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PubMed2002

A markov chain approach to calculate r(0) in stochastic epidemic models.

R(0) has been defined as "The expected number of secondary infections originated by a "typical" infective individual when introduced into a population of susceptibles", and it is perhaps the single most important parameter in epidemic models. A general framework to calculate R(0) that can be applied to complicated stochastic epidemic models that may include demography, several strains, latent or carrier-like states, with or without density-dependent parameters is introduced. This framework helps us to understand the concept of a "typical" infective individual used in the deterministic definition of R(0). The method is illustrated with applications to several epidemic models, including some in which it has been found that the disease may persist even if R(0)<1. It is shown that although the probability of extinction is difficult to calculate in these latter cases, it is possible to give general conditions on the parameters under which eventual extinction is certain.

Algorithms↗
PubMed2002

The role of colony organization on pathogen transmission in social insects.

Social organisms are especially vulnerable to pathogens due to the homogeneity of the colony, and the close proximity and extensive interactions among its members. However, the social organization of these groups also offers the potential to provide an effective barrier against the transmission of pathogens within the colony. Social insects with their elaborate colony organizations provide an ideal model system to develop and test this hypothesis. While the different elements of colony organization are generally assumed to be products of ergonomic selection, in this paper we address how the same elements could influence the transmission of pathogens. By developing a simple model, we explore how three parameters of colony organization, division of labor, interaction network and colony demography could influence the transmission of pathogens. We find that heterogeneity among individuals in terms of division of labor alone has little effect on the spread of an infection in the colony and the scenario is indistinguishable from one in which all the individuals are homogeneous. However, division of labor, combined with heterogeneity in the interaction network and demographic schedules reduce the spread of an infection.

Animals↗
PubMed2002

Soldier production strategy in lower termites: from young instars or old instars?

We constructed a mathematical model to explain from which instars soldiers should be produced to maximize the growth rate of a termite colony. The model is based on the demography of the lower termite's colony: many of them feed inside the nest. The model predicts the following: (1) When the colony is young (it still has enough food and needs a high ratio of soldiers to workers) it will produce soldiers from young instars. (2) When the colony is old (it does not have enough food any more and needs a lower ratio of soldiers to workers) it will produce soldiers from old instars. This prediction fits well with the fact investigated by past empirical researches. With the samples of colonies of Neotermes koshunensis, we measured the antennal joint number of soldiers of each colony. We plotted the antennal joint number (1) of the average and (2) of the maximum one for each colony, against the total number of colony members. For both in each colony, we confirmed the consistency between the model and the samples: as the colony matures, it produces soldiers from older instars. The model also explains why higher termites produce soldiers from fixed instars. Their life style, in which they go outside to collect food, does not require a change in their soldier production strategy.

Aging↗
PubMed2003

Migration frequency and the persistence of host-parasitoid interactions.

This paper analyses the effect of migration frequency on the stability and persistence of a host-parasitoid system in a two-patch environment. The hosts and parasitoids are allowed to move from one patch to the other a certain number of times within a generation. When this number is low, i.e. when the time-scales associated with migration and demography are of the same order, host-parasitoid interactions are usually not persistent. When this number is high, however, persistence is more likely. Moreover, in this situation, aggregation methods can be used to simplify the proposed initial model into an aggregated model describing the dynamics of both the total host and parasitoid populations. Analysis of the aggregated model shows that the system reaches a stable steady state for some regions of the parameter domain. Persistence occurs when the movement of the parasitoids is asymmetrical, i.e. they move preferentially to one of the two patches. We show that the growth rate of the host population is a key parameter in determining which migration strategies of the parasitoids lead to persistent host-parasitoid interactions.

Animals↗
PubMed1996

Demographic Models for Subdivided Populations: The Renewal Equation Approach

Le Bras (Theor. Popul. Biol. 2, 100-121, 1971) and Rogers (Demography 11, 473-481, 1974), in two neglected papers, have generalized to the multisite case the Euler-Lotka renewal equation and demographic characteristics such as age structure and reproductive value. The purpose of this paper is twofold: first, to restate the multisite renewal equation in the matrix context; second, to derive results on age structure, net reproduction rate, generation time, and sensitivities, as generalizations of the one site case. The potential of this approach for population biology is illustrated using a model of a black-headed gull Larus ridibundus population.

Journal Article↗
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↗
PubMed2001

Effects of population size and metapopulation dynamics on a mating-system polymorphism.

The evolutionary dynamics of neutral alleles under the Wright-Fisher model are well understood. Similarly, the effect of population turnover on neutral genetic diversity in a metapopulation has attracted recent attention in theoretical studies. Here we present the results of computer simulations of a simple model that considers the effects of finite population size and metapopulation dynamics on a mating-system polymorphism involving selfing and outcrossing morphs. The details of the model are based on empirical data from dimorphic populations of the annual plant Eichhornia paniculata, but the results are also of relevance to species with density-dependent selfing rates in general. In our model, the prior selfing rate is determined by two alleles segregating at a single diploid locus. After prior selfing occurs, some remaining ovules are selfed through competing self-fertilisation in finite populations as a result of random mating among gametes. Fitness differences between the mating-system morphs were determined by inbreeding depression and pollen discounting in a context-dependent manner. Simulation results showed evidence of frequency dependence in the action of pollen discounting and inbreeding depression in finite populations. In particular, as a result of selfing in outcrossers through random mating among gametes, selfers experienced a "fixation bias" through drift, even when the mating-system locus was selectively neutral. In a metapopulation, high colony turnover generally favoured the fixation of the outcrossing morph, because inbreeding depression reduced opportunities for colony establishment by selfers through seed dispersal. Our results thus demonstrate that population size and metapopulation processes can lead to evolutionary dynamics involving pollen and seed dispersal that are not predicted for large populations with stable demography.

Alleles↗
PubMed2002

The metapopulation dynamics of an infectious disease: tuberculosis in possums.

An SEI metapopulation model is developed for the spread of an infectious agent by migration. The model portrays two age classes on a number of patches connected by migration routes which are used as host animals mature. A feature of this model is that the basic reproduction ratio may be computed directly, using a scheme that separates topography, demography, and epidemiology. We also provide formulas for individual patch basic reproduction numbers and discuss their connection with the basic reproduction ratio for the system. The model is applied to the problem of spatial spread of bovine tuberculosis in a possum population. The temporal dynamics of infection are investigated for some generic networks of migration links, and the basic reproduction ratio is computed-its value is not greatly different from that for a homogeneous model. Three scenarios are considered for the control of bovine tuberculosis in possums where the spatial aspect is shown to be crucial for the design of disease management operations.

Animal Migration↗
PubMed1985

Behavioral effects of alcohol in sons of alcoholics.

This paper describes a search for genetically mediated biological factors that contribute to a predisposition toward alcoholism. Healthy, nonalcoholic, drinking men aged 21 to 25 who have a close alcoholic relative were selected as the family history positive (FHP) or higher risk subjects and matched with lower risk controls (family history negative, or FHN) on demography, drinking and smoking history, and height to weight ratio. Comparisons of this higher and lower risk population revealed no significant differences in selected personality attributes, and an alcohol challenge resulted in identical patterns of blood alcohol over time. However, the FHP men developed higher levels of the first breakdown product of ethanol, acetaldehyde, after drinking, demonstrated a less intense intoxication than the FHN controls despite identical blood alcohol concentrations, and tended to show a lower amplitude of a positive brain wave sent at about 300 msec in an event-related potential paradigm. These results may be clues to help investigators identify genetic markers of a predisposition toward alcoholism.

Adult↗
PubMed1994

Alcohol sensitivity and dependence.

The relationship between the intensity of response to alcohol and the future development of alcohol abuse or dependence was evaluated in 454 18- to 25-year-old drinking but not alcohol dependent subjects. This sample consisted of sons of alcohol dependent fathers and family history negative controls matched on demography as well as alcohol and substance use histories. In response to an alcohol challenge at approximately age 20, 40% of the sons of alcoholics but less than 10% of the controls demonstrated less alcohol-related changes in subjective feelings, physiological measures, and motor performance. An average of 9.3 years later, a successful follow-up of 100% of the first 223 individuals revealed that the diminished response to alcohol was a potent predictor of future alcohol abuse or dependence. Neither the family history of alcoholism nor the intensity of response to alcohol at age 20 predicted drug dependence or major psychiatric disorders during the follow-up period.

Adolescent↗
PubMed1994

Genetic structure of a population with social structure and migration.

Long-tailed macaques (Macaca fascicularis) live in social groups consisting of resident adult females and their offspring, and immigrant males. Subadult males leave their birth group, and might establish themselves as reproducing males in another group. Females do not leave their birth group. Such a social pattern might have consequences for the genetic differentiation between groups and the genetic relationships within groups. In a field study of long-tailed macaques (Macaca fascicularis) in Ketambe, Sumatra, Indonesia, blood samples were taken from individuals in seven adjacent social groups. Electrophoretic analysis showed 17 blood proteins and enzymes to be polymorphic, allowing the computation of heterozygosities and of the F-statistics. Of the F-statistics, F(IS) indicates the deviation from Hardy-Weinberg equilibrium averaged over local populations, FST indicates the differentiation in allele frequency between local populations, and F(IT) indicates the deviation from Hardy-Weinberg equilibrium over the total population. In a computer simulation of the population of long-tailed macaques using many loci with many neutral alleles, F(IS) and FST values proved to be characteristic for a certain demography and life history of the population, and proved not to depend upon the number of alleles or level of heterozygosity. FST values found in the simulation were compatible to those found in the field; in the simulation, values for F(IS) and F(IT) were consistently negative. The explanation for the negative F(IS) appears to be that genetic drift causes differentiation in allele frequencies between groups, and that due to this differentiation, allele frequencies differ between resident females and immigrant males, leading to offspring with an excess of heterozygotes (negative F(IS)) relative to the expectation based upon the overall allele frequency. The excess of heterozygotes might imply that slightly deleterious alleles are protected from selection. A population with a social structure and differential migration of the sexes is liable to accumulate deleterious recessives and, as a consequence, to be very sensitive to inbreeding on disruption of the social structure, as for instance in zoos.

Animals↗
PubMed1987

The genetic consequences of primate social organization: a review of macaques, baboons and vervet monkeys.

Primates, as long-lived, iteroparous, socially complex mammals, offer the opportunity to assess the effects of behavior and demography on genetic structure. Because it is difficult to obtain tissue samples from wild primate populations, research in this area has largely been confined to terrestrial and semi-terrestrial old world monkeys (e.g., rhesus and Japanese macaques, vervets and several subspecies of baboons). However, these species display a multi-male, multi-female social structure commonly found in many other primate and non-primate mammals. Electrophoretic analyses of blood proteins from individually recognized and/or marked wild Himalayan rhesus monkeys, themselves the subject of long-term behavioral and demographic research, have begun to reveal the genetic consequences of such phenomena as social group fission, male-limited dispersion, non-consanguineous mating patterns, and agonistically defined male dominance. Specifically, rhesus social groups, consisting primarily of clusters of maternal relatives, appear to be non-random samples of a population's genotypes and genes. The genetic effects of social group fission are highly dependent on each group's size, demographic structure, and average degree of relatedness. In all cases fission contributes to the degree of intergroup genetic differentiation. Male-limited dispersion appears both to retard genetic differentiation between social groups and to lead to mating patterns that result in an avoidance of consanguinity. Groups, therefore, appear to be genetically outbred. Comparing these results with studies of other free-ranging or wild cercopithecines allows several generalizations: (a) genetic variation seems to be evenly distributed throughout each local population of multi-male social groups; (b) social groups, however, because they contain clusters of relatives, are distinctive in their specific frequencies of genes; (c) the degree of genetic differentiation between a population's social groups, because of the effects of social group fission and non-deterministic forms of male dispersal, is somewhat greater than expected on the basis of migration rates alone; and (d) the asymmetrical pattern of dispersion with respect to sex effectively precludes inbreeding in any one social group or the population as a whole. These observations have important implications for understanding the unusually rapid rates of evolution among the primates.

Animals↗
PubMed1992

An AIDS model with distributed incubation and variable infectiousness: applications to i.v. drug users in Latium, Italy.

An AIDS model with distributed incubation and variable infectiousness is considered and simulated via a second-order numerical method. The method is applied to the HIV epidemic among IV drug users in the Latium region of Italy, using available data on the length of the incubation period before the onset of AIDS, on the infectivity of infected individuals during that period, and on the demography of drug users. The contact rate is adjusted to match the actual number of AIDS cases. The sensitivity of the model to uncertainties in the parameters is finally investigated, by performing several simulations.

Acquired Immunodeficiency Syndrome↗
PubMed1992

Heterozygote deficiency, population substructure and their implications in DNA fingerprinting.

Substructured populations exhibit an overall deficiency of heterozygosity whose proportional magnitude depends on the nature of substructuring, i.e., the number of subpopulations (s), their time of divergence (t) from the ancestral population, and the rate of gene flow amongst them (m). Since apparent heterozygote deficiency could be caused by many factors other than population substructuring, one must examine the nature of substructuring that could produce the observed extent of heterozygote deficiency, in order to infer the substructuring from an observed heterozygote deficiency. Using the equivalence of proportional heterozygote deficiency and the coefficient of gene differentiation (GST), we can generate isolines of GST as functions of s, t (in units of 2Ne generations, Ne being the effective population size) and m. Analytical results suggest that large GST values cannot be reached by substructuring alone, unless the number of subpopulations are large and they remain isolated over a long period of time. Application of the theory to population data on six variable number of tandem repeats (VNTR) loci in US Caucasians and US Blacks demonstrates that the observed heterozygote deficiencies at these loci cannot be explained by substructuring within these populations alone. This is so because such large values of GST (3%-10%) would require an absence of gene exchange between the subpopulations and a divergence time from each other of at least 25,000 years ago, neither of which is compatible with the demography and ethnohistory of US Caucasians and Blacks. In contrast, the inability to detect extreme-sized alleles and/or incomplete resolution of nearly similar-sized alleles following Southern gel electrophoresis could easily explain the observed heterozygote deficiencies. The implications of these results are discussed in the context of the forensic use of DNA-typing data, and justify the employment of population genetic principles in forensic genetics.

Black People↗
PubMed1995

Granisetron (Kytril): a survey of use in clinical practice in Switzerland.

Data on the use of intravenous grainsetron (Kytril) were collected during a surveillance exercise amongst Swiss oncologists. The data were analysed to ascertain how grainsetron was used, and to document the incidence of adverse experiences in a clinical setting. Forty-nine oncologists at 40 Swiss centres were surveyed for their use of granisetron for the prevention and treatment of chemotherapy-induced nausea and vomiting. All were advised to follow the Swiss prescribing instructions for granisetron. They were invited to return data on patient demography, chemotherapy duration, granisetron dosing and adverse experiences. From 285 patients it was deduced that the mean daily dose of granisetron was 1.3 ampoules (3.9 mg) and the median daily dose was 1 ampoule (3 mg). The average number of doses of granisetron per patient per session was 3.8. There were 44 reports of adverse experiences by 34 patients, the most common report being headache. The survey confirmed that the large majority of patients undergoing chemotherapy required only a single dose of granisetron per day, and that the adverse experience profile was good.

Adolescent↗
PubMed1996

Clinical validity of major depression-endogenous subtype in adolescent patients.

This paper explored the validity of the distinction between endogenous and nonendogenous forms of major depression in a clinical sample of adolescents, average age 13.6 years. The criteria used to establish this distinction were features of the disorder itself and the external criteria of demography, family history and psychosocial stressors. Endogenous subjects had a more severe form of depressive disorder than nonendogenous subjects. However, there were no significant differences between the depressed groups on any of the external criteria. The distinction between endogenous and nonendogenous major depression in early and mid-adolescence appears therefore to be quantitative rather than qualitative.

Adolescent↗
PubMed1980

Ethnic specificity in the relative minority use and staffing of community mental health centers.

One of the mechanisms proposed for remedying underuse of community mental health centers by ethnic minorities is the increased employment of these minorities on community mental health center staff. The relative use and staffing rates by minorities were examined separately for four minorities: blacks, Spanish Americans, Asian Americans, and American Indians. There were statistically reliable moderate relationships between relative use and staffing rates for particular minorities, but somewhat lower relationships between relative use by one minority and staffing by other minorities. No general pattern was found for all minorities either for a different relationship between utilization and staffing for males and females or for a sex specificity; rather, different patterns appear for different minorities. Service-area demography appears to have no consistent effect on these relationships.

Adult↗
PubMed1989

Using demographic data for ambulatory health care planning: a dental health practice management model.

The use of sociodemographic data in planning ambulatory health services is discussed and illustrated. Five global indices are identified as important for establishing contours of need within local community areas: social class, population heterogeneity, resident mobility, family organization, and general stress factors. Knowledge of sociodemographic distributions within a given community can serve as an adjunct for rational decision making in planning and placement of ambulatory health care services. It can also establish a means for evaluating whether extant health services reach their intended targets via comparisons to the social demography of patients receiving care in private practices or public clinics. Such analyses are germane to ambulatory health care practitioners in both the public and private sector.

Ambulatory Care↗