The population dynamics of acquired immunity to helminth infection: experimental and natural transmission

Exploring the impact of infection-induced immunity on the transmission of Schistosoma japonicum in hilly and mountainous environments in China

Schistosomiasis has long been a threat to villagers in hilly and mountainous areas of southwestern China where the intermediate snail host is abundant. In recent years our group has focused on the development and parameterization of a community-level mathematical model of S. japonicum transmission that accounts for the role of environmental determinants of transmission intensity in Sichuan Province. To date the model has not incorporated acquired immunity. A review of previous epidemiologic data from our study area in Sichuan suggested modeling of acquired immunity as a function of history of infection. To explore the potential impacts on the dynamics of transmission, a mathematical representation of acquired immunity was incorporated, and parameterized based on this epidemiological evidence. It is shown through simulation that the effect of immunity is to reduce the rate of worm development and thereby lower the endemic level significantly. The effect was more striking at increasing levels of a village'tm)s basic reproductive number. Further, residual immunity modestly alters the threshold of external parasite input necessary to trigger re-emergence of transmission and its subsequent rate of development. Despite limitations in our quantitative knowledge of the immunity function, these findings, along with the uncertainties in transmission dynamics at low infection levels, underscore the need for improved diagnostic methods for disease control, especially in potentially re-emergent settings.

Animal model of Nippostrongylus brasiliensis and Heligmosomoides polygyrus

Animal models of Nippostrongylus brasiliensis and Heligmosomoides polygyrus infection are powerful tools for the investigation of the basic biology of immune responses and protective immunity. In particular, they model the induction and maintenance of Th2 type immune responses and exhibit all the requisite hallmarks of CD4 T cell-dependent IgE production, eosinophilia, mastocytosis, and mucus production. This chapter describes simple, cost-effective techniques for using and maintaining these easy-to-work-with parasites in the context of a modern laboratory.

Modelling parasitism and predation of mosquitoes by water mites

Parasitism and predation are two ecological interactions that can occur simultaneously between two species. This is the case of Culicidae (Insecta: Diptera) and water mites (Acari: Hydrachnidia). The larva mites are~parasites of aquatic and semiaquatic insects, and deutonymphs and adults are predators of insect larvae and eggs. Since several families of water mites are associated with mosquitoes there is an interest in the potential use of these mites as biological control agents. The aim of this paper is to use mathematical modelling and analysis to assess the impact of predation and parasitism in the mosquito population. We propose a system of ordinary differential equations to model the interactions among the larval and adult stages of mosquitoes and water mites. The model exhibits three equilibria: the first equilibrium point corresponds to the state where the two species are absent, the second one to the state where only mosquitoes are present (water mites need insects to complete their life cycle), and the third one is the coexistence equilibrium. We analyze conditions for the asymptotic stability of equilibria, supported by analytical and numerical methods. We discuss the different scenarios that appear when we change the parasitism and predation parameters. High rates of parasitism and moderate predation can drive two species to a stable coexistence.

Immunoepidemiology of human schistosomes: taking the theory into the field

Much is known about human immune responses to schistosome infection, but it has proved difficult to determine the impact of these responses on schistosome epidemiology in the field. In this paper, Mark Woolhouse compares epidemiological patterns from field data with the predictions o f simple mathematical models of different immunological processes. The comparison gives some indications as to which types of immune response may be important, and of their strength and duration. The results are consistent with a significant impact of a process similar to 'concomitant' immunity, with a possible role for anti-fecundity effects.

A pocket guide to host-parasite models

Mathematical models have been used to describe the population dynamics of a wide range of host-parasite interactions. Mick Roberts here discusses mathematical models for the dynamics of helminth endoparasites of non-human mammalian hosts, paying particular attention to the density-dependent factors that regulate the parasite populations, and the interaction between parasite and wild or feral animal host populations.

Modelling of parasite populations: gastrointestinal nematode models

This paper surveys models of nematode parasites of veterinary importance. A distinction is drawn between generic models which are usually simple formulations applicable to whole classes of parasite and specific models which are often more complex and designed to address questions concerning a particular species. Most of the models considered employ a deterministic framework. Four main groups are considered: generic models of trichostrongylid infection of domestic ruminants, specific models of trichostrongylid infection of domestic ruminants, specific models of experimental laboratory infections of rodents, and a specific model of nematode infections in wildlife.

Population biology of the parasitic phase of trichostrongylid nematode parasites of cattle and sheep

This paper reviews the previous mathematical and conceptual models for the parasitic phase of a range of trichostrongylid nematode parasites of cattle and sheep. It reassesses the results of single and trickle infection experiments and suggests as a working hypothesis that the common trichostrongylids are essentially identical with respect to the processes that determine their survivorship in the host. Parasite abundance in the parasitic phase is explained in terms of immune exclusion, which acts on recently ingested third stage larvae, and mortality of established (fifth stage) parasites. The functional forms used to describe immune exclusion and the mortality of fifth stage worms are defined, respectively, as a declining sigmoid and an asymptotic curve.

A theoretical framework for the immunoepidemiology of helminth infection

Field studies of parasitic helminths in endemically infected human communities have provided quantitative information on the relationships between parasite burdens, immune responses and age. There are considerable difficulties in the interpretation of these immunoepidemiological data due to the complexities of the biological processes generating the observed patterns. In this paper simple mathematical models are used to explore the expected patterns of variation with host age in parasite burdens, the aggregation of parasites among hosts, levels of immune response, and the correlation between parasite burdens and immune responses. These relationships reflect rates of infection, rates of parasite mortality, the strength of the immune response, and the duration of immunological memory. The models generate some complex and counterintuitive patterns. The analysis suggests that some of these patterns might serve to (i) distinguish effects due to acquired immunity from effects due to age-dependent exposure, (ii) identify potentially protective immune responses, and (iii) identify the parasite stages important in the development of acquired immunity. The results imply that previous analyses of immunoepidemiological data may have been overly simplistic and, especially, that patterns believed to be inconsistent with protective immunity may have been incorrectly interpreted.

The relationship between Trichuris trichiura transmission intensity and the age-profiles of parasite-specific antibody isotypes in two endemic communities

The present study compares parasite-specific antibody responses in two Caribbean communities with high and low levels of Trichuris trichiura transmission. The age-dependency of antibody levels suggest that IgG1 and IgG2 levels relate to the current intensity of infection (as assessed by density of eggs in stool (e.p.g.) and reflect the age-intensity profile at the population level. IgG4, IgE and IgA levels persist into early adulthood and the subsequent decline is gradual. In the low transmission area, lower infection levels are reflected in lower parasite-specific antibody levels (of all isotypes) in the community as a whole. Despite a significantly greater past experience of infection in the high transmission area, antibody levels are not maintained at significantly higher levels throughout adulthood. The production of IgA appears to require a threshold for triggering, and a vigorous IgA response is maintained into early adulthood only in the high transmission village where peak intensity is greatest and the age-convexity of intensity is most marked. Experimental and theoretical studies focusing on the dynamic nature of host-helminth interactions in hosts exposed to high and low infection levels, and the putative role of acquired immunity, are discussed in relation to the data presented.

Modelling coccidial infection in chickens: emphasis on vaccination by in-feed delivery of oocysts

Trickle immunization of poultry by incorporating wildtype strains of coccidia in the feed has been shown to be an effective and practical means of controlling coccidiosis. Here we develop a mathematical model of the life cycle of Eimeria tenella and estimate the model parameters from extant experimental studies. Numerical solutions of the model compare well with experiment. The model provides quantitative estimates of the required trickle immunization hitherto only available after extensive time-consuming trials.

Immuno-epidemiology of human geohelminthiasis: ecological and immunological determinants of worm burden

The morbidity and transmission dynamics of geohelminthiases are determined by the patterns of infection intensity in the community. Understanding the determinants of these patterns requires a combination of field, laboratory and theoretical study. Studies of age-specific reinfection, and of the phenomenon of predisposition, indicate that the major determinant of convex age-intensity profiles and of heterogeneity in infection intensity is the rate of establishment of infection, rather than the rate of adult worm mortality. The rate of establishment is, in turn, determined by exposure to, and protection from, infection. The evidence indicates that exposure, at least to the orally-transmitted geohelminths, varies with age and is highly heterogeneous between hosts. The immune response in geohelminthiasis is vigorous, parasite-specific, heterogeneous between hosts, and both age and infection dose dependent, but has yet to be convincingly shown to be protective. Since the immune response it itself a function of exposure, unravelling the interaction between ecology and immunology as determinants of geohelminth worm burden will require simultaneous assessment of both processes via immunoepidemiological study.

The population dynamics of acquired immunity to Heligmosomoides polygyrus in the laboratory mouse: strain, diet and exposure

An experiment was designed to investigate aspects of the population dynamics of acquired immunity to Heligmosomoides polygyrus in laboratory mice. The influence of host strain (CBA or NIH), rate of exposure (5 or 40 L3/mouse/2 weeks) and diet (3 or 16% protein w/w) on the population dynamics of repeated infection and the response to a standard challenge infection were investigated. The time delay between the end of the period of repeated infection and the subsequent challenge (between 1 and 24 weeks) had no effect on worm recovery. The effects of both exposure and diet were significant and similar whether assessed on the basis of the dynamics of repeated infection or response to challenge: low rates of exposure and low dietary protein were both associated with low levels of acquired immunity. Mouse strain was the most important determinant of worm recovery after challenge, but had no significant effect on the degree to which parasite population growth was constrained by acquired immunity during repeated infection. It is suggested that both CBA and NIH mice raise immune responses which act on parasite survival, but that only NIH mice raise responses operative against larval establishment.