Variation and polymorphism in helminth parasites

Somatic Dimorphism in Cercariae of a Bird Schistosome

Phenotypic polymorphism is a commonly observed phenomenon in nature, but extremely rare in free-living stages of parasites. We describe a unique case of somatic polymorphism in conspecific cercariae of the bird schistosome Trichobilharzia sp. “peregra”, in which two morphs, conspicuously different in their size, were released from a single Radix balthica snail. A detailed morphometric analysis that included multiple morphological parameters taken from 105 live and formalin-fixed cercariae isolated from several naturally infected snails provided reliable evidence for a division of all cercariae into two size groups that contained either large or small individuals. Large morph (total body length of 1368 and 1339 μm for live and formalin-fixed samples, respectively) differed significantly nearly in all morphological characteristics compared to small cercariae (total body length of 976 and 898 μm for live and formalin samples, respectively), regardless of the fixation method. Furthermore, we observed that small individuals represent the normal/commonly occurring phenotype in snail populations. The probable causes and consequences of generating an alternative, much larger phenotype in the parasite infrapopulation are discussed in the context of transmission ecology as possible benefits and disadvantages facilitating or preventing the successful completion of the life cycle.

Advances in the Development of Anti-Haemonchus contortus Vaccines: Challenges, Opportunities, and Perspectives

The gastrointestinal nematode parasite Haemonchus contortus (H. contortus) is a resident of tropical and subtropical regions worldwide that imposes significant production losses, economic losses, and animal health issues in the small ruminant industry, particularly sheep and goats. Considerable efforts have been made to understand how immunity is elicited against H. contortus infection. Various potential vaccine antigens have been tested by different methods and strategies applied in animal models, and significant progress has been made in the development of vaccines against H. contortus. This review highlighted and shared the knowledge about the current understanding of host immune responses to H. contortus and ongoing challenges in the development of a protective, effective, and long-lasting vaccine against H. contortus infection. We have also pinpointed some achievements and failures in the development and testing of vaccines, which will establish a road map for future research directions to explore new effective vaccine candidates for controlling and preventing H. contortus infection.

Recent Progress in the Development of Liver Fluke and Blood Fluke Vaccines

Liver flukes (Fasciola spp., Opisthorchis spp., Clonorchis sinensis) and blood flukes (Schistosoma spp.) are parasitic helminths causing neglected tropical diseases that result in substantial morbidity afflicting millions globally. Affecting the world’s poorest people, fasciolosis, opisthorchiasis, clonorchiasis and schistosomiasis cause severe disability; hinder growth, productivity and cognitive development; and can end in death. Children are often disproportionately affected. F. hepatica and F. gigantica are also the most important trematode flukes parasitising ruminants and cause substantial economic losses annually. Mass drug administration (MDA) programs for the control of these liver and blood fluke infections are in place in a number of countries but treatment coverage is often low, re-infection rates are high and drug compliance and effectiveness can vary. Furthermore, the spectre of drug resistance is ever-present, so MDA is not effective or sustainable long term. Vaccination would provide an invaluable tool to achieve lasting control leading to elimination. This review summarises the status currently of vaccine development, identifies some of the major scientific targets for progression and briefly discusses future innovations that may provide effective protective immunity against these helminth parasites and the diseases they cause.

Low allelic diversity in vaccine candidates genes from different locations sustain hope for Fasciola hepatica immunization

Fasciola hepatica is a trematode parasite that causes fasciolosis in animals and humans. Fasciolosis is usually treated with triclabendazole, although drug-resistant parasites have been described in several geographical locations. An alternative to drug treatment would be the use of a vaccine, although vaccination studies that have been performed mainly in ruminants over the last 30 years, show high variability in the achieved protection and are not yet ready for commercialisation. Since F. hepatica exhibits a high degree of genomic polymorphism, variation in vaccine efficacy could be attributed, at least partially, to phenotypic differences in vaccine candidate sequences amongst parasites used in the challenge infections. To begin to address this issue, a collection of F. hepatica isolates from geographically dispersed regions, as well as parasites obtained from vaccination trials performed against a field isolate from Uruguay and the experimentally maintained South Gloucester isolate (Ridgeway Research, UK), were compiled to establish a F. hepatica Biobank. These collected isolates were used for the genetic analysis of several vaccine candidates that are important in host-parasite interactions and are the focus of the H2020 PARAGONE vaccine project (https://www.paragoneh2020.eu/), namely FhCL1, FhCL2, FhPrx, FhLAP and FhHDM. Our results show that F. hepatica exhibits a high level of conservation in the sequences encoding each of these proteins. The consequential low variability in these vaccine candidates amongst parasites from different geographical regions reinforces the idea that they would be suitable immunogens against liver fluke isolates worldwide.

Variation in the ITS-1 and ITS-2 rRNA genomic regions of Cytauxzoon felis from bobcats and pumas in the eastern United States and comparison with sequences from domestic cats

Cytauxzoon felis, a tick-borne protozoan parasite, is the causative agent of cytauxzoonosis in domestic cats in the United States. The natural reservoir for this parasite is the bobcat (Lynx rufus), which typically does not develop clinical signs. Although not likely important reservoirs, C. felis has also been detected in pumas (Puma concolor) in Florida and Louisiana. Recent studies suggest that specific genotypes of C. felis that circulate in domestic cats may be associated with variable clinical outcomes and specific spatial locations. In the current study, we investigated the intraspecific variation of the C. felis internal transcribed spacer (ITS)-1 and ITS-2 rRNA regions from 145 wild felids (139 bobcats and six pumas) from 11 states (Florida, Georgia, Kansas, Kentucky, Louisiana, Missouri, North Carolina, North Dakota, South Carolina, Oklahoma, and Pennsylvania). Unambiguous ITS-1 and ITS-2 data were obtained for 144 and 112 samples, respectively, and both ITS-1 and ITS-2 sequences were obtained for 111 (77%) samples. For the ITS-1 region, sequences from 65 samples collected from wild felids were identical to those previously reported in domestic cats, while the other 79 sequences were unique. C. felis from 45 bobcats and one puma had ITS-1 sequences identical to the most common sequence reported from domestic cats. Within the ITS-2 region, sequences from 49 bobcats were identical to those previously reported in domestic cats and 63 sequences were unique (with some occurring in more than one bobcat). The most common ITS-2 sequence from domestic cats was also common in wild felids (31 bobcats and a puma). Samples from three pumas from Florida and two bobcats from Missouri had a 40- or 41-bp insert in the ITS-2 similar to one described previously in a domestic cat from Arkansas. Additionally, a previously undescribed 198- or 199-bp insert was detected in the ITS-2 sequence from four bobcats. Collectively, based on combined ITS-1 and ITS-2 sequences, five different genotypes were detected in the wild felids. Genotype ITSa was the most common genotype (11 bobcats and one puma) and fewer numbers of ITSb, ITSe, ITSg, and ITSi were detected in bobcats. These data indicate that, based on ITS-1 and ITS-2 sequences, numerous C. felis strains may circulate in wild felids.

Influence of anti-filarial chemotherapy strategies on the genetic structure of Wuchereria bancrofti populations

Lymphatic filarial (LF) parasites have been under anti-filarial drug pressure for more than half a century. Currently, annual mass drug administration (MDA) of diethylcarbamazine (DEC) or ivermectin in combination with albendazole (ALB) have been used globally to eliminate LF. Long-term chemotherapies exert significant pressure on the genetic structure of parasitic populations. We investigated the genetic variation among 210 Wuchereria bancrofti populations that were under three different chemotherapy strategies, namely MDA with DEC alone (group I, n = 74), MDA with DEC and ALB (group II, n = 60) and selective therapy (ST) with DEC (group III, n = 34) to understand the impact of these three drug regimens on the parasite genetic structure. Randomly amplified polymorphic DNA profiles were generated for the three groups of parasite populations; the gene diversity, gene flow and genetic distance values were determined and phylogenetic trees were constructed. Analysis of these parameters indicated that parasite populations under ST with a standard dose of DEC (group III) were genetically more diverse (0.2660) than parasite populations under MDA with DEC alone (group I, H = 0.2197) or with DEC + ALB (group II, H = 0.2317). These results indicate that the MDA may reduce the genetic diversity of W. bancrofti populations when compared to the genetic diversity of parasite populations under ST.

Acquired immune heterogeneity and its sources in human helminth infection

Similarities in the immunobiology of different parasitic worm infections indicate that co-evolution of humans and helminths has shaped a common anti-helminth immune response. However, recent in vitro and immuno-epidemiological studies highlight fundamental differences and plasticity within host-helminth interactions. The 'trade-off' between immunity and immunopathology inherent in host immune responses occurs on a background of genetic polymorphism, variable exposure patterns and infection history. For the parasite, variation in life-cycle and antigen expression can influence the effector responses directed against them. This is particularly apparent when comparing gastrointestinal and tissue-dwelling helminths. Furthermore, insights into the impact of anti-helminthic treatment and co-infection on acquired immunity suggest that immune heterogeneity arises not from hosts and parasites in isolation, but also from the environment in which immune responses develop. Large-scale differences observed in the epidemiology of human helminthiases are a product of complex host-parasite-environment interactions which, given potential for exposure to parasite antigens in utero, can arise even before a parasite interacts with its human host. This review summarizes key differences identified in human acquired immune responses to nematode and trematode infections of public health importance and explores the factors contributing to these variations.

Variation in the hatching behaviour of Nematodirus battus: polymorphic bet hedging?

Previous work on the transmission dynamics of Nematodirus battus, an important nematode parasite of farmed ruminants in temperate regions, suggests that it operates a bet-hedging strategy. Hatching of cold-sensitised eggs is concentrated in spring, while alternative hatching of non-cold-sensitised eggs in autumn mitigates the risk of poor conditions for hatching in spring or host absence during peak larval availability. Isolates from Scotland showed much less propensity to hatch without chilling than the previously characterised isolate from southern England. Nematodirus battus eggs from a hill farm in Scotland showed intermediate proportions of non-chilled hatching, perhaps related to unpredictability of climate at higher altitudes. Geographic polymorphism in larval behaviour appears to be present in the form of differing chilling requirements for egg hatching. Since bet-hedging through trait diversification is a plausible and demonstrated strategy for coping with environmental unpredictability, it is a likely target for adaptation to climate change. Predictions of disease epidemiology in a changing climate should incorporate parasite adaptation, but further theoretical and empirical characterisations of likely evolutionary responses are needed before this is possible for the most economically important systems.

Intraspecific epitopic variation in a carbohydrate antigen exposed on the surface of Trichostrongylus colubriformis infective L3 larvae

The carbohydrate larval antigen, CarLA, is present on the exposed surface of all strongylid nematode infective L3 larvae tested, and antibodies against CarLA can promote rapid immune rejection of incoming Trichostrongylus colubriformis larvae in sheep. A library of ovine recombinant single chain Fv (scFv) antibody fragments, displayed on phage, was prepared from B cell mRNA of field-immune sheep. Phage displaying scFvs that bind to the surface of living exsheathed T. colubriformis L3 larvae were identified, and the majority of worm-binding scFvs recognized CarLA. Characterization of greater than 500 worm surface binding phage resulted in the identification of nine different anti-CarLA scFvs that recognized three distinct T. colubriformis CarLA epitopes based on blocking and additive ELISA. All anti-CarLA scFvs were specific to the T. colubriformis species of nematode. Each of the three scFv epitope classes displayed identical Western blot recognition patterns and recognized the exposed surface of living T. colubriformis exsheathed L3 larvae. Surprisingly, each of the anti-CarLA scFvs was able to bind to only a subset of worms. Double-labelling indirect immunofluorescence revealed that the three classes of anti-CarLA scFvs recognize distinct, non-overlapping, T. colubriformis sub-populations. These results demonstrate that individual T. colubriformis L3 larvae display only one of at least three distinct antigenic forms of CarLA on their surface at any given time, and suggest that antigenic variation within CarLA is likely a mechanism of immune evasion in strongylid nematodes.

Strongylid nematode worm parasites currently infect hundreds of millions of people, and most farmed animals, causing enormous morbidity and economic loss. These parasites commonly produce chronic gastrointestinal infections that are highly refractory to immune clearance mechanisms. Mucosal antibodies against a carbohydrate surface antigen (CarLA) can cause rapid expulsion of incoming larval nematodes. Sheep develop strong anti-strongylid immunity following long-term grazing on contaminated pasture. From these sheep, we identified and characterized recombinant antibodies that recognize CarLA on living L3 stage infective larvae of the strongylid parasite, Trichostrongylus colubriformis. The selected antibodies are specific only to larvae of the T. colubriformis species and, surprisingly, recognize only a subset of these worms. Three different anti-CarLA antibody classes were found and each recognizes different, non-overlapping subsets of worms which, together, comprise virtually the entire population. These results are the first demonstration of “intraspecific epitopic variation” within strongylid nematodes and suggest that these parasites have a mechanism that permits the surface presentation of at least three different antigenic forms of CarLA to avoid immune clearance.

Exploring the immunology of parasitism--from surface antigens to the hygiene hypothesis

Helminth immunology is a field which has changed beyond recognition in the past 30 years, transformed not only by new technologies from cDNA cloning to flow cytometry, but also conceptually as our definition of host immune pathways has matured. The molecular revolution defined key nematode surface and secreted antigens, and identified candidate immunomodulators that are likely to underpin parasites' success in eluding immune attack. The immunological advances in defining cytokine networks, lymphocyte subsets and innate cell recognition have also made a huge impact on our understanding of helminth infections. Most recently, the ideas of regulatory immune cells, in particular the regulatory T cell, have again overturned older thinking, but also may explain immune hyporesponsiveness observed in chronic helminth diseases, as well as the link to reduced allergic reactions observed in human and animal infections. The review concludes with a forward look to where we may make future advances towards the final eradication of helminth diseases.

Predisposition to ascariasis: patterns, mechanisms and implications

Ascaris lumbricoides, the human roundworm, is a remarkably infectious and persistent parasite. It is a member of the soil-transmitted helminths or geohelminths and infects in the order of 1472 million people worldwide. Despite, its high prevalence and wide distribution it remains along with its geohelminth counterparts, a neglected disease. Ascariasis is associated with both chronic and acute morbidity, particularly in growing children, and the level of morbidity assessed as disability-adjusted life years is about 10·5 million. Like other macroparasite infections, the frequency distribution ofA. lumbricoidesis aggregated or overdispersed with most hosts harbouring few or no worms and a small proportion harbouring very heavy infections. Furthermore, after chemotherapeutic treatment, individuals demonstrate consistency in the pattern of re-infection with ascariasis, described as predisposition. These epidemiological phenomena have been identified, in a consistent manner, from a range of geographical locations in both children and adults. However, what has proved to be much more refractory to investigation has been the mechanisms that contribute to the observed epidemiological patterns. Parallel observations utilizing human subjects and appropriate animal model systems are essential to our understanding of the mechanisms underlying susceptibility/resistance to ascariasis. Furthermore, these patterns ofAscarisintensity and re-infection have broader implications with respect to helminth control and interactions with other important bystander infections.

Age-related and infection intensity-related shifts in antibody recognition of defined protein antigens in a schistosome-exposed population

BACKGROUND

This study compared patterns of recognition of defined Schistosoma haematobium adult worm antigens by serum antibodies from schistosome-exposed Zimbabweans aged 5-18 years.

METHODS

The population was stratified by age and infection intensity into 9 groups within which serum specimens were pooled and used to screen for protein recognition by 2-dimensional Western blotting. Recognized proteins were identified by electrospray ionizing tandem mass spectrometry.

RESULTS

A total of 71 antigens were recognized by >or=1 of the serum pools. The recognition varied distinctly with host age and infection intensity, with some isoform-specific responses. The repertoire of antigens recognized increased with age, peaking in the oldest participants whose had no or mild-to-moderate infection intensity. The intensity of antigen recognition also increased with age, peaking in the oldest participants with the heaviest infection intensity.

CONCLUSIONS

The recognition of specific schistosome antigens, both in terms of the diversity of antigens recognized and the intensity of antigen recognition, increased with duration of exposure to infection, supporting the hypothesis that the slow development of schistosome-acquired immunity is due to the slow accumulation of responsiveness to relevant parasite antigens.

Taeniid history, natural selection and antigenic diversity: evolutionary theory meets helminthology

Large sets of nucleotide sequence data of parasitic helminths have been accumulated in the past two decades. Our ability to improve the health of people and animals using this knowledge has not increased proportionally, however. Evolutionary biology provides the background to understand how parasites adapt to their hosts, and computational molecular biology offers the tools to infer the mechanisms involved. The study of antigenic diversity, a way for parasites to overcome host defenses against parasites, has been neglected in helminths, yet such a study could contribute to the development of more efficient drugs, diagnostic tests and vaccines. This review focuses on the study of adaptive evolution as the cause of antigenic diversity in tapeworms and its potential applications.

HOST AVAILABILITY AND THE EVOLUTION OF PARASITE LIFE-HISTORY STRATEGIES

Parasites exploit an inherently patchy resource, their hosts, which are discrete entities that may only be available for infection within a relatively short time window. However, there has been little consideration of how heterogeneities in host availability may affect the phenotypic or genotypic composition of parasite populations or how parasites may evolve to cope with them. Here we conduct a selection experiment involving an entomopathogenic nematode (Steinernema feltiae) and show for the first time that the infection rate of a parasite can evolve rapidly to maximize the chances of infecting within an environment characterized by the rate of host availability. Furthermore, we show that the parasite's infection rate trades off with other fitness traits, such as fecundity and survival. Crucially, the outcome of competition between strains with different infection strategies depends on the rate of host availability; frequently available hosts favor "fast" infecting nematodes, whereas infrequently available hosts favor "slow" infecting nematodes. A simple evolutionarily stable strategy (ESS) analysis based on classic epidemiological models fails to capture this behavior, predicting instead that the fastest infecting phenotype should always dominate. However, a novel model incorporating more realistic, discrete bouts of host availability shows that strain coexistence is highly likely. Our results demonstrate that heterogeneities in host availability play a key role in the evolution of parasite life-history traits and in the maintenance of phenotypic variability. Parasite life-history strategies are likely to evolve rapidly in response to changes in host availability induced by disease management programs or by natural dynamics in host abundance. Incorporating parasite evolution in response to host availability would therefore enhance the predictive ability of current epidemiological models of infectious disease.

Developments and hurdles in generating vaccines for controlling helminth parasites of grazing ruminants

As a direct consequence of rising drug resistance among common nematodes of grazing animals, efforts toward state-of-the-art vaccine development have clearly intensified in recent years, fuelled primarily by the advent of newer technologies in gene discovery, by advancements in antigen identification, characterisation and production. In this regard, it is appropriate to review progress that has been made in generating helminth vaccines and in particular, vaccines against common nematodes of production animals for consumption. In like manner, it is prudent to evaluate barriers that have hindered progress in the past and continue to present obstacles that must be solved when utilizing and depending on host immunity to attenuate parasitic infections.

Crystal Structure of the Complex between the Monomeric Form of Toxoplasma gondii Surface Antigen 1 (SAG1) and a Monoclonal Antibody that Mimics the Human Immune Response

Toxoplasma gondii, the intracellular parasite responsible for toxoplasmosis infects more than one-third of the world population and can be life-threatening for fetuses and immunocompromised patients. The surface protein SAG1 is an important immune target, which provides a strong immune response against the invasive tachyzoite while the other forms of the parasite, devoid of SAG1 at their surface, are multiplying. In addition to this role as a "hot spot" decoy, SAG1 is predicted to act as an adhesin during host-cell attachment through its binding to proteoglycans. To begin to understand the relationships between SAG1 epitopes and the ligand-binding site, we have solved the crystal structure of the monomeric form of T.gondii SAG1 complexed to a Fab derived from a monoclonal antibody raised against tachyzoite particles. This antibody competes strongly with human Toxoplasma-specific sera, suggesting that its epitope is part of an immunodominant region present on the surface of SAG1. The structure reveals that this conformational epitope, located within the SAG1 N-terminal domain, does not overlap with the proposed ligand-binding pocket. This study provides the first structural description of the monomeric form of SAG1, and significant insights into its dual role of adhesin and immune target during parasite infection.

High polymorphism in genes encoding antigen B from human infecting strains of Echinococcus granulosus

Echinococcus granulosus antigen B (AgB) is encoded by a gene family and is involved in the evasion of the host immune response. E. granulosus exists as a number of strains (G1-G10) that differ in biological characteristics. We used PCR-SSCP followed by DNA sequencing to evaluate sequence variation and transcription profile of AgB in 5 E. granulosus strains. Twenty-four genomic sequences were isolated and clustered in 3 groups related to 2 of the 5 reported AgB genes. AgB4 genes were present in almost all strains, whereas AgB2 were present as functional genes exclusively in G1/G2 cluster, and as non-functional genes in G5 and the G6/G7 cluster, suggesting inter-strain variation. The AgB transcription patterns, analysed by RT-PCR, showed that AgB2 and AgB4 genes were transcribed in G1, while only the AgB4 gene was transcribed in G7 strain. Cysts from the same strain or cluster shared more genomic and cDNA variants than cysts from different strain or cluster. The level of nucleotide and deduced amino acid sequence variation observed is higher than that reported so far for coding genes of other helminths. Neutrality was rejected for AgB2 genes. These data show the genetic polymorphism of antigen-coding genes among genetically characterized strains of E. granulosus.

mRNA sequences for Haemonchus contortus intestinal cathepsin B-like cysteine proteases display an extreme in abundance and diversity compared with other adult mammalian parasitic nematodes

Cathepsin B-like cysteine protease (cbl) genes produce the most abundant mRNAs ( approximately 16%) detected in the adult female intestine of the parasitic nematode Haemonchus contortus. CBL enzymes appear to digest host proteins and are vaccine candidates for immune control of H. contortus and potentially other parasitic nematodes. Hence, it is important to quantify the extent of diversity of H. contortuscbl genes. Here, expressed sequence tags (ESTs) were used to assess both the size and diversity of the H. contortuscbl gene family. Contig analysis of 686 cbl ESTs from a USA isolate resolved 123 clusters. ESTs were grouped into discrete sets and analyzed using an additive model. Discovery of new cbl clusters increased with each set and reached a terminal rate of about 1 per 10 ESTs. The extreme diversity was unique to cbls relative to other genes investigated and was ascribed to specific cbl clades. Sixty percent of cbl clusters from a UK isolate were shared with those identified in the USA isolate, suggesting conservation of cbl gene repertoires across regions, although minor to moderate geographic variation cannot be excluded. Sequence comparisons also suggested high potential for antigenic diversity among CBL proteins, which is relevant to vaccine strategies. Compared to other parasitic nematodes of mammals, the extreme abundance and diversity of intestinal cbl transcripts appear to be relative specializations for H. contortus. Therefore, adaptations related to nutrient acquisition may vary markedly among these parasitic nematodes.

Prevalence, intensity, and differential development of Pseudodelphis oligocotti (Nematoda: Dracunculoidea) in sympatric fish hosts of the northeastern Pacific coast

Counter to expectations of coevolved parasite-host relationships, parasites frequently infect hosts that never contribute to their reproduction, making the identification of a parasite's true host-specificity problematic. Pseudodelphis oligocotti (Nematoda: Dracunculoidea) infects several coastal Pacific fishes, but its course of development appears highly variable, suggesting that incidence does not reflect effective host range. To determine the host range of P. oligocotti and describe its relationship to various potential hosts, 24 fish species were examined from several British Columbia localities for prevalence, intensity, and extent and tissue location of parasite development. Pseudodelphis oligocotti infects 9 species of fishes from 5 orders, of which penpoint gunnel, Apodichthys flavidus, showed the highest prevalence and intensity, up to 80% and 19 (+/- 17.1 SD) worms per host, respectively. Although subadult and adult P. oligocotti occurred in all 9 fishes, larvigerous P. oligocotti only occurred in A. flavidus and rarely in the northern clingfish, Gobiesox maeandricus. Infective first-stage larvae were recovered from gill tissue of A. flavidus. Thus, at most only 2 of the 9 host species infected by P. oligocotti actually contribute to its transmission. The occurrence of P. oligocotti in diverse hosts may be accounted for by the parasite's indiscriminant mode of transmission via ingestion of free-living intermediate copepod hosts, where highly exposed or more suitable fishes (or both) are closely related by diet and microhabitat. This study demonstrates how parasite transmission and host ecology can greatly affect observed host range and ultimately its potential for expansion.

Phase and antigenic variation in bacteria

Phase and antigenic variation result in a heterogenic phenotype of a clonal bacterial population, in which individual cells either express the phase-variable protein(s) or not, or express one of multiple antigenic forms of the protein, respectively. This form of regulation has been identified mainly, but by no means exclusively, for a wide variety of surface structures in animal pathogens and is implicated as a virulence strategy. This review provides an overview of the many bacterial proteins and structures that are under the control of phase or antigenic variation. The context is mainly within the role of the proteins and variation for pathogenesis, which reflects the main body of literature. The occurrence of phase variation in expression of genes not readily recognizable as virulence factors is highlighted as well, to illustrate that our current knowledge is incomplete. From recent genome sequence analysis, it has become clear that phase variation may be more widespread than is currently recognized, and a brief discussion is included to show how genome sequence analysis can provide novel information, as well as its limitations. The current state of knowledge of the molecular mechanisms leading to phase variation and antigenic variation are reviewed, and the way in which these mechanisms form part of the general regulatory network of the cell is addressed. Arguments both for and against a role of phase and antigenic variation in immune evasion are presented and put into new perspective by distinguishing between a role in bacterial persistence in a host and a role in facilitating evasion of cross-immunity. Finally, examples are presented to illustrate that phase-variable gene expression should be taken into account in the development of diagnostic assays and in the interpretation of experimental results and epidemiological studies.

Phenotypic plasticity in adult worms of Schistosoma mansoni (Trematoda:Schistosomatidae) evidenced by brightfield and confocal laser scanning microscopies

A comparative morphometric study was performed to identify host-induced morphological alterations in Schistosoma mansoni adult worms. A wild parasite population was obtained from a naturally infected rodent (Nectomys squamipes) and then recovered from laboratory infected C3H/He mice. Furthermore, allopatric worm populations maintained for long-term under laboratory conditions in Swiss Webster mice were passed on to N. squamipes. Suckers and genital system (testicular lobes, uterine egg, and egg spine) were analyzed by a digital system for image analysis. Confocal laser scanning microscopy (CLSM) showed details of the genital system (testicular lobes, vitelline glands, and ovary) and the tegument just below the ventral sucker. Significant morphological changes (p < 0.05) were detected in male worms in all experimental conditions, with no significant variability as assessed by CLSM. Significant changes (p < 0.05) were evident in females from the wild population related to their ovaries and vitelline glands, whereas allopatric females presented differences only in this last character. We conclude that S. mansoni worms present the phenotypic plasticity induced by modifications in the parasite's microenvironment, mainly during the first passage under laboratory conditions.

Pathogenesis and host responses in human onchocerciasis: impact of Onchocerca filariae and Wolbachia endobacteria

Onchocerca volvulus is a tissue-invasive parasitic nematode causing skin and eye pathology in human onchocerciasis. The filariae habour abundant intracellular Wolbachia bacteria, now recognised as obligatory symbionts, and therefore emerging as a novel target for chemotherapy. Recent research demonstrates that both the filariae and endobacteria contribute to the pathogenesis of onchocerciasis, and molecules have been identified that promote inflammatory or counter-inflammatory immune mechanisms, divert the host's immune response or procure evasion of the parasite.

Control of filarial infections: not the beginning of the end, but more research is needed

PURPOSE OF REVIEW

Infections with the filarial nematodes affect more than 150 million people mainly in the tropics. The very successful efforts to control filarial infections, however, have to be sustained by new tools that require long-term commitment to research. This review, focusing on reports from 2002 and 2003, highlights recent advances in research on immunology, understanding of pathogenesis and drug development in lymphatic filariasis and onchocerciasis research with potential relevance to the generation of new tools for control.

RECENT FINDINGS

Dramatic improvement has been achieved in the control of lymphatic filariasis and onchocerciasis by vector control and mass treatment with microfilaricidal drugs. Additional tools that could help in regional elimination or, ultimately, eradication of filariasis may arise from the development of new drugs or a vaccine. Research into the immune responses mediating protection or pathology has provided new insights into the pathways that lead to effector function and immunosuppression, such as T regulatory responses, as well as into genetic predispositions from the host's side, and to the identification of vaccine candidates that show protection in animal models. Recognition of the role the Wolbachia endosymbionts may play in activating the innate immune system has altered our understanding of immunopathology of filariasis and adverse reactions to microfilaricidal drugs. Wolbachia spp. have also proven to be suitable targets for the development of a long-term sterilizing or potentially macrofilaricidal drug.

SUMMARY

This review summarizes recent developments in the control of filariasis, in particular lymphatic filariasis and onchocerciasis, as well as in modern research into the immunity of filariasis and new drug development that could lead to additional tools necessary for sustained success in filariasis control.