The over-dispersion between cells of chromosomal aberrations.
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Chromosome Aberrations↗
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The dynamics of infection of Tribolium confusum by Hymenolepis diminuta: the influence of infective-stage density and spatial distribution.
The mean parasite burden of a population of Tribolium confusum is shown to rise to a plateau as the exposure density of infective eggs of Hymenolepis diminuta increases. The level of this plateau is shown to be dependent on the nutritional status of the host population, being depressed from approximately 18 cysticeroids/beetle in hosts which have been starved prior to experimentation, to approximately 2 cysticercoids/beetle in satiated hosts. A simple model is used to describe the shape of this infection functional response in terms of the predator-prey interaction between hosts (T. confusum) and parasite infective stages (H. diminuta eggs). The distribution of successful infections/host is shown to be over-dispersed, even when hosts are exposed to infective stages arranged in a uniform spatial pattern. The over-dispersion of parasite numbers/host is shown to become more severe as the spatial pattern of infective stages changes from under-dispersed, through random, to over-dispersed. Experimental results are discussed in relation to the dynamics of parasite-host interactions, in which infection takes place by host ingestion of a free-living infective stage.
Feeding Behavior↗
Covariance in parasite burdens: the effect of predisposition to infection.
Recently the phenomenon of predisposition to helminth infection has been reported in a number of studies: those individuals which are heavily infected before treatment with an anthelmintic tend also to acquire heavy parasite burdens following a period of reinfection. This correlation between parasite burdens in initial and reinfections is generated by differences between hosts in their exposure to infective stages and in their susceptibility to infection. Inter-host differences in these factors also generate the aggregated or over-dispersed parasite distributions that are usually observed. This paper uses probability theory to predict the correlation between initial and reinfection parasite burdens assuming that those inter-host differences which generate over-dispersion remain constant for a given individual between initial and reinfection periods. The predicted correlation is considerably greater than is observed in most published data sets. Over-dispersion is thus generated by variability between hosts which has components that remain constant between initial and reinfection and also components which vary between infection periods. The model is modified to account for those two sources of variability, and the result applied to published data to determine the contributions made by short and long-term factors to the observed distributions.
Analysis of Variance↗
Frequency distribution of Brugia malayi microfilariae in human populations.
This study examines the effects of host age and sex on the frequency distribution of Brugia malayi infections in the human host. Microfilarial (mf) counts for a large data base on the epidemiology of brugian filariasis in Shertallai, Kerala, South India are analysed. Frequency distributions of microfilarial counts partitioned by age are successfully described by zero-truncated negative binomial distributions, fitted by maximum likelihood. This analysis provides estimates of the proportion of mf-positive individuals who are identified as negative due to sampling errors, allowing the construction of corrected mf age-prevalence curves, which indicate that the observed prevalence may under-estimate the true figures by between 18 and 47%. There is no evidence from these results for a decrease in the degree of over-dispersion of parasite frequency distributions with host age, such as might be produced by the acquired immunity to infection. This departure from the pattern in bancroftian filariasis (where there is evidence of such decreases in over-dispersion; Das et al. 1990) is discussed in terms of the long history of filariasis control (and consequently low infection prevalence) in Shertallai.
Adolescent↗
A divide and conquer strategy for recapitulating whole genome 3D structure using Hi-C data.
The three dimensional (3D) spatial organization of the genome is closely linked to biological functions and can be captured by Hi-C assays through interrogating genome-wide chromatin interactions. Methodologies for inferring 3D structures from Hi-C data summarized as a two-dimensional (2D) contact matrix can be broadly placed within the paradigms of optimization-based and sampling-based. Many optimization-based methods are capable of constructing whole genome 3D structures but do not account for spatial dependency in the 2D data matrix nor cell heterogeneity in bulk Hi-C data, which provide an average over millions of cells. Sampling-based methods, on the other hand, are probabilistic model-based and can account for not only dependency, heterogeneity, but also other features inherent in Hi-C data, such as over-dispersion and sparsity. However, whole-genome 3D structure recapitulation is too computationally expensive for sampling-based methods, while chromosome-by-chromosome strategies for sampling-based methods ignore important information on inter-chromosomal contacts. To address these issues, we propose the truncated Random effect EXpression-cut and paste (tREX-cap) method, which applies the tREX model within a divide and conquer strategy. The resulting method inherits the good data-feature-cognizant properties of tREX and, in the meantime, can efficiently infer the whole genome 3D structure. We demonstrate the performance of tREX-cap through an extensive simulation study and analyses of a Hi-C lymphoblastoid dataset and a Hi-C IMR90 dataset.
Humans↗
Coping with extra Poisson variability in the analysis of factors influencing vaginal ring expulsions.
The vaginal ring is one method of contraception currently under investigation and development by the Human Reproduction Programme of WHO. One factor that reduces its acceptability is that some women experience involuntary expulsions of the ring. WHO have collected prospective data on 1005 women from 19 centres worldwide to investigate various aspects of ring use including factors that might affect expulsions. For each subject the data include number of expulsions, age, parity, ponderal index, number of days of ring use and health care centre. To model the factors influencing expulsion, the number of expulsions was first considered as a Poisson variate with mean directly proportional to the log of the duration of ring use. Such a model gave a poor description of the data and it appeared that the number of expulsions was considerably over-dispersed for an assumed Poisson variate. This paper examines a variety of strategies for coping with the resulting lack of fit of the model. It turns out that a truncated Poisson model gives a better description of the expulsion data and that health care centre and parity influence the rates.
Contraceptive Devices, Female↗
The cytogenetic effect of bleomycin on human peripheral lymphocytes in vitro and in vivo.
The cytogenetic effect of bleomycin (BLM) in human lymphocytes was studied after exposure to different doses during the G0 and G2 phases. BLM produced a marked specific effect on the cell cycle. The main aberration types after exposure in tg0 were dicentrics and deletions; and after exposure in G2, open chromatid breaks. A linear dose--response was calculated for all these aberration types as well as for the number of aberrant cells. In the G2 experiments, partially and totally pulverized cells also increased linearly with dose. The intercellular distributions of the most frequent aberration types after exposure in G0 and G2--the dicentrics and chromatid breaks, respectively--showed over-dispersion. These results show that the cytogenetic effect of BLM may be compared with that of densely ionizing irradiation. Preliminary results of chromosome analysis of three cancer patients in the course of BLM therapy showed effects similar to those in the G0 experiments.
Bleomycin↗
Epidemiology of hydatid disease in Sardinia: a study of fertility of cysts in sheep.
Hydatidosis, caused by Echinococcus granulosus, is a cyclozoonotic disease of economic significance in Sardinia. The life-cycle involves stray and sheep dogs as definitive hosts and sheep, pigs, goats and cattle as intermediate hosts. The most important intermediate host is sheep, due to home slaughtering with ready access of the viscera to dogs. This survey was undertaken in 1987 to ascertain the epidemiological significance of sheep in maintaining the life-cycle. A total of 700 (91.3%) of 767 sheep harboured hydatid cysts. The frequency distribution of the number of hydatid cysts was over-dispersed. Of 497 infected sheep, 7.6% had fertile cysts, 75.7% sterile cysts and 16.7% fertile + sterile cysts.
Animals↗
Experimental studies of infection dynamics: infection of the definitive host by the cercariae of Transversotrema patialense.
The number of cercariae of Transversotrema patialense which attach to the fish host Brachydanio rerio, during a fixed exposure period, is shown to be directly proportional to cercarial density within an experimental infection arena. The distribution of successful infections/host is shown to change from a random pattern to an over-dispersed form as cercarial exposure density or duration of host exposure to infection increases. A stochastic simulation model is used to demonstrate that small differences in host susceptibility to infection, within a population of hosts, can generate patterns of dispersion in parasite numbers/host similar to those observed in the experimental studies. Differences in host behaviour, during the period of exposure to infection, are thought to generate variability in host susceptibility to cercarial infection.
Animals↗
Population dynamics of snail infection by miracidia.
The essential biological features of snail infection by miracidia are incorporated into a simple model which describes the rate of change with respect to time of the number of miracidial infections/host. The model is based on the assumption that the net rate of infection is directly proportional to the density of both miracidia and hosts. Empirical evidence is provided to support this assumption. The basic framework of the model is expanded to take into account demographic stochasticity in infection and is used to predict the percentage of snails that become infected after exposure to a known number of miracidia for a set period of time. The influence of miracidial mortalities and age-dependent infectivity are examined and theoretical predictions are compared with a range of experimental results. Underlying heterogeneity in the distribution of the number of infections/snail is shown to generate an artifactual decrease in infection rates as exposure density rises, if rate estimation procedures are based on an assumption of randomness. Empirical evidence is presented to illustrate the generation of over-dispersion in the number of miracidial infections/snail under tightly controlled laboratory conditions, using supposedly homogeneous snail populations. Biological causes for underlying patterns of heterogeneity are discussed in relation to snail susceptibility to infection and 'attractiveness' to infective stages.
Animals↗
Distribution patterns of Opisthorchis viverrini within a human community.
This paper describes patterns of infection with Opisthorchis viverrini within a human community assessed by egg count, parasite-specific antibody levels and worm burden determined by expulsion chemotherapy. All three measurements increased significantly with age, reaching a plateau in young adults. There were no significant sex-associated differences in the prevalence or intensity of infection, antibody levels, or in the frequency of heavy worm burdens. A striking 81.5% of the total Opisthorchis population and 74% of the total egg output were expelled by the most heavily infected 10% of the humans sampled. This extreme over-dispersion, together with recent findings that hepatobiliary disease frequently accompanies heavy infections, argues that the priority for community-based parasite control programmes should be to maximize treatment and health promotion efforts toward heavily infected individuals.
Adolescent↗
Heterogeneities in water contact patterns and the epidemiology of Schistosoma haematobium.
Variations in the amount of water contact made by individuals and in the amount of water contact made at different sites may have significant impacts on patterns of human schistosome infection. Previous studies have reported variations in the rate of water contact and differences in the sites used between age/sex classes, but there is limited information on variations in individual water contact behaviour. In this paper we report and analyse observations of essentially all water contacts made over a two week period by all individuals in a rural community in eastern Zimbabwe. The mean rate of water contact was 0.43 contacts/person/day. These data were over-dispersed, ranging from zero to 3.3 contacts/person/day; 90% of contacts were made by only 37% of the population. Contact rates were related to age (highest in 8 to 10-year-olds) but not sex, with substantial variation unaccounted for by these variables. Age and sex classes differed in types of water-related activities and the time of day of contact. A greater diversity of sites was used by children than by adults and by males than by females. Individual contact rates were correlated with intensities of infection, although the risk of infection per contact was estimated to be highest in 2 to 4-year-old children and higher for males than females. Five contact sites were used during the study period, with more than 50% of contacts occurring at just 2 sites. Different age and sex classes used different sites and there were additional site-related differences in types of activity and the time of day of use. The implications of these water contact patterns for schistosome epidemiology are discussed. In particular the results provide strong quantitative support for control programmes aimed at heavily used sites (e.g. focal mollusciciding) or at the minority of individuals making most water contacts (e.g. targeted chemotherapy).
Adolescent↗
Epidemiology and immunology of Necator americanus infection in a community in Papua New Guinea: humoral responses to excretory-secretory and cuticular collagen antigens.
Baseline data from an immuno-epidemiological study of hookworm infection in a rural village in Madang Province, Papua New Guinea are reported. Necator americanus was found to be the commonest helminth infection, with a prevalence of near 100% and intensity of 40 worms per host in adults. Enterobius vermicularis, Ascaris lumbricoides and Trichuris trichiura were also present, at prevalences of 53, 10 and 3% respectively; Ancylostoma duodenale was absent. The frequency distribution of N. americanus was highly over-dispersed, and was well described by a negative binomial distribution with aggregation parameter, k, of 0.370. Intensity of infection was significantly related to host age, but did not differ between the sexes. Haemoglobin levels and haematocrit values were indicative of anaemia in the community, but were unrelated to hookworm infection. Levels of antibodies (IgG, IgA and IgM combined) against adult Necator cuticular collagen and excretory-secretory (ES) products were determined. Serum concentrations of the two types of antibody were significantly correlated with each other. Significant positive correlations were found between anti-ES antibody levels and hookworm egg production, and between anti-collagen antibody levels and host age. It is suggested that the level of anti-collagen antibodies may reflect cumulative exposure to infection, whereas levels of anti-ES antibodies may be more dependent on current worm burden. No evidence was found to suggest that either antibody response is important in regulating parasite population growth. Similarly, the presence of a positive correlation between eosinophil concentration and infection intensity in adults indicates that eosinophilia reflects, rather than determines, the host's worm burden.
Adolescent↗
Long-term dynamics of natural populations of Schistosoma mansoni among Rattus rattus in patchy environment.
Dynamics of natural populations of Schistosoma mansoni were studied during 8 consecutive years among Rattus rattus populations from 8 transmission sites of the marshy forest focus of Guadeloupe (French West Indies). The schistosome population is over-dispersed (k = 0.119) within the murine hosts and ecological factors linked to the patchy environment may be responsible for such aggregated distribution. Analysis of the spatio-temporal variations in prevalences, intensities and abundances showed limited variations of the infection during the 8 years at the level of the whole parasite population but great spatial heterogeneity at the level of local schistosome populations. Inter-populational genetic variability linked to the degree of adaptation of this human parasite to the murine host may explain differences in transmission dynamics between the local populations of S. mansoni.
Animals↗
Genetic linkage disequilibrium of deleterious mutations in threatened mammals.
The impact of negative selection against deleterious mutations in endangered species remains underexplored. Recent studies have measured mutation load by comparing the accumulation of deleterious mutations, however, this method is most effective when comparing within and between populations of phylogenetically closely related species. Here, we introduced new statistics, LDcor, and its standardized form nLDcor, which allows us to detect and compare global linkage disequilibrium of deleterious mutations across species using unphased genotypes. These statistics measure averaged pairwise standardized covariance and standardize mutation differences based on the standard deviation of alleles to reflect selection intensity. We then examined selection strength in the genomes of seven mammals. Tigers exhibited an over-dispersion of deleterious mutations, while gorillas, giant pandas, and golden snub-nosed monkeys displayed negative linkage disequilibrium. Furthermore, the distribution of deleterious mutations in threatened mammals did not reveal consistent trends. Our results indicate that these newly developed statistics could help us understand the genetic burden of threatened species.
Animals↗
Distribution of sibship size in families manifesting a genetically determined disorder.
The effect on sibship size distribution of the birth of a child with a genetical defect is considered for several different conditions. Family size continues to be over-dispersed in such cases, rather than showing any sign of reduced variation, though theoretical expectations about the correlation between numbers of normal and affected children are not well-supported by the data.
Chromosome Aberrations↗
The detection of genetical influences on human family size.
The genetics of human family size is not readily susceptible to analysis. Some of the problems in its investigation are discussed, together with methods for overcoming them. In particular, the problem of obtaining a satisfactory model which will account for both the low heritability of family size and its over-dispersed distribution is examined in detail.
Family↗
Ecological aspects of nematode parasites of introduced salmonids from Valdivia river basin, Chile.
Between 1986 and 1987, 666 fishes distributed among the following species introduced in Chile, and from different sectors of the Valdivia river basin (39 degrees 30'-40 degrees 00'S, 73 degrees 30'-71 degrees 45'W), were examined: 348 Salmo trutta, 242 Salmo gairdneri, 24 Cyprinus carpio and 52 Gambusia affinis holbrooki. The presence of Camallanus corderoi and Contracaecum sp. in S. gairdneri and of C. corderoi in S. trutta is recorded in Chile for the first time. Cyprinus carpio and G. a. holbrooki did not present infections by nematodes. The prevalence and mean intensity of the infections by nematodes presented significant differences among some sectors of the Valdivia river basin. In general, the prevalence and intensity of the infections by C. corderoi were greater than those by Contracaecum sp. The infections in S. gairdneri were higher than in S. trutta. The sex of the hosts had no influence on the prevalence and intensity of the infections by both nematodes. The length of the hosts did have an influence, except in the case of the infections by Contracaecum sp. in S. gairdneri. The infrapopulations of both nematode species showed over-dispersion in most cases. The diet of the examined salmonids suggests that they would become infected principally through the consumption of autochthonous fishes.
Animals↗