Transmission and burden and the impact of temperature on two species of vertically transmitted microsporidia

Microsporidia are unusual amongst eukaryotic parasites in that they utilize both vertical and horizontal transmission and vertically transmitted species can cause sex ratio distortion in their host. Here we study vertical transmission in two species of feminising microsporidia, Nosema granulosis and Dictyocoela duebenum, infecting a single population of the crustacean host Gammarus duebeni and measure the effect of temperature on parasite transmission and replication. N. granulosis was vertically transmitted to 82% of the host embryos and D. duebenum was transmitted to 72% of host embryos. For both parasites, we report relatively low parasite burdens in developing host embryos. However, the parasites differ in their pattern of replication and burden within developing embryos. Whilst N. granulosis undergoes replication during host development, the burden of D. duebenum declines, leading us to propose that parasite dosage and feminisation efficiency underlie the different parasite frequencies in the field. We also examine the effect of temperature on parasite transmission and replication. Temperature does not affect the percentage of young that inherit the infection. However, low temperatures inhibit parasite replication relative to host cell division, resulting in a reduction in parasite burden in infected embryos. The reduced parasite burden at low temperatures may underpin reduced feminization at low temperatures and so limit the spread of sex ratio distorters through the host population.     

Host factors limiting monogenean infections: a case study

Comprehensive field data on polystomatid monogeneans record low prevalence and intensity of infection and suggest that worm burdens in this group are strongly regulated: thus, in the majority of Polystoma species infecting anuran amphibians mean abundance is typically less than one parasite/host. There is circumstantial evidence that the dominant control is attributable to host factors which over-ride variations in transmission success. This review provides a brief summary of information on Pseudodiplorchis americanus, a parasite of the desert toad, Scaphiopus couchii, and then focuses in detail on the spectrum of factors regulating infrapopulations of Protopolystoma xenopodis, a parasite of the aquatic Xenopus laevis. Infection levels of adult worms and their contribution to transmission are regulated by external environmental factors (especially temperature), by host factors (including behaviour and population density), and by a range of parasite factors including intra- and inter-specific competitive interactions and variations in intrinsic characters, especially survivorship and reproductive output. In addition to these factors whose primary effect is to modulate transmission rates, there is a major attrition in parasite numbers between invasion and maturity (3 months post-infection). Long-term laboratory experiments on the Xenopus laevis/Protopolystoma xenopodis interaction demonstrate a powerful acquired immune response. Primary infection is characterised by a high prevalence of established adult worms but the success of subsequent challenge infection is greatly reduced, leading to low prevalence and extended pre-patent period. In the small proportion of hosts supporting a second infection of adult parasites, surviving burdens are small (one to two worms/host) and show reduced egg production. These results provide an explanation for the low burdens encountered in field studies: a majority of adult X. laevis in natural populations are likely to exhibit strong, relatively long-term, post-infection immunity after the loss of a previous infection.     

Larval helminths in intermediate hosts: does competition early in life determine the fitness of adult parasites?

Density-dependent effects on parasite fitness have been documented from adult helminths in their definitive hosts. There have, however, been no studies on the cost of sharing an intermediate host with other parasites in terms of reduced adult parasite fecundity. Even if larval parasites suffer a reduction in size, caused by crowding, virtually nothing is known about longer-lasting effects after transmission to the definitive host. This study is the first to use in vitro cultivation with feeding of adult trematodes to investigate how numbers of parasites in the intermediate host affect the size and fecundity of adult parasites. For this purpose, we examined two different infracommunities of parasites in crustacean hosts. Firstly, we used experimental infections of Maritrema novaezealandensis in the amphipod, Paracalliope novizealandiae, to investigate potential density-dependent effects in single-species infections. Secondly, we used the crab, Macrophthalmus hirtipes (Ocypodidae), naturally infected by the trematodes, M. novaezealandensis and Levinseniella sp., the acanthocephalan, Profilicollis spp., and an acuariid nematode. These four helminths all develop and grow in their crustacean host before transmission to their bird definitive host by predation. In experimental infections, we found an intensity-dependent establishment success, with a decrease in the success rate of cercariae developing into infective metacercariae with an increasing dose of cercariae applied to each amphipod. In natural infections, we found that M. novaezealandensis-metacercariae achieved a smaller volume, on average, when infrapopulations of this parasite were large. Small metacercariae produced small in vitro-adult worms, which in turn produced fewer eggs. Crowding effects in the intermediate host thus were expressed at the adult stage in spite of the worms being cultured in a nutrient-rich medium. Furthermore, excystment success and egg-production in M. novaezealandensis in naturally infected crabs were influenced by the number of co-occurring Profilicollis cystacanths, indicating interspecific interactions between the two species. Our results thus indicate that the infracommunity of larval helminths in their intermediate host is interactive and that any density-dependent effect in the intermediate host may have lasting effects on individual parasite fitness.     

Patterns of intermediate host use and levels of association between two conflicting manipulative parasites.

For many parasites with complex life cycles, manipulation of intermediate host phenotypes is often regarded as an adaptation to increase the probability of successful transmission. This phenomenon creates opportunities for either synergistic or conflicting interests between different parasite species sharing the same intermediate host. When more than one manipulative parasite infect the same intermediate host, but differ in their definitive host, selection should favour the establishment of a negative association between these manipulators. Both Polymorphus minutus and Pomphorhynchus laevis exploit the amphipod Gammarus pulex as intermediate host but differ markedly in their final host, a fish for P. laevis and a bird for P. minutus. The pattern of host use by these two conflicting manipulative parasites was studied. Their incidence and intensity of infection and their distribution among G. pulex were first examined by analysing three large samples of gammarids collected from the river Tille, Eastern France. Both parasites had low prevalence in the host population. However, temporal fluctuation in the level of parasitic infection was observed. Overall, prevalence of both parasite species was higher in male than in female G. pulex. We then assessed the degree of association between the two parasites among their intermediate hosts, using two different methods: a host-centred measure and a parasite-centred measure. Both measures gave similar results; showing random association between the two acanthocephalan species in their intermediate hosts. We discuss our results in relation to the selective forces and ecological constraints that may determine the pattern of association between conflicting manipulative parasites.     

Parasite age-intensity relationships in red-spotted newts: does immune memory influence salamander disease dynamics?

Acquired immune memory in vertebrates influences transmission and persistence of infections, with consequences for parasite dynamics at both the individual and population levels. The potential impact of acquired immunity is of particular interest for salamanders, whose acquired immune systems are thought to be less effective than those of frogs and other tetrapods. One way to examine the importance of acquired immunity to parasite dynamics at the population level is by examining the relationship between host age and parasite infection intensity. Acquired immunity reduces infection rates in older animals, causing decreased parasite intensity in older age classes and leading to curvilinear age-intensity relationships for persistent parasites and convex age-intensity relationships for transient parasites. We used age-intensity relationships to look for the signature of acquired immunity for 12 parasite taxa of red-spotted newts (Notophthalmus viridescens), using data from a 2-year parasitological survey of six newt populations. We estimated ages from snout-vent length (SVL) based on the relationship between SVL and skeletochronologically-derived ages in a subset of newts. We found evidence of acquired immunity to two parasite taxa, bacterial pathogens and the protist Amphibiocystidium viridescens, whose convex age-intensity relationships could not be easily explained by alternative mechanisms. Our results suggest that the acquired immune response of newts is sufficient to influence the dynamics of at least some parasites.     

Targeting of host cell lineages by vertically transmitted, feminising microsporidia

Feminising microsporidian parasites are transmitted vertically from generation to generation of their crustacean hosts. Little is known about the mechanisms underpinning vertical transmission, in particular, parasite transmission to the host gonad during host development. Here, we investigate the burden and distribution of two species of vertically transmitted, feminising microsporidia (Dictyocoela duebenum and Nosema granulosis) during early embryogenesis (zygote to eight-cells) of the Gammarus duebeni host. Parasite burden differs between the two parasites with N. granulosis being higher by a factor of 10. Whilst D. duebenum replicates during the first few host cell divisions, there is no increase in N. granulosis burden. Only merogonic parasite stages were observed in the host embryo. Distribution of both parasites was non-random from the two-cell embryo stage, indicating biased parasite segregation at host cell division. Dictyocoela duebenum burden was low in the germline and somatic gonad progenitor cells but was highest in the ectoderm precursors, leading us to propose that the parasite targets these cells and then secondarily infects the gonad later in host development. Targeting by N. granulosis was less specific although there was a persistent bias in parasite distribution throughout host cell divisions. Parasite burden was highest in the ectoderm precursors as well as the germline progenitors leading us to suggest that, in addition to using the ectodermal route, N. granulosis may also target germline directly. Biased segregation will be adaptive for these parasites as it is likely to lead to efficient transmission and feminisation whilst minimising virulence in the host.     

Endemic infection reduces transmission potential of an epidemic parasite during co-infection

Endemic, low-virulence parasitic infections are common in nature. Such infections may deplete host resources, which in turn could affect the reproduction of other parasites during co-infection. We aimed to determine whether the reproduction, and therefore transmission potential, of an epidemic parasite was limited by energy costs imposed on the host by an endemic infection. Total lipids, triacylglycerols (TAG) and polar lipids were measured in cockroaches (Blattella germanica) that were fed ad libitum, starved or infected with an endemic parasite, Gregarina blattarum. Reproductive output of an epidemic parasite, Steinernema carpocapsae, was then assessed by counting the number of infective stages emerging from these three host groups. We found both starvation and gregarine infection reduced cockroach lipids, mainly through depletion of TAG. Further, both starvation and G. blattarum infection resulted in reduced emergence of nematode transmission stages. This is, to our knowledge, the first study to demonstrate directly that host resource depletion caused by endemic infection could affect epidemic disease transmission. In view of the ubiquity of endemic infections in nature, future studies of epidemic transmission should take greater account of endemic co-infections.     

Variation for host range within and among populations of the parasitic plant Striga hermonthica

Striga hermonthica is an angiosperm parasite that causes substantial damage to a wide variety of cereal crop species, and to the livelihoods of subsistence farmers in sub-Saharan Africa. The broad host range of this parasite makes it a fascinating model for the study of host-parasite interactions, and suggests that effective long-term control strategies for the parasite will require an understanding of the potential for host range adaptation in parasite populations. We used a controlled experiment to test the extent to which the success or failure of S. hermonthica parasites to develop on a particular host cultivar (host resistance/compatibility) depends upon the identity of interacting host genotypes and parasite populations. We also tested the hypothesis that there is a genetic component to host range within individual S. hermonthica populations, using three rice cultivars with known, contrasting abilities to resist infection. The developmental success of S. hermonthica parasites growing on different rice-host cultivars (genotypes) depended significantly on a parasite population by host-genotype interaction. Genetic analysis using amplified fragment length polymorphism (AFLP) markers revealed that a small subset of AFLP markers showed 'outlier' genetic differentiation among sub-populations of S. hermonthica attached to different host cultivars. We suggest that, this indicates a genetic component to host range within populations of S. hermonthica, and that a detailed understanding of the genomic loci involved will be crucial in understanding host-parasite specificity and in breeding crop cultivars with broad spectrum resistance to S. hermonthica.     

Vector-borne parasites decrease host mobility: a field test of freeze or flee behaviour of willow ptarmigan

Transmission mode has been suggested to be a strong predictor of virulence. According to theory, the transmission of vector-borne parasites should be less dependent on host mobility than directly transmitted parasites. This could select for increased exploitation of host resources in parasites transmitted by vectors, which may be manifested as higher virulence. Here, we test the prediction that there is an association between transmission mode and the effect on host mobility by comparing parasite infection levels and mobility in willow ptarmigan (Lagopus lagopus L.). We examined the endoparasite infracommunities of individual hosts to obtain annual, quantitative data on four vector-transmitted species (Leucocytozoon lovati, Trypanosoma avium, Haemoproteus mansoni and microfilaria), two directly transmitted species (Trichostrongylus tenuis and Eimeria sp.) and two species with indirect life cycles (Hymenolepis microps and Parionella urogalli). We then used observed variations in freeze-or-flee responses of individual willow ptarmigan to assess whether parasite intensities were related to scored freezing responses. From a field data set covering a period of 9 years from a single area, we found that stronger freezing responses were associated with higher intensities of vector-borne parasites, especially with higher intensities of the haemosporidian L. lovati. Freezing responses were not associated with parasites transmitted in other ways. Thus, high intensities of vector-borne parasites tended to reduce host movements, while parasites with other transmission modes did not.     

Interactions among co-infecting parasite species: a mechanism maintaining genetic variation in parasites?

Individuals of free-living organisms are often infected simultaneously by a community of parasites. If the co-infecting parasites interact, then this can add significantly to the diversity of host genotypexparasite genotype interactions. However, interactions between parasite species are usually not examined considering potential variation in interactions between different strain combinations of co-infecting parasites. Here, we examined the importance of interactions between strains of fish eye flukes Diplostomum spathaceum and Diplostomum gasterostei on their infectivity in naive fish hosts. We assessed the infection success of strains of both species in single-strain exposures and in co-exposures with a random strain of the other species. Parasite infection success did not consistently increase or decrease in the co-exposure treatment, but depended on the combinations of co-infecting parasite strains. This disrupted the relative infectivity of D. spathaceum strains observed in single-strain exposures. The infection success of D. gasterostei strains was independent of exposure type. These results suggest that interactions among parasite species may be strain specific and potentially promote maintenance of genetic polymorphism in parasite populations.     

Host manipulation by parasites in the world of dead-end predators: adaptation to enhance transmission?

Trophically transmitted parasites often alter their intermediate host's phenotype, thereby predisposing the hosts to increased predation. This is generally considered a parasite strategy evolved to enhance transmission to the next hosts. However, the adaptive value of host manipulation is not clear as it may be associated with costs, such as increased susceptibility to predators that are unsuitable next hosts for the parasites. We examined the ratio between the benefits and costs of host manipulation for transmission success of Acanthocephalus lucii (Acanthocephala), a parasite that alters the hiding behaviour and pigmentation of its isopod hosts. We experimentally compared the susceptibility of infected and uninfected isopods to predation by perch (Perca fluvialis; definitive host of the parasite) and dragonfly larvae (dead end). We found that the parasite predisposed the isopods to predation by both predators. However, the increased predation vulnerability of the infected isopods was higher towards perch. This suggests that, despite the costs due to non-host predation, host manipulation may still be advantageous for the parasite.     

Expression of parasite genetic variation changes over the course of infection: implications of within-host dynamics for the evolution of virulence

How infectious disease agents interact with their host changes during the course of infection and can alter the expression of disease-related traits. Yet by measuring parasite life-history traits at one or few moments during infection, studies have overlooked the impact of variable parasite growth trajectories on disease evolution. Here we show that infection-age-specific estimates of host and parasite fitness components can reveal new insight into the evolution of parasites. We do so by characterizing the within-host dynamics over an entire infection period for five genotypes of the castrating bacterial parasite Pasteuria ramosa infecting the crustacean Daphnia magna. Our results reveal that genetic variation for parasite-induced gigantism, host castration and parasite spore loads increases with the age of infection. Driving these patterns appears to be variation in how well the parasite maintains control of host reproduction late in the infection process. We discuss the evolutionary consequences of this finding with regard to natural selection acting on different ages of infection and the mechanism underlying the maintenance of castration efficiency. Our results highlight how elucidating within-host dynamics can shed light on the selective forces that shape infection strategies and the evolution of virulence.     

Social network structure and parasite infection patterns in a territorial reptile, the tuatara (Sphenodon punctatus)

We investigated whether the parasite load of an individual could be predicted by its position in a social network. Specifically, we derived social networks in a solitary, territorial reptile (the tuatara, Sphenodon punctatus), with links based on the sharing of space, not necessarily synchronously, in overlapping territories. Tuatara are infected by ectoparasitic ticks (Amblyomma sphenodonti), mites (Neotrombicula spp.) and a blood parasite (Hepatozoon tuatarae) which is transmitted by the tick. We recorded the location of individual tuatara in two study plots twice daily during the mating season (March) in 2 years (2006 and 2007) on Stephens Island, New Zealand. We constructed weighted, directed networks to represent pathways for parasite transmission, where nodes represented individual tuatara and edges connecting the nodes represented the extent of territory overlap among each pair of individuals. We considered a network-based hypothesis which predicted that the in-strength of individuals (the sum of edge weights directed towards a node) in the derived network would be positively related to their parasite load. Alternatively, if the derived social network did not reflect actual parasite transmission, we predicted other factors such as host sex, size or territory size may better explain variation in parasite infection patterns. We found clear positive relationships between the in-strength of tuatara and their tick loads, and infection patterns with tick-borne blood parasites. In particular, the extent that individuals were connected to males in the network consistently predicted tick loads of tuatara. However, mite loads of tuatara were significantly related to host sex, body size and territory size, and showed little association with network measures. The results suggest that the pathway of transmission of parasites through a population will depend on the transmission mechanism of the parasite, but that social networks provide a powerful predictive tool for some parasites.     

The within-host dynamics of African trypanosome infections

African trypanosomes are single-celled protozoan parasites that are capable of long-term survival while living extracellularly in the bloodstream and tissues of mammalian hosts. Prolonged infections are possible because trypanosomes undergo antigenic variation—the expression of a large repertoire of antigenically distinct surface coats, which allows the parasite population to evade antibody-mediated elimination. The mechanisms by which antigen genes become activated influence their order of expression, most likely by influencing the frequency of productive antigen switching, which in turn is likely to contribute to infection chronicity. Superimposed upon antigen switching as a contributor to trypanosome infection dynamics is the density-dependent production of cell-cycle arrested parasite transmission stages, which limit the infection while ensuring parasite spread to new hosts via the bite of blood-feeding tsetse flies. Neither antigen switching nor developmental progression to transmission stages is driven by the host. However, the host can contribute to the infection dynamic through the selection of distinct antigen types, the influence of genetic susceptibility or trypanotolerance and the potential influence of host-dependent effects on parasite virulence, development of transmission stages and pathogenicity. In a zoonotic infection cycle where trypanosomes circulate within a range of host animal populations, and in some cases humans, there is considerable scope for a complex interplay between parasite immune evasion, transmission potential and host factors to govern the profile and outcome of infection.     

Malaria parasite liver stages render host hepatocytes susceptible to mitochondria-initiated apoptosis

Intracellular eukaryotic parasites and their host cells constitute complex, coevolved cellular interaction systems that frequently cause disease. Among them, Plasmodium parasites cause a significant health burden in humans, killing up to one million people annually. To succeed in the mammalian host after transmission by mosquitoes, Plasmodium parasites must complete intracellular replication within hepatocytes and then release new infectious forms into the blood. Using Plasmodium yoelii rodent malaria parasites, we show that some liver stage (LS)-infected hepatocytes undergo apoptosis without external triggers, but the majority of infected cells do not, and can also resist Fas-mediated apoptosis. In contrast, apoptosis is dramatically increased in hepatocytes infected with attenuated parasites. Furthermore, we find that blocking total or mitochondria-initiated host cell apoptosis increases LS parasite burden in mice, suggesting that an anti-apoptotic host environment fosters parasite survival. Strikingly, although LS infection confers strong resistance to extrinsic host hepatocyte apoptosis, infected hepatocytes lose their ability to resist apoptosis when anti-apoptotic mitochondrial proteins are inhibited. This is demonstrated by our finding that B-cell lymphoma 2 family inhibitors preferentially induce apoptosis in LS-infected hepatocytes and significantly reduce LS parasite burden in mice. Thus, targeting critical points of susceptibility in the LS-infected host cell might provide new avenues for malaria prophylaxis.     

Invasion success of Fibrillanosema crangonycis, n.sp., n.g.: a novel vertically transmitted microsporidian parasite from the invasive amphipod host Crangonyx pseudogracilis

Parasitism is known to be an important factor in determining the success of biological invasions. Here we examine Crangonyx pseudogracilis, a North American amphipod invasive in the United Kingdom and describe a novel microsporidium, Fibrillanosema crangonycis n.sp., n.g. The primary site of infection is the female gonad and the parasite is transovarially transmitted to the eggs. PCR screening reveals a female bias in the distribution of parasites (96.6% of females, N=29; 22.2% of males, N=27), which is indicative of host sex ratio distortion. The morphological and molecular characterisations of this new microsporidium place it outside all currently established genera. On the basis of these differences, we erect the new genus Fibrillanosema n.g. While F. crangonycis is morphologically identical to uncharacterised microsporidia from populations of North American amphipods, it is distinct from microsporidia found in European populations of amphipods. These data support the hypothesis that vertically transmitted parasites may be selectively retained during invasion events. Furthermore where vertical transmission is combined with host sex ratio distortion these parasites may directly enhance host invasion success through increased rates of population growth.     

Molecular data suggest that microsporidian parasites in freshwater snails are diverse

Microsporidian parasites infect almost all invertebrate and vertebrate hosts and have significant effects on individual and population fitness. Phylogenetic analysis demonstrates that the phylum is highly divergent and that some lineages show strong associations with host taxa. We here examine the diversity and distribution of parasites in gastropod molluscs to test for host-parasite co-association. 16 populations representing 10 species of freshwater snails were screened using microsporidian specific small subunit rDNA primers. Four novel microsporidian parasite sequences were detected within populations of three host species from the genera Bulinus, Biomphalaria and Planorbis. Prevalence ranged from 5 to 84%. Phylogenetic analysis of these novel sequences reveals that they group together as a paraphyletic assemblage in the microsporidian tree basal to the two lineages containing the genera Encephalitozoon and Nosema. Preliminary observation of one microsporidian infection, show parasites distributed in all tissue systems of Bulinus globosus. However, infection is most prevalent in the digestive gland while also in the egg sacs, suggesting that the microsporidium is using a mixed strategy of horizontal and vertical transmission in this population.     

Establishment of the new genus Paranosema based on the ultrastructure and molecular phylogeny of the type species Paranosema grylli Gen. Nov., Comb. Nov. (Sokolova, Selezniov, Dolgikh, Issi 1994), from the cricket Gryllus bimaculatus Deg

The ultrastructure of the microsporidian parasite Nosema grylli, which parasitizes primarily fat body cells and haemocytes of the cricket Gryllus bimaculatus (Orthoptera, Gryllidae) is described. All observed stages (meront, meront/sporont transitional stage ("second meront"), sporont, sporoblast, and spore) are found in direct contact with the host cell cytoplasm. Nuclei are diplokaryotic during almost all stages of the life cycle, but a brief stage with one nucleus containing an abundance of electron-dense material is observed during a "second merogony." Sporogony is disporous. Mature spores are ovocylindrical in shape and measure 4.5+/-0.16micromx2.2+/-0.07 microm (n=10) on fresh smears and 3.3+/-0.06 micromx1.4+/-0.07 microm (n=10) on ultrathin sections. Spores contain 15-18 coils of an isofilar polar filament arranged in one or two layers. Comparative phylogenetic analysis using rDNA shows N. grylli to be closely related to another orthopteran microsporidian, Nosema locustae, and to Nosema whitei from the confused flour beetle, Tribolium confusum. Antonospora scoticae, a parasite of the communal bee Andrena scotica, is a sister taxon to these three Nosema species. The sequence divergence and morphological traits clearly separate this group of "Nosema" parasites from the "true" Nosema clade containing Nosema bombycis. We therefore propose to change the generic name of N. grylli and its close relative N. locustae to Paranosema n. comb. We leave N. whitei in former status until more data on fine morphology of the species are obtained.     

The role of moulting in parasite defence

Parasitic infections consist of a succession of steps during which hosts and parasites interact in specific manners. At each step, hosts can use diverse defence mechanisms to counteract the parasite's attempts to invade and exploit them. Of these steps, the penetration of parasites into the host is a key step for a successful infection and the epithelium is the first line of host defence. The shedding of this protective layer (moulting) is a crucial feature in the life cycle of several invertebrate and vertebrate taxa, and is generally considered to make hosts vulnerable to parasites and predators. Here, we used the crustacean Daphnia magna to test whether moulting influences the likelihood of infection by the castrating bacterium Pasteuria ramosa. This parasite is known to attach to the host cuticula before penetrating into its body. We found that the likelihood of successful parasite infection is greatly reduced if the host moults within 12 h after parasite exposure. Thus, moulting is beneficial for the host being exposed to this parasite. We further show that exposure to the parasite does not induce hosts to moult earlier. We discuss the implications of our findings for host and parasite evolution and epidemiology.     

H-2 complex influences cytokine gene expression in Leishmania infantum-infected macrophages

This work aims to study the influence of H-2 locus in the control of Leishmania infantum infection by evaluating whether cytokine responses by host macrophages of different H-2 haplotype are differentially regulated, either induced or actively impaired during parasite growth and replication. This study shows that macrophages of "non-cure" phenotype (H-2(d)) are more susceptible to infection with virulent L. infantum promastigotes. Virulent parasites lead to impaired IL-12 and inhibited TNF-alpha expression. The degree of parasite virulence is an important contributing factor to differences detected in cytokine expression. Virulent parasites also induced TGF-beta, a deactivating cytokine that is known to suppress Th-1 type responses, thus allowing the parasite to subvert antimicrobial activity and increase its chances of survival. Depending on specific host haplotype, cells differentially respond to infection since TNF-alpha expression is inhibited and TGF-beta is enhanced by macrophages of "non-cure" phenotype, thus perhaps determining their degree of susceptibility in this strain of mice.