Changes in parasite aggregation with age: a discrete infection model

Haemogregarine (Apicomplexa: Adeleorina) infection in Vanderhaege's toad-headed turtle, Mesoclemmys vanderhaegei (Chelidae), from a Brazilian Neotropical savanna region

Knowledge of blood parasites in Brazilian chelonians is limited, since they have been recorded in only six species. Mesoclemmys vanderhaegei (Bour) is a freshwater turtle with a wide geographic distribution in Brazil, but there is little information about its natural history. This paper reports on a study of the prevalence and infection intensity of a haemogregarine in two subpopulations of M. vanderhaegei. The study was conducted in two areas of Cerrado in the Upper Paraguay River basin in the state of Mato Grosso, Brazil, between November 2010 and August 2013. Ninety-five (53%) of the 179 turtles captured were positive for haemogregarine parasites. The parasitic forms observed were two morphotypes of intraerythrocytic gametocytes. The prevalence differed between size classes, increasing significantly according to the animals' body size. There was no significant difference between prevalence and sex, or between sampling periods. The mean parasite intensity was 9 parasites/2,000 erythrocytes (0.45%) and the parasite population presented an aggregated distribution, with an aggregation index of 19 and discrepancy of 0.772. This is the first record of a hemoparasite in the freshwater turtle M. vanderhaegei.

Dynamics and effects of Ligula intestinalis (L.) infection in the native fish Barbus callensis Valenciennes, 1842 in Algeria

The dynamics of the emergence, duration, and decline phases in epizootic cycles are well known for humans and some crops, but they are poorly understood for host-parasite systems in the wild. Parasites may be particularly insidious as they are often introduced unintentionally, simultaneously with their hosts, and later transferred to species in the new location. Here we investigate the epizootic dynamics of the tapeworm Ligula intestinalis in the Hamiz reservoir, Algeria, and explore its effects on the cyprinid fish Barbus callensis. Regular sampling was conducted from October 2005 to February 2008 with intermittent surveys carried out until 2010. Five percent of the 566 specimens of B. callensis that were caught were infected, with the maximum number of parasites found in spring. There was no obvious difference in weight between uninfected fish and infected ones, and infection did not affect fish condition. However, infected fish were significantly longer than uninfected fish and had inhibited gonad development. The proportion of infected fish caught was significantly higher in year 1 and by the second winter, infection collapsed to zero. The Ligula infection thus appeared to have minimal ecological effects and be of a temporary nature, thus exhibiting an epizootic cycle. Taken together, our data indicates that this infection declined or even failed during our study period. Failure may be due to the specific genetic strain of Ligula, but invasive carp may also have been influential in both the introduction and subsequent decline of this parasite.

On the variance mean ratio in models of parasite distributions

We study the variance-to-mean ratio of the distributions of parasites among hosts for some models of parasite infection, using the cohort approach. We consider a model with density dependence in parasite mortality, and two different formulations of disease induced host mortality. We show that the distributions of parasites, conditional on host survival, converge to quasi-stationary distributions as host age increases. When there is density dependence in parasite mortality, the limiting variance-to-mean ratio is less than 1 (an ‘under-dispersed’ distribution). In contrast, the two modes of disease induced host mortality show that either over- or underdispersed distributions may result.

On the variance mean ratio in models of parasite distributions

We study the variance-to-mean ratio of the distributions of parasites among hosts for some models of parasite infection, using the cohort approach. We consider a model with density dependence in parasite mortality, and two different formulations of disease induced host mortality. We show that the distributions of parasites, conditional on host survival, converge to quasi-stationary distributions as host age increases. When there is density dependence in parasite mortality, the limiting variance-to-mean ratio is less than 1 (an ‘under-dispersed’ distribution). In contrast, the two modes of disease induced host mortality show that either over- or underdispersed distributions may result.

An explicit immunogenetic model of gastrointestinal nematode infection in sheep

Gastrointestinal nematodes are a global cause of disease and death in humans, wildlife and livestock. Livestock infection has historically been controlled with anthelmintic drugs, but the development of resistance means that alternative controls are needed. The most promising alternatives are vaccination, nutritional supplementation and selective breeding, all of which act by enhancing the immune response. Currently, control planning is hampered by reliance on the faecal egg count (FEC), which suffers from low accuracy and a nonlinear and indirect relationship with infection intensity and host immune responses. We address this gap by using extensive parasitological, immunological and genetic data on the sheep–Teladorsagia circumcincta interaction to create an immunologically explicit model of infection dynamics in a sheep flock that links host genetic variation with variation in the two key immune responses to predict the observed parasitological measures. Using our model, we show that the immune responses are highly heritable and by comparing selective breeding based on low FECs versus high plasma IgA responses, we show that the immune markers are a much improved measure of host resistance. In summary, we have created a model of host–parasite infections that explicitly captures the development of the adaptive immune response and show that by integrating genetic, immunological and parasitological understanding we can identify new immune-based markers for diagnosis and control.

Spatial and seasonal factors are key determinants in the aggregation of helminths in their definitive hosts: Pseudamphistomum truncatum in otters (Lutra lutra)

Parasites are typically aggregated within their host populations. The most heavily infected hosts are frequently cited as targets for optimal disease control. Yet a heavily infected individual is not necessarily highly infective and does not automatically contribute a higher proportion of infective parasitic stages than a host with fewer parasites. Here, Pseudamphistomum truncatum (Opisthorchiida) parasitic infection within the definitive otter host (Lutra lutra) is used as a model system. The hypothesis tested is that variation in parasite abundance, aggregation and egg production (fecundity, as a proxy of host infectivity) can be explained by abiotic (season and region) or biotic (host age, sex and body condition) factors. Parasite abundance was affected most strongly by the biotic factors of age and body condition, such that adults and otters with a higher condition index had heavier infections than sub-adults or those with a lower condition index, whilst there were no significant differences in parasite abundance among the seasons, regions (ecological regions defined by river catchment boundaries) or host sexes. Conversely, parasite aggregation was affected most strongly by the abiotic factors of season and region, which were supported by four different measures of parasite aggregation (the corrected moment estimate k, Taylor's Power Law, the Index of Discrepancy D, and Boulinier's J). Pseudamphistomum truncatum was highly aggregated within otters, with aggregation stronger in the Midlands (England) and Wales than in the southwestern region of the United Kingdom. Overall, more parasites were found in fewer hosts during the summer, which coincides with the summer peak in parasite fecundity. Combined, these data suggest that (i) few otters carry the majority of P. truncatum parasites and that there are more infective stages (eggs) produced during summer; and (ii) abiotic factors are most influential when describing parasite aggregation whilst biotic factors have a greater role in defining parasite abundance. Together, parasite abundance, aggregation and fecundity can help predict which hosts make the largest contribution to the spread of infectious diseases.

Occurrence of parasitic copepods in Carangid fishes from Parangipettai, Southeast coast of India

In the present study, 68 fishes were infested out of 544 specimens examined from six different species of Carangid fishes which were collected from Parangipettai coastal waters. Eight species of parasitic copepods were found on gill filaments, body surface and nasal capsule regions. The maximum prevalence was recorded in Carangoides malabaricus (22.5 %) and minimum was noticed in (2.4 %) Selaroides leptolepis. The intensity of infection ranged from 1 to 1.2. Thus, considerable variation in the respiratory area was observed owing to the attachment of parasites in the infected fishes. Caligus sp. and C. epidemicus parasites were attached to body surface and only one Sphyriid sp. parasites were found in nasal capsule region. It is very difficult to estimate the actual harm to fish caused by the presence of parasites; if this is uneasy in cultured fish, it is almost impossible in feral fish populations. It should also be emphasized that the presence of a parasite does not necessarily imply manifestation of a disease. In aquaculture, some parasites are able to reproduce rapidly and heavily infect a large proportion of fish which may lead to diseases with significant economic consequences.

Modelling parasite aggregation: disentangling statistical and ecological approaches

The overdispersion in macroparasite infection intensity among host populations is commonly simulated using a constant negative binomial aggregation parameter. We describe an alternative to utilising the negative binomial approach and demonstrate important disparities in intervention efficacy projections that can come about from opting for pattern-fitting models that are not process-explicit. We present model output in the context of the epidemiology and control of soil-transmitted helminths due to the significant public health burden imposed by these parasites, but our methods are applicable to other infections with demonstrable aggregation in parasite numbers among hosts.

Protective immunity to Schistosoma haematobium infection is primarily an anti-fecundity response stimulated by the death of adult worms

Protective immunity against human schistosome infection develops slowly, for reasons that are not yet fully understood. For many decades, researchers have attempted to infer properties of the immune response from epidemiological studies, with mathematical models frequently being used to bridge the gap between immunological theory and population-level data on schistosome infection and immune responses. Here, building upon earlier model findings, stochastic individual-based models were used to identify model structures consistent with observed field patterns of Schistosoma haematobium infection and antibody responses, including their distributions in cross-sectional surveys, and the observed treatment-induced antibody switch. We found that the observed patterns of infection and antibody were most consistent with models in which a long-lived protective antibody response is stimulated by the death of adult S. haematobium worms and reduces worm fecundity. These findings are discussed with regard to current understanding of human immune responses to schistosome infection.

Does the early frog catch the worm? Disentangling potential drivers of a parasite age-intensity relationship in tadpoles

The manner in which parasite intensity and aggregation varies with host age can provide insights into parasite dynamics and help identify potential means of controlling infections in humans and wildlife. A significant challenge is to distinguish among competing mechanistic hypotheses for the relationship between age and parasite intensity or aggregation. Because different mechanisms can generate similar relationships, testing among competing hypotheses can be difficult, particularly in wildlife hosts, and often requires a combination of experimental and model fitting approaches. We used field data, experiments, and model fitting to distinguish among ten plausible drivers of a curvilinear age–intensity relationship and increasing aggregation with host age for echinostome trematode infections of green frogs. We found little support for most of these proposed drivers but did find that the parsimonious explanation for the observed age–intensity relationship was seasonal exposure to echinostomes. The parsimonious explanation for the aggregated distribution of parasites in this host population was heterogeneity in exposure. A predictive model incorporating seasonal exposure indicated that tadpoles hatching early or late in the breeding season should have lower trematode burdens at metamorphosis, particularly with simulated warmer climates. Application of this multi-pronged approach (field surveys, lab experiments, and modeling) to additional parasite–host systems could lead to discovery of general patterns in the drivers of parasite age–intensity and age–distribution relationships.

Seasonal dynamics of Diplectanum grouperi parasitism on wild versus cultured groupers, Epinephelus spp., and the linkage between infestation and host species phylogeny

In this article, we examined the influences of the polycultured potential hosts on monogenean seasonality and the possible linkage between the infestations on different species of hosts and host phylogeny. The seasonality of Diplectanum grouperi , the dominant species, on wild versus cultured groupers, Epinephelus spp., was analyzed in Daya Bay, South China Sea, between April 2008 and January 2009. The prevalence, mean intensity, abundance, and variance/mean ratio were calculated for each species of host under polycultured and wild conditions. Except for the overall prevalence, which was slightly higher in autumn than that in summer under wild conditions, the mean intensity and variance were highest in summer, decreasing slightly in autumn to lowest levels in winter or spring. The infection level (prevalence, mean intensity, and abundance) was correlated with changes in water temperature during the sampling period, with a peak in summer, with the exception of E. awoara in autumn under wild conditions. The prevalence, intensity, and mean intensity of D. grouperi on Epinephelus spp. in the wild were much lower than those in experimental (mixed species) culture ponds. The correlation between the molecular phylogeny of 5 species of Epinephelus and the dendrogram based on the susceptibility to D. grouperi was not significant, which infers that variable susceptibilities of these Epinephelus species cannot be revealed by phylogenetic relationships determined from mitochondrial 16S rDNA and Cyt b gene sequences.

Spatial distribution of enteroparasites among school children from Guarapuava, State of Paraná, Brazil

The most common infections in human beings are caused by intestinal parasites. They can lead to a number of harmful effects, which could include, among others, intestinal obstruction, malnutrition, iron deficiency anemia, diarrhea, and poor absorption. In Brazil, enteroparasites are one of the main public health issues. The present study aims at analyzing the distribution and frequency of enteroparasite occurrence in 635 children from seven community schools in the city of Guarapuava, Paraná (PR). In addition, we used similarity and diversity indices to analyze the parasite community. We found 475 samples with at least one parasite showing that 75.27% of children had enteroparasites. A smaller fraction (26.73%) of children harbored several parasites (multiparasitism), especially Giardia duodenalis (56%), and Ascaris lumbricoides (18%). Statistical analysis showed that three (out of seven) children communities had higher similarity in frequency and amount of parasites. Our results suggest that the children studied were highly infected by enteroparasites. These levels of infestation could be related to several factors, such as climate, social and economic conditions and characteristics of the parasites.

Microfilarial distribution of Loa loa in the human host: population dynamics and epidemiological implications

Severe adverse events (SAEs) following ivermectin treatment may occur in people harbouring high Loa loa microfilarial (mf) densities. In the context of mass ivermectin distribution for onchocerciasis control in Africa, it is crucial to define precisely the geographical distribution of L. loa in relation to that of Onchocerca volvulus and predict the prevalence of heavy infections. To this end, we analysed the distribution of mf loads in 4183 individuals living in 36 villages of central Cameroon. Mf loads were assessed quantitatively by calibrated blood smears, collected prior to ivermectin distribution. We explored the pattern of L. loa mf aggregation by fitting the (zero-truncated) negative binomial distribution and estimating its overdispersion parameter k by maximum likelihood. The value of k varied around 0.3 independently of mf intensity, host age, village and endemicity level. Based on these results, we developed a semi-empirical model to predict the prevalence of heavy L. loa mf loads in a community given its overall mf prevalence. If validated at the continental scale and linked to predictive spatial models of loiasis distribution, this approach would be particularly useful for optimizing the identification of areas at risk of SAEs and providing estimates of populations at risk in localities where L. loa and O. volvulus are co-endemic.

Density dependence and overdispersion in the transmission of helminth parasites

The influence of density-dependent processes on the transmission of parasitic helminths is determined by both the severity of the regulatory constraints and the degree of parasite overdispersion among the host population. We investigate how overdispersed parasite distributions among humans influence transmission levels in both directly- and indirectly-transmitted nematodes (Ascaris lumbricoides and Onchocerca volvulus). While past work has assumed, for simplicity, that density dependence acts on the average worm load, here we model density-dependence as acting on individual parasite burdens before averaging across hosts. A composite parameter, which we call the effective transmission contribution, is devised to measure the number of transmission stages contributed by a given worm burden after incorporating over-dispersion in adult worm mating probabilities and other density-dependent mechanisms. Results indicate that the more overdispersed the parasite population, the greater the effect of density dependence upon its transmission dynamics. Strong regulation and parasite overdispersion make the relationship between mean worm burden and its effective contribution to transmission highly non-linear. Consequently, lowering the intensity of infection in a host population using chemotherapy may produce only a small decline in transmission (relative to its initial endemic level). Our analysis indicates that when parasite burden is low, intermediate levels of parasite clustering maximize transmission. Implications are discussed in relation to existing control programmes and the spread of anthelmintic resistance.

The relationships between the burden of adult parasites, host age and the microfilarial density in human onchocerciasis

We investigate the relationship between the microfilarial density in the skin and the burden of adult female Onchocerca volvulus by analysing pre-control nodulectomy data which allow for a direct approach, independent of exposure. The data of 169 patients in Burkina Faso and 182 patients in Liberia represent savannah and forest onchocerciasis in West Africa, respectively. Whereas in Burkina Faso, a saturating relationship between microfilarial density and worm burden suggests the operation of density-dependent processes within human hosts, the Liberian data show a linear relationship implying no density dependence. The differences may derive from differences between both parasite strains, i.e. the savannah or the forest strain of O. volvulus. Consistently for both parasite strains and independent of the worm burden, the microfilarial density increases with host age emphasising the concept of the acquisition of immunological tolerance. In male hosts in Liberia, the microfilarial density increases stronger with the worm burden than in female hosts, whereas such sex-specific differences cannot be found in Burkina Faso. In the methodological part of this investigation, we suggest the beta-distribution to be most appropriate for describing variability in microfilarial densities and we present an approach to consider the uncertainty in the adult parasite burden which cannot be determined precisely in helminth infections. Implications of density dependence are discussed with respect to immunological processes in the human host and with respect to the success of control programs. The relationships described show that regulatory processes between the parasite and the human host are multi-dimensional, operating within a high degree of biological variability.

Genetics of susceptibility to human helminth infection

Recent studies have shown that host genetics is an important determinant of the intensity of infection and morbidity due to human helminths. Epidemiological studies of a number of parasite species have shown that the intensity of infection (worm burden) is a heritable phenotype. The proportion of variance in human worm burden explained by genetic effects varies from 0.21 to 0.44. Human genome scans have identified a locus responsible for controlling Schistosoma mansoni infection intensity on chromosome 5q31-q33, and loci controlling Ascaris lumbricoides intensity on chromosomes 1 and 13, although the genes involved have not yet been identified. There is also evidence for genetic control of pathology due to S. mansoni, and linkage has been reported to a region containing the gene for the interferon-gamma receptor 1 subunit. There is some evidence for genetic control of filarial infection, though little information on filarial disease. Association studies have provided evidence for major histocompatibility complex control of pathology in schistosomiasis and onchocerciasis. Recent candidate gene studies suggest a role of other immune response genes in controlling helminth infection and pathology, but require replication. Identification of the genetic loci involved may be important in the understanding of helminth epidemiology and the mechanisms of resistance and pathology.

Immunity, antigenic heterogeneity, and aggregation of helminth parasites

Empirical studies of helminth parasites reveal that the distribution of parasite burdens in their host populations is highly aggregated. This aggregation is fundamental to the ecology and epidemiology of helminth parasites. Results from a stochastic model predict that aggregation of helminth parasites is inversely related to the intensity of host immunity. Aggregation also decreases with antigenic heterogeneity and increases with heterogeneity in transmissibility among parasite strains. It is also found that the degree of aggregation is greater when immunity affects parasite fecundity than when immunity acts on host susceptibility. Potential relevance of this result for assessing the influence of vaccines that target either host susceptibility or parasite fecundity on the level of aggregation and consequent effects on drug resistance and disease prevalence are discussed.

Aggregation, stability, and oscillations in different models for host-macroparasite interactions

Aggregation is generally recognized as an important factor in the dynamics of host-macroparasite interactions and it has been found relevant in stabilizing the dynamics toward an equilibrium coexistence. In this paper we review the models of Anderson and May (1978, J. Anim. Ecol. 47, 219-247, 249-267) and compare them with some more recently developed models, which incorporate explicit mechanisms (multiple infections or host heterogeneity) for generating aggregation and different degrees of mathematical accuracy. We found that the stabilization yielded by aggregation depends strongly on the mechanism producing the aggregation: multiple infections are much less stabilizing than when aggregation is assumed to be fixed from the outside, while the opposite holds for host heterogeneity. We also give analytical estimates of the period of oscillations occurring when the equilibrium is unstable. Finally, we explore in these models the role of aggregation in host regulation and in determining a threshold value for parasite establishment.

[Implications of spatial aggregation of parasites for the population dynamics in host-parasite interaction]

Some aspects of the widely observed over-dispersed pattern of the distribution of parasites within the host population are examined. It has been established in the parasitological literature that most hosts usually harbour few parasites, while only few hosts harbour a large proportion of the parasite population. Factors that may influence the pattern of distribution of parasites, the relation between the level of parasite aggregation and the prevalence of infection, and changes in this level of aggregation as a function of host age are analysed. Factors which determine the diversity of species in parasite communities are presented, and aspects of exploitative and interference competition among parasites and their relations with biological control procedures are also considered. Attention is also focused on the regulatory and destabilizing processes influencing the dynamic behaviour of host-parasite population interactions.