Mixed inoculations of a microsporidian parasite with horizontal and vertical infections

Wide geographic distribution of overlooked parasites: Rare Microsporidia in Gammarus balcanicus, a species complex with a high rate of endemism

Parasites and other symbionts deeply influence host organisms, and no living organism can be considered to have evolved independent of its symbionts. The first step towards understanding symbiotic influences upon host organisms is a strong supporting knowledge of parasite/symbiont diversity. Parasites of freshwater amphipods are diverse, with Microsporidia being a major group. These intracellular parasites impact gammarid fitness in different ways, ranging from reduced fitness to increased fecundity. Many Microsporidia have been recorded using molecular data, with multiple taxa pending formal taxonomic description. While some parasites are common, others are known only through sporadic records of single infections. In this study, we focus on rare/sporadic microsporidian infections within Gammarus balcanicus, a host species complex with a high level of endemism. In addition to enriching our knowledge on Microsporidia parasite diversity in amphipod hosts, we test whether these symbionts are specific to G. balcanicus or if they are the same taxa infecting other gammarid species. Of 2231 hosts from 87 sites, we catalogued 29 sequences of “rare” Microsporidia clustering into 19 haplogroups. These haplogroups cluster into 11 lineages: four pre-described taxa (Cucumispora roeselum, C. ornata, C. dikerogammari and Enterocytospora artemiae) and seven ‘Molecular Operational Taxonomic Units’, which are known from previously published studies to infect other European amphipod species. Our study significantly widens the geographic range of these Microsporidia and expands the known spectrum of hosts infected. Our results suggest that these parasites are ancient infections of European gammarids. For some host-parasite systems, we hypothesize that the common parasite ancestors that infected the hosts' common ancestors, diversified alongside host diversification. For others, we observe Microsporidia taxa with wide host ranges that do not follow host phylogeny.

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Microsporidia infect a wide range of host but are well studied for the most abundant parasite taxa.

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Amphipods are common microsporidian hosts but knowledge on parasite diversity remains partial.

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Gammarus balcanicus species complex is a perfect host to study evolutionary history of rare Microsporidia.

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All Microsporidia found in G. balcanicus are ancient infections in the European gammarids.

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Some Microsporidia co-diversified with the host, while the others did not follow the host phylogeny.

Identification and characterization of three microsporidian genera concurrently infecting a silkworm, Bombyx mori, in Brazil

Microsporidia are important entomopathogens known for infecting insects such as the silkworm (Bombyx mori) thus impairing global silk production. This study aimed to identify and characterize the microsporidia isolated from a diseased larva of silkworm, collected from a sericulture farm in southern Brazil. Identification was performed by phylogenetic analysis of the nucleotide sequences of the SSU rRNA genes. Characterization was performed by analyzing spore sizes, tissue tropism, internal and external symptoms, and pathogenicity against B. mori. Microsporidia belonging to three different genera were identified, namely, Endoreticulatus, Nosema and Tubulinosema. After inoculation of the mixed spores of the microsporidian isolates into B. mori larvae, a high prevalence of Tubulinosema spp. was observed. This isolate showed high prevalence on the silk glands and a late mortality, initially of around 10% until the 20th day post-inoculation but reaching 91.5% upon pupation. Therefore, we demonstrated that Tubulinosema spp. causes chronic infection with slow pathogenicity. We identified for the first time three different microsporidians concurrently infecting B. mori in Brazil. Tubulinosema is of particular interest because of its potential threat to silk production; it affects the formation of silk glands in B. mori while not presenting distinguishable external symptoms or causing the immediate death of these insects. Further studies focusing on this species, mainly regarding its life cycle within the host and the sublethal effects of surviving individuals, demonstrate the importance of describing it as a new species and improving the characterization of the disease in order to prevent its spread.

QTL mapping of a natural genetic polymorphism for long-term parasite persistence in Daphnia populations

Knowing the determinants of the geographic ranges of parasites is important for understanding their evolutionary ecology, epidemiology and their potential to expand their range. Here we explore the determinants of geographic range in the peculiar case of a parasite species – the microsporidian Hamiltosporidium tvaerminnensis – that has a limited geographic distribution in a wide-spread host – Daphnia magna. We conducted a quantitative trait loci (QTLs) analysis with monoclonal F2D. magna populations originating from a cross between a susceptible northern European genotype and a resistant central European genotype. Contrary to our expectations, long-term persistence turned out to be a quantitative trait across the F2 offspring. Evidence for two QTLs, one epistatic interaction and for further minor QTL was found. This finding contrasts markedly with the previously described bimodal pattern for long-term parasite persistence in natural host genotypes across Europe and leaves open the question of how a quantitative genetic trait could determine the disjunct geographic distribution of the parasite across Europe.

Multiple infections, kin selection and the evolutionary epidemiology of parasite traits

The coinfection of a host by several parasite strains is known to affect selective pressures on parasite strategies of host exploitation. I present a general model of coinfections that ties together kin selection models of virulence evolution and epidemiological models of multiple infections. I derive an analytical expression for the invasion fitness of a rare mutant in a population with an arbitrary distribution of the multiplicity of infection (MOI) across hosts. When a single mutation affects parasite strategies in all MOI classes, I show that the evolutionarily stable level of virulence depends on a demographic average of within-host relatedness across all host classes. This generalization of previous kin selection results requires that within-host parasite densities do not vary between hosts. When host exploitation strategies are allowed to vary across classes, I show that the strategy of host exploitation in a focal MOI class depends on the relative magnitudes of parasite reproductive values in the focal class and in the next. Thus, in contrast to previous findings, lower within-host relatedness in competitive parasite interactions can potentially correspond to either higher or lower levels of virulence.

New insights on the genetic diversity of the honeybee parasiteNosema ceranaebased onmultilocussequence analysis

The microsporidian parasiteNosema ceranaeis a common pathogen of the Western honeybee (Apis mellifera) whose variable virulence could be related to its genetic polymorphism and/or its polyphenism responding to environmental cues. Since the genotyping ofN. ceranaebased on unique marker sequences had been unsuccessful, we tested whether amultilocusapproach, assessing the diversity of ten genetic markers – encoding nine proteins and the small ribosomal RNA subunit – allowed the discrimination betweenN. ceranaevariants isolated from singleA. melliferaindividuals in four distant locations. High nucleotide diversity and allele content were observed for all genes. Most importantly, the diversity was mainly present within parasite populations isolated from single honeybee individuals. In contrast the absence of isolate differentiation precluded anytaxadiscrimination, even through amultilocusapproach, but suggested that similar populations of parasites seem to infect honeybees in distant locations. As statistical evolutionary analyses showed that the allele frequency is under selective pressure, we discuss the origin and consequences ofN. ceranaeheterozygosity in a single host and lack of population divergence in the context of the parasite natural and evolutionary history.

Probing mixed-genotype infections I: extraction and cloning of infections from hosts of the trypanosomatid Crithidia bombi

We here present an efficient, precise and reliable method to isolate and cultivate healthy and viable single Crithidia bombi cells from bumblebee faeces using flow cytometry. We report a precision of >99% in obtaining single trypanosomatid cells for further culture and analysis (“cloning”). In the study, we have investigated the use of different liquid media to cultivate C. bombi and present an optimal medium for obtaining viable clones from all tested, infected host donors. We show that this method can be applied to genotype a collection of clones from natural infections. Furthermore, we show how to cryo-preserve C. bombi cells to be revived to become infective clones after at least 4 years of storage. Considering the high prevalence of infections in natural populations, our method provides a powerful tool in studying the level and diversity of these infections, and thus enriches the current methodology for the studies of complex host-parasite interactions.

Probing mixed-genotype infections II: high multiplicity in natural infections of the trypanosomatid, Crithidia bombi, in its host, Bombus spp

Mixed-genotype infections have major consequences for many essential elements of host-parasite interactions. With genetic exchange between co-infecting parasite genotypes increased diversity among parasite offspring and the emergence of novel genotypes from infected hosts is possible. We here investigated mixed- genotype infections using the host, Bombus spp. and its trypanosome parasite Crithidia bombi as our study case. The natural infections of C. bombi were genotyped with a novel method for a representative sample of workers and spring queens in Switzerland. We found that around 60% of all infected hosts showed mixed-genotype infections with an average of 2.47±0.22 (S.E.) and 3.65±1.02 genotypes per worker or queen, respectively. Queens, however, harboured up to 29 different genotypes. Based on the genotypes of co-infecting strains, these could be putatively assigned to either ‘primary’ and ‘derived’ genotypes - the latter resulting from genetic exchange among the primary genotypes. High genetic relatedness among co-infecting derived but not primary genotypes supported this scenario. Co-infection in queens seems to be a major driver for the diversity of genotypes circulating in host populations.

To be or not to be solitary: Phytophthora infestans' dilemma for optimizing its reproductive fitness in multiple infections

The success of parasitic life lies in an optimal exploitation of the host to satisfy key functions directly involved in reproductive fitness. Resource availability generally decreases over time with host mortality, but also during multiple infections, where different strains of parasite share host resources. During multiple infections, the number of parasite strains and their genetic relatedness are known to influence their reproductive rates. Using infections of the potato plant Solanum tuberosum with the parasite Phytophthora infestans, we set up an experimental design to separate dose effects (double- vs. single-site infections) from genetic relatedness (different vs. identical genotypes) on the reproductive fitness of competing parasite genotypes. We showed the existence of two basic response patterns--increase or decrease in reproductive fitness in multiple infections- depending on the parasite genotype. In all cases, the intensity of the response of any genotype depended on the genotype of the competing strain. This diversity of responses to multiple infections is probably maintained by the fluctuating frequencies of multiple infections in nature, arising from variations in disease pressure over the course of an epidemic and between successive epidemics. It allows a rapid response of parasitic populations to changing environments, which are particularly intense in agricultural systems.

Microsporidia infecting Apis mellifera: coexistence or competition. Is Nosema ceranae replacing Nosema apis?

Nosema ceranae has been suggested to be replacing Nosema apis in some populations of Apis mellifera honeybees. However, this replacement from one to the other is not supported when studying the distribution and prevalence of both microsporidia in professional apiaries in Spanish territories (transverse study), their seasonal pattern in experimental hives with co-infection or their prevalence at individual level (either in worker bees or drones). Nevertheless, N.ceranae has shown to present a higher prevalence at all the studied levels that could indicate any advantage for its development over N.apis or that it is more adapted to Spanish conditions. Also, both microsporidia show a different pattern of preference for its development according to the prevalence in the different Spanish bioclimatic belts studied. Finally, the fact that all analyses were carried out using an Internal PCR Control (IPC) newly developed guarantees the confidence of the data extracted from the PCR analyses. This IPC provides a useful tool for laboratory detection of honeybee pathogens.

Converging seasonal prevalence dynamics in experimental epidemics

Background

Regular seasonal changes in prevalence of infectious diseases are often observed in nature, but the mechanisms are rarely understood. Empirical tests aiming at a better understanding of seasonal prevalence patterns are not feasible for most diseases and thus are widely lacking. Here, we set out to study experimentally the seasonal prevalence in an aquatic host-parasite system. The microsporidian parasite Hamiltosporidium tvärminnensis exhibits pronounced seasonality in natural rock pool populations of its host, Daphnia magna with a regular increase of prevalence during summer and a decrease during winter. An earlier study was, however, unable to test if different starting conditions (initial prevalence) influence the dynamics of the disease in the long term. Here, we aim at testing how the starting prevalence affects the regular prevalence changes over a 4-year period in experimental populations.

Results

In an outdoor experiment, populations were set up to include the extremes of the prevalence spectrum observed in natural populations: 5% initial prevalence mimicking a newly invading parasite, 100% mimicking a rock pool population founded by infected hosts only, and 50% prevalence which is commonly observed in natural populations in spring. The parasite exhibited similar prevalence changes in all treatments, but seasonal patterns in the 100% treatment differed significantly from those in the 5% and 50% treatments. Populations started with 5% and 50% prevalence exhibited strong and regular seasonality already in the first year. In contrast, the amplitude of changes in the 100% treatment was low throughout the experiment demonstrating the long-lasting effect of initial conditions on prevalence dynamics.

Conclusions

Our study shows that the time needed to approach the seasonal changes in prevalence depends strongly on the initial prevalence. Because individual D. magna populations in this rock pool metapopulation are mostly short lived, only few populations might ever reach a point where the initial conditions are not visible anymore.

The expression of virulence during double infections by different parasites with conflicting host exploitation and transmission strategies

In many natural populations, hosts are found to be infected by more than one parasite species. When these parasites have different host exploitation strategies and transmission modes, a conflict among them may arise. Such a conflict may reduce the success of both parasites, but could work to the benefit of the host. For example, the less-virulent parasite may protect the host against the more-virulent competitor. We examine this conflict using the waterflea Daphnia magna and two of its sympatric parasites: the blood-infecting bacterium Pasteuria ramosa that transmits horizontally and the intracellular microsporidium Octosporea bayeri that can concurrently transmit horizontally and vertically after infecting ovaries and fat tissues of the host. We quantified host and parasite fitness after exposing Daphnia to one or both parasites, both simultaneously and sequentially. Under conditions of strict horizontal transmission, Pasteuria competitively excluded Octosporea in both simultaneous and sequential double infections, regardless of the order of exposure. Host lifespan, host reproduction and parasite spore production in double infections resembled those of single infection by Pasteuria. When hosts became first vertically (transovarilly) infected with O. bayeri, Octosporea was able to withstand competition with P. ramosa to some degree, but both parasites produced less transmission stages than they did in single infections. At the same time, the host suffered from reduced fecundity and longevity. Our study demonstrates that even when competing parasite species utilize different host tissues to proliferate, double infections lead to the expression of higher virulence and ultimately may select for higher virulence. Furthermore, we found no evidence that the less-virulent and vertically transmitting O. bayeri protects its host against the highly virulent P. ramosa.

Cytological and molecular description of Hamiltosporidium tvaerminnensis gen. et sp. nov., a microsporidian parasite of Daphnia magna, and establishment of Hamiltosporidium magnivora comb. nov

We describe the new microsporidium Hamiltosporidium tvaerminnensis gen. et sp. nov. with an emphasis on its ultrastructural characteristics and phylogenetic position as inferred from the sequence data of SSU rDNA, alpha- and beta-tubulin. This parasite was previously identified as Octosporea bayeri Jírovec, 1936 and has become a model system to study the ecology, epidemiology, evolution and genomics of microsporidia - host interactions. Here, we present evidence that shows its differences from O. bayeri. Hamiltosporidium tvaerminnensis exclusively infects the adipose tissue, the ovaries and the hypodermis of Daphnia magna and is found only in host populations located in coastal rock pool populations in Finland and Sweden. Merogonial stages of H. tvaerminnensis have isolated nuclei; merozoites are formed by binary fission or by the cleaving of a plasmodium with a small number of nuclei. A sporogonial plasmodium with isolated nuclei yields 8 sporoblasts. Elongated spores are generated by the most finger-like plasmodia. The mature spores are polymorphic in shape and size. Most spores are pyriform (4·9–5·6×2·2–2·3 μm) and have their polar filament arranged in 12–13 coils. A second, elongated spore type (6·8–12·0×1·6–2·1 μm) is rod-shaped with blunt ends and measures 6·8–12·0×1·6–2·1 μm. The envelope of the sporophorous vesicle is thin and fragile, formed at the beginning of the sporogony. Cytological and molecular comparisons with Flabelliforma magnivora, a parasite infecting the same tissues in the same host species, reveal that these two species are very closely related, yet distinct. Moreover, both cytological and molecular data indicate that these species are quite distant from F. montana, the type species of the genus Flabelliforma. We therefore propose that F. magnivora also be placed in Hamiltosporidium gen. nov.

Transmission of wild-type and recombinant HaSNPV among larvae of Helicoverpa armigera (Lepidoptera: Noctuidae) on cotton

Horizontal transmission of insect viruses is a key factor in their cycling in agro-ecosystems. Here we study the transmission of the baculovirus HaSNPV among larvae of Helicoverpa armigera (Hübner) in cotton. Transmission of three HaSNPV genotypes was studied from larvae infected with a single virus genotype and from larvae infected with two different genotypes. Genotypes included a wild-type virus, an ecdysteroid UDP-glucosyltransferase (egt) deletion mutant (HaSNPV-LM2) with slightly enhanced speed of kill, and an egt-negative genotype that expresses a neurotoxin gene derived from the scorpion Androctonus australis Hector (HaSNPV-4A). The latter genotype has a substantially increased speed of kill. In three field experiments, the wild-type and egt deletion virus variants and a mixture of the two had similar rates of transmission. Transmission increased with density of infector insects and decreased with time lapsed since the inoculation of the infector larvae. Transmission of the neurotoxin expressing virus was lower than that of the other two genotypes in a glasshouse experiment. The studied genotypes of HaSNPV have significant differences in time to kill and virus yield, but we found no significant differences in rates of virus transmission at the crop level in the case of the egt deletion variant HaSNPV-LM2. Transmission of the transgenic virus genotype HaSNPV-4A was significantly reduced. Overall, differences in transmission between virus genotypes were subtler, and more difficult to detect with statistical significance, than effects of other factors, such as density of infectors and time delay between release of infectors and recipient caterpillars on the plant.

Presence of multiparasite infections within individual colonies of leaf-cutter ants

Host-parasite dynamics can be altered when a host is infected by multiple parasite genotypes. The different strains of parasite are expected to compete for the limited host resources, potentially affecting the survival and reproduction of the host as well as the infecting parasites. Fungus-growing ants, including the well-known leaf-cutters, are an emerging model system for studying the evolution and ecology of symbiosis and host-parasite dynamics. We examine whether the fungus gardens of leaf-cutter ants can be simultaneously infected by multiple strains of the fungal pathogen Escovopsis. Intensive sampling of Escovopsis was conducted from individual gardens, as well as between different garden chambers within individual colonies of leaf-cutting ants. Isolates obtained were genotyped by DNA sequencing. We found that, minimally, 67% of the individual colonies of the leaf-cutter ant genera Atta and Acromyrmex and 50% of the At. colombica garden chambers studied were simultaneously infected by multiple distinct Escovopsis strains. Experimental challenges showed that different Escovopsis strains do not exhibit obvious antagonism toward each other, suggesting that coinfecting strains of the parasite do not engage in interference competition, although interactions were not studied at the cellular level. Further research is needed to understand interparasite interactions between coinfecting Escovopsis strains and to understand the impact of multiparasite infections on the survival of leaf-cutter ant gardens.

Intensive fish farming and the evolution of pathogen virulence: the case of columnaris disease in Finland

Ecological changes affect pathogen epidemiology and evolution and may trigger the emergence of novel diseases. Aquaculture radically alters the ecology of fish and their pathogens. Here we show an increase in the occurrence of the bacterial fish disease Flavobacterium columnare in salmon fingerlings at a fish farm in northern Finland over 23 years. We hypothesize that this emergence was owing to evolutionary changes in bacterial virulence. We base this argument on several observations. First, the emergence was associated with increased severity of symptoms. Second, F. columnare strains vary in virulence, with more lethal strains inducing more severe symptoms prior to death. Third, more virulent strains have greater infectivity, higher tissue-degrading capacity and higher growth rates. Fourth, pathogen strains co-occur, so that strains compete. Fifth, F. columnare can transmit efficiently from dead fish, and maintain infectivity in sterilized water for months, strongly reducing the fitness cost of host death likely experienced by the pathogen in nature. Moreover, this saprophytic infectiousness means that chemotherapy strongly select for strains that rapidly kill their hosts: dead fish remain infectious; treated fish do not. Finally, high stocking densities of homogeneous subsets of fish greatly enhance transmission opportunities. We suggest that fish farms provide an environment that promotes the circulation of more virulent strains of F. columnare. This effect is intensified by the recent increases in summer water temperature. More generally, we predict that intensive fish farming will lead to the evolution of more virulent pathogens.

The influence of clonal diversity and intensity-dependence on trematode infections in an amphipod

Individual animals are often infected not only by different parasite species, but also by multiple genotypes of the same parasite species. Genetic relatedness among parasites sharing a host is expected to modulate their strategies of resource exploitation, growth and virulence. We experimentally examined the effects that genetic diversity and infection intensity had on host mortality, infectivity and growth of the marine trematode Maritrema novaezealandensis in amphipod hosts. The presence of 2 versus 1 parasite genotype during infection did not influence subsequent host mortality, had different effects on infectivity among genotypes and did not influence growth or variation in parasite growth. Density-dependent growth reductions revealed that the number of parasites infecting a host was more important than their genetic relatedness. Temperature, host size, and host sex influenced the degree to which density-dependent factors affected parasite growth. Our results suggest that the effects of parasite relatedness vary among parasite genotypes in this trematode species, and reveal that many factors play an important role during parasite development and transmission.

Mode of transmission and the evolution of arbovirus virulence in mosquito vectors

The traditional assumption that vector-borne pathogens should evolve towards a benign relationship with their arthropod vectors has been challenged on theoretical grounds and empirical evidence. However, in the case of arboviruses (arthropod-borne viruses), although a number of investigators have reported experimental evidence for virus-induced vector mortality, others have failed to detect any significant impact. Whether this variation in the observed level of arbovirus virulence depends on biological traits or experimental design is unclear. Here, we perform a meta-analysis of studies across a range of mosquito-virus systems to show that, overall, arboviruses do reduce the survival of their mosquito vectors, but that the magnitude of the effect depends on the vector/virus taxonomic groups and the mode of virus transmission. Alphaviruses were associated with highest virulence levels in mosquitoes. Horizontal transmission (intrathoracic inoculation or oral infection) was correlated with significant virus-induced mortality, whereas a lack of adverse effect was found for Aedes mosquitoes infected transovarially by bunyaviruses-a group of viruses characterized by high natural rates of vertical transmission in their enzootic vectors. Our findings are consistent with the general prediction that vertically transmitted pathogens should be less virulent than those transmitted horizontally. We conclude that varying degrees of virulence observed among vector-virus systems probably reflect different selective pressures imposed on arboviruses that are primarily transmitted horizontally versus vertically.

Genetic diversity of Daphnia magna populations enhances resistance to parasites

The diversity-disease hypothesis states that decreased genetic diversity in host populations increases the incidence of diseases caused by pathogens (= monoculture effect) and eventually influences ecosystem functioning. The monoculture effect is well-known from crop studies and may be partially specific to the artificial situation in agriculture. The effect received little attention in animal populations of different diversities. Compared with plants, animals are mobile and exhibiting social interactions. We followed the spread of a microsporidian parasite in semi-natural outdoor Daphnia magna populations of low and high genetic diversity. We used randomly selected, naturally occurring host genotypes. Host populations of low diversity were initially monoclonal, while the host populations of high diversity started with 10 genotypes per replicate. We found that the parasite spread significantly better in host populations of low diversity compared with host populations of high diversity, independent of parasite diversity. The difference was visible over a 3-year period. Host genotypic diversity did not affect host population density. Our experiment demonstrated a monoculture effect in independently replicated semi-natural zooplankton populations, indicating that the monoculture effect may be relevant beyond agriculture.

The genotype specific competitive ability does not correlate with infection in natural Daphnia magna populations

BACKGROUND

Different evolutionary hypotheses predict a correlation between the fitness of a genotype in the absence of infection and the likelihood to become infected. The cost of resistance hypothesis predicts that resistant genotypes pay a cost of being resistant and are less fit in the absence of parasites. The inbreeding-infection hypothesis predicts that the susceptible individuals are less fit due to inbreeding depression.

METHODS AND RESULTS

Here we tested if a host's natural infection status was associated with its fitness. First, we experimentally confirmed that cured but formerly infected Daphnia magna are genetically more susceptible to reinfections with Octosporea bayeri than naturally uninfected D. magna. We then collected from each of 22 populations both uninfected and infected D. magna genotypes. All were treated against parasites and kept in their asexual phase. We estimated their relative fitness in an experiment against a tester genotype and in another experiment in direct competition. Consistently, we found no difference in competitive abilities between uninfected and cured but formerly infected genotypes. This was the case both in the presence as well as in the absence of sympatric parasites during the competition trials.

CONCLUSIONS

Our data do not support the inbreeding-infection hypothesis. They also do not support a cost of resistance, however ignoring other parasite strains or parasite species. We suggest as a possible explanation for our results that resistance genes might segregate largely independently of other fitness associated genes in this system.

Horizontal transmission success of Nosema bombi to its adult bumble bee hosts: effects of dosage, spore source and host age

Parasite transmission dynamics are fundamental to explaining the evolutionary epidemiology of disease because transmission and virulence are tightly linked. Horizontal transmission of microsporidian parasites, e.g. Nosema bombi, may be influenced by numerous factors, including inoculation dose, host susceptibility and host population heterogeneity. Despite previous studies of N. bombi and its bumble bee hosts, neither the epidemiology nor impact of the parasite are as yet understood. Here we investigate the influence N. bombi spore dosage (1000 to 500,000 spores), spore source (Bombus terrestris and B. lucorum isolates) and host age (2- and 10-day-old bees) have on disease establishment and the presence of patent infections in adult bumble bees. Two-day-old bees were twice as susceptible as their 10-day-old sisters, and a 5-fold increase in dosage from 100,000 to 500,000 spores resulted in a 20-fold increase in the prevalence of patent infections. While intraspecific inoculations were 3 times more likely to result in non-patent infections there was no such effect on the development of patent infections. These results suggest that host-age and dose are likely to play a role in N. bombi's evolutionary epidemiology. The relatively low levels of horizontal transmission success are suggestive of low virulence in this system.

Growth behaviour of two Trypanosoma cruzi strains in single and mixed infections: In vitro and in the intestinal tract of the blood-sucking bug, Triatoma brasiliensis

Competition and cooperation are well-recognized biological phenomena, even among parasites. Co-infection of parasites in a single host leads to several outcomes, one being competition for a limited resource. Here, the behaviour of mixed infection was evaluated using two isolates of Trypanosoma cruzi, previously typed as belonging to genotypes TcI and TcII. The growth in vitro and in the different compartments of the gut of Triatoma brasiliensis was studied. In vitro growth showed that MDID/BR/1999/M1 (TcI) has a doubling time of 19.5h and MIDID/BR/1999/JCPD4 (TcII) of 9.6h, while the mixed infection group presented a doubling time of 13.9h. In vivo, three groups of infection were done: M1/TcI, JCPD4/TcII and mixed infection (50% of each strain), respectively. All comparisons among the groups were done using the Kruskal-Wallis non-parametric test. The data showed that the in vitro culture of mixed populations has a similar pattern to the growth of M1/TcI, apparently suggesting a positively selection for M1/TcI strain, in axenic culture. In the gut of the insects, M1/TcI isolate and mixed infections colonized predominantly the rectal wall and rectal lumen, in contrast to the JCPD4/TcII isolate, which was found mainly colonizing the small intestine. According to the isolates investigated, it could be concluded that the doubling time was not determinant factor for the final composition of a co-infection. Moreover, mixed infections resulted in a homogenous distribution of the parasites, comparing to the isolates studied separately. Apparently, in the gut of the bugs, the simultaneous presence of JCPD4/TcII isolate resulted in an improvement of the number of parasites from M1/TcI.

Apparent seasonality of parasite dynamics: analysis of cyclic prevalence patterns

Seasonal disease dynamics are common in nature, but their causes are often unknown. Our case study provides insight into the cyclic prevalence pattern of the horizontally and vertically transmitted microsporidium Octosporea bayeri in its Daphnia magna host. Data from several populations over a four year period revealed a regular prevalence increase during summer and a decrease over winter when hosts underwent diapause. Prevalence also decreased after summer diapause indicating that the decline is causally linked to diapause rather than to winter conditions. Experiments showed that host diapause itself can explain a certain proportion of the decline. The decline further depends on the environmental conditions during diapause: infected resting eggs suffered from higher mortality under experimental winter than under experimental summer diapause conditions. Investigating the mechanisms of prevalence increase after diapause, the parasite was found to survive winter outside its host, enabling horizontal infection of susceptible hosts in the following growing season. Allowing for horizontal transmission in experimental host populations resulted in a steep prevalence increase, while excluding it led to a pronounced decline. Thus, the apparent seasonality in O. bayeri prevalence is characterized by a decline during host diapause followed by horizontal spread of the parasite during the host's asexual growth phase.