A new approach to the treatment of filariasis

Population dynamics of Wolbachia bacterial endosymbionts in Brugia malayi

The human filarial nematode Brugia malayi contains an endosymbiotic bacterium, Wolbachia. We used real-time quantitative polymerase chain reaction (QPCR) and microscopy to investigate the population dynamics of the bacterium-nematode association. Two Wolbachia (wsp and ftsZ) and one nematode (gst) genes were amplified from all life-cycle stages of B. malayi and results expressed as gene copies per worm and as Wolbachia/nematode ratios. Since the genes were single copy and there was one genome per Wolbachia, the gene copy numbers were equivalent to the numbers of bacteria. These were similar in microfilariae and the mosquito-borne larval stages (L2 and L3), with the lowest ratios of Wolbachia/nematode DNA. However, within 7 days of infection of the mammalian host, bacteria had increased 600-fold and the bacteria/worm ratio was the highest of all life-cycle stages. The rapid multiplication continued throughout L4 development, so that the major period of bacterial population growth occurred within 4 weeks of infection of the definitive host. Microscopy confirmed that there were few bacteria in mosquito-derived L3 but many, in large groups, in L4 collected 9 and 21 days after infection. In adult male worms up to 15 months of age, the bacterial populations were maintained, whilst in females, bacteria numbers increased as the worms matured and as the ovary and embryonic larval stages became infected. These results support the hypothesis that the bacteria are essential for larval development in the mammalian host and for the long-term survival of adult worms.

Mapping the presence of Wolbachia pipientis on the phylogeny of filarial nematodes: evidence for symbiont loss during evolution

Wolbachia pipientis is a bacterial endosymbiont associated with arthropods and filarial nematodes. In filarial nematodes, W. pipientis has been shown to play an important role in the biology of the host and in the immuno-pathology of filariasis. Several species of filariae, including the most important parasites of humans and animals (e.g. Onchocerca volvulus, Wuchereria bancrofti and Dirofilaria immitis) have been shown to harbour these bacteria. Other filarial species, including an important rodent species (Acanthocheilonema viteae), which has been used as a model for the study of filariasis, do not appear to harbour these symbionts. There are still several open questions about the distribution of W. pipientis in filarial nematodes. Firstly the number of species examined is still limited. Secondly, it is not clear whether the absence of W. pipientis in negative species could represent an ancestral characteristic or the result of a secondary loss. Thirdly, several aspects of the phylogeny of filarial nematodes are still unclear and it is thus difficult to overlay the presence/absence of W. pipientis on a tree representing filarial evolution. Here we present the results of a PCR screening for W. pipientis in 16 species of filariae and related nematodes, representing different families/subfamilies. Evidence for the presence of W. pipientis is reported for five species examined for the first time (representing the genera Litomosoides, Litomosa and Dipetalonema); original results on the absence of this bacterium are reported for nine species; for the remaining two species, we have confirmed the absence of W. pipientis recently reported by other authors. In the positive species, the infecting W. pipientis bacteria have been identified through 16S rDNA gene sequence analysis. In addition to the screening for W. pipientis in 16 species, we have generated phylogenetic reconstructions based on mitochondrial gene sequences (12S rDNA; COI), including a total of 28 filarial species and related spirurid nematodes. The mapping of the presence/absence of W. pipientis on the trees generated indicates that these bacteria have possibly been lost during evolution along some lineages of filarial nematodes.

The role of endosymbiotic Wolbachia bacteria in filarial disease

In this review, we describe the pathogenic role of Wolbachia endosymbiotic bacteria in filarial diseases, focusing on the host innate immune responses to filarial and Wolbachia products. A description of the host pathogen recognition and early inflammatory responses including TLR4-mediated signalling, chemokine and cytokine responses and inflammatory cell recruitment is provided from human studies and from animal models of filarial disease. Finally, the impact of the discovery and characterization of Wolbachia on filarial research and treatment programmes is discussed.

Phylogenomics of the reproductive parasite Wolbachia pipientis wMel: a streamlined genome overrun by mobile genetic elements

The complete sequence of the 1,267,782 bp genome of Wolbachia pipientis wMel, an obligate intracellular bacteria of Drosophila melanogaster, has been determined. Wolbachia, which are found in a variety of invertebrate species, are of great interest due to their diverse interactions with different hosts, which range from many forms of reproductive parasitism to mutualistic symbioses. Analysis of the wMel genome, in particular phylogenomic comparisons with other intracellular bacteria, has revealed many insights into the biology and evolution of wMel and Wolbachia in general. For example, the wMel genome is unique among sequenced obligate intracellular species in both being highly streamlined and containing very high levels of repetitive DNA and mobile DNA elements. This observation, coupled with multiple evolutionary reconstructions, suggests that natural selection is somewhat inefficient in wMel, most likely owing to the occurrence of repeated population bottlenecks. Genome analysis predicts many metabolic differences with the closely related Rickettsia species, including the presence of intact glycolysis and purine synthesis, which may compensate for an inability to obtain ATP directly from its host, as Rickettsia can. Other discoveries include the apparent inability of wMel to synthesize lipopolysaccharide and the presence of the most genes encoding proteins with ankyrin repeat domains of any prokaryotic genome yet sequenced. Despite the ability of wMel to infect the germline of its host, we find no evidence for either recent lateral gene transfer between wMel and D. melanogaster or older transfers between Wolbachia and any host. Evolutionary analysis further supports the hypothesis that mitochondria share a common ancestor with the α-Proteobacteria, but shows little support for the grouping of mitochondria with species in the order Rickettsiales. With the availability of the complete genomes of both species and excellent genetic tools for the host, the wMel–D. melanogaster symbiosis is now an ideal system for studying the biology and evolution of Wolbachia infections.

The genome sequence of Wolbachia provides insights into the origins of mitochondria, as well as the ecology and evolution of endosymbiosis

Culture and phenotypic characterization of a Wolbachia pipientis isolate

The recent isolation of Wolbachia pipientis in the continuous cell line Aa23, established from eggs of a strain of the Asian tiger mosquito Aedes albopictus, allowed us to perform extensive characterization of the isolate. Bacterial growth could be obtained in C6/36, another A. albopictus cell line, at 28 degrees C and in a human embryonic lung fibroblast monolayer at 28 and 37 degrees C, confirming that its host cell range is broader than was initially thought. The bacteria were best visualized by Diff-Quik and May-Grünwald-Giemsa staining. Proteins from 213 to 18 kDa with two major protein bands of 65 and 25 kDa were observed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. By Western blotting with specific polyclonal mouse and rabbit antisera, dominant immunoreactive antigens were found at approximately 100, 80, and 30 kDa. The genome size was calculated to be 1,790 +/- 17 kb by pulsed-field gel electrophoresis. The sequence of the citrate synthase gene (gltA) of W. pipientis was determined by gene walking. Its position in the phylogenetic tree constructed with gltA confirmed that found in a phylogenetic tree constructed with 16S rRNA genes and that it belongs in the alpha subgroup of the class Proteobacteria and that it is closely related to but independent from the genera Ehrlichia, Anaplasma, and Neorickettsia.

Possible pathogenic pathways in the adverse clinical events seen following ivermectin administration to onchocerciasis patients

BACKGROUND: Reactions are commonly associated with the chemotherapy of onchocerciasis. However unmanageable reactions are uncommon when ivermectin (Mectizan(R)) is used for the treatment of this infection, and this drug has proved to be a great improvement over previously used agents. Serious adverse events (SAE) nevertheless have occurred, and there is considerable concern about the negative effect such events may have on mass drug administration programs.This paper reviews the basic pathogenic mechanisms that can be involved in the destruction of microfilaria by chemotherapeutic agents. A central challenge to filarial chemotherapy is the need to remove parasites from biologically sensitive tissues, a more difficult medical challenge than eliminating nematodes from the gastrointestinal tract.Explanations for the etiology of the serious adverse reactions occurring with ivermectin treatment in specific geographic areas where there is coincident heavy Loa loa infections are hampered by a lack of specific pathological case material. Ways to investigate these possibilities are reviewed. Possible pathogenic mechanisms include embolic vascular pathology accompanied by local inflammation, blood brain barrier mdr1 abnormalities, and genetic predisposition to excessive inflammatory responses. CONCLUSION: It is important to keep ivermectin, and all its associated adverse clinical events, in perspective with the many other chemotherapeutic agents in general use - many of which produce serious adverse events even more frequently than does ivermectin. Currently available evidence indicates that the pathogenesis of the Loa-associated adverse reactions are probably related to inflammatory responses to microfilariae in specific tissues. However, the possibility of genetic predispositions to pathology should also be considered.

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.

Wolbachia in the inflammatory pathogenesis of human filariasis

Filarial nematodes cause some of the most debilitating diseases in tropical medicine. Recent studies, however, have implicated the parasites' endosymbiotic Wolbachia bacteria, rather than the nematode, as the cause of inflammatory-mediated filarial disease. Soluble extracts of a variety of filarial species stimulate innate inflammatory responses, which are absent or reduced when using extracts derived from species either devoid of bacteria, or those cleared of bacteria by antibiotics. Characterization of the molecular nature of the bacterial derived inflammatory stimulus points toward an endotoxin-like activity that is dependent on the pattern recognition receptors CD14 and TLR4 and can be inhibited by lipid A antagonists. TLR4 dependent inflammation has been shown to occur in the systemic inflammatory adverse reaction to Brugia malayi following anti-filarial chemotherapy and in the development of neutrophil-mediated ocular inflammation in a mouse model of river blindness. The development of acute and severe inflammatory responses in people infected with Brugia malayi and Onchocerca volvulus is associated with the release of Wolbachia into the blood following death or damage of the worms after anti-filarial chemotherapy. Together these studies suggest that Wolbachia are the principal cause of acute inflammatory filarial disease. Accumulated exposure to acute episodes of inflammation may also underlie the development of chronic filarial pathology. The use of antibiotic therapy to target Wolbachia of filarial parasites may therefore provide a means to prevent the development of filarial pathology.

Obligatory symbiotic Wolbachia endobacteria are absent from Loa loa

BACKGROUND: Many filarial nematodes harbour Wolbachia endobacteria. These endobacteria are transmitted vertically from one generation to the next. In several filarial species that have been studied to date they are obligatory symbionts of their hosts. Elimination of the endobacteria by antibiotics interrupts the embryogenesis and hence the production of microfilariae. The medical implication of this being that the use of doxycycline for the treatment of human onchocerciasis and bancroftian filariasis leads to elimination of the Wolbachia and hence sterilisation of the female worms. Wolbachia play a role in the immunopathology of patients and may contribute to side effects seen after antifilarial chemotherapy. In several studies Wolbachia were not observed in Loa loa. Since these results have been doubted, and because of the medical significance, several independent methods were applied to search for Wolbachia in L. loa. METHODS: Loa loa and Onchocerca volvulus were studied by electron microscopy, histology with silver staining, and immunohistology using antibodies against WSP, Wolbachia aspartate aminotransferase, and heat shock protein 60. The results achieved with L. loa and O. volvulus were compared. Searching for Wolbachia, genes were amplified by PCR coding for the bacterial 16S rDNA, the FTSZ cell division protein, and WSP. RESULTS: No Wolbachia endobacteria were discovered by immunohistology in 13 male and 14 female L. loa worms and in numerous L. loa microfilariae. In contrast, endobacteria were found in large numbers in O. volvulus and 14 other filaria species. No intracellular bacteria were seen in electron micrographs of oocytes and young morulae of L. loa in contrast to O. volvulus. In agreement with these results, Wolbachia DNA was not detected by PCR in three male and six female L. loa worms and in two microfilariae samples of L. loa. CONCLUSIONS: Loa loa do not harbour obligatory symbiotic Wolbachia endobacteria in essential numbers to enable their efficient vertical transmission or to play a role in production of microfilariae. Exclusively, the filariae cause the immunopathology of loiasis is patients and the adverse side effects after antifilarial chemotherapy. Doxycycline cannot be used to cure loiais but it probably does not represent a risk for L. loa patients when administered to patients with co-infections of onchocerciasis.

Evidence against Wolbachia symbiosis in Loa loa

BACKGROUND: The majority of filarial nematode species are host to Wolbachia bacterial endosymbionts, although a few including Acanthocheilonema viteae, Onchocerca flexuosa and Setaria equina have been shown to be free of infection. Comparisons of species with and without symbionts can provide important information on the role of Wolbachia symbiosis in the biology of the nematode hosts and the contribution of the bacteria to the development of disease. Previous studies by electron microscopy and PCR have failed to detect intracellular bacterial infection in Loa loa. Here we use molecular and immunohistological techniques to confirm this finding. METHODS: We have used a combination of PCR amplification of bacterial genes (16S ribosomal DNA [rDNA], ftsZ and Wolbachia surface protein [WSP]) on samples of L. loa adults, third-stage larvae (L3) and microfilariae (mf) and immunohistology on L. loa adults and mf derived from human volunteers to determine the presence or absence of Wolbachia endosymbionts. Samples used in the PCR analysis included 5 adult female worms, 4 adult male worms, 5 mf samples and 2 samples of L3. The quality and purity of nematode DNA was tested by PCR amplification of nematode 5S rDNA and with diagnostic primers from the target species and used to confirm the absence of contamination from Onchocerca sp., Mansonella perstans, M. streptocerca and Wuchereria bancrofti. Immunohistology was carried out by light and electron microscopy on L. loa adults and mf and sections were probed with rabbit antibodies raised to recombinant Brugia malayi Wolbachia WSP. Samples from nematodes known to be infected with Wolbachia (O. volvulus, O. ochengi, Litomosoides sigmodontis and B. malayi) were used as positive controls and A. viteae as a negative control. RESULTS: Single PCR analysis using primer sets for the bacterial genes 16S rDNA, ftsZ, and WSP were negative for all DNA samples from L. loa. Positive PCR reactions were obtained from DNA samples derived from species known to be infected with Wolbachia, which confirmed the suitability of the primers and PCR conditions. The quality and purity of nematode DNA samples was verified by PCR amplification of 5S rDNA and with nematode diagnostic primers. Additional analysis by 'long PCR' failed to produce any further evidence for Wolbachia symbiosis. Immunohistology of L. loa adults and mf confirmed the results of the PCR with no evidence for Wolbachia symbiosis. CONCLUSION: DNA analysis and immunohistology provided no evidence for Wolbachia symbiosis in L. loa.

Wolbachia pipientis growth kinetics and susceptibilities to 13 antibiotics determined by immunofluorescence staining and real-time PCR

Wolbachia spp. are strict intracellular bacteria that infect a wide range of arthropods and filarial nematodes. Filarial nematodes are important causes of human diseases. There is increasing evidence that Wolbachia spp. influence important functions in the biology of the hosts, specifically, infertility. Preliminary experiments with humans and animals have suggested that antibiotics with activity against Wolbachia may help to treat filariasis. In this study, we determined using a real-time quantitative PCR assay the growth kinetics of a strain of Wolbachia pipientis from a mosquito grown in Aa23 cells. The doubling time was estimated to be 14 h. We then determined the susceptibilities of this strain to 13 antibiotics by two methods: an immunofluorescent-antibody test and a real-time quantitative PCR assay. Both techniques gave similar results. Doxycycline and rifampin were the most effective compounds, with MICs of 0.125 and 0.06 to 0.125 micro g/ml, respectively. Fluoroquinolones were less effective, with MICs of 2 to 4 micro g/ml for ciprofloxacin, 2 micro g/ml for ofloxacin, and 1 micro g/ml for levofloxacin. beta-Lactams (penicillin G, amoxicillin, ceftriaxone) were not effective at concentrations up to 128 micro g/ml. The MIC of erythromycin was >32 micro g/ml, whereas that of telithromycin was 8 micro g/ml. Other antibiotic compounds were bacteriostatic only at high concentrations, including gentamicin, co-trimoxazole, and thiamphenicol. The real-time PCR assay was a convenient and reliable technique for determination of the antibiotic susceptibilities of WOLBACHIA: It may help in the future to simplify antibiotic susceptibility testing of strict intracellular pathogens.

Antibiotic therapy in murine filariasis (Litomosoides sigmodontis): comparative effects of doxycycline and rifampicin on Wolbachia and filarial viability

The symbiosis of filarial nematodes and rickettsial Wolbachia endobacteria has been exploited as a target for antibiotic therapy of filariasis. Depletion of Wolbachia after tetracycline treatment results in filarial sterility because of interruption of embryogenesis and inhibits larval development and adult worm viability. The aim of this study was to investigate if antibiotic intervention of BALB/c mice infected with the rodent filaria Litomosoides sigmodontis with rifampicin or the combination of rifampicin and doxycycline can be used to shorten the treatment period. Both regimens, when given over a period of 14 days initiated with infection, were sufficient to deplete Wolbachia as evidenced by immunohistology and semiquantitative PCR. Worm development and filarial load were significantly reduced in experiments followed up until 63 days p.i. The therapy inhibited embryogenesis and led to filarial sterility. In contrast, treatment with doxycycline alone for 21 days led only to a modest reduction of Wolbachia, filarial growth retardation, worm viability and fertility. In conclusion, the combination of antirickettsial drugs could be used as a suitable tool to explore the minimum duration of therapy required for the depletion of Wolbachia in parasitized hosts subsequent to the onset of patency in human and animal filariasis and the prevention of adverse reactions in human infections.

Doxycycline in the treatment of human onchocerciasis: Kinetics of Wolbachia endobacteria reduction and of inhibition of embryogenesis in female Onchocerca worms

Recently, experts have warned that mass treatment with ivermectin alone may not interrupt the transmission of Onchocerca. Hence, additional drugs are needed, such as antibiotics acting on symbiotic endobacteria of the filariae, the causative agents of onchocerciasis. Based on animal experiments, human onchocerciasis was treated with doxycycline, and preliminary observations published in 2001 in The Lancet showed sterility in female worms by depletion and marked reduction in symbiotic Wolbachia endobacteria from the filariae. Here, a detailed kinetic analysis of the features of the worms, following administration or not of doxycycline to the patients is reported. Sixty-three onchocerciasis patients in Ghana were treated with 100 mg doxycycline daily for 6 weeks and 2 or 6 months later with ivermectin. Onchocercomas were extirpated 2, 6, 11 and 18 months after the onset of treatment and the filariae were examined by immunohistology and PCR. The analysis showed: (i) progressive depletion of Wolbachia from adult worms and microfilariae by doxycycline over a period of 6 months; (ii) inhibition of embryogenesis by doxycycline after 6 months with respect to all embryo stages followed by decline in microfilariae after 11 months; (iii) reduction in spermatozoa in the female genital tract by doxycycline, whereas spermiogenesis was only partly reduced after 11 and 18 months; (iv) no relevant macro- or microfilaricidal activity; (v) depletion/marked reduction in endobacteria and inhibition of embryogenesis were sustained until 18 months after doxycycline and 12 months after co-administration of ivermectin; (vi) no severe adverse side effects were seen. Due to its long-lasting inhibition of embryogenesis, doxycycline presents an additional strategy for the treatment of onchocerciasis and control of Onchocerca microfilariae transmission. Extension of the existing registration will not require much time or high cost. Treatment of individual patients can be considered immediately.

Wolbachia endosymbiotic bacteria of filarial nematodes. A new insight into disease pathogenesis and control

Filarial nematodes are parasitic worms that cause some of the most devastating of all tropical diseases such as elephantiasis and river blindness. Studies on the inflammatory pathogenesis of filarial disease have shown that endotoxin-like activity derived from endosymbiotic Wolbachia bacteria is the major inflammatory stimulus of filarial nematodes. Wolbachia appear to have evolved as essential symbionts of their filarial nematode hosts. Antibiotic depletion of bacteria shows that they are required for normal fertility and development of the worm and may even protect the parasites from host immunity. In addition to the uncovering of a fascinating symbiotic relationship, this discovery means we can now consider using antibiotics as a new approach to the treatment of filarial diseases.

The role of endosymbiotic Wolbachia bacteria in the pathogenesis of river blindness

Parasitic filarial nematodes infect more than 200 million individuals worldwide, causing debilitating inflammatory diseases such as river blindness and lymphatic filariasis. Using a murine model for river blindness in which soluble extracts of filarial nematodes were injected into the corneal stroma, we demonstrated that the predominant inflammatory response in the cornea was due to species of endosymbiotic Wolbachia bacteria. In addition, the inflammatory response induced by these bacteria was dependent on expression of functional Toll-like receptor 4 (TLR4) on host cells.