Host-parasite Population Research Articles

Mathematical model of the population dynamics of brood parasitism: what is the benefit for the host?

The problem of how the system of a brood-parasite with parasite-accepting host (i.e. a host that accepts a parasite’s eggs) exists in stable equilibrium, is considered. We analyse a mathematical model of host-parasite population dynamics for this system, corresponding to certain cases of brood-parasitic animals where brood-parasitization causes drastic failure of the reproduction of the brood-parasitized host, and discuss the possible benefit to the persistence of a parasite-accepting host from the viewpoint of population dynamics.

Modelling patterns of parasite aggregation in natural populations: trichostrongylid nematode-ruminant interactions as a case study.

The characteristically aggregated frequency distribution of macroparasites in their hosts is a key feature of host-parasite population biology. We begin with a brief review of the theoretical literature concerning parasite aggregation. Though this work has illustrated much about both the sources and impact of parasite aggregation, there is still no definite analysis of both these aspects. We then go on to illustrate the use of one approach to this problem--the construction of Moment Closure Equations (MCEs), which can be used to represent both the mean and second moments (variances and covariances) of the distribution of different parasite stages and phenomenological measures of host immunity. We apply these models to one of the best documented interactions involving free-living animal hosts--the interaction between trichostrongylid nematodes and ruminants. The analysis compares patterns of variability in experimental infections of Teladorsagia circumcincta in sheep with the equivalent wildlife situation--the epidemiology of T. circumcincta in a feral population of Soay sheep on St Kilda, Outer Hebrides. We focus on the relationship between mean parasite load and aggregation (inversely measured by the negative binomial parameter, k) for cohorts of hosts. The analysis and empirical data indicate that k tracks the increase and subsequent decline in the mean burden with host age. We discuss this result in terms of the degree of heterogeneity in the impact of host immunity or parasite-induced mortality required to shorten the tail of the parasite distribution (and therefore increase k) in older animals. The model is also used to analyse the relationship between estimated worm and egg counts (since only the latter are often available for wildlife hosts). Finally, we use these results to review directions for future work on the nature and impact of parasite aggregation.

Behavioral stabilization of host-parasite population dynamics

We demonstrate that individual behavior can stabilize classical (Nicholson-Bailey) host-parasite population dynamics. Our model assumes that hosts can be divided into at least two phenotypes and that parasites either do not attack one of the phenotypes or attack them facultatively. The former case corresponds to a behavioral refuge (Hassell 1978) and it is known that other kinds of refuges lead to stability of population dynamics. Behavioral refuges can stabilize the population dynamics in the same way that spatial refuges do. When parasites attack hosts facultatively within the year, strange attractors may arise in the year-to-year population dynamics, in response to the nonlinear nature of the facultative response to the distribution of host densities.

Competition between zidovudine-sensitive and zidovudine-resistant strains of HIV.

To investigate competitive interactions between zidovudine-sensitive and resistant strains of HIV within the context of host-parasite population dynamic interactions between CD4+ cells and HIV. A mathematical model of the population dynamics of CD4+ cells, sensitive HIV and resistant HIV is developed. The model is analysed numerically and analytically and model predictions are compared with previously published data on population dynamics of HIV and CD4+ cells in patients receiving zidovudine. A threshold result describing the critical dose of zidovudine above which resistant HIV will out-compete sensitive HIV is derived, as are expressions describing the critical effective doses for the eradication of sensitive and resistant strains. Numerical simulations of the dynamics of the shift from the pre-treatment, equilibrium to the treatment equilibrium are presented and an analytic expression approximating the time taken until virus growth restarts is derived. It is shown that competition between strains of virus is the important factor determining which type of virus will eventually start to grow during the course of zidovudine treatment, but host-parasite interactions are the important determinant of when viral resurgence occurs. Although resistant strains are observed after prolonged treatment with zidovudine, this model suggests that it is the growing supply of uninfected CD4+ cells which causes the eventual upsurge in viral burden.

Parasitic castration: host species preferences, size-selectivity and spatial heterogeneity.

This study investigates host-parasite population dynamics in a marine intertidal community of three barnacle host species (Balanus glandula, Chthamalus fissus andC. dalli). Our paper addresses the following questions: (1) Does prevalence (percentage parasitism) differ among the three host species? (2) What are the spatial and temporal population dynamics within the community? and (3) Does the parasite exhibit size-selective behaviour in any of the three host species? Significant differences in prevalence were found among the three host species; the parasitic castrator (Hemioniscus balani) most heavily infected the least abundant host. Parasitism occurred throughout the year and also showed significant spatial variation.H. balani showed size-selective parasitism inC. fissus, but not inC. dalli. Consequently, the population effects of parasitic castration inC. fissus depend both upon the host population size structure and the intensity of the parasite's size-selectivity.

The adaptive significance of sexuality.

A theory of sexuality and polymorphism is proposed in which diversity at the molecular level is the adaptive response of multicellular organisms to the challenge of microparasites that have smaller genomes, shorter generation times and which can evolve more quickly than their hosts. The theory has implications for genetically homogenized crops and other cultivated plants as well as for immunology. A different function of sexuality is proposed for microorganisms that reproduce both asexually and sexually. Several possible experimental tests are discussed. Mathematical modelling techniques are outlined qualitatively and compared with game-theoretical methods which may be interpreted as simplifications of population dynamics of polymorphic host-parasite populations are referenced.

Systematic temporal changes in host susceptibility to infection: demographic mechanisms.

Simple mathematical models are developed to examine the influence of variability in host susceptibility to infection, on the dynamics of host-parasite population interactions. When hosts differ in their innate susceptibility (at birth), to infection by a specific parasite, the average susceptibility of the host population as a whole may show systematic changes through time. Such patterns may arise as a result of demographic factors associated with the interaction between host and parasite populations, in the absence of inheritance mechanisms (a genetic component) or acquired resistance (an immunological component). The general significance of this observation is discussed in terms of the coevolution of host-parasite associations.

The ecological basis of parasite control: Nematodes

In veterinary parasitology the major ecological interest is in production ecology, which concentrates on those aspects of the interaction of host, parasite and environment which determine the size of the host's parasite population and thus the risk to the production process. Control aims at reducing this population by breaking the life cycle in two ways: 1. (a)at the host level — by anthelmintic treatment to eliminate the parasite and perhaps more importantly to remove the source of environmental contamination 2. (b) at the environmental level — by segregating susceptible hosts from the infective stage of the parasite. The application of control measures requires an understanding of the parasite population pattern in the host and in the environment, and of the factors influencing the population. If these factors can be separated out and quantified by the use of systems analysis then the host—parasite system can be analysed and modelled. Such a model could be used in simulation studies on control strategies, and to indicate productive lines of research. The adult parasite population in the host and egg production are largely controlled by immunological factors, in particular the peri-parturient rise in egg output, and these physiological factors are difficult to quantify. However, the development and transmission of the external larval population can be extensively subdivided and studied experimentally, and utilisable data is becoming increasingly available for modelling purposes. The key factors governing the infective population are temperature and moisture and these have been used to develop models of the population pattern in the external environment. More sophisticated models have been constructed to predict population size as well as pattern and this approach is likely to dominate future ecological studies.

The regulation of host population growth by parasitic species.

SummaryThe nature of parasitism at the population level is defined in terms of the parasite's influence on the natural intrinsic growth rate of its host population. It is suggested that the influence on this rate is related to the average parasite burden/host and hence to the statistical distribution of parasites within the host population.Theoretical models of host–parasite associations are used to assess the regulatory influence of parasitic species on host population growth. Model predictions suggest that three specific groups of population processes are of particular importance: over-dispersion of parasite numbers/host, density dependence in parasite mortality or reproduction and parasite-induced host mortality that increases faster than linearly with the parasite burden. Other population mechanisms are shown to have a destabilizing influence, namely: parasite-induced reduction in host reproductive potential, direct parasite reproduction within the host and time delays in the development of transmission stages of the parasite.These regulatory and destabilizing processes are shown to be commonly observed features of natural host-parasite associations. It is argued that interactions in the real world are characterized by a degree of tension between these regulatory and destabilizing forces and that population rate parameter values in parasite life-cycles are very far from being a haphazard selection of all numerically possible values. It is suggested that evolutionary pressures in observed associations will tend to counteract a strong destabilizing force by an equally strong regulatory influence. Empirical evidence is shown to support this suggestion in, for example, associations between larval digeneans and molluscan hosts (parasite-induced reduction in host reproductive potential counteracted by tight density-dependent constraints on parasite population growth), and interactions between protozoan parasites and mammalian hosts (direct parasite reproduction counteracted by a well-developed immunological response by the host).The type of laboratory and field data required to improve our understanding of the dynamical properties of host–parasite population associations is discussed and it is suggested that quantitative measurement of rates of parasite-induced host mortality, degrees of over-dispersion, transmission rates and reproductive and mortality rates of both host and parasite would provide an important first step. The value of laboratory work in this area is demonstrated by reference to studies which highlight the regulatory influence of parasitic species on host population growth.

Regulation and Stability of Host-Parasite Population Interactions: II. Destabilizing Processes

Regulation and Stability of Host-Parasite Population Interactions: II. Destabilizing Processes

Regulation and Stability of Host-Parasite Population Interactions: I. Regulatory Processes

Regulation and Stability of Host-Parasite Population Interactions: I. Regulatory Processes

Genetic Change in Host-Parasite Populations

Although the total amount of factual genetic information relating to population changes in host-parasite systems is small, the recent literature in phytopathology abounds with interpretation and discussion of events that have taken place or may have taken place at the population level. Accordingly, this review does not catalog the detailed results of a large number of studies. Its purpose is to review, in the light of the theory of population genetics, the mainly speculative literature that is accumulating. Because the existing observational and experimental data pertain almost exclusively to parasitic systems in which host populations have been manipulated by man, this review centers mainly on changes that have oc­ curred, or may occur, in parasite populations as a direct consequence of human intervention. The process by which pathogen populations change as a direct result of human activity was referred to by Johnson ( 17) as man-guided evolution. The need for accurate information that would have relevance to this interpretation was referred to in an earlier review (27). That need still exists. There is also a pressing need for adoption of a clearly defined set of terms (a problem recently discussed in another connection by Nelson, 26). It is also important to distinguish between organisms that reproduce asexually (either as haploids or diploids) and those that do not. Population genetics has traditionally dealt with those that do not repro­ duce asexually. Relatively little attention has been directed to organisms such as the rusts, mildews, and other parasites that undergo repeated cycles of asexual division which are only occasionally interrupted by nuclear fusion and meiosis, and in which genetic recombination may occur only occasionally or not at all.

The population biology of the acanthocephalan Pomphorhynchus laevis (Müller) in the River Avon

Regular samples of Gammarus pulex and dace Leuciscus leuciscus and occasional grayling Thymallus thymallus and chub L.cephalus were examined from the River Avon, Hampshire, for the presence of the acanthocephalan Pomphorhynchus leavis. The parasite only occurred in medium sized Gammarus due to lower probability of contact with small gammarids and stunted growth and selective mortality amongst older infected ones. No cycles in incidence or development of the parasite in G.pulex were observed. The parasite infected gammarids and grew in all months, and cystacanths were available throughout the year. Despite seasonal feeding activity and dietary preferences, fish fed on Gammarus and acquired infections in all months. Dispersion of P.laevis within the fish population was related to host feeding behaviour. No evidence of seasonal cycles in incidence or intensity of infection in fish was found, and observed monthly changes in the parasite population were related to changes in size structure of the host sample. In dace and grayling P.laevis grew little and matured only in summer, but in chub it grew and produced acanthors all year. The parasite population in fish appeared to be in a state of dynamic equilibrium and gain and loss of parasites took place throughout the year with the level of infection at any moment being determined primarily by the feeding behaviour of the host. This relationship between host diet, water temperature and parasite population size is discussed, and P.laevis in the R. Avon compared with other localities and other parasites.

Interactions in Insect Populations

Temperature differentially affects the toleration, developmental, and response physiology of host and parasite species and therefore modifies the co-actions of host-parasite populations. This example of the influence of a physical agent on host-parasite interaction indicates that the changing physical environment can control the types and intensities of many of the biotic interactions that occur in natural communities.

Experimental Host-Parasite Populations

Experimental Host-Parasite Populations

A Survey of Parasites of the Larch Sawfly (Pristiphora erichsonii (Hartig)) in Manitoba and Saskatchewan

Records indicate that the present outbreak of the larch sawfly in Manitoba and Saskatchewan began about 1938 in the Spruce Woods-Riding Mountain area of Manitoba. Since then, it has spread in all directions where the principal host tree, larch, Larix laricina (Du Roi) K. Koch, occurs. It now includes nearly all of Manitoba and Saskatchewan, northwestern Ontario, part of northeastern Alberta, and northern Minnesota. In 1944, an annual survey of parasites that attack larvae of the larch sawfly was begun by the Forest Biology Laboratory, Winnipeg. The purposes of the survey were to determine (a) the principal species of parasites, (b) their abundance and effectiveness, and (c) .host-parasite population trends.

Simplified Method for the Study of Interacting Host-Parasite Populations

EcologyVolume 35, Issue 2 p. 292-293 Article Simplified Method for the Study of Interacting Host-Parasite Populations Stanley C. Flanders, Stanley C. FlandersSearch for more papers by this author Stanley C. Flanders, Stanley C. FlandersSearch for more papers by this author First published: 01 April 1954 https://doi.org/10.2307/1931129AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. Volume35, Issue2April 1954Pages 292-293 RelatedInformation

The Effect of Temperature on an Insect Host‐Parasite Population

EcologyVolume 30, Issue 2 p. 113-134 Article The Effect of Temperature on an Insect Host-Parasite Population Thomas Burnett, Thomas BurnettSearch for more papers by this author Thomas Burnett, Thomas BurnettSearch for more papers by this author First published: 01 April 1949 https://doi.org/10.2307/1931181Citations: 55AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume30, Issue2April 1949Pages 113-134 RelatedInformation