Biomedical subjects
Publications and source records attributed to Pierre Auger.
A model illustrating Darwin's divergence principle is presented. It shows how competition for resources can explain evolutionary branching. It is based on an assumption of degenerate competition. Mathematically, it is a partial differential equation with an integral term, and it describes a new mechanism of self-organization.
We consider a host-parasitoid system with individuals moving on a square grid of patches. We study the effects of increasing movement frequency of hosts and parasitoids on the spatial dynamics of the system. We show that there exists a threshold value of movement frequency above which spatial synchrony occurs and the dynamics of the system can be described by an aggregated model governing the total population densities on the grid. Numerical simulations show that this threshold value is usually small. This allows using the aggregated model to make valid predictions about global host-parasitoid spatial dynamics.
This work presents a predator-prey Lotka-Volterra model in a two patch environment. The model is a set of four ordinary differential equations that govern the prey and predator population densities on each patch. Predators disperse with constant migration rates, while prey dispersal is predator density-dependent. When the predator density is large, the dispersal of prey is more likely to occur. We assume that prey and predator dispersal is faster than the local predator-prey interaction on each patch. Thus, we take advantage of two time scales in order to reduce the complete model to a system of two equations governing the total prey and predator densities. The stability analysis of the aggregated model shows that a unique strictly positive equilibrium exists. This equilibrium may be stable or unstable. A Hopf bifurcation may occur, leading the equilibrium to be a centre. If the two patches are similar, the predator density dependent dispersal of prey has a stabilizing effect on the predator-prey system.
To better understand the role of habitat quality and boundaries on population dynamics at the landscape scale, we develop a model combining a spatially implicit approach, a spatial population Leslie-type model and an implicit model of habitat fragmentation. An original approach of elasticity permits to identify which types of element and boundary influence the most population viability according to the wood fragmentation degree. The studied species is a corridor forest insect sensitive to fragmentation (Abax parallelepipedus, Coleoptera, Carabidae). We show that a single large patch of wood is better than several small patches for the population viability.
The authors conducted a population-based case-control study of 1,068 incident leukemia cases and 5,039 controls aged 20-74 years during 1994-1997 to examine the association between exposure to drinking water chlorination disinfection by-products and adult leukemia risk in Canada. Residence and drinking water source histories and data from municipal water supplies were used to estimate individual chlorination disinfection by-product exposure according to water source, chlorination status, and chlorination disinfection by-product levels during the 40-year period before the interview. The analysis included 686 cases and 3,420 controls for whom water quality information was available for at least 30 of these years. Increased risk of chronic myeloid leukemia was associated with increasing years of exposure to different chlorination disinfection by-product indexes, with an adjusted odds ratio of 1.72 (95% confidence interval: 1.01, 3.08) for the highest exposure duration to total trihalomethanes of more than 40 microg/liter. In contrast, the risk of the other studied leukemia subtypes was found to decrease with increasing years of exposure to chlorination disinfection by-products. A protective effect was noted for chronic lymphoid leukemia (odds ratio = 0.60, 95 percent confidence interval: 0.41, 0.87) associated with the highest exposure duration to total trihalomethanes of more than 40 microg/liter. More studies with long-term exposure measures and large enough to evaluate leukemia subtypes are needed to further understanding of the issue.
Most classical prey-predator models do not take into account the behavioural structure of the population. Usually, the predator and the prey populations are assumed to be homogeneous, i.e. all individuals behave in the same way. In this work, we shall take into account different tactics that predators can use for exploiting a common self-reproducing resource, the prey population. Predators fight together in order to keep or to have access to captured prey individuals. Individual predators can use two behavioural tactics when they encounter to dispute a prey, the classical hawk and dove tactics. We assume two different time scales. The fast time scale corresponds to the inter-specific searching and handling for the prey by the predators and the intra-specific fighting between the predators. The slow time scale corresponds to the (logistic) growth of the prey population and mortality of the predator. We take advantage of the two time scales to reduce the dimension of the model and to obtain an aggregated model that describes the dynamics of the total predator and prey densities at the slow time scale. We present the bifurcation analysis of the model and the effects of the different predator tactics on persistence and stability of the prey-predator community are discussed.
OBJECTIVES: To evaluate the impact of active smoking, obesity, and dietary intakes on the risk of adult leukemia. METHODS: We analysed data obtained from a population-based case-control study conducted in eight Canadian provinces. Risk estimates were generated by applying multivariate logistic regression methods to 1068 incident histologically confirmed leukemia cases and 5039 controls aged 20-74. RESULTS: We found a statistically significant increased risk for acute myeloid leukemia (AML) associated with active smoking, with a clear dose-response relationship and an adjusted odds ratio (OR) of 1.5 (95% confidence interval [CI]=1.1-2.0) for heavy smokers reporting more than 20 pack-years of cigarette smoking. We also observed positive associations with the highest body mass index (BMI) for AML, chronic myeloid leukemia, and chronic lymphoid leukemia with a significant dose-response relationship. No association with leukemia was observed for the intake of fruits and vegetables, and the effect of active smoking on adult leukemia risk was not modified by fruits and/or vegetables consumption or obesity. However, the positive risk for AML associated with active smoking disappeared among subjects with high BMI (> or =30 kg/m2). CONCLUSIONS: Our study contributes to the accumulating evidence linking AML and active smoking, and provides some evidence that obesity increases the risk of most of the adult leukemia subtypes.
This work presents two stock-effort dynamical models describing the evolution of a fish population growing and moving between two fishing zones, on which it is harvested by a fishing fleet, distributed on the two zones. The first model corresponds to the case of constant displacement rates of the fishing effort, and the second one to fish stock-dependent displacement rates. In equations of the fishing efforts, a control function is introduced as the proportion of the revenue to be invested, for each fleet. The stabilizability analysis of the aggregated model, in the neighborhood of the equilibrium point, enables the determination of a Lyapunov function, which ensures the existence of a stabilizing discontinuous feedback for this model. This enables us to control the system and to lead, in an uniform way, any solution of this system towards this desired equilibrium point.
BACKGROUND: The role of environmental tobacco smoke (ETS) in the causation of lung and breast cancer has been repeatedly evaluated over recent years. In contrast, its impact on the risk of adult leukemia has received little attention. METHODS: We used the lifetime residential and occupational ETS exposure histories from a population-based sample of 1068 incident and histologically confirmed adult leukemia cases and 5039 population controls age 20 to 74 years to evaluate the relationship between ETS exposure and adult leukemia risk among nonsmokers in Canada. The duration of exposure and smoker-years index were used as indices of ETS exposure. We restricted our analysis to the 266 case and 1326 control subjects who reported being lifetime nonsmokers and provided residential ETS exposure history for at least 75% of their lifetime. RESULTS: No association was found for most leukemia subtypes, and in particular for acute myeloid leukemia. In contrast, the risk for chronic lymphocytic leukemia was clearly associated with ETS exposure, with an adjusted odds ratio of 2.3 (95% confidence interval = 1.2-4.5) for more than 83 smoker-years of residential exposure and 2.4 (1.3-4.3) for more than 72 smoker-years of occupational exposure. There was a dose-response relationship for chronic lymphocytic leukemia with both indices of exposure. Risk was not higher with recent exposure, using time-window-exposure analyses. CONCLUSIONS: Regular long-term ETS exposure may be a risk factor for chronic lymphocytic leukemia.
This paper deals with the study of a predator-prey model in a patchy environment. Prey individuals moves on two patches, one is a refuge and the second one contains predator individuals. The movements are assumed to be faster than growth and predator-prey interaction processes. Each patch is assumed to be homogeneous. The spatial heterogeneity is obtained by assuming that the demographic parameters (growth rates, predation rates and mortality rates) depend on the patches. On the predation patch, we use a Lotka-Volterra model. Since the movements are faster that the other processes, we may assume that the frequency of prey and predators become constant and we would get a global predator-prey model, which is shown to be a Lotka-Volterra one. However, this simplified model at the population level does not match the dynamics obtained with the complete initial model. We explain this phenomenom and we continue the analysis in order to give a two-dimensional predator-prey model that gives the same dynamics as that provided by the complete initial one. We use this simplified model to study the impact of spatial heterogeneity and movements on the system stability. This analysis shows that there is a globally asymptotically stable equilibrium in the positive quadrant, i.e. the spatial heterogeneity stabilizes the equilibrium.
We present a model of predator and prey grouping strategies using game theory. As predators respond strategically to prey behavior and vice versa, the model is based on a co-evolution approach. Focusing on the "many eyes-many mouths" trade-off, this model considers the benefits and costs of being in a group for hunting predators and foraging prey: predators in a group have more hunting success than solitary predators but they have to share the prey captured; prey in a group face a lower risk of predation but greater competition for resources than lone prey. The analysis of the model shows that the intersections of four curves define distinct areas in the parameter space, corresponding to different strategies used by predators and prey at equilibrium. The model predictions are in accordance with empirical evidence that an open habitat encourages group living, and that low risks of predation favor lone prey. Under some conditions, continuous cycling of the relative frequencies of the different strategies may occur. In this situation, the proportions of grouped vs. solitary predators and prey oscillate over time.
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.
Biological systems are composed of different levels of organization. Usually, one considers the atomic, molecular, cellular, individual, population, community and ecosystem levels. These levels of organization also correspond to different levels of observation of the system, from microscopic to macroscopic, i.e., to different time and space scales. The more microscopic the level is, the faster the time scale and the smaller the space scale are. The dynamics of the complete system is the result of the coupled dynamical processes that take place in each of its levels of organization at different time scales. Variables aggregation methods take advantage of these different time scales to reduce the dimension of mathematical models such as a system of ordinary differential equations. We are going to study the dynamics of a system which is hierarchically organized in the sense that it is composed of groups of elements that can be themselves divided into further smaller sub-groups and so on. The hierarchical structure of the system results from the fact that the intra-group interactions are assumed to be larger than inter-group ones. We present aggregation methods that allow one to build a reduced model that governs a few global variables at the slow time scale.
We developed a multiregion matrix population model to explore how the demography of a hypothetical brown trout population living in a river network varies in response to different spatial scenarios of cadmium contamination. Age structure, spatial distribution, and demographic and migration processes are taken into account in the model. Chronic or acute cadmium concentrations affect the demographic parameters at the scale of the river range. The outputs of the model constitute population-level end points (the asymptotic population growth rate, the stable age structure, and the asymptotic spatial distribution) that allow comparing the different spatial scenarios of contamination regarding the demographic response at the scale of the whole river network. An analysis of the sensitivity of these end points to lower order parameters enables us to link the local effects of cadmium to the global demographic behavior of the brown trout population. Such a link is of broad interest in the point of view of ecotoxicological management.
BACKGROUND: Diabetes worsens the prognosis of patients with ischemic cardiovascular disease (ICVD). Increased body weight and abdominal obesity have been shown to increase the risk of diabetes in people without ICVD. Such a relationship has not been assessed in patients with ICVD who may have a different occurrence due to their disease and medications. OBJECTIVE: To examine the risk of developing diabetes among patients with ICVD according to body mass index (BMI), waist-to-hip ratio and waist circumference METHODS: Anthropometric measurements were done in 4699 men and 1187 women with ICVD (mean age 66 years) and without known diabetes at entry to the Heart Outcomes Prevention Evaluation (HOPE) study. During the median 4.5-year follow-up, a diagnosis of diabetes was reported in 261 (4.4%) participants. RESULTS: There was a positive and graded association between increased BMI, waist circumference and waist-to-hip ratio, and the risk of developing diabetes (P for trends <0.0001). After adjusting for all baseline characteristic differences including medications, the relative risk of developing diabetes after the first 40th percentile of each anthropometric measure increased by 12% (95% CI 9% to 15%) for every 1 kg/m2 increase in BMI; the relative risk increased by 5% (95% CI 4% to 6%) for every 1 cm increase in waist circumference and by 38% (95% CI 18% to 61%) for every 0.1 unit increase in waist-to-hip ratio. CONCLUSION: In patients with ICVD, increased BMI, waist-to-hip ratio and particularly waist circumference constitute independent risk factors for development of diabetes.
As the result of the complexity inherent in nature, mathematical models employed in ecology are often governed by a large number of variables. For instance, in the study of population dynamics we often deal with models for structured populations in which individuals are classified regarding their age, size, activity or location, and this structuring of the population leads to high dimensional systems. In many instances, the dynamics of the system is controlled by processes whose time scales are very different from each other. Aggregation techniques take advantage of this situation to build a low dimensional reduced system from which behavior we can approximate the dynamics of the complex original system. In this work we extend aggregation techniques to the case of time dependent discrete population models with two time scales where both the fast and the slow processes are allowed to change at their own characteristic time scale, generalizing the results of previous studies. We propose a non-autonomous model with two time scales, construct an aggregated model and give relationships between the variables governing the original and the reduced systems. We also explore how the properties of strong and weak ergodicity, regarding the capacity of the system to forget initial conditions, of the original system can be studied in terms of the reduced system.
BACKGROUND: Increased antibiotic resistance of common bacteria is attributed in part to the widespread use of various antibiotic agents. Prophylactic and therapeutic antibiotic treatments are routinely used in cardiac surgical units, and it is no surprise that methicillin-resistant Staphylococcus aureus infection is becoming a major cause of surgical infections in cardiac patients. METHODS: We reviewed our experience with patients who underwent cardiac surgery and experienced infection caused by methicillin-resistant Staphylococcus aureus. Between 1992 and 2000 at the Montreal Heart Institute, 39 patients had methicillin-resistant Staphylococcus aureus surgical infections, and 13,199 patients underwent cardiac surgery. The yearly incidence of methicillin-resistant Staphylococcus aureus infection, the relative risk of acute mediastinitis and of superficial wound infections or other types of methicillin-resistant Staphylococcus aureus infection episodes, and the effect of preventive measures were analyzed. RESULTS: The annual incidence of methicillin-resistant Staphylococcus aureus acute mediastinitis decreased from 0.37% (5/1321) of cardiac patients in 1992 and 0.44% (6/1355) in 1993 to 0% between 1994 and 1997, 0.13% (2/1528) in 1999, and 0% (0/1700) in 2000. The total incidence of methicillin-resistant Staphylococcus aureus infection, including mediastinitis, superficial and deep sternal and leg wound infection, and all systemic infection episodes ranged from 0.68% of patients in 1992 and 0.96% of patients in 1993 to 0.46% of patients in 1999 and 0.53% of patients in 2000. The relative risk of severe mediastinal methicillin-resistant Staphylococcus aureus infection to all other methicillin-resistant Staphylococcus aureus infection episodes decreased from 1.65 in 1992 to 0.41 in 1999 and 0 in 2000. Beginning in 1993, all patients given a diagnosis methicillin-resistant Staphylococcus aureus infection and all nasal carriers of methicillin-resistant Staphylococcus aureus were strictly isolated on the surgical unit, and vancomycin was used as the prophylactic antibiotic agent for cardiac surgery in these patients. Moreover, since 1998, all patients admitted in the hospital were screened, and nasal carriers were isolated and treated with topical antibiotic ointment. CONCLUSION: Mediastinal and other infections caused by methicillin-resistant Staphylococcus aureus have a significant morbidity in cardiac surgical patients. After an outbreak of methicillin-resistant Staphylococcus aureus mediastinal infections, several preventive measures to control methicillin-resistant Staphylococcus aureus contamination of surgical patients were implemented (nasal screening, preventive isolation, application of mupirocin, prophylaxis with vancomycin and alcohol gels) and were effective in decreasing the incidence of methicillin-resistant Staphylococcus aureus infection and mediastinitis after cardiac surgery.
OBJECTIVE: To equip medical practitioners with up-to-date scientific and medical information on the health effects of exposure to fungi in indoor air, clinical evaluation of these health problems, and possible preventive measures. QUALITY OF EVIDENCE: MEDLINE was searched from 1985 to 2000 using the MeSH words mould (mold), fungus, indoor air, and health effects. Nearly all studies were case reports, case-control studies, and cross-sectional studies. Evidence of an association between respiratory problems and the presence of fungi and dampness is strong. MAIN MESSAGE: Recent well designed studies and literature reviews indicate that exposure to dampness and fungi in indoor air brings on or exacerbates asthma and other respiratory complaints. More studies are required, however, before a definite conclusion on other potential effects of such exposure (such as systemic and long-term effects and pulmonary hemorrhage in infants) is possible. The various health problems that can result from exposure to dampness and fungi in indoor air make such exposure unacceptable from a public health perspective. Physicians are important in treating and preventing such problems; various resources are available to help them. CONCLUSION: Even though some scientific issues remain to be resolved concerning the health effects of exposure to dampness and fungi in indoor air, family physicians can identify potential problems and refer patients to appropriate resources.