Biomedical subjects

M J Nauta

Publications and source records attributed to M J Nauta.

8 recordsLinked to original sources

PubMed2004

Disease burden in The Netherlands due to infections with Shiga toxin-producing Escherichia coli O157.

Surveys carried out between 1990 and 2000 indicated that the incidence of STEC O157-associated gastroenteritis in The Netherlands was 1250 cases/year (median), of which 180 visited a general practitioner, 40 are reported and 0.6 are fatal, mainly in the elderly. There are approximately 20 cases of STEC O157-associated haemolytic-uraemic syndrome (HUS) per year, mainly in children. There are 2.5 HUS patients per year who develop end-stage renal disease (ESRD). There are an estimated 2 HUS-related and 0.5 ESRD-related fatalities per year. The mean disease burden associated with STEC O157 in the Dutch population is 116 (90% confidence interval 85-160) Disability Adjusted Life Years (DALYs) per year. Mortality due to HUS (58 DALYs), and ESRD (21 DALYs) and dialysis due to ESRD (21 DALYs) constitute the main determinants of disease burden. Sensitivity analysis indicates that uncertainty associated with model assumptions did not have a major effect on these estimates.

Adolescent↗

PubMed2001

Estimation of animal-level prevalence from pooled samples in animal production.

Often, the prevalence of an infection in the animal-production sector is determined at the group level. The prevalence at animal level (p) gives more-precise information on the infection status of the sector. This paper shows that pooled-sample data together with mathematical models allow for estimation of p. For this, model assumptions have to be made on the variation of p between groups separated in space and/or time. Formulas were derived for four models that were based on different assumptions. Model 1 assumed that p has the same value for all groups. Models 2-4 assumed that some of the groups were not infected. In addition, model 2 assumed that p has the same value for all infected groups; model 3 assumed that for an infected group, p is equal to either p(1) or p(2); and model 4 assumed that p was Beta distributed among infected groups. The models were applied to data sets on Salmonella infection in broiler flocks, including serotype data dominated by S. Hadar and S. Paratyphi B, var. Java. Based on likelihood-ratio tests, models 3 and 4 consistently fitted significantly better to the data. The applicability of model 4 is numerically bounded, related to the shape of the Beta distribution of p. Model calculations show that flock-level prevalence of Salmonella is much higher after than before slaughter. This difference (which possibly is related to different types of samples) is much smaller at the animal level. An important result of the estimation of p is that it in turn allows for an estimation of the proportion of false-negative groups--which is important in estimating the effect of veterinary or public-health measures.

Animals↗

PubMed2000

A model for evaluating intervention strategies to control salmonella in the poultry meat production chain.

A model of the transmission of salmonella through the poultry meat production chain is developed, to predict the effects of intervention strategies for salmonella control. The model first describes the situation before intervention in terms of salmonella prevalences at flock level and some transmission parameters. After single control measures are translated into effects on these transmission parameters, the effects of sets of control measures (intervention strategies), can be calculated with the model. As research data are lacking, the model input parameters were derived from expert opinion. As an example, the effects of two intervention strategies proposed for the Dutch poultry industry are predicted. A sensitivity analysis is performed to indicate where the most effective control measures may be expected. Additionally, the reliability of the model predictions is studied by an uncertainty analysis. The use of the model as a tool for policy makers deciding about salmonella control strategies is discussed.

Animal Husbandry↗

PubMed1998

Human exposure to Mycobacterium paratuberculosis via pasteurised milk: a modelling approach.

Paratuberculosis is a disease of cattle caused by infection with Mycobacterium paratuberculosis, and it has been suggested that this bacterium may also play a role in the aetiology of Crohn's disease in humans. M paratuberculosis is shed in the milk and may be able to survive pasteurisation. Therefore, people may be exposed to it by the consumption of pasteurised milk. The risk of such exposure has been analysed using a modelling approach and the model has been used to evaluate the effects of intervention measures at different points in the potential route of transmission. On the basis of data from the literature and expert opinion, an initial point estimate of the exposure level of about 0-5 cfu/litre pasteurised milk was derived, mainly due to milk from clinically affected animals. The model indicates the need for quantitative data on variations in the shedding rates of M paratuberculosis in faeces and milk, and the levels of faecal contamination of milk. Such data are essential for a proper analysis of potential exposure, and may result in a 100-fold increase in the estimated median level of exposure.

Animals↗

PubMed1996

Constraints on allele size at microsatellite loci: implications for genetic differentiation.

Microsatellites are promising genetic markers for studying the demographic structure and phylogenetic history of populations. We present theoretical arguments indicating that the usefulness of microsatellite data for these purposes may be limited to a short time perspective and to relatively small populations. The evolution of selectively neutral markers is governed by the interaction of mutation and random genetic drift. Mutation pressure has the inherent tendency to shift different populations to the same distribution of alleles. Hence, mutation pressure is a homogenizing force, and population divergence is caused by random genetic drift. In case of allozymes or sequence data, the diversifying effect of drift is typically orders of magnitude larger than the homogenizing effect of mutation pressure. By a simple model, we demonstrate that the situation may be different for microsatellites where mutation rates are high and the range of alleles is limited. With the help of computer simulations, we investigate to what extent genetic distance measures applied to microsatellite data can nevertheless yield useful estimators for phylogenetic relationships or demographic parameters. We show that predictions based on microsatellite data are quite reliable in small populations, but that already in moderately sized populations the danger of misinterpretation is substantial.

Alleles↗

PubMed1993

Evolutionary dynamics of spore killers.

Spore killing in ascomycetes is a special form of segregation distortion. When a strain with the Killer genotype is crossed to a Sensitive type, spore killing is expressed by asci with only half the number of ascospores as usual, all surviving ascospores being of the Killer type. Using population genetic modeling, this paper explores conditions for invasion of Spore killers and for polymorphism of Killers, Sensitives and Resistants (which neither kill, nor get killed), as found in natural populations. The models show that a population with only Killers and Sensitives can never be stable. The invasion of Killers and stable polymorphism only occur if Killers have some additional advantage during the process of spore killing. This may be due to the effects of local sib competition or some kind of "heterozygous" advantage in the stage of ascospore formation or in the short diploid stage of the life cycle. This form of segregation distortion appears to be essentially different from other, well-investigated forms, and more field data are needed for a better understanding of spore killing.

Biological Evolution↗

PubMed1992

Evolution of reproductive systems in filamentous ascomycetes. II. Evolution of hermaphroditism and other reproductive strategies.

The evolution of different reproductive systems in filamentous ascomycetes is studied in a population genetic model. These fungi differ essentially from higher plants and animals because mating types can exist in addition to male and female gametes, and the conidia serve as both male gametes and asexual spores; moreover, selfing is genetically equivalent to asexual reproduction in these haploid organisms. A variable fitness of ascospore production is predicted as the explanation for the evolution of two systems that abundantly exist in nature: hermaphroditism in heterothallic species and the formation of both asexual and sexual spores in homothallic species. Imperfect fungi will evolve if sexual spores do not show a remarkably higher fitness than asexual spores.

Ascomycota↗

PubMed1987

Family distances and human lymphocyte antigens.

We compared family distances of homozygotes and heterozygotes for HLA-A and -B. When matched on number of inhabitants per birthplace, no significant differences were found. However, when homozygotes were compared with heterozygotes from larger birthplaces, homozygotes showed significantly smaller family distances in the grandparental generation. We suggest that matching for population size of birthplace and the choice of the geographic study area are important factors in studies of family distances.

Gene Frequency↗