Drug resistance to schistosomicides and other anthelmintics of medical significance

Genetic Diversity of Schistosoma haematobium in Qena Governorate, Upper Egypt

Introduction

Schistosomiasis is an important neglected tropical disease (NTD) in several developing countries. Praziquantel is the principle and efficacious chemotherapeutic agent that has been used to treat schistosomiasis for decades. Unfortunately, emerging resistance to praziquantel with accompanying reduced efficacy is reported in some localities. Hence, genetic diversity among parasite populations is of significant interest in assessing the effects of selective pressure generated by praziquantel therapy that might result in encouraging the emergence of new genotypes that are either non-susceptible or drug-resistant. The present study aimed to investigate the genetic diversity of Schistosoma haematobium among human populations using the RAPD technique to help clarify disease epidemiology and transmission.

Materials and Methods

S. haematobium eggs were isolated from 50 of 134 patients from four different localities in Qena Governorate, Upper Egypt. These patients complained of terminal hematuria and burning micturition. Samples were used for molecular analysis using RAPD-PCR primers (A02, A07, A09, A10).

Results

Twenty S. haematobium isolates (40%) were amplified using the selected RAPD primers. Amplification patterns of these isolates showed distinct variation in the size and number of amplified fragments, indicating high genetic variation among these isolates.

Conclusion

To the best of our knowledge, this study is the first to characterize the genetic diversity of S. haematobium in human populations in Upper Egypt. Future studies on a larger geographic scale involving many districts in Upper Egypt should be encouraged. Information from such a study would provide better insight into clonal lineages of S. haematobium in this endemic area. In turn, understanding transmission of the parasite may have a major role in establishing control strategies for urogenital schistosomiasis in Upper Egypt.

Immunomodulatory effects of curcumin treatment on murine schistosomiasis mansoni

Curcumin is a polyphenol derived from the dietary spice turmeric. It has been shown to regulate numerous transcription factors, cytokines, adhesion molecules, and enzymes that have been linked to inflammation. In addition to inhibiting the growth of a variety of pathogens, curcumin has been shown to have nematocidal activity. The present study was designed to evaluate the schistosomicidal activity of curcumin in vivo as well as immunomodulation of granulomatous inflammation and liver pathology in acute schistosomiasis mansoni. Mice were infected each with 80 Schistosoma (S.) mansoni cercariae and injected intraperitoneally with curcumin at a total dose of 400mg/kg body weight. Curcumin was effective in reducing worm and tissue-egg burdens, hepatic granuloma volume and liver collagen content by 44.4%, 30.9%, 79%, and 38.6%, respectively. Curcumin treatment restored hepatic enzymes activities to the normal levels and enhanced catalase activity in the liver tissue of infected mice. Moreover, hepato-spleenomegaly and eosinophilia induced by S. mansoni infection were largely improved with curcumin treatment. Infected mice treated with curcumin showed low serum level of both interleukin (IL)-12 and tumor necrosis factor alpha (TNF-alpha), but IL-10 level was not significantly altered. Specific IgG and IgG1 responses against both soluble worm antigen (SWAP) and soluble egg antigen (SEA) were augmented with curcumin treatment, but IgM and IgG2a responses were not significantly changed. In conclusion, curcumin treatment modulates cellular and humoral immune responses of infected mice and lead to a significant reduction of parasite burden and liver pathology in acute murine schistosomiasis mansoni.

Dose-confirmation studies of the cestocidal activity of pyrantel pamoate paste in horses

Dose confirmation studies of the cestocidal activity of pyrantel pamoate paste were conducted at two sites in North America during 2001. Horses with naturally-acquired cestode infections were identified by detection of typical Anoplocephala spp. eggs in feces collected between 7 and 92 days prior to treatment. Twenty and 22 horses were enrolled at Site 1 (Urbana, IL) and Site 2 (Knoxville, TN), respectively. Candidate horses were acclimated to study conditions for 14 days, ranked by length of interval since coprologic confirmation, and allocated randomly to one of two treatment groups: (T1) pyrantel pamoate paste 13.2mg pyrantel base per kilogram body weight administered orally, and (T2) untreated controls. Individual doses of pyrantel pamoate paste were prepared on the basis of contemporaneous body weights and administered to Group T1 horses on Day 0. Trained personnel monitored the animals at regular intervals after treatment to detect potential adverse reactions. Horses were euthanatized and necropsied 10-12 days after treatment. The contents of the large and small intestines were collected, and the walls of each organ were rinsed with water and inspected. Attached cestodes were recovered and preserved in 10% formalin. The intestinal contents and rinsed ingesta were washed over a #10-mesh (2mm aperture) sieve and tapeworms were extracted and preserved. Recovered cestodes were counted and examined at 1-4x magnification for identification to genus and species. At Site 1, specimens of Anoplocephala perfoliata were recovered from seven of 10 control horses, and from one of 10 horses treated with pyrantel pamoate. Mean cestode numbers were 4.52 in the control group and 0.07 for treated horses. At Site 2, cestodes were found in 10 of 11 controls (mean 26.2) and in five of 11 horses (mean 1.2) treated with pyrantel pamoate. In both studies, Group T1 means were significantly lower than the control group (P<0.005). The calculated efficacies were 98.4 and 95.5% at Sites 1 and 2, respectively. In two dose-confirmation studies, a single, oral treatment of pyrantel pamoate paste (19.13% w/w pyrantel base) at 13.2mg/kg was >or=95.5% effective against A. perfoliata in naturally-infected horses.

Voltage-gated calcium channel subunits from platyhelminths: potential role in praziquantel action

Voltage-gated calcium (Ca2+) channels provide the pathway for Ca2+ influxes that underlie Ca2+ -dependent responses in muscles, nerves and other excitable cells. They are also targets of a wide variety of drugs and toxins. Ca2+ channels are multisubunit protein complexes consisting of a pore-forming alpha(1) subunit and other modulatory subunits, including the beta subunit. Here, we review the structure and function of schistosome Ca2+ channel subunits, with particular emphasis on variant Ca2+ channel beta subunits (Ca(v)betavar) found in these parasites. In particular, we examine the role these beta subunits may play in the action of praziquantel, the current drug of choice against schistosomiasis. We also present evidence that Ca(v)betavar homologs are found in other praziquantel-sensitive platyhelminths such as the pork tapeworm, Taenia solium, and that these variant beta subunits may thus represent a platyhelminth-specific gene family.

Heterogeneous model of schistosomiasis transmission and long-term control: the combined influence of spatial variation and age-dependent factors on optimal allocation of drug therapy

Prior field studies and modelling analyses have individually highlighted the importance of age-specific and spatial heterogeneities on the risk for schistosomiasis in human populations. As long-term, large-scale drug treatment programs for schistosomiasis are initiated in subSaharan Africa and elsewhere, optimal strategies for timing and distribution of therapy have yet to be fully defined on the working, district-level scale, where strong heterogeneities are often observed among sublocations. Based on transmission estimates from recent field studies, we develop an extended model of heterogeneous schistosome transmission for distributed human and snail population clusters and age-dependent behaviour, based on a 'mean worm burden + snail infection prevalence' formulation. We analyse its equilibria and basic reproduction patterns and their dependence on the underlying transmission parameters. Our model allows the exploration of chemotherapy-based control strategies targeted at high-risk behavioural groups and localities, and the approach to an optimal design in terms of cost. Efficacy of the approach is demonstrated for a model environment having linked, but spatially-distributed, populations and transmission sites.

Are Ca2+ channels targets of praziquantel action?

Praziquantel is the current drug of choice for the control of schistosomiasis. It is highly effective against all species of schistosomes and shows minimal adverse effects. Though introduced for the treatment of schistosomiasis more than 20 years ago, the mode of action of praziquantel remains to be elucidated. This review will focus on advances in defining the molecular target of praziquantel action, with particular emphasis on recent work indicating an important role for voltage-gated calcium channels.

Immunogenicity and Vaccine Potential of Dipeptidic Multiple Antigen Peptides from Schistosoma mansoni Glyceraldehyde 3-Phosphate Dehydrogenase

Six peptides, A, B1, B, C, D and E, derived from the primary sequence of Schistosoma mansoni glyceraldehyde 3-phosphate dehydrogenase (SG3PDH) were selected based on lowest homology to human glyceraldehyde 3-P dehydrogenase (G3PDH), multimerized in dipeptidic multiple antigen peptide (D-MAP) constructs and used for the immunization of BALB/c mice. Tetrabranched D-MAPs A-B, B1-C and D-E and the bis-D-MAP B-E elicited strong cell-mediated and antibody responses against immunogen, unit peptides and their cognate sequences in the native and denatured protein. D-MAP A-B induced protection against challenge infection. Immunization with D-MAP B1-C failed to affect the challenge worm parameters, probably because peptides B1 and C, previously shown to elicit immune responses associated with increase and decrease in challenge worm fertility, respectively, induced immune responses with opposing effects when combined in a D-MAP construct. A similar suggestion may explain the failure of D-MAP D-E to protect the host against challenge infection. In contrast, immunization with D-MAP B-E resulted in robust protection of the host, possibly because it contains peptides known to evoke immune responses associated with a significant decrease in challenge worm burden and fertility. The data together suggest that the specificity, not the quantity, of the induced immune responses is the determining factor for the efficacy of synthetic peptide-based vaccine for schistosomiasis.

In vitro and in vivo studies on the bioactivity of a ginger (Zingiber officinale) extract towards adult schistosomes and their egg production

The bioactivity of an ethyl acetate extract of ginger (Zingiber officinale) towards Schistosoma mansoni adult pairs, both cultured in vitro and in vivo in laboratory mice, was investigated by monitoring worm mortality and fecundity. In vitro, a concentration of 200 mg l(-1) of extract killed almost all worms within 24 h. Male worms seemed more susceptible than female under these conditions. Cumulative egg output of surviving worm pairs in vitro was considerably reduced when exposed to the extract. For example, after 4 days of exposure to 50 mg l(-1), cumulative egg output was only 0.38 eggs per worm pair compared with 36.35 for untreated worms. In vivo efficacy of the extract was tested by oral and subcutaneous delivery of 150 mg kg(-1) followed by assessment of worm survival and fecundity. Neither delivery route produced any significant reduction in worm numbers compared with untreated controls. Worm fecundity was assessed in vivo by cumulative egg counts per liver at 55 days post infection with mice treated subcutaneously. Such infections showed egg levels in the liver of about 2000 eggs per worm pair in 55 days, in both treated and control mice, with no significant difference between the two groups. To ensure that density-dependent effects did not confound this analysis, a separate experiment demonstrated no such influence on egg output per worm pair, at intensities between 1 and 23 worms per mouse.

Human T- and B-cell responses to Schistosoma mansoni recombinant glyceraldehyde 3-phosphate dehydrogenase correlate with resistance to reinfection with S. mansoni or Schistosoma haematobium after chemotherapy

Recently we reported that human T- and B-cell recognition of a 42-kDa protein (p42) in soluble extracts of adult Schistosoma mansoni worms correlates with resistance to reinfection with S. mansoni or S. haematobium. Amino acid microsequencing of p42 revealed that it consists predominantly of schistosome glyceraldehyde 3-phosphate dehydrogenase (SG3PDH). We have expressed SG3PDH in Escherichia coli and purified the recombinant protein in a soluble and enzymatically active form. Recombinant SG3PDH (rSG3PDH) reacted with human monospecific antibodies to p42. Lymphoproliferation and production of interleukin-4 and gamma interferon (IFN-gamma) after in vitro stimulation with rSG3PDH and serum isotype responses to rSG3PDH were examined in individuals with extremes of resistance and susceptibility to reinfection after treatment of previous S. mansoni or S. haematobium infection. Lymphoproliferation and IFN-gamma production in response to rSG3PDH and the presence of serum immunoglobulin G1 (IgG1), IgG3, and IgA antibodies to rSG3PDH generally characterized individuals who are resistant to reinfection after chemotherapy. The data indicate that T- and B-cell immune reactivity to rSG3PDH correlates with resistance to reinfection, confirming previous studies identifying SG3PDH as a target of protective immunity in humans, and suggest that SG3PDH should be investigated as a possible vaccine for human schistosomiasis.

Drug resistance in human helminths: current situation and lessons from livestock

In this review the available reports on drug resistance in human helminths, particularly hookworms and schistosomes, are critically analyzed. The experiences with helminths of livestock are then reviewed, in particular the factors contributing to the development of anthelmintic resistance, the mechanisms and genetics of resistance to various anthelmintic classes, and the methods available for detection. These experiences appear to be worryingly similar and relevant to the potential development of drug resistance in human helminths. Recommendations to reduce its risks are suggested.

Drug resistance in human helminths: current situation and lessons from livestock

In this review the available reports on drug resistance in human helminths, particularly hookworms and schistosomes, are critically analyzed. The experiences with helminths of livestock are then reviewed, in particular the factors contributing to the development of anthelmintic resistance, the mechanisms and genetics of resistance to various anthelmintic classes, and the methods available for detection. These experiences appear to be worryingly similar and relevant to the potential development of drug resistance in human helminths. Recommendations to reduce its risks are suggested.

Randomly amplified polymorphic DNA (RAPD) polymerase chain reaction assay for identification of Schistosoma mansoni strains sensitive or tolerant to anti-schistosomal drugs

The genetic differences between Schistosoma mansoni strains from different geographic areas that were reportedly resistant or sensitive to anti-schistosomal drugs were studied with randomly amplified polymorphic DNA (RAPD) and simple sequence repeat (SSR) polymerase chain reaction (PCR) assays. Of the 20 RAPD primers we chose, 19 showed the capacity to produce a medium to high level of amplification and 6 revealed difference PCR bands between drug-resistant and drug-sensitive strains. One particular primer, 5'-CAGCGACAAG-3', showed 2 major difference bands between praziquantel (PZQ)-resistant and PZQ-sensitive strains from the endemic area of Egypt. These results demonstrate that defined sequence primers could be applied as a useful tool for differentiating drug-resistant and -sensitive schistosome parasites in the field.

Evidence against rapid emergence of praziquantel resistance in Schistosoma haematobium, Kenya

We examined the long-term efficacy of praziquantel against Schistosoma haematobium, the causative agent of urinary schistosomiasis, during a school-based treatment program in the Msambweni area of Coast Province, Kenya, where the disease is highly endemic. Our results, derived from treating 4,031 of 7,641 children from 1984 to 1993, indicate substantial year-to- year variation in drug efficacy. However, the pattern of this variation was not consistent with primary or progressive emergence of praziquantel resistance. Mathematical modeling indicated that, at current treatment rates, praziquantel resistance will likely take 10 or more years to emerge.

The control of the human filariases

There is a growing momentum for the global control and elimination of the major human filariases as public health problems worldwide. The renewed optimism for undertaking this objective reflects the development of simple and potentially cost-effective strategies for mass drug delivery in onchocerciasis and the availability of new extremely effective drug combinations to treat infection, and new methods of morbidity control in lymphatic filariasis. It also reflects the development and current availability of very effective diagnostic, surveillance and control modeling tools for both parasites. Control programming will also be aided by our greater understanding of the biology of transmission, host immunity and disease pathogenesis.

Control strategies for human intestinal nematode infections

In recent years significant progress has been made in understanding the ecology, epidemiology and related morbidity and development of new tools for the control of soil-transmitted helminths. Such knowledge has recognized the impact of helminth infections on the health of infected groups and has created a rational basis for their control. Schoolchildren harbour some of the most intense helminthic infections, which produce adverse effects on health, growth and scholastic performance. However, although great effort has been put into targeting school-age children, women of child-bearing age and pre-school children are two other groups at high risk of morbidity due to intestinal nematode infections. Highly effective and safety-tested, single-dose anthelminthic drugs are now available, permitting periodical deworming of schoolchildren and other high-risk groups at affordable prices. Four anthelminthics against all intestinal nematodes are included in the WHO Essential Drug List (albendazole, levamisole, mebendazole and pyrantel). Recently ivermectin has also been registered for use against Strongyloides stercoralis in humans. Several well-monitored country experiences have shown that chemotherapy-based control of morbidity due to soil-transmitted helminths is possible and highly cost-effective.

Schistosome resistance to praziquantel

Praziquantel (PZQ) is the drug of choice for the treatment of human schistosomiasis. In 1994, it was first demonstrated that by sustained drug pressure on a Schistosoma mansoni strain in laboratory conditions resistance to PZQ can develop. Studies in Senegal and Egypt, both schistosomiasis endemic areas, have found that there are schistosome strain(s) that are tolerant to PZQ. In this article evidence from laboratory and field studies regarding the existence of PZQ resistance or tolerant schistosome strain(s) will be examined.

Evaluation of the schistosomicidal efficacy of liposome-entrapped oxamniquine

Oxamniquine (OXA) was successfully encapsulated in small unilamellar vesicles using a pH gradient method. This procedure led to a high drug encapsulation efficiency (> 85%) at a drug to lipid molar ratio of 1/10. Moreover, these liposomes were found to retain encapsulated OXA efficiently under dialysis conditions at 37 degrees C. Liposome-entrapped OXA (LOXA), OXA, and empty liposomes were tested against Schistosoma mansoni in a murine model. LOXA produced a significant reduction of the worm burden compared to the other preparations, when inoculated by subcutaneous route (s.c.) with 10 mg OXA/kg animal one day before the infection, and 3, 7, and 14 days after. However, LOXA was not effective when given 7 days before, or 35 days after infections. OXA, in the free form, was effective in relation to the untreated group, only when administered 3 days after the infection. Maximum effect of LOXA, with 97% reduction of the parasite number, was observed when the preparation was given s.c. one day before the infection. On the other hand, LOXA inoculated intraperitoneally one day before the infection didn't show any reduction of the parasite count. It can be concluded that LOXA is more effective than OXA for the treatment of experimental schistosomiasis, particularly when administered subcutaneously at a time close to the infection.

Schistosome resistance to praziquantel: Fact or artifact?

Praziquantel is the current drug of choice for human schistosomiasis. Recent reports from laboratory and field studies concerning reduced praziquantel efficacy against Schistosoma mansoni have generated some controversy. The prevailing question is whether the emergence of strains of schistosome resistant to praziquantel is a fact, or an artifact resulting from erroneous field or laboratory experimentation. In this article, Padraic Fallon, Liang-feng Tao, Magdi Ismail and James Bennett examine the available evidence for schistosome resistance to praziquantel. Contributory factors to the schistosomicidal activity of praziquantel, which may interfere with evaluation of drug efficacy or resistance, are also considered.