apparently greater influence in younger age groups are discussed.

Adolescent↗
PubMed1987

Evidence for predisposition in humans to infection with Ascaris, hookworm, Enterobius and Trichuris in a South Indian fishing community.

Studies of patterns of reinfection with four species of intestinal nematodes (Ascaris, hookworm, Trichuris and Enterobius) in 174 individual patients following chemotherapeutic treatment revealed statistical evidence for predisposition to heavy or light infection (relative to the average level in the overall population). Analyses of associations between the abundances of the four species of nematodes within a combined sample of 525 worm burdens showed significant correlations between 5 out of the 6 possible pair-wise comparisons between species. The relevance of these results to the design of control programmes based on chemotherapeutic application is discussed.

Adolescent↗
PubMed1987

The distribution and abundance of Enterobius vermicularis in a South Indian fishing community.

The distribution and abundance of Enterobius vermicularis in a fishing community in South India, as determined by counting worms expelled following mass anthelmintic chemotherapy, was examined in samples of patients stratified by age, sex and family grouping. The results of a worm expulsion study in January 1984 are compared with those of a second expulsion programme in November 1984, following an 11-month period of reinfection. The prevalence of Enterobius infection was consistently high in all age groups of both males and females. A comparison of the overall prevalence and intensity of infection in the January and November surveys revealed significant increases in both the percent infected and the mean number of worms harboured in November relative to the initial level. No significant trends in the intensity of Enterobius according to host age or sex were detected in either survey. The frequency distribution of Enterobius was found to be highly aggregated in the population as a whole and within age groups. At both sampling dates, the most heavily infected 25% of the community harboured over 90% of the total pinworms recovered. There was a significant pattern within most age groups for individuals to re-acquire worm burdens of a similar magnitude to their initial pre-treatment burdens. Heavy infections were found to be associated with household. A selective or targetted approach to treatment of heavily infected individuals or households is likely to be an efficient means of reducing parasite abundance on a community-wide basis. It must be stressed, however, that treatment must be applied at frequent intervals in order to achieve a long-term reduction in pinworm abundance and to avoid the possibility of increasing the intensity of infection, as was observed in this community.

Adolescent↗
PubMed1987

The regulation of Ostertagia ostertagi populations in calves: the effect of past and current experience of infection on proportional establishment and parasite survival.

A mathematical model of the parasitic phase of the life-cycle of Ostertagia ostertagi in calves is described. The model is used in the re-analysis of previously published data from a long-term trickle infection experiment in which groups of calves were infected daily with graded doses of 3rd-stage (L3) larvae. The results of the analysis are consistent with the hypothesis that the observed changes in the intensity of infection in the calves were the result of a decline in the proportional establishment of ingested L3 larvae, and a rise in the death rate of the 5th-stage worms as the duration of exposure to infection increased. The proportion of ingested L3 larvae that become established in the mucosa can be described as an exponential decay function of the duration of the infection. Within the range of trickle intensities investigated, the function appears to vary independently of the level of exposure to infection. In contrast, the rate of mortality of the 5th-stage worms appears to be an increasing linear function of the total cumulative number of 3rd-stage larvae administered.

Animals↗