Host immune responses are necessary for density dependence in nematode infections

Within-host and external environments differentially shape β-diversity across parasite life stages

Uncovering drivers of community assembly is a key aspect of learning how biological communities function. Drivers of community similarity can be especially useful in this task as they affect assemblage-level changes that lead to differences in species diversity between habitats. Concepts of β-diversity originally developed for use in free-living communities have been widely applied to parasite communities to gain insight into how infection risk changes with local conditions by comparing parasite communities across abiotic and biotic gradients. Factors shaping β-diversity in communities of immature parasites, such as larvae, are largely unknown. This is a key knowledge gap as larvae are frequently the infective life-stage and understanding variation in these larval communities is thus key for disease prevention. Our goal was to uncover links between β-diversity of parasite communities at different life stages; therefore, we used gastrointestinal nematodes infecting African buffalo in Kruger National Park, South Africa, to investigate within-host and extra-host drivers of adult and larval parasite community similarity. We employed a cross-sectional approach using PERMANOVA that examined each worm community at a single time point to assess independent drivers of β-diversity in larvae and adults as well as a longitudinal approach with path analysis where adult and larval communities from the same host were compared to better link drivers of β-diversity between these two life stages. Using the cross-sectional approach, we generally found that intrinsic, within-host traits had significant effects on β-diversity of adult nematode communities, while extrinsic, extra-host variables had significant effects on β-diversity of larval nematode communities. However, the longitudinal approach provided evidence that intrinsic, within-host factors affected the larval community indirectly via the adult community. Our results provide key data for the comparison of community-level processes where adult and immature stages inhabit vastly different habitats (i.e. within-host vs. abiotic environment). In the context of parasitism, this helps elucidate host infection risk via larval stages and the drivers that shape persistence of adult parasite assemblages, both of which are useful for predicting and preventing infectious disease.

Ascaridia galli eggs obtained from fresh excreta, worm uteri or worms cultured in artificial media differ in embryonation capacity and infectivity

Ascaridia galli infection models use eggs isolated from chicken excreta, worm uteri and worms cultured in artificial media. The aim of this study was to compare the infectivity of A. galli eggs isolated from these sources under two infection regimens. A 3 × 2 factorial arrangement was employed to test the infectivity of A. galli eggs from the three sources and two modes of infection (single or trickle infection). One hundred and fifty-six Isa-Brown one day-old cockerels randomly assigned to the six treatment groups (n = 26) were orally infected with embryonated A. galli eggs obtained from the three A. galli egg sources (worm uteri, excreta or eggs shed in vitro) administered either as single dose of 300 eggs at one day-old or trickle infected with 3 doses of 100 eggs over the first week of life. Twenty-two negative control birds remained uninfected. Eggs obtained from cultured worms or excreta exhibited a higher embryonation capacity (P = 0.003) than eggs obtained from worm uteri. There were higher worm establishment (infectivity) rates from embryonated eggs originating from cultured worms and worm uteri compared with eggs obtained from fresh excreta (P < 0.0001). Trickle infection resulted in a significantly higher total worm burden (P = 0.002), establishment rate (P = 0.002) and excreta egg counts (EEC, P = 0.025) than single infection. Worm length was greater in birds infected with embryonated eggs from excreta than from uteri or cultured worms (P < 0.0001). However, mode of infection did not affect worm length (P = 0.719) and weight (P = 0.945). A strong significant positive linear correlation was observed between EECs and female worm counts at 12 weeks of post infection sampling (r = 0.75; P < 0.0001). Body weight of birds was negatively correlated with both worm burden (r = - 0.21; P < 0.01) and EEC (r = - 0.20; P < 0.05) at 12 weeks post infection. In conclusion, our results show that eggs shed by cultured worms or isolated from worm uteri had greater infective capacity than eggs harvested from excreta and that trickle rather than bolus infection resulted in higher worm establishment. These factors should be taken into account when considering artificial infection protocols for A. galli.

The effect of different intensities of the ectoparasitic salmon lice (Lepeophtheirus salmonis) on Atlantic salmon (Salmo salar)

The effect of different intensities of the ectoparasitic salmon lice (Lepeophtheirus salmonis) on stress, growth and the expression of immune and wound healing transcripts in the skin of Atlantic salmon (Salmo salar) was investigated. Lice infection success and survival were similar at the chalimus and preadult stage in the low and high dose group, but infection success and survival were significantly lower in the high than in the low dose group at the adult stage. The expression of investigated transcripts was not correlated to lice intensities, but several of them were significantly differently expressed locally in the skin at the site of lice attachment in infected fish compared to controls. This included an up-regulation of pro-inflammatory markers at the site of lice attachment (e.g., interleukin 1-beta, interleukin 8 and the acute phase protein serum amyloid A), a reduction of markers of adaptive immunity (cluster of differentiation 8-alpha and immunoglobulin M) and decreased expression of the anti-inflammatory cytokine interleukin 10.

Host effects on the free-living stages of Haemonchus contortus

A group of 5 lambs (Host 1-5) was infected with the same batch of Haemonchus contortus and after patency individual faecal samples were collected, separately incubated at 23 °C for 14 days and third stage larvae collected through Baermannisation. Life-history traits were compared between larvae from different hosts: the length of the larvae was measured by microscope image analysis, larval survival in water at 35 °C, larval susceptibility to ivermectin (EC₅₀) in a migration assay, the proportion of larvae exsheathing in vitro and the proportion establishing to the adult stage in young lambs. For all traits there were significant differences between the host animals, with larvae from specific hosts following a consistent pattern of displaying the highest or lowest trait results. Compared with larvae from Host 1 the larvae from Host 5 were () shorter (741-692 μm, p < 0.05), had a longer median survival at 35 °C (3.6-6.4 days, p < 0.05), were less susceptible to ivermectin (EC₅₀ of 1.2 v 4.5 μM, p < 0.05), exsheathed to a lesser degree (83.6-58 %, p < 0.05), but showed a higher establishment rate in the consecutive host (15.2-31.4 %, p < 0.05). Regarding the survival time, anthelmintic susceptibility (under most commercial farming practices) and establishment rate as indicators for fitness, the parasites populating Host 5 produced progeny of higher fitness. The findings indicate that the host animal of the parental parasite generation has a significant effect on the parasite progeny.

Complex relationships between physiological stress and endoparasite infections in natural populations

Short-term elevation of glucocorticoids (GCs) is one of the major physiological mechanisms by which vertebrates cope with challenging environmental or social factors (stressors). However, when exposure to stressors occurs repeatedly or over a prolonged period of time, animals may experience chronic elevation of GCs, which reduces the immune response efficiency and can lead to higher intensity of parasitic infection. Here, we used invasive gray squirrels Sciurus carolinensis introduced in Northern Italy and their 2 most prevalent gastrointestinal parasites, the nematode Strongyloides robustus and coccidia of the genus Eimeria, as a model to investigate relationships among macroparasite infection and concentrations of fecal glucocorticoid metabolites (FGMs), an integrated measure of circulating GCs. Our results revealed an association of FGMs with infection by St. robustus, but not with coccidia. Individuals with higher FGMs appear to be responsible for the greatest St. robustus egg shedding within gray squirrel populations, thus possibly acting as superspreaders. However, FGMs were negatively associated with adult St. robustus, suggesting that the abundance of adults of this nematode species does not induce elevation in FGMs, but is only affected by it through immune-mediated effects on its fecundity. Finally, the relationship between St. robustus (both eggs and adult parasites) and FGMs was not linear, suggesting that only high levels of physiological stress influence parasite infection. Our findings highlight that the direction and magnitude of the stress-infection relationship may depend not only on the specific host-parasite system, but also on the different life stages of the same parasite.

Spillover of an alien parasite reduces expression of costly behaviour in native host species

Understanding the effects of invasive alien species (IAS) on native host-parasite relationships is of importance for enhancing ecological theory and IAS management. When IAS and their parasite(s) invade a guild, the effects of interspecific resource competition and/or parasite-mediated competition can alter existing native host-parasite relationships and the dependent biological traits such as native species' behaviour. We used a natural experiment of populations of native red squirrels Sciurus vulgaris that were colonized by the alien grey squirrel Sciurus carolinensis, comparing repeated measurements of red squirrel parasite infection and personality with those taken in sites where only the native species occurred. We explored two alternative hypotheses: (a) individual differences in personality traits (activity and/or sociability) of native red squirrel positively affect the probability of macroparasite spillover and thus the likelihood to acquire the alien's parasitic helminth Strongyloides robustus; (b) the combined effects of grey squirrel presence and parasite infection result in a reduction of costly personality traits (activity and/or exploration). Using data from 323 arena tests across three experimental (native species and IAS) and three control sites (only native species), we found negative correlations between native species' activity and infection with S. robustus in the sites invaded by the alien species. Activity was also negatively correlated with infection by its native helminth Trypanoxyuris sciuri but only when grey squirrels were present, while in the red-only sites there was no relationship of T. sciuri infection with any of the personality traits. Moreover, individuals that acquired S. robustus during the study reduced their activity after infection, while this was not the case for animals that remained uninfected. Our results show that parasite-mediated competition is costly, reducing activity in individuals of the native species, and altering the native host-native parasite relationships.

Host food quality and quantity differentially affect Ascogregarina barretti parasite burden, development and within-host competition in the mosquito Aedes triseriatus

Host condition depends in large part on the quality and quantity of available food and heavily influences the outcome of parasite infection. Although parasite fitness traits such as growth rate and size may depend on host condition, whether host food quality or quantity is more important to parasite fitness and within-host interactions is poorly understood. We provided individual mosquito hosts with a standard dose of a gregarine parasite and reared mosquitoes on two food types of different quality and two quantities. We measured host size, total parasite count and area, and average size of parasites within each treatment. Food quality significantly influenced the number of parasites in a host; hosts fed a low-quality diet were infected with more parasites than those provided a high-quality diet. In addition, we found evidence of within-host competition; there was a negative relationship between parasite size and count though this relationship was dependent on host food quality. Host food quantity significantly affected total parasite area and parasite size; lower food quantity resulted in smaller parasites and reduced overall parasite area inside the host. Thus both food quality and quantity have the potential to influence parasite fitness and population dynamics.

Association of MHC class II haplotypes with reduced faecal nematode egg count and IgA activity in British Texel sheep

Nematode infection is one of the principal diseases suffered by sheep and the class II region of the MHC has been repeatedly associated with differences in susceptibility and resistance to infection. The aim of this study was to examine the association of MHC class II haplotypes in a flock of Texel sheep with faecal egg counts and antibody responsiveness. Two haplotypes carried the DRB1*11:01 allele which has previously been associated with reduced egg counts in Scottish Blackface and Suffolk sheep. One of the two haplotypes was associated with reduced egg counts in the Texel breed, and both haplotypes were associated with reduced IgA activity against an extract from fourth-stage larvae. The reduced IgA activity is probably a consequence of reduced numbers of fourth-stage larvae in sheep carrying the resistance allele. The association of specific MHC alleles with reduced egg counts, reduced worm numbers and decreased IgA activity provides a mechanism for the density-dependent regulation of parasite growth and fecundity.

Changes in parasite traits, rather than intensity, affect the dynamics of infection under external perturbation

Understanding the mechanisms that generate complex host-parasite interactions, and how they contribute to variation between and within hosts, is important for predicting risk of infection and transmission, and for developing more effective interventions based on parasite properties. We used the T. retortaeformis (TR)-rabbit system and developed a state-space mathematical framework to capture the variation in intensity of infection and egg shedding in hosts infected weekly, then treated with an anthelminthic and subsequently re-challenged following the same infection regime. Experimental infections indicate that parasite intensity accumulates more slowly in the post-anthelminthic phase but reaches similar maximum numbers. By contrast, parasite EPG (eggs per gram of feces) shed from rabbits in the post-treatment phase is lower and less variable through time. Inference based on EPG alone suggests a decline in parasite intensity over time. Using a state-space model and incorporating all sources of cross-sectional and longitudinal data, we show that while parasite intensity remains relatively constant in both experimental phases, shedding of eggs into the environment is increasingly limited through changes in parasite growth. We suggest that host immunity directly modulates both the accumulation and the growth of the parasite, and indirectly affects transmission by limiting parasite length and thus fecundity. This study provides a better understanding of how within-host trophic interactions influence different components of a helminth population. It also suggests that heterogeneity in parasite traits should be addressed more carefully when examining and managing helminth infections in the absence of some critical data on parasite dynamics.

Breaking beta: deconstructing the parasite transmission function

Transmission is a fundamental step in the life cycle of every parasite but it is also one of the most challenging processes to model and quantify. In most host–parasite models, the transmission process is encapsulated by a single parameter β. Many different biological processes and interactions, acting on both hosts and infectious organisms, are subsumed in this single term. There are, however, at least two undesirable consequences of this high level of abstraction. First, nonlinearities and heterogeneities that can be critical to the dynamic behaviour of infections are poorly represented; second, estimating the transmission coefficient β from field data is often very difficult. In this paper, we present a conceptual model, which breaks the transmission process into its component parts. This deconstruction enables us to identify circumstances that generate nonlinearities in transmission, with potential implications for emergent transmission behaviour at individual and population scales. Such behaviour cannot be explained by the traditional linear transmission frameworks. The deconstruction also provides a clearer link to the empirical estimation of key components of transmission and enables the construction of flexible models that produce a unified understanding of the spread of both micro- and macro-parasite infectious disease agents.

This article is part of the themed issue ‘Opening the black box: re-examining the ecology and evolution of parasite transmission’.

The Immunology of Wild Rodents: Current Status and Future Prospects

Wild animals' immune responses contribute to their evolutionary fitness. These responses are moulded by selection to be appropriate to the actual antigenic environment in which the animals live, but without imposing an excessive energetic demand which compromises other component of fitness. But, exactly what these responses are, and how they compare with those of laboratory animals, has been little studied. Here, we review the very small number of published studies of immune responses of wild rodents, finding general agreement that their humoral (antibody) responses are highly elevated when compared with those of laboratory animals, and that wild rodents' cellular immune system reveals extensive antigenic exposure. In contrast, proliferative and cytokine responses of ex vivo-stimulated immune cells of wild rodents are typically depressed compared with those of laboratory animals. Collectively, these responses are appropriate to wild animals' lives, because the elevated responses reflect the cumulative exposure to infection, while the depressed proliferative and cytokine responses are indicative of effective immune homeostasis that minimizes immunopathology. A more comprehensive understanding of the immune ecology of wild animals requires (i) understanding the antigenic load to which wild animals are exposed, and identification of any key antigens that mould the immune repertoire, (ii) identifying immunoregulatory processes of wild animals and the events that induce them, and (iii) understanding the actual resource state of wild animals, and the immunological consequences that flow from this. Together, by extending studies of wild rodents, particularly addressing these questions (while drawing on our immunological understanding of laboratory animals), we will be better able to understand how rodents' immune responses contribute to their fitness in the wild.

Sources of variability in the measurement of Ascaris lumbricoides infection intensity by Kato-Katz and qPCR

Background

Understanding and quantifying the sources and implications of error in the measurement of helminth egg intensity using Kato-Katz (KK) and the newly emerging “gold standard” quantitative polymerase chain reaction (qPCR) technique is necessary for the appropriate design of epidemiological studies, including impact assessments for deworming programs.

Methods

Repeated measurements of Ascaris lumbricoides infection intensity were made from samples collected in western Kenya using the qPCR and KK techniques. These data were combined with data on post-treatment worm expulsions. Random effects regression models were used to quantify the variability associated with different technical and biological factors for qPCR and KK diagnosis. The relative precision of these methods was compared, as was the precision of multiple qPCR replicates.

Results

For both KK and qPCR, intensity measurements were largely determined by the identity of the stool donor. Stool donor explained 92.4% of variability in qPCR measurements and 54.5% of observed measurement variance for KK. An additional 39.1% of variance in KK measurements was attributable to having expelled adult A. lumbricoides worms following anthelmintic treatment. For qPCR, the remaining 7.6% of variability was explained by the efficiency of the DNA extraction (2.4%), plate-to-plate variability (0.2%) and other residual factors (5%). Differences in replicate measurements by qPCR were comparatively small. In addition to KK variability based on stool donor infection levels, the slide reader was highly statistically significant, although it only explained 1.4% of the total variation. In a comparison of qPCR and KK variance to mean ratios under ideal conditions, the coefficient of variation was on average 3.6 times larger for KK highlighting increased precision of qPCR.

Conclusions

Person-to-person differences explain the majority of variability in egg intensity measurements by qPCR and KK, with very little additional variability explained by the technical factors associated with the practical implementation of these techniques. qPCR provides approximately 3.6 times more precision in estimating A. lumbricoides egg intensity than KK, and could potentially be made more cost-effective by testing each sample only once without diminishing the power of a study to assess population-level intensity and prevalence.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2164-y) contains supplementary material, which is available to authorized users.

Study on the tolerance and adaptation of rats to Angiostrongylus cantonensis infection

Angiostrongylus cantonensis (A. cantonensis) is the most common infectious agent causing eosinophilic meningitis. As an important food-borne parasitic disease, angiostrongyliasis cantonensis is an emerging infectious disease which brings severe harm to central nerve system of human. Rat, one of the few permissive hosts of A. cantonensis known to date, plays an indispensable role in the worm's life cycle. However, the tolerance and adaptation of rat to A. cantonensis infection is rarely understood. In this study, we infected rats with different numbers the third stage larvae (L3) of A. cantonensis and explored their tolerance through analysis on survival curve, neurological function score, and detection of pathological damages in organs including the brain, lung, and heart of the animals. Results indicated that rats' survival condition worsens, and body weight dropped more significantly as more worms were used for infection. Death appeared in groups infected with 80 and more A. cantonesnsis per rat. Morris water maze revealed that the neurological function of rats damaged gradually with increasing infection number of A. cantonensis larvae. When the number of infected parasite exceeded 240 per animal, rats showed significant neurological impairments. Collection of A. cantonensis from rat lung after 35 days of infection implied an upper limit for worm entry, and the average length of worm was inversely proportional to the infection amount, while the ratio between female and male worms was positively related to the infection number. The degree of pulmonary and cardiac inflammation was proportional to the infection number of A. cantonensis. Meanwhile, there existed considerable amount of adult worms in rat's right atrium and right ventricle, leading to a right heart myocardial inflammation. The present study firstly reports the tolerance and adaptation of rat, a permissive host of A. cantonensis to its infection, which will not only provide accurate technical parameters for maintaining A. cantonensis life cycle under laboratory conditions but also help unveil the underlying mechanism of the distinct pathological outcomes in the permissive and non-permissive hosts with A. cantonensis infection.

Parasite fecundity decreases with increasing parasite load in the salmon louse Lepeophtheirus salmonis infecting Atlantic salmon Salmo salar

Aggregation is common amongst parasites, where a small number of hosts carry a large proportion of parasites. This could result in density-dependent effects on parasite fitness. In a laboratory study, we explored whether parasite load affected parasite fecundity and survival, using ectoparasitic salmon lice (Lepeophtheirus salmonis Krøyer, 1837) infecting Atlantic salmon (Salmo salar) hosts. We found a significant reduction in fecundity with higher parasite load, but no significant effect on survival. Together with previous findings, this suggests that stronger competition amongst female lice under high parasite load is a more likely explanation than increased host immune response.

Meta-analysis of the parasitic phase traits of Haemonchus contortus infection in sheep

BACKGROUND

The parasitic nematode Haemonchus contortus shows highly variable life history traits. This highlights the need to have an average estimate and a quantification of the variation around it to calibrate epidemiological models.

METHODS

This paper aimed to quantify the main life history traits of H. contortus and to identify explanatory factors affecting these traits using a powerful method based on a systematic review and meta-analysis of current literature. The life history traits considered are: (i) the establishment rate of ingested larvae; (ii) the adult mortality rate; (iii) the fertility (i.e. the number of eggs laid/female/day); and (iv) fecundity of female worms (i.e. the number of eggs per gram of faeces).

RESULTS

A total of 37 papers that report single experimental infection with H. contortus in sheep and published from 1960 to 2015, were reviewed and collated in this meta-analysis. This encompassed 115 experiments on 982 animals. Each trait was analysed using a linear model weighted by its inverse variance. The average (± SE) larval establishment rate was 0.24 ± 0.02, which decreased as a function of the infection dose and host age. An average adult mortality rate of 0.021 ± 0.002) was estimated from the literature. This trait varied as a function of animal age, breed and protective response due to prior exposure to the parasite. Average female fertility was 1295.9 ± 280.4 eggs/female/day and decreased in resistant breeds and previously infected hosts. Average faecal egg count at necropsy was 908.5 ± 487.1 eggs per gram of faeces and varied as a function of infection duration and host resistance. The average sex ratio of H. contortus was 0.51 ± 0.006.

CONCLUSION

This work is the first systematic review to summarise the available information on the parasitic phase of H. contortus in sheep. The results of the meta-analysis provide robust estimates of life history traits for parametrization of epidemiological models, their expected variation according to experimental factors, and provides correlations between these.

Parasitological and transcriptomic comparison of Strongyloides ratti infections in natural and in suboptimal permissive hosts

The nematode genus Strongyloides consists of fairly species-specific small intestinal parasites of various vertebrates, among them the human pathogen S. stercoralis. Between the parthenogenetic parasitic generations these worms can also form single facultative sexual free-living generations. In addition to their primary hosts, several species can also live more or less well in other permissive hosts, which are sometimes not very closely related with the normal host. For example, S. stercoralis can also infect dogs and non-human primates. Here we compare the infection and reproductive success over time and the gene expression profiles as determined by quantitative sequencing of S. ratti parasitizing in its natural host rat and in the permissive host gerbil. We show that in gerbils fewer infective larvae successfully establish in the host, but those that do accomplish this survive and reproduce for longer and produced a higher proportion of males during the first two month of infection. Globally, the gene expression profiles in the two hosts are very similar. Among the relatively few differentially expressed genes, astacin-like and acetylcholinesterase genes are prominently represented. In the future it will be interesting to see if these changes in the suboptimal host are indeed ecologically sensible responses to the different host.

Major histocompatibility complex class II DAB alleles associated with intestinal parasite load in the vulnerable Chinese egret (Egretta eulophotes)

The maintenance of major histocompatibility complex (MHC) polymorphism has been hypothesized to result from many mechanisms such as rare-allele advantage, heterozygote advantage, and allele counting. In the study reported herein, 224 vulnerable Chinese egrets (Egretta eulophotes) were used to examine these hypotheses as empirical results derived from bird studies are rare. Parasite survey showed that 147 (65.63%) individuals were infected with 1-3 helminths, and 82.31% of these infected individuals carried Ascaridia sp. Using asymmetric polymerase chain reaction technique, 10 DAB1, twelve DAB2, and three DAB3 exon 2 alleles were identified at each single locus. A significant association of the rare allele Egeu-DAB2*05 (allele frequency: 0.022) with helminth resistance was found for all helminths, as well as for the most abundant morphotype Ascaridia sp. in the separate analyses. Egeu-DAB2*05 occurred frequently in uninfected individuals, and individuals carrying Egeu-DAB2*05 had significantly lower helminth morphotypes per individual (HMI) (the number of HMI) and the fecal egg count values. Further, the parasite infection measurements were consistently lower in individuals with an intermediate number of different alleles in the duplicated DAB loci. Significantly, heterozygosity within each DAB locus was not correlated with any parasite infection measurements. These results indicate that the diversity in MHC Egeu-DAB gene is associated with intestinal parasite load and maintained by pathogen-driven selection that probably operate through both the rare-allele advantage and the allele counting strategy, and suggest that Egeu-DAB2*05 might be a valuable indicator of better resistance to helminth diseases in the vulnerable Chinese egret.

Evaluation of gastrointestinal transit after infection with different loads of Strongyloides venezuelensis in rats

The aim was to correlate the gastrointestinal transit profile in rats, evaluated by a biomagnetic technique, in response to infection with different loads of Strongyloides venezuelensis. Eggs per gram, intestinal number of worms and fecundity, and also gastric emptying time, cecum arrival time, small intestinal transit time and stool weight were determined. Assessments occurred at 0 (control), 3, 6, 9, 12, 15, 18 and 21 days post infection (dpi) with three infective loads (400, 2000, and 10,000 L). Gastric emptying was faster (p=0.0001) and the intestinal transit was significantly slower (p=0.001) during the infection time course. Also, linear mixed-effects models showed significantly changes in small intestinal transit after three parasite load over time. Cecum arrival was not influenced by infection time course or parasite load. As indirect effect, stool weight decreased accompanied a strong oviposition peak at 9 dpi in 400 L and 2000 L. In several motor function instances, neuromuscular dysfunction persists after mucosal inflammation has decreased. Our approach could be very helpful to evaluate gastrointestinal motor abnormalities in vivo after parasite infection. Despite parasitological data progressively decreased after 15 dpi, small intestinal transit worse over time and according to burden.

Density-dependent regulation of fecundity in Syngamus trachea infrapopulations in semi-naturally occurring ring-necked pheasants (Phasianus colchicus) and wild Carrion Crows (Corvus corone)

Previous work has highlighted increased opportunities for the transmission of Syngamus trachea within pheasant release pens, due in part to high levels of environmental contamination around communal areas. Despite this, the distribution of adult worms within their definitive hosts is not significantly different from predicted distributions under Taylor's power law. Therefore, density-dependent processes are probably acting to regulate S. trachea population dynamics. Patterns of nematode fecundity were investigated in a semi-naturally occurring population of ring-necked pheasants (Phasianus colchicus) and a wild population of carrion crows (Corvus carone). Worm length was a reliable indicator of nematode fecundity, and a negative association between mean worm length and mean worm burden was identified within both the species. The stunting of worms at greater parasite densities was present in both immunologically naïve and previously exposed pheasants, so is unlikely to be a function of age-dependent acquired immunity. Interestingly, the effect of parasite crowding in the crow population explained more of the variation in mean worm length, apparently driven by a greater mean worm burden when compared with pheasants. The findings of the present study suggest that fecundity is a function of parasite density, i.e. parasite-mediated competition and not host-mediated heterogeneities in immunocompetence.

Strongyloides ratti and S. venezuelensis - rodent models of Strongyloides infection

Strongyloides spp. are common parasites of vertebrates and two species, S. ratti and S. venezuelensis, parasitize rats; there are no known species that naturally infect mice. Strongyloides ratti and S. venezuelensis overlap in their geographical range and in these regions co-infections appear to be common. These species have been widely used as tractable laboratory systems in rats as well as mice. The core biology of these two species is similar, but there are clear differences in aspects of their within-host biology as well as in their free-living generation. Phylogenetic evidence suggests that S. ratti and S. venezuelensis are the result of two independent evolutionary transitions to parasitism of rats, which therefore presents an ideal opportunity to begin to investigate the basis of host specificity in Strongyloides spp.

High prevalence of Ancylostoma ceylanicum hookworm infections in humans, Cambodia, 2012

Preventative chemotherapy without community hygiene and animal health programs may be leading to emergence of this zoonosis.

Ancylostoma ceylanicum, a hookworm of canids and felids in Asia, is becoming the second most common hookworm infecting humans. In 2012, we investigated the prevalence and infection dynamics of and risk factors for hookworm infections in humans and dogs in a rural Cambodian village. Over 57% of the population was infected with hookworms; of those, 52% harbored A. ceylanicum hookworms. The greatest intensities of A. ceylanicum eggs were in persons 21-30 years of age. Over 90% of dogs also harbored A. ceylanicum hookworms. Characterization of the cytochrome oxidase-1 gene divided isolates of A. ceylanicum hookworms into 2 groups, 1 containing isolates from humans only and the other a mix of isolates from humans and animals. We hypothesize that preventative chemotherapy in the absence of concurrent hygiene and animal health programs may be a factor leading to emergence of A. ceylanicum infections; thus, we advocate for a One Health approach to control this zoonosis.

The crowding effect in Ancylostoma ceylanicum: density-dependent effects on an experimental model of infection

This study compared the course of Ancylostoma ceylanicum infection in hamsters infected with different inocula and the consequences for the host and helminth populations. The average of adult worms recovered, according to the number of third stage larva used, were 28.0, 24.8, 24.6, and 24.8% to inocula size of 25 L3, 75 L3, 125 L3, and 250 L3, respectively. The size of the inoculum did not affect the establishment, survival, or fecundity of adult helminths. Reductions in the red blood cell and hemoglobin levels in the infected group were inversely proportional to the number of white blood cells. Moreover, differential cell counting revealed a positive correlation between the worm load and leucocyte numbers. The humoral response against excretion-secretion antigens was more robust and sensitive compared with the response against crude extract, with no direct linear correlation with the number of worms. The effect of the population density was more evident in females.

Nematode-coccidia parasite co-infections in African buffalo: Epidemiology and associations with host condition and pregnancy

Co-infections are common in natural populations and interactions among co-infecting parasites can significantly alter the transmission and host fitness costs of infection. Because both exposure and susceptibility vary over time, predicting the consequences of parasite interactions on host fitness and disease dynamics may require detailed information on their effects across different environmental (season) and host demographic (age, sex) conditions. This study examines five years of seasonal health and co-infection patterns in African buffalo (Syncerus caffer). We use data on two groups of gastrointestinal parasites, coccidia and nematodes, to test the hypothesis that co-infection and season interact to influence (1) parasite prevalence and intensity and (2) three proxies for host fitness: host pregnancy, host body condition, and parasite aggregation. Our results suggest that season-dependent interactions between nematodes and coccidia affect the distribution of infections. Coccidia prevalence, coccidia intensity and nematode prevalence were sensitive to factors that influence host immunity and exposure (age, sex, and season) but nematode intensity was most strongly predicted by co-infection with coccidia and its interaction with season. The influence of co-infection on host body condition and parasite aggregation occurred in season-dependent manner. Co-infected buffalo in the early wet season were in worse condition, had a less aggregated distribution of nematode parasites, and lower nematode infection intensity than buffalo infected with nematodes alone. We did not detect an effect of infection or co-infection on host pregnancy. These results suggest that demographic and seasonal variation may mediate the effects of parasites, and their interactions, on the distribution and fitness costs of infection.

Density related effects on lifetime fecundity of Heterakis gallinarum in chickens

Background

Density related effects, both inverse density- and density-dependent, contribute to regulating population dynamics of parasites. We investigated whether density related effects are directly controlling lifetime fecundity of Heterakis gallinarum.

Methods

Daily total numbers of H. gallinarum eggs in faeces samples (N = 1365) from chickens (N = 39) were quantified starting from 3 weeks (wk) post-infection (p.i.). The birds were necropsied 8 wk p.i., and intensity and demographic characteristics of infrapopulations were determined. Density related effects on cumulative egg excretion (CEE), lifetime fecundity and worm length were investigated with a segmented regression analysis.

Results

For CEE, lifetime fecundity and female worm length, we determined highly similar parasite intensity thresholds (52–54 worms), which separated infrapopulations for influences of inverse density- and density dependence. CEE increased as parasite intensity increased up to an intensity of 52 worms. After this threshold, the relationship followed more of a horizontal line indicating impaired worm fecundity at higher parasite intensities. Lifetime fecundity was enhanced linearly in infrapopulations with up to 54 worms, but thereafter decreased gradually with increasing infrapopulation size. Female worm length increased linearly with elevating parasite intensity up to a threshold of 54 worms and thereafter declined with a rate of -0.014 mm for each additional worm. Lifetime fecundity and female worm length did not significantly differ between infrapopulations below and above the thresholds (P > 0.05). Lifetime fecundity was positively associated with the percentage of male worms (r = 0.44; P < 0.001), but negatively with absolute deviation from the theoretically expected sex-ratio in the infrapopulations (r = -0.56; P = 0.005). These relationships were stronger in infrapopulations below the threshold (r = 0.51 and -0.61, respectively), and were not significantly different from zero in the infrapopulations above the threshold (P > 0.05).

Conclusions

Egg production of H. gallinarum is regulated by the effects of both inverse density- and density-dependent mechanisms, which result in similar average lifetime fecundity below or above intensity thresholds. In infrapopulations below the intensity thresholds, inverse density dependence effects on lifetime fecundity appear to result partly from sex-ratio fluctuations and impaired mating success of the nematode.

Faecal egg counts from field experiment reveal density dependence in helminth fecundity: Strongyloides robustus infecting grey squirrels (Sciurus carolinensis)

Investigation of endo-macroparasite infections in living animals relies mostly on indirect methods aimed to detect parasite eggs in hosts' faeces. However, faecal flotation does not provide quantitative information on parasite loads, whereas faecal egg count (FEC) techniques may not give reliable estimates of parasite intensity, since egg production may be affected by density-dependent effects on helminth fecundity. We addressed this issue using Eastern grey squirrels (Sciurus carolinensis) and their gastrointestinal nematode Strongyloides robustus to assess the performance of coprological techniques and to investigate factors affecting parasite fecundity. We compared results of gut examination, flotation and McMaster FECs in 65 culled grey squirrels. Sensitivity and specificity of flotation were 81.2% (Confidence Interval, CI 54.3-95.9%) and 85.7% (CI 72.7-94.1%), respectively, resulting in low positive predictive values when infection prevalence is low. Individual parasite fecundity (no. of eggs/adult female worm) was negatively affected by S. robustus intensity, leading to a non-linear relationship between parasite load and eggs/gram of faeces (EPG). As a consequence, whereas flotation may be a valid method to perform the first screening of infection status, FECs are not a reliable method to estimate S. robustus intensity, since diverse values of EPG may correspond to the same number of parasites. Neither the amount of analysed faeces nor the season had any effect on EPG, indicating that the observed reduction in helminth fecundity is likely caused exclusively by density-dependent processes such as competition among worms or host immune response.

Physalopterine nematodes in Australian reptiles: interactions and patterns of infection

Spirurid nematodes (family Physalopteridae) are widespread as adults or as encysted larvae in many species of Australian reptiles. Fifteen species of physalopterine nematodes (subfamily Physalopterinae) in the genera Kreisiella, Abbreviata and Skrjabinoptera infect more than 40 species of reptile in the five families Agamidae, Varanidae, Gekkonidae, Scincidae and Elapidae. Four species of nematode are host-species specific, six are host-family specific to varanid lizards, and three to agamid lizards. Larger species of reptile support a higher prevalence and abundance of nematodes, and often support multiple infections with more than one species, with the potential for interspecific competition. Geographic distribution of nematodes is partially limited by host distribution, and by climatic factors, mainly precipitation and temperature. There are strong positive and negative associations between several pairs of nematodes. Two species of nematode with the most pronounced muscular development at the anterior end, Abbreviata tumidocapitis and Abbreviata glebopalmae, only occur concurrently, and in low numbers, with species of nematode without these morphological features, suggesting differences in feeding in the hosts’ stomachs. A combination of host specificity, geographic distribution and habitat, climatic factors and feeding organ morphology are factors that probably reduce the potential for interspecific competition. There is no evidence that concurrent infections affect either prevalence or abundance of nematodes, or cause discernible pathological changes to their hosts.

Patterns and processes in parasite co-infection

Co-infection of individual hosts by multiple parasite species is a pattern that is very commonly observed in natural populations. Understanding the processes that generate these patterns poses a challenge. For example, it is difficult to discern the relative roles of exposure and susceptibility in generating the mixture and density of parasites within hosts. Yet discern them we must, if we are to design and deliver successful medical interventions for co-infected populations. Here, we synthesise an emergent understanding of how processes operate and interact to generate patterns of co-infection. We consider within-host communities (or infracommunities) generally, that is including not only classical parasites but also the microbiota that are so abundant on mucosal surfaces and which are increasingly understood to be so influential on host biology. We focus on communities that include a helminth, but we expect similar inferences to pertain to other taxa. We suggest that, thanks to recent research at both the within-host (e.g. immunological) and between-host (e.g. epidemiological) scales, researchers are poised to reveal the processes that generate the observed distribution of parasite communities among hosts. Progress will be facilitated by using new technologies as well as statistical and experimental tools to test competing hypotheses about processes that might generate patterns in co-infection data. By understanding the multiple interactions that underlie patterns of co-infection, we will be able to understand and intelligently predict how a suite of co-infections (and thus the host that bears them) will together respond to medical interventions as well as other environmental changes. The challenge for us all is to become scholars of co-infections.

Slaving and release in co-infection control

BACKGROUND

Animal and human infection with multiple parasite species is the norm rather than the exception, and empirical studies and animal models have provided evidence for a diverse range of interactions among parasites. We demonstrate how an optimal control strategy should be tailored to the pathogen community and tempered by species-level knowledge of drug sensitivity with use of a simple epidemiological model of gastro-intestinal nematodes.

METHODS

We construct a fully mechanistic model of macroparasite co-infection and use it to explore a range of control scenarios involving chemotherapy as well as improvements to sanitation.

RESULTS

Scenarios are presented whereby control not only releases a more resistant parasite from antagonistic interactions, but risks increasing co-infection rates, exacerbating the burden of disease. In contrast, synergisms between species result in their becoming epidemiologically slaved within hosts, presenting a novel opportunity for controlling drug resistant parasites by targeting co-circulating species.

CONCLUSIONS

Understanding the effects on control of multi-parasite species interactions, and vice versa, is of increasing urgency in the advent of integrated mass intervention programmes.

Evolutionary divergence of the swim bladder nematode Anguillicola crassus after colonization of a novel host, Anguilla anguilla

Background

Anguillicola crassus, a swim bladder nematode naturally parasitizing the Japanese eel, was introduced about 30 years ago from East Asia into Europe where it colonized almost all populations of the European eel. We conducted a common garden experiment under a reciprocal transfer design infecting both European and Japanese eels with populations of A. crassus from Germany, Poland and Taiwan. We tested, whether differences in infectivity, developmental dynamics and reproductive output between the European and Asian parasite populations occur while harboured in the specimens of native and colonized eel host, and if these differences are genetically based or are plastic responses to the new environment.

Results

Under common garden conditions an evolutionary change in the both European parasite populations of A. crassus compared with their Taiwanese conspecifics was observed for infectivity and developmental dynamics, but not for reproductive output. When infecting the European eel, current European populations of the parasite were less infective and developed faster than their Taiwanese conspecifics. In the reciprocally infected Japanese eel the genetically induced differences between the parasite strains were less apparent than in the European eel but higher infectivity, faster development and higher larval mortality of the European parasite populations could be inferred.

Conclusions

The differences in infectivity and developmental dynamics between European and Taiwanese populations of A. crassus found in our study suggest rapid genetic divergence of this parasite after a successful host switch in Europe.

Characterization of genes with a putative key role in the parasitic lifestyle of the nematode Strongyloides ratti

Parasitic nematodes are significant pathogens of humans and other animals. The molecular and genetic basis of animal parasitism is not yet fully understood. Strongyloides spp. are a genus of gastrointestinal nematodes of which species infect approximately 100&#x2013;200 million people worldwide. S. ratti is a natural parasite of the rat, and a useful and amenable laboratory model. Previous EST and microarray analyses of the S. ratti life cycle have identified genes whose expression was specific, or biased, to the parasitic adult stage, suggesting that they may play a key role in parasitism in this species. Here we have further investigated the expression of these genes (by RT-PCR) throughout the S. ratti life-cycle. We produced recombinant proteins in vitro for a subset of these genes, which were used in Western blot analyses to investigate the distribution of the gene products among different stages of the S. ratti life cycle. We tested the efficacy of these recombinant proteins as anti-S. ratti vaccines. One of the proteins was detected in the excretory&#x002F;secretory products of the parasitic stages.

Explaining patterns of infection in free-living populations using laboratory immune experiments

The host response to different helminth species can vary and have different consequences for helminth persistence. Often these differences are generated by changes in the dynamics and intensity of the immune components against parasites with distinct life history strategies. We examined the immune response of rabbits to primary infections of the gastrointestinal nematodes Trichostrongylus retortaeformis and Graphidium strigosum under controlled conditions for 120 days post-challenge. Results showed that rabbits developed a robust and effective immune response against T. retortaeformis and abundance quickly decreased in the duodenum and was completely cleared in the remaining sections of the small intestine within 4 months. Infected individuals exhibited an initial strong inflammatory response (IFN-γ), IL-4 expression also increased and was coupled to a rapid serum and mucus IgG and IgA and eosinophilia. Strong IL-4, serum IgA and IgG responses and eosinophilia were also observed against G. strigosum. However, parasite abundance remained consistently high throughout the infection, and this was associated with relatively low mucus antibodies. These findings suggest that immunity plays a key role in affecting the abundance of these nematodes, and different immune mechanisms are involved in regulating the dynamics of each infection and their long-term persistence in free-living host populations.

Interactive effects of protein nutrition, genetic growth potential and Heligmosomoides bakeri infection pressure on resilience and resistance in mice

The ability of animals to cope with an increasing parasite load, in terms of resilience and resistance, may be affected by both nutrient supply and demand. Here, we hypothesized that host nutrition and growth potential interact and influence the ability of mice to cope with different parasite doses. Mice selected for high (ROH) or low (ROL) body weight were fed a low (40 g/kg; LP) or high (230 g/kg; HP) protein diet and infected with 0, 50, 100, 150, 200 or 250 L3 infective Heligmosomoides bakeri larvae. ROH-LP mice grew less at doses of 150 L3 and above, whilst growth of ROH-HP and of ROL mice was not affected by infection pressure. Total worm burdens reached a plateau at doses of 150L3, whilst ROH mice excreted fewer worm eggs than ROL mice. Serum antibodies increased with infection dose and ROH mice were found to have higher parasite-specific IgG1 titres than ROL mice. In contrast, ROL had higher total IgE titres than ROH mice, only on HP diets. The interaction between host nutrition and growth potential appears to differentially affect resilience and resistance in mice. However, the results support the view that parasitism penalises performance in animals selected for higher growth.

The influence of MHC and immunoglobulins a and e on host resistance to gastrointestinal nematodes in sheep

Gastrointestinal nematode parasites in farmed animals are of particular importance due to their effects on production. In Australia, it is estimated that the direct and indirect effects of parasite infestation cost the animal production industries hundreds of millions of dollars each year. The main factors considered by immunologists when studying gastrointestinal nematode infections are the effects the host's response has on the parasite, which immunological components are responsible for these effects, genetic factors involved in controlling immunological responses, and the interactions between these forming an interconnecting multilevel relationship. In this paper, we describe the roles of immunoglobulins, in particular IgA and IgE, and the major histocompatibility complex in resistance to gastrointestinal parasites in sheep. We also draw evidence from other animal models to support the involvement of these immune components. Finally, we examine how IgA and IgE exert their influence and how methods may be developed to manage susceptible animals.

Response of the Strongyloides ratti transcriptome to host immunological environment

The immunological environment experienced by parasitic nematodes varies greatly between hosts and is particularly influenced by whether or not a host has been previously infected. How a parasitic nematode responds to these different environments is poorly understood, but may allow a parasite to ameliorate the adverse effects of host immunity on parasite fitness. Here we use a microarray approach to identify genes in the parasitic nematode Strongyloides ratti that exhibit differential transcription between different rat host immunological environments, and between replicate lines of S. ratti selected for either early or late reproduction. We hypothesise that such genes may be used by this species to cope with and respond to its host environment. Our results showed that, despite large phenotypic differences between S. ratti adults from different immunological environments, the S. ratti transcriptome exhibited a relatively stable pattern of expression. Thus, differential expression amongst treatments was limited to a small proportion of transcripts and generally involved only modest fold changes. These transcripts included a group of collagen genes up-regulated in parasites early in an infection, and in immunised host environments, which may be related to protection against the damage caused to a parasite by host immune responses. We found that later in an infection, a number of genes associated with muscle function and repair were up-regulated in immunised host environments; these may help parasites maintain their position in the host intestine. Differences in transcription between selection lines of S. ratti were only observed in immunised hosts and included genes associated with the response to the host's immunological environment.

The relative importance of host characteristics and co-infection in generating variation in Heligmosomoides polygyrus fecundity

We examined the relative importance of intrinsic host factors and microparasite co-infection in generating variation in Heligmosomoides polygyrus fecundity, a parameter that serves as a proxy for infectiousness. We undertook extensive trapping of Apodemus flavicollis, the yellow-necked mouse in the woodlands of the Italian Alps and recorded eggs in utero from the dominant nematode species H. polygyrus, and tested for the presence of five microparasite infections. The results showed that sex and breeding status interact, such that males in breeding condition harboured more fecund nematodes than other hosts; in particular, worms from breeding males had, on average, 52% more eggs in utero than worms from non-breeding males. In contrast, we found a weak relationship between intensity and body mass, and no relationship between intensity and sex or intensity and breeding condition. We did not find any evidence to support the hypothesis that co-infection with microparasites contributed to variation in worm fecundity in this system. The age-intensity profiles for mice singly-infected with H. polygyrus and those co-infected with the nematode and at least one microparasite were both convex and not statistically different from each other. We concluded that intrinsic differences between hosts, specifically with regard to sex and breeding condition, contribute relatively more to the variation in worm fecundity than parasite co-infection status.

Immunity-mediated regulation of fecundity in the nematode Heligmosomoides polygyrus--the potential role of mast cells

Previous studies have shown that host immunity regulates the fecundity of nematodes. The present study was aimed at clarifying the reversible nature of fecundity in response to changes of immunological status and to determine which effector cells are responsible for compromising fecundity in Heligmosomoides polygyrus. Enhanced fecundity was observed in immunocompromised SCID and nu/nu mice compared to those in the corresponding wild-type mice, with significantly fewer numbers of intrauterine eggs produced in the wild-type than in the immunodeficient mice. When 14-day-old adult worms from BALB/c mice were transplanted into naïve BALB/c mice, their fecundity increased significantly as early as 24 h post-transplantation, but not when they were transferred into immune mice, suggesting the plastic and reversible nature of fecundity in response to changes in host immunological status. In mast cell-deficient W/W(v) mice, nematode fecundity was significantly higher than in mast cell-reconstituted W/W(v) or +/+ mice. The serum levels of the mast-cell protease mMCP1 were markedly increased in the wild-type as well as the mast cell-reconstituted W/W(v), but not in the W/W(v), SCID, or nu/nu mice during infection. These findings raise the interesting possibility that certain activities of mast cells, either directly or indirectly, regulate parasite fecundity during infection.

Identification of novel aspartic proteases from Strongyloides ratti and characterisation of their evolutionary relationships, stage-specific expression and molecular structure

BACKGROUND

Aspartic proteases are known to play an important role in the biology of nematode parasitism. This role is best characterised in blood-feeding nematodes, where they digest haemoglobin, but they are also likely to play important roles in the biology of nematode parasites that do not feed on blood. In the present work, we investigate the evolution and expression of aspartic proteases in Strongyloides ratti, which permits a unique comparison between parasitic and free-living adult forms within its life-cycle.

RESULTS

We identified eight transcribed aspartic protease sequences and a further two genomic sequences and compared these to homologues in Caenorhabditis elegans and other nematode species. Phylogenetic analysis demonstrated a complex pattern of gene evolution, such that some S. ratti sequences had a one-to-one correspondence with orthologues of C. elegans but that lineage-specific expansions have occurred for other aspartic proteases in these two nematodes. These gene duplication events may have contributed to the adaptation of the two species to their different lifestyles. Among the set of S. ratti aspartic proteases were two closely-related isoforms that showed differential expression during different life stages: ASP-2A is highly expressed in parasitic females while ASP-2B is predominantly found in free-living adults. Molecular modelling of the ASP-2 isoforms reveals that their substrate specificities are likely to be very similar, but that ASP-2B is more electrostatically negative over its entire molecular surface than ASP-2A. This characteristic may be related to different pH values of the environments in which these two isoforms operate.

CONCLUSIONS

We have demonstrated that S. ratti provides a powerful model to explore the genetic adaptations associated with parasitic versus free-living life-styles. We have discovered gene duplication of aspartic protease genes in Strongyloides and identified a pair of paralogues differentially expressed in either the parasitic or the free-living phase of the nematode life-cycle, consistent with an adaptive role for aspartic proteases in the evolution of nematode parasitism.

Consequences of infection pressure and protein nutrition on periparturient resistance to Teladorsagia circumcincta and performance in ewes

The consequences of protein nutrition on the degree of periparturient relaxation of immunity to nematode parasites in sheep may be more pronounced at higher levels of infection pressure. Here, we investigated interactive effects of metabolizable protein (MP) nutrition and infection pressure on resistance and lactational performance of ewes. Twin-rearing ewes were trickle infected with either 1000, 5000 or 10,000 infective Teladorsagia circumcincta larvae and fed either at 0.8 (low protein, LP) or 1.3 (high protein, HP) times their estimated MP requirement. Expected interactions between feeding treatment and infection pressure were not observed. Periparturient relaxation of immunity, as indicated by variation in faecal egg counts, was higher in LP ewes than in HP ewes and FEC showed an inverse relationship with infection pressure indicating possible density dependency effects on worm fecundity. Plasma pepsinogen concentration linearly increased with infection pressure. Daily total nematode egg excretion, assessed at week three of lactation, was not significantly affected by infection pressure but was reduced by 65% in HP ewes compared to LP ewes. MP supplementation improved lamb performance but had little effect on ewe body weight and plasma protein concentrations, whilst lactational performance, as judged from lamb performance, tended to be reduced with increased infection pressure. The results suggest periparturient MP supplementation to ewes reduces nematode egg excretion independent of infection pressure and improves lactational performance of parasitized ewes even in the presence of moderate MP scarcity.

The effect of infection history on the fitness of the gastrointestinal nematode Strongyloides ratti

SUMMARY

Hosts in nature will often acquire infections by different helminth species over their lifetime. This presents the potential for new infections to be affected (particularly via the host immune response) by a host's history of previous con- or hetero-specific infection. Here we have used an experimental rat model to investigate the consequences of a history of primary infection with either Nippostrongylus brasiliensis, Strongyloides venezuelensis or S. ratti on the fitness of, and immunological response to, secondary infections of S. ratti. We found that a history of con-specific, but not hetero-specific, infection reduced the survivorship of S. ratti; the fecundity of S. ratti was not affected by a history of either con- or hetero-specific infections. We also found that a history of con-specific infection promoted Th2-type responses, as shown by increased concentrations of total IgE, S. ratti-specific IgG1, rat mast cell protease II (RMCPII), IL4 (but decreased concentrations of IFNgamma) produced by mesenteric lymph node cells in response to S. ratti antigen. Additionally, S. ratti-specific IgG1 was positively related to the intensity of both primary and secondary infections of S. ratti. Hetero-specific primary infections were only observed to affect the concentration of total IgE and RMCPII. The overall conclusion of these experiments is that the major immunological effect acting against an infection is induced by the infection itself and that there is little effect of prior infections of the host.

Density-dependent effects on the weight of female Ascaris lumbricoides infections of humans and its impact on patterns of egg production

BACKGROUND

Ascaris lumbricoides exhibits density-dependent egg production, a process which has a marked impact on both the transmission dynamics and the stability of the parasite population. Evidence suggests that the egg production of female Ascaris is also associated with the size of the worm. If worm size is mediated by density-dependent processes then the size of female worms may have a causal impact upon patterns of Ascaris egg production.

RESULTS

We analyse data collected from a cohort of human hosts, and demonstrate that the per host mean weight (a proxy for size) of female Ascaris is dependent on the number of infecting females (worm burden) following a pattern of initial facilitation followed by limitation. Applying a negative binomial (NB) generalized linear model (GLM) and a zero-inflated negative binomial (ZINB) model we confirm that the per host female mean weight is significantly associated with per host egg production. Despite these associations, the mean weight of female Ascaris has little causal impact on patterns of density-dependent egg output. The ZINB model is able to account for the disproportionately large number of zero egg counts within the data and is shown to be a consistently better fit than the NB model. The probability of observing a zero egg count is demonstrated as being negatively associated with both female worm burden and female mean weight.

CONCLUSION

The mean weight of female Ascaris is statistically significantly associated with egg output, and follows a consistent pattern of facilitation preceding limitation with increasing female worm burden. Despite these relationships, incorporation of female Ascaris mean weight into models of egg output has little effect on patterns of density dependence. The ZINB model is a superior fit to the data than the NB model and provides additional information regarding the mechanisms that result in a zero egg count. The ZINB model is shown to be a useful tool for the analysis of individual-based egg output data.

Effects of host characteristics and parasite intensity on growth and fecundity ofTrichostrongylus retortaeformisinfections in rabbits

Host-mediated responses and parasite density-dependent processes can have a major influence on the growth and fecundity of parasitic nematodes. However, host characteristics and parasite intensity consistently change during the course of an infection and these could affect worm length and number of eggs in a non-constant way. We used a free-living population of rabbits naturally infected withTrichostronglyus retortateformisand examined how adult nematode length and the number of eggsin uterowere affected by host characteristics (i.e. age, sex, breeding status) and parasite intensity, in a seasonal environment, between 2004 and 2007. Nematode length and number of eggsin uterodecreased exponentially with host age; in contrast, parasite intensity increased, peaked in juveniles and decreased in older hosts. These patterns were consistent between rabbit cohorts. A negative relationship was observed between parasite intensity and nematode length, as well as number of eggs. Nematode length was strongly affected by nematode sex and host age, while the number of eggs was mainly influenced by nematode length. The direct influence of host-mediated effects appeared quantitatively more important than parasite density dependence in controlling length and egg production in naturally infected wild rabbits. However, their relative contribution changed during the course of the infection such that, while host immunity still influenced worm numbers, the direct effect of density-dependent interactions contributed the most at high parasite intensities.

Modelling heterogeneity and the impact of chemotherapy and vaccination against human hookworm

There is a growing emphasis on the development of vaccines against helminths (worms), and mathematical models provide a useful tool to assess the impact of new vaccines under a range of scenarios. The present study describes a stochastic individual-based model to assess the relative impact of chemotherapy and vaccination against human hookworm infection and investigates the implications of potential correlations between risk of infection and vaccine efficacy. Vaccination is simulated as a reduction in susceptibility to infection and the model includes population heterogeneities and dynamical waning of protection. To help identify appropriate measures of vaccine impact, we present a novel framework to quantify the vaccine impact on the infection-associated morbidity and introduce a measure of symmetry to study the correspondence between reduction in intensity and reduction in morbidity. Our modelling shows that, in high-transmission settings, the greatest impact of vaccination will be attained when vaccine efficacy is the greatest among individuals harbouring the heaviest worm burdens, and that the decline of morbidity primarily depends on the level of protection attained in the most at risk 8-12% of the population. We also demonstrate that if risk of infection and vaccine protection are correlated, there is not always a direct correspondence between the reduction in worm burden and in morbidity, with the precise relationship varying according to transmission setting.