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At least 235 records · Page 13Linked to original sources
PubMed2003

Quantifying BSE control by calculating the basic reproduction ratio R0 for the infection among cattle.

The safety of using meat and bone meal (MBM) in mammal feed was studied in view of BSE, by quantifying the risk of BSE transmission through different infection routes. This risk is embodied in the basic reproduction ratio R(0) of the infection, i.e. the average number of new infections induced by one initial infection. Only when R(0) is below 1, will the disease die out with certainty and the population will become free from BSE. Unfortunately this is a slow process due to the slow progression of the disease. We calculate R(0) explicitly from basic ingredients taking several different transmission routes into account. Several of the basic ingredients are functions of age or of infection-age. We also calculate the exponential growth rate r in terms of the same basic ingredients. Next we quantify the ingredients from available data and compute the effects on R(0) of various scenario's for controlling BSE, with examples for the UK and the Netherlands.

Age Factors↗
PubMed2003

[Scheme for dispersion analysis for evaluating heritability of reproductive traits in medicinal leech Hirudo medicinalis L].

ANOVA designs for estimating heritabilities of fecundity traits in Hirudo medicinalis including batch size, the number of juveniles per cocoon, and juvenile weight. Accounting for reproduction mode of this species, different types of kinship were identified, which were taken into account in the ANOVA designs: juveniles from one batch and one cocoon were considered respectively full sibs and polyzygotic twins. Variation components were analyzed in the following kinship groups: for batch size, in full sibs; for the number of juveniles per cocoon, between sibships, full sins, and replicates; and for juvenile weight, in full sibs and polyzygotic twins. Using these designs, heritabilities of basic reproductive traits of H. medicinalis were obtained: for batch size, h2 = 0.35-0.40, H2 = 0.40-0.45; for the number of juveniles per cocoon, h2 = 0.33-0.36, H2 = 0.36-0.39; and for juvenile weight, h2 = 0.40-0.44, H2 = 0.44-0.48.

Analysis of Variance↗
PubMed2000

An explicit approach to evolutionarily stable dispersal strategies: no cost of dispersal.

The evolution of dispersal is examined by looking at evolutionarily stable strategies (ESS) for dispersal parameters in discrete time multisite models without any cost of dispersal. ESS are investigated analytically, based on explicit results on sensitivity analysis of matrix models. The basic model considers an arbitrary number of sites and a single age class. An ESS for dispersal parameters is obtained when the spatial reproductive values, calculated at the density-dependent population equilibrium, are equal across sites. From this basic formulation, one derives equivalently that all local populations should be at equilibrium in the absence of migration, and that dispersal between sites should be balanced, i.e., the numbers of individuals arriving to and leaving a site are equal. These results are then generalized to a model with several age classes. Equal age-specific reproductive values do not however imply balanced dispersal in this case. Our results generalize to any number of sites and age classes those available ¿M. Doebeli, Dispersal and dynamics, Theoret. Popul. 47 (1995) 82 for two sites and one age class.

Age Distribution↗
PubMed1990

Loglinear models, sexual behavior and HIV: epidemiological implications of heterosexual transmission.

An analysis is presented of sexual behaviour data from a "random" sample (n = 780) of the adult population of England and Wales. Sex stratified frequency distributions of the reported number of sex partners/time were analysed for heterosexuals, demographic characteristics associated with the number of sex partners/time were identified, and epidemiological parameters (the basic reproductive rate of HIV and the doubling time of the epidemic) were calculated. These analyses suggest that the size of the group at risk for acquiring the virus by heterosexual transmission may be large and that the age of first sexual intercourse (for males and females) is decreasing in younger cohorts. Members of the potential heterosexual at-risk group may be identified by demographic variables such as marital status (males and females) and age (females only), but not by socioeconomic class. The epidemiological implications of our results for the heterosexual transmission of HIV are discussed.

Adolescent↗
PubMed2006

Salmonella Dublin infection in young dairy calves: transmission parameters estimated from field data and an SIR-model.

In this study we used field data collected from October 2001 to January 2002 to estimate number of days of faecal excretion of Salmonella Dublin bacteria and time to seroconversion in infected calves below the age of 180 days. Based on these estimates all calves in four endemically infected dairy herds were grouped into the following infection states: susceptible (S), infectious (I) and resistant/recovered (R). Resistant calves had either acquired maternal antibodies through colostrum or they have recovered from previous infection and had a high level of antibodies directed against Salmonella Dublin possibly protecting them from becoming infected again until the level of antibodies had decreased to sufficiently low levels. Using the antibody measurements and faecal excretion periods, it was possible to assign the most likely infection state to each calf per week of the study period. Estimates of transmission parameter, beta, were obtained from a generalised linear model relating the number of new infections to the proportion of susceptible and infectious calves per week. From beta, the reproduction ratio R at steady state and the basic reproduction ratio R(0) were estimated for each herd and across herds. The R(0) denotes the average number of new infections caused by one infectious individual that is introduced to a fully susceptible population. The point estimates for R(0) ranged from 1.1 to 2.7 in the study herds. However, the confidence intervals were wide. Data were too limited to show possible significant differences in the parameters between the study herds. However, the tendency in the data suggested that there may be important differences. Across herds the R(0) was close to two suggesting that on average one infectious calf will produce two new infectious calves when introduced into a fully susceptible population under typical Danish dairy production systems. Further, the analyses indicated that environmental contamination from infectious calves plays an important role in transmitting Salmonella Dublin between calves.

Animals↗
PubMed2003

Analyzing bioterror response logistics: the case of smallpox.

To evaluate existing and alternative proposals for emergency response to a deliberate smallpox attack, we embed the key operational features of such interventions into a smallpox disease transmission model. We use probabilistic reasoning within an otherwise deterministic epidemic framework to model the 'race to trace', i.e., attempting to trace (via the infector) and vaccinate an infected person while (s)he is still vaccine-sensitive. Our model explicitly incorporates a tracing/vaccination queue, and hence can be used as a capacity planning tool. An approximate analysis of this large (16 ODE) system yields closed-form estimates for the total number of deaths and the maximum queue length. The former estimate delineates the efficacy (i.e., accuracy) and efficiency (i.e., speed) of contact tracing, while the latter estimate reveals how congestion makes the race to trace more difficult to win, thereby causing more deaths. A probabilistic analysis is also used to find an approximate closed-form expression for the total number of deaths under mass vaccination, in terms of both the basic reproductive ratio and the vaccination capacity. We also derive approximate thresholds for initially controlling the epidemic for more general interventions that include imperfect vaccination and quarantine.

Bioterrorism↗
PubMed1983

Reproductive suppression among female mammals: implications for biomedicine and sexual selection theory.

Female mammals experience a very high and often unappreciated rate of reproductive failure. Among human pregnancies alone, over 50 per cent fail between conception and parturition, and the majority of these failures are unexplained. These findings present important problems for evolutionary theory as well as for health care practices. This paper addresses these high rates of reproductive failure among mammals, by extending the work of a number of evolutionary biologists regarding the reproductive consequences of environmental adversity. The basic model upon which we elaborate, termed the Reproductive Suppression Model, argues that females can optimize their lifetime reproductive success by suppressing reproduction when future conditions for the survival of offspring are likely to be sufficiently better than present ones as to exceed the costs of the suppression itself. These costs are a function of reproductive time lost and the direct phenotypic effects of the suppression itself. To evaluate the benefits and costs of suppression, the following types of cues should be assessed: the female's physical and mental health, her stage of reproduction, the physical and genetic status of her offspring, and the external conditions at the time of birth. We also examine various issues of social suppression, whereby the conditions for survival of offspring are a function of the reproduction and support of other group members. Under such conditions, some females may be able to improve current conditions for reproduction by suppressing the reproduction of others. Field data from our own work are presented, describing socially mediated reproductive competition among continuously breeding female yellow baboons and among female hoary marmots. Social suppression in other mammals is also evaluated, including that in human beings, and we conclude with some implications of the Reproductive Suppression Model for sexual selection theory regarding female-female reproductive competition, as well as human health care.

Abortion, Spontaneous↗
PubMed2000

NIH Funding and Career Development Opportunities for Women in Reproductive Immunology.

The National Institutes of Health (NIH) has a number of mechanisms available to support research, research training and research career development. The various institutes, centers and special offices within the NIH have, as part of their missions, a diverse array of basic and clinical research goals. Much of the scientific spectrum of reproductive immunology, however, falls within the mission of the National Institute of Child Health and Human Development (NICHD). There have been a number of recent innovations in the preparation, review and funding of NIH grant applications. Specific opportunities, such as fellowships, career awards, and new investigator status, are available for investigators at different career stages. A wealth of information is available on the NIH website at www.nih.gov. NIH staff are available to answer your questions and to provide administrative guidance in preparing grant applications.

Female↗
PubMed2006

The effect of contact heterogeneity and multiple routes of transmission on final epidemic size.

Heterogeneity in the number of potentially infectious contacts amongst members of a population increases the basic reproduction ratio (R(0)) and markedly alters disease dynamics compared to traditional mean-field models. Most models describing transmission on contact networks only account for one specific route of transmission. However, for many infectious diseases multiple routes of transmission exist. The model presented here captures transmission through a well defined network of contacts, complemented by mean-field type transmission amongst the nodes of the network that accounts for alternative routes of transmission. The impact of these combined transmission mechanisms on the final epidemic size is investigated analytically. The analytic predictions for the purely mean-field case and the transmission through the network-only case are confirmed by individual-based network simulations. There is a critical transmission potential above which an increased contribution of the mean-field type transmission increases the final epidemic size while an increased contribution of the transmission through the network decreases it. Below the critical transmission potential the opposite effect is observed.

Communicable Diseases↗
PubMed2004

Can the clock be turned back on ovarian aging?

A basic tenet of reproductive biology is that female mammals are born with a set number of germ cells (oocytes). A new study by Johnson et al. challenges these beliefs by demonstrating that the postnatal mouse ovary contains actively dividing germ cells. These findings have implications for prolonging the reproductive life span of women.

Aging↗
PubMed1997

Viral dynamics of primary viremia and antiretroviral therapy in simian immunodeficiency virus infection.

Mathematical modeling of viral replication dynamics, based on sequential measurements of levels of virion-associated RNA in plasma during antiretroviral treatment, has led to fundamental new insights into human immunodeficiency virus type 1 pathogenesis. We took advantage of the simian immunodeficiency virus (SIV)-infected macaque model to perform detailed measurements and mathematical modeling during primary infection and during treatment of established infection with the antiretroviral drug (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA). The calculated clearance half-life for productively infected cells during resolution of the peak viremia of primary infection was on the order of 1 day, with slightly shorter clearance half-lives calculated during PMPA treatment. Viral reproduction rates upon discontinuation of PMPA treatment after 2 weeks were approximately twofold greater than those obtained just prior to initiation of treatment in the same animals, likely reflecting accumulation of susceptible target cells during treatment. The basic reproductive ratio (R0) for the spread of SIV infection in vivo, which represents the number of productively infected cells derived from each productively infected cell at the beginning of infection, was also estimated. This parameter quantifies the extent to which antiviral therapy or vaccination must limit the initial spread of virus to prevent establishment of chronic disseminated infection. The results thus provide an important guide for efforts to develop vaccines against SIV and, by extension, human immunodeficiency virus.

AIDS Vaccines↗
PubMed1996

Mathematical modeling to estimate efficacy of postmilking teat disinfection in split-udder trials of dairy cows.

A mathematical model was used to estimate the efficacy of postmilking teat disinfection from observations in split-udder trials with natural exposure. Data were studied from an outbreak of Staphylococcus aureus IMI during a split-udder trial in a commercial herd with low SCC. The efficacy of postmilking teat disinfection was similar when calculated based on incidence density rates or on transmission rates of IMI in dipped and control quarters. If, however, first and subsequent S. aureus IMI in a cow were not assumed to be independent and were therefore treated separately in the models, the efficacy of post-milking teat disinfection was calculated as being higher with the modeling procedure. The analysis using mathematical modeling, which includes the effect of the number of existing IMI on the number of new IMI, is presented and discussed. This analysis also allows estimation of the basic reproduction ratio. The impact of postmilking teat disinfection on transmission of pathogens is quantified, and proposals for additional preventive measures can be generated. We concluded that efficacy estimations from split-udder trials, assuming quarters to be independent observations, might underestimate the effect of postmilking teat disinfectants on udder pathogens.

Animals↗
PubMed1991

The population dynamics of nematode infections of ruminants: periodic perturbations as a model for management.

We propose and analyse a model for the dynamics of directly transmitted nematode infections of ruminants which allows for the long-term effects of the annual removal of hosts. The model is a simple continuous-time formulation which captures the principal features of parasite transmission and the acquisition of immunity to infection by the host. Analysis of the simplest version of the model, which assumes a constant host population through time, indicates that its equilibrium is locally stable for feasible biological parameters. The regular removal of hosts involved in most management strategies is modelled in terms of periodic perturbations. The analysis indicates that the periodic removal of parasitized hosts corresponds effectively to a reduction in the basic reproductive rate of infection. We also demonstrate that the system exhibits a dramatic peak in parasite numbers during each grazing season at its dynamic equilibrium. This phenomenon (which is characteristically observed in real systems) is discussed in terms of the balance between acquired immunity and periodic perturbations of the parasite population.

Animals↗
PubMed1988

The population biology of parasite-induced changes in host behavior.

The ability of parasites to change the behavior of infected hosts has been documented and reviewed by a number of different authors (Holmes and Bethel, 1972; Moore, 1984a). This review attempts to quantify the population dynamic consequences of this behavior by developing simple mathematical models for the most frequently recorded of such parasite life cycles. Although changes in the behavior of infected hosts do occur for pathogens with direct life cycles, they are most commonly recorded in the intermediate hosts of parasites with complex life cycles. All the changes in host behavior serve to increase rates of transmission of the parasites between hosts. In the simplest case the changes in behavior increase rates of contact between infected and susceptible conspecific hosts, whereas in the more complex cases fairly sophisticated manipulations of the host's behavioral repertory are achieved. Three topics are dealt with in some detail: (1) the behavior of the insect vectors of such diseases as malaria and trypanosomiasis; (2) the intermediate hosts of helminths whose behavior is affected in such a way as to make them more susceptible to predation by the definitive host in the life cycle; and (3) the behavior and fecundity of molluscs infected with asexually reproducing parasitic flatworms. In each case an expression is derived for R0, the basic reproductive rate of the parasite when first introduced into the population. This is used to determine the threshold numbers of definitive and intermediate hosts needed to maintain a population of the pathogen. In all cases, parasite-induced changes in host behavior tend to increase R0 and reduce the threshold number of hosts required to sustain the infection. The population dynamics of the interaction between parasites and their hosts are then explored using phase plane analyses. This suggests that both the parasite and intermediate host populations may show oscillatory patterns of abundance. When the density of the latter is low, parasite-induced changes in host behavior increase this tendency to oscillate. When intermediate host population densities are high, parasite population density is determined principally by interactions between the parasites and their definitive hosts, and changes in the behavior of intermediate hosts are less important in determining parasite density. Analysis of these models also suggests that both asexual reproduction of the parasite within a host and parasite-induced reduction in host fecundity may be stabilizing mechanisms when they occur in the intermediate hosts of parasite species with indirect life cycles.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Controlling HIV in Africa: effectiveness and cost of an intervention in a high-frequency STD transmitter core group.

Since 1985, a population of over 1,000 predominantly HIV-positive female prostitutes residing in a low-income area of Nairobi, has been enrolled in a sexually transmitted disease (STD)/HIV control programme. The major elements of the programme include the diagnosis and treatment of conventional STD, and the promotion of condom use to prevent the transmission of HIV and other sexually transmitted infections. Using estimates of numbers of HIV-seropositive prostitutes, numbers of sexual contacts, susceptibility of clients to HIV, HIV transmission efficiency, rates of condom use and the basic reproductive rate of HIV infection in Kenya, we estimate that the programme is responsible for preventing between 6,000 and 10,000 new cases of HIV infection per year among clients and contacts of clients. The total annual operating cost of the programme is approximately US$77,000 or between US$8.00 and US$12.00 for each case of HIV infection prevented. Programmes to reduce the transmission of HIV and other sexually transmitted infections which are targeted at high-frequency STD transmitters, such as prostitutes, can be effective and relatively inexpensive to undertake. More such programmes should be developed and evaluated in different settings.

Acquired Immunodeficiency Syndrome↗
PubMed1996

Population dynamics of bovine herpesvirus 1 infection in a dairy herd.

An induced outbreak of a bovine herpesvirus 1 (BHV1) infection in a dairy herd is described. The outbreak was induced by injecting three BHV1 seropositive cows with dexamethasone. Within 7 weeks all seronegative cows had seroconverted. Also some seropositive animals showed a significant increase in serum antibody titre. Using these data, parameters of population dynamics such as R0, the basic reproduction ratio, could be estimated. The basic reproduction ratio is a threshold value describing infection dynamics in a population. This parameter is defined as the average number of secondary cases generated by one primary case in a wholly susceptible population of defined density. In this population R0 was estimated to be at least 7. The importance of these findings, and implications for eradication of BHV1 are discussed.

Animals↗
PubMed2005

Floral gene resources from basal angiosperms for comparative genomics research.

BACKGROUND: The Floral Genome Project was initiated to bridge the genomic gap between the most broadly studied plant model systems. Arabidopsis and rice, although now completely sequenced and under intensive comparative genomic investigation, are separated by at least 125 million years of evolutionary time, and cannot in isolation provide a comprehensive perspective on structural and functional aspects of flowering plant genome dynamics. Here we discuss new genomic resources available to the scientific community, comprising cDNA libraries and Expressed Sequence Tag (EST) sequences for a suite of phylogenetically basal angiosperms specifically selected to bridge the evolutionary gaps between model plants and provide insights into gene content and genome structure in the earliest flowering plants. RESULTS: Random sequencing of cDNAs from representatives of phylogenetically important eudicot, non-grass monocot, and gymnosperm lineages has so far (as of 12/1/04) generated 70,514 ESTs and 48,170 assembled unigenes. Efficient sorting of EST sequences into putative gene families based on whole Arabidopsis/rice proteome comparison has permitted ready identification of cDNA clones for finished sequencing. Preliminarily, (i) proportions of functional categories among sequenced floral genes seem representative of the entire Arabidopsis transcriptome, (ii) many known floral gene homologues have been captured, and (iii) phylogenetic analyses of ESTs are providing new insights into the process of gene family evolution in relation to the origin and diversification of the angiosperms. CONCLUSION: Initial comparisons illustrate the utility of the EST data sets toward discovery of the basic floral transcriptome. These first findings also afford the opportunity to address a number of conspicuous evolutionary genomic questions, including reproductive organ transcriptome overlap between angiosperms and gymnosperms, genome-wide duplication history, lineage-specific gene duplication and functional divergence, and analyses of adaptive molecular evolution. Since not all genes in the floral transcriptome will be associated with flowering, these EST resources will also be of interest to plant scientists working on other functions, such as photosynthesis, signal transduction, and metabolic pathways.

Biodiversity↗