Detection of praziquantel resistance in schistosomes

Natural Products Are a Promising Source for Anthelmintic Drug Discovery

Parasitic nematodes infect almost all forms of life. In the human context, parasites are one of the major causative factors for physical and intellectual growth retardation in the developing world. In the agricultural setting, parasites have a great economic impact through a reduction in livestock performance or control cost. The main method of controlling these devastating conditions is the use of anthelmintic drugs. Unfortunately, there are only a few anthelmintic drug classes available in the market and significant resistance has developed in most of the parasitic species of livestock. Therefore, development of new anthelmintics with different modes of action is critical for sustainable parasitic control in the future. The drug development pipeline is broadly limited to two types of molecules, namely synthetic compounds and natural plant products. Compared to synthetic compounds, natural products are highly diverse, and many have historically proven valuable in folk medicine to treat various gastrointestinal ailments. This review focus on the use of traditional knowledge-based plant extracts in the development of new therapeutic leads, the approaches used as screening techniques, and common bottlenecks and opportunities in plant-based anthelmintic drug discovery.

Ethnopharmacological evaluation of schistosomicidal and cercaricidal activities of some selected medicinal plants from Ghana

Background

The adulticidal and cercaricidal activities of five Ghanaian medicinal plants, namely, Phyllanthus amarus, Vernonia amygdalina, Azadirachta indica, Morinda lucida and Nauclea latifolia against S. mansoni were evaluated in this study. Six weeks old ICR mice (n = 25) were percutaneously infected with S. mansoni cercariae. Nine weeks later, infected mice (n = 5) were anaesthetised and perfused for adult S. mansoni. Cercariae were treated with different concentrations (1000, 500, 250, 125, 62.5, 31.25 μg/mL) of methanolic extracts of the experimenting plants in triplicates. Adult S. mansoni incopula were also treated with same concentrations of each extract or 20 μg/mL praziquantel. The cercariae and adult worms were observed at time intervals for 180 min and 120 h to assess mortality and viability respectively. Additionally, 9-week cercariae-infected mice (4 groups of 5 mice) were treated with either 500 mg/kg po A. indica or V. amygdalina, 400 mg/kg po praziquantel or distilled water for 14 days. The mice were euthanized after adult worms were recovered from them. The liver was processed and histologically examined for granuloma formations.

Results

All the plants exhibited varying cercaricidal and adulticidal activities against S. mansoni in a time and concentration-dependent manner. A. indica (3 h IC₅₀ = 27.62 μg/mL) and V. amygdalina (3 h IC₅₀ = 35.84 μg/mL) exerted the highest cercaricidal activity. Worm recovery after treatment with V. amygdalina, A. indica and praziquantel in vivo was 48.8%, 85.1 % and 59.9 % respectively (p < 0.05). A. indica and V. amydalina-treated mice recorded lesser mean liver and spleen weights compared to untreated groups (p < 0.05).

Conclusion

A. indica demonstrated the highest cercaricidal and alduticidal activities in vitro, whereas V. amygdalina exhibited the most potent aldulticidal activity in vivo. This study could provide baseline information which can be used to develop plant-based alternative commercial drugs against S. mansoni.

Efficacy of praziquantel in the treatment of Schistosoma haematobium infection among school-age children in rural communities of Abeokuta, Nigeria

Background

Chemotherapy with praziquantel (PZQ) has been the cornerstone of schistosomiasis control over the last two decades. Being the only available drug for the treatment of over 200 million people worldwide, continuous monitoring of PZQ efficacy under the pressure of widespread use is therefore advocated.

Methods

The efficacy of taking two doses of oral PZQ for the treatment of Schistosoma haematobium was examined among school children in Nigeria. Urine specimens were collected from 350 school children and examined using the filtration technique. Blood was collected for packed cell volume (PCV) estimation, and the weight and height of each child were estimated. S. haematobium egg positive pupils were treated with two oral doses of PZQ at 40 mg/kg with a four-week interval in between. Drug efficacy was determined based on the egg reduction rate (ERR).

Results

Among 350 school children, 245 (70.0%) – of which 132 were males and 113 were females, with an age range of 4 to 15 years – were diagnosed with S. haematobium. All the 245 infected children received a single oral dose of 40 mg/kg PZQ twice with a four-week interval in between and were followed up for 12 weeks. At four, eight and twelve weeks post treatment, the ERR was 57.1%, 77.6% and 100%, respectively. The ERR was significantly higher among the children with a light infection compared to those with a heavy infection. One hundred and twenty-one children were egg negative at four weeks post treatment, among which 1 (6.3) and 120 (52.4%) had heavy and light infections, respectively. Following the second round of treatment, the cure rate at eight weeks and twelve weeks was 85.3% and 100%, respectively.

Conclusion

This study demonstrated the efficacy of taking two doses of oral PZQ for the treatment of urinary schistosomiasis among school children in Nigeria.

DNA vaccines against tropical parasitic diseases

Parasitic diseases caused by protozoan and helminth parasites are among the leading causes of morbidity and mortality in tropical and subtropical regions of the world. Unfortunately, at present, there is no vaccine against any human parasitic disease. Conventional vaccine methods have largely failed against parasitic infections. This is due, in part, to the complexity of the parasite life cycle, the ability of the parasite to evade the immune system, and difficulties in identifying and eliciting the desired protective immune responses. The discovery of DNA vaccines has renewed hope for vaccine development against parasites. In the last decade, DNA vaccines were successful in inducing at least partial protection against several parasitic diseases. This review discusses the latest developments in DNA vaccines against tropical parasitic diseases.

Is there reduced susceptibility to praziquantel in Schistosoma japonicum? Evidence from China

Praziquantel is widely used for the treatment of human schistosomiasis. However, in recent years, there has been increasing concern about the resistance of Schistosoma species to praziquantel. The study described here was designed to evaluate the current susceptibility to praziquantel in S. japonicum in China. During the non-transmission period of schistosomiasis, a random sample of 4760 subjects from the main endemic foci of China were examined using parasitological stool examination. In total, 584 subjects were identified as being infected with S. japonicum, with a prevalence rate of 12.27%. Among them, 565 stool-egg-positive subjects were treated with praziquantel in a single oral dose of 40 mg/kg. Six weeks post-treatment, among the 505 villagers re-examined, 480 (95.05%) had no detectable S. japonicum eggs. Twenty-one subjects still excreting eggs after the first treatment were treated with praziquantel for the second time. All stool samples, including those from those participants with second treatment were re-examined 6 weeks after the second treatment, and no stool-egg-positives were found. The results indicate that the current efficacy of praziquantel against S. japonicum is still high and has not changed after more than 2 decades of repeated, expanded chemotherapy in the main endemic areas of China. It is suggested that no evidence of tolerance or resistance to praziquantel in S. japonicum was detected in China.

Modelling environmentally-mediated infectious diseases of humans: transmission dynamics of schistosomiasis in China

Macroparasites of humans are sensitive to a variety of environmental variables, including temperature, rainfall and hydrology, yet current comprehension of these relationships is limited. Given the incomplete mechanistic understanding of environment-disease interactions, mathematical models that describe them have seldom included the effects of time-varying environmental processes on transmission dynamics and where they have been included, simple generic, periodic functions are usually used. Few examples exist where seasonal forcing functions describe the actual processes underlying the environmental drivers of disease dynamics. Transmission of human schistosomes, which involves multiple environmental stages, offers a model for applying our understanding of the environmental determinants of the viability, longevity, infectivity and mobility of these stages to controlling disease in diverse environments. Here, a mathematical model of schistosomiasis transmission is presented which incorporates the effects of environmental variables on transmission. Model dynamics are explored and several key extensions to the model are proposed.

Remote sensing and geographic information systems in the spatial temporal dynamics modeling of infectious diseases

Similar to species immigration or exotic species invasion, infectious disease transmission is strengthened due to the globalization of human activities. Using schistosomiasis as an example, we propose a conceptual model simulating the spatio-temporal dynamics of infectious diseases. We base the model on the knowledge of the interrelationship among the source, media, and the hosts of the disease. With the endemics data of schistosomiasis in Xichang, China, we demonstrate that the conceptual model is feasible; we introduce how remote sensing and geographic information systems techniques can be used in support of spatio-temporal modeling; we compare the different effects caused to the entire population when selecting different groups of people for schistosomiasis control. Our work illustrates the importance of such a modeling tool in supporting spatial decisions. Our modeling method can be directly applied to such infectious diseases as the plague, lyme disease, and hemorrhagic fever with renal syndrome. The application of remote sensing and geographic information systems can shed light on the modeling of other infectious disease and invasive species studies.

Cystoscopy in the diagnosis and follow-up of urinary schistosomiasis in Brazilian soldiers returning from Mozambique, Africa

The assessment of urinary schistosomiasis in individuals coming from endemic areas often requires diagnostic resources not used in areas of exposure in order to determine complications or to establish more precise criteria of cure. Cystoscopy and 24-hour urine examination were performed, after treatments with praziquantel 40 mg/kg body weight, single dose, on 25 Brazilian military men who were part of a United Nations peace mission to Mozambique in 1994. The median age of the individuals was 29 years and all presented a positive urine parasitological exam. The alterations detected by cystoscopy were hyperemia and granulomas in the vesical submucosa in 59.1% of the individuals and only granulomas in 40.9%. A vesical biopsy revealed granulomas in all patients and viable eggs in 77.3% even after a period during which the patients no longer excreted eggs in urine. Cystoscopy after treatment, followed by biopsy and histopathological evaluation, performed in areas where the evolution of the disease can be better monitored, was found to be a safe criterion of parasitological cure.

Hepatic and intestinal schistosomiasis after orthotopic liver transplant

Schistosomiasis affects 200 to 250 million people worldwide. Hepatic schistosomiasis is a well-recognized cause of chronic liver disease and portal hypertension. There are no previous reports of schistosomiasis post liver transplantation. We report on 2 cases of schistosomiasis in liver transplant recipients--a case of gastric schistosomiasis and a case of hepatic schistosomiasis. A discussion of the pathology of schistosomal infection and a rationale for screening potential liver transplant recipients from endemic areas follows.

Therapeutic failure of praziquantel in the treatment of Schistosoma haematobium infection in Brazilians returning from Africa

Several cases of therapeutic failure of praziquantel used for the treatment of urinary schistosomiasis have been reported. Alternative drugs, like niridazol and metrifonate, have shown a lower therapeutic effect and more side effects than praziquantel. Twenty-six Brazilian military men (median age of 29 years) with a positive urine parasitological exam who were part of a United Nation peace mission in Mozambique in 1994 were treated with 40 mg/kg body weight praziquantel, single dose. They swam in Licungo river (Mocuba city, Mozambique) during the weekends. After this, they presented haematuria, dysuria, polakiuria, and lumbar pain. Control cystoscopy examinations carried out between 6 and 24 months after each treatment (including two additional treatments at a minimum interval of 6 months) revealed the presence of viable eggs. Granulomas in the vesical submucosa were observed in 46.2% (12/26) of the individuals. A vesical biopsy confirmed the presence of granulomas in all of these patients and the presence of viable eggs in 34.3% (9/26) of individuals who no longer excreted eggs in urine. The eggs filled with miracidia showed characteristics of viability. Histopathological examination using different strains demonstrated therapeutic failure and the need for repeated treatment. In this study, we demonstrated a low efficacy of praziquantel in the treatment of schistosomiasis haematobia, and the necessity of the urinary bladder biopsy as criterion of cure.

A multi-group model of Schistosoma japonicum transmission dynamics and control: model calibration and control prediction

Previously we formulated a quantitative model to characterize site-specific schistosomiasis transmission. In this paper, we present a procedure to calibrate the model to data collected in endemic villages of south-western Sichuan, China, with the objective of reducing parametric uncertainty to allow the model to describe local transmission with relative confidence. A Bayesian approach using local epidemiological data and expert opinion is employed to calibrate the model. Results indicate that, after calibration, the output uncertainty is reduced substantially. The calibrated model is then used for prediction of the effects of different intervention options. Simulations reflect a bimodal transmission in both human (early summer and early fall) and snail (late summer and late fall) infections in this area, for which there is some field evidence. Also shown in the simulations are relatively high reinfection rates following chemotherapy in these endemic villages. These results suggest that a sustainable control strategy is essential in reducing transmission, and that transmission can be reduced by chemotherapy, focal snail (e.g. snail clusters) control, and egg control. Our work demonstrates the feasibility of characterizing site-specific schistosomiasis transmission using a mathematical model and a calibration approach that integrates diverse field data, and the use of the calibrated model to design control strategies.

Tomocomd-Cardd, a novel approach for computer-aided ? rational? drug design: I. Theoretical and experimental assessment of a promising method for computational screening and in silico design of new anthelmintic compounds

In this work, the TOMOCOMD-CARDD approach has been applied to estimate the anthelmintic activity. Total and local (both atom and atom-type) quadratic indices and linear discriminant analysis were used to obtain a quantitative model that discriminates between anthelmintic and non-anthelmintic drug-like compounds. The obtained model correctly classified 90.37% of compounds in the training set. External validation processes to assess the robustness and predictive power of the obtained model were carried out. The QSAR model correctly classified 88.18% of compounds in this external prediction set. A second model was performed to outline some conclusions about the possible modes of action of anthelmintic drugs. This model permits the correct classification of 94.52% of compounds in the training set, and 80.00% of good global classification in the external prediction set. After that, the developed model was used in virtual in silico screening and several compounds from the Merck Index, Negwer's handbook and Goodman and Gilman were identified by models as anthelmintic. Finally, the experimental assay of one organic chemical (G-1) by an in vivo test coincides fairly well (100%) with model predictions. These results suggest that the proposed method will be a good tool for studying the biological properties of drug candidates during the early state of the drug-development process.

Atom, atom-type and total molecular linear indices as a promising approach for bioorganic and medicinal chemistry: theoretical and experimental assessment of a novel method for virtual screening and rational design of new lead anthelmintic

Helminth infections are a medical problem in the world nowadays. In this paper a novel atom-level chemical descriptor has been applied to estimate the anthelmintic activity. Total and local linear indices and linear discriminant analysis were used to obtain a quantitative model that discriminates between anthelmintic and non-anthelmintic drug-like compounds. The discriminant model has an accuracy of 90.11% in the training set, with a high Matthews' correlation coefficient (MCC=0.80). To assess the robustness and predictive power of the obtained model, internal (leave-n-out) and external validation process was performed. The QSAR model correctly classified 88.55% of compounds in this external prediction set, yielding a MCC of 0.77. Another LDA model was carried out to outline some conclusions about the possible modes of action of anthelmintic drugs. It has an accuracy of 93.50% in the training set, and 80.00% in the external prediction set. After that, the developed model was used in the virtual--in silico--screening and several compounds from the Merck Index, Negwer's Handbook and Goodman and Gilman were identified by the model as anthelmintic. Finally, the experimental assay of an organic chemical (a furylethylene derivative) by an in vivo test permits us to carry out an assessment of the model. An accuracy of 100% with the theoretical predictions was observed. These results suggest that the proposed method will be a good tool for studying the biological properties of drug candidates during the early state of the drug-development process.

A DNA-prime/protein-boost vaccination regimen enhances Th2 immune responses but not protection following Schistosoma mansoni infection

DNA immunization represents a promising vaccine strategy that has been reasonably successful, and will likely play an even greater role in vaccine development as these vaccines continue to be improved. We have developed a partially protective DNA vaccine against schistosome infection based on a 23-kDa integral membrane protein, Sm23. The focus of this study was to compare immunogenicity and efficacy of vaccination regimens utilizing Sm23 DNA vaccine alone vs. regimens that utilized both Sm23 DNA and Sm23 in recombinant protein form. We found that priming and boosting with the Sm23 DNA construct (Sm23-pcDNA) resulted in a significant level of protection against challenge infection (36-44%). In contrast, altering this protocol by changing the boost from Sm23 DNA to boosting with recombinant Sm23 protein (rSm23) formulated in aluminium hydroxide (alum) failed to induce a significant reduction in worm burdens. Similarly, mice primed and boosted with the rSm23 in alum also did not develop significant levels of protection against challenge infection. We hypothesize that the differences in the ability to drive protective immunity using the DNA prime-DNA boost strategy and the inability to do so when recombinant Sm23 in alum was substituted for Sm23 DNA is due to driving of different immune responses. In support of this, we found that mice primed and boosted with Sm23-pcDNA had Th1-type immune responses characterized by low anti-Sm23 IgG1 : IgG2a antibody isotype ratios, whereas mice boosted with rSm23 had higher IgG1 : IgG2a ratios. In addition, priming and boosting with rSm23 elicited mainly IgG1 antibodies with no detectable IgG2a, indicative of a polarized Th2-type immune response. Thus, similar to our earlier work, the results of this study show that protective vaccination using Sm23 is associated with a Th1 immune response, and efficacy is diminished using protocols that diminish this Th1 bias. In our study, this was likely due to the use of the Th2-driving adjuvant alum, and future studies are planned where we will compare the protective efficacy of rSm23 administered with Th1-type adjuvants.

Immunogenicity of polymerizable synthetic peptides derived from a vaccine candidate against schistosomiasis: the asparaginyl endopeptidase (Sm32)

The asparaginyl endopeptidase (Sm32) is expressed in the gastrodermal cells of the schistosome gut and in the head glands of the cercariae. Possibly, Sm32 hydrolyzes pro-proteins involved in the degradation of host hemoglobin [Parasitol. Today 12 (1996) 125]. Preliminary evidences using an Sj32/Sm32 murine vaccine have shown a profound effect on oviposition and worm burden [Chin. J. Schist. Control. 7 (1995) 72; Bull. Human Med. Univ. 24 (1999) 225; Vaccine 20 (2002) 439]. The importance of Sm32 as a novel vaccine candidate is based on the possibility of preventing the maturation of other cathepsins and/or preventing schistosome skin invasion. We studied the immunogenicity of polymerizable peptides derived from Sm32 to select potential protective epitopes. Sm32 prediction of T and B epitopes and homology studies with human legumain were performed. Among the variety of factors that influence the antibody response, we specifically examined the effect of: (i) genetic background of mouse strain, inbred (C57BL/6) versus outbred (Swiss) mice; and (ii) vaccination with a single peptide versus pool of peptides. Swiss mice raised antibodies to three different regions of the Sm32, as tested by the Multiple Antigen Blot Assay (MABA): 182-215 (peptides IMT-70 and 72), 244-273 (IMT-64) and 336-355 (IMT-66). None of these regions were immunogenic for C57BL/6. On the contrary, other peptides, IMT-4 (21-40), IMT-12 (101-120) and IMT-26 (292-313) were highly immunogenic for this inbred strain. Only Swiss mice immunized with a single peptide (IMT-64 and 72) or with three different pools of IMT-peptides (Pool A-II: 14, 16, 18, 70, 72, 89; pool A-III: 22, 64, 24, 26, 28 and pool A-V: 64, 66, 28, 70, 72) recognized the original protein in a crude extract of the worm antigen by Western blot. Peptides IMT-64, 14 and 26 were responsible for this recognition. In general, the vaccination with pool of peptides was more immunogenic for both mouse strains. Predicted B cell epitopes, with hydrophilicity scores over +10 (IMT-12, 64, 26) were always immunogenic after either single or combined peptide vaccination. Sm32 sequences 41-80 (IMT-6 and 8), 141-160 (IMT-16) and 182-215 (IMT-70 and 72) were nearly identical to the corresponding human legumain regions and should be excluded from the human vaccine. We can conclude that the regions of Sm32 that were recognized by antibodies of mice immunized with polymerizable peptides depended on the mice strain and on the hydrophilicity score of the peptides.

Adult worm tegumental damage and egg-granulomas in praziquantel-resistant and -susceptible Schistosoma mansoni treated in vivo

The effect of treatment with praziquantel (PZQ) on the tegument of adult Schistosoma mansoni worms and on liver egg-granulomas has been examined in mice infected with PZQ-resistant and -susceptible parasite isolates. Two PZQ-resistant S. mansoni isolates, one selected by passage in the laboratory under drug pressure and one from Senegal established from eggs excreted by an uncured patient, were compared with PZQ-susceptible control isolates. Scanning electron microscopic observations on the tegument of Schistosoma adult worms treated in vivo with PZQ showed that more severe damage was inflicted by PZQ on susceptible worms than on drug-resistant worms. Observations on the pathology of Schistosoma egg-granulomas in the liver of infected mice after treatment with PZQ indicated that eggs from susceptible control isolates were more sensitive to PZQ than those from drug-resistant isolates.

Resistance of Schistosoma mansoni to praziquantel: is there a problem?

Evidence for resistance to praziquantel (PZQ) in Schistosoma mansoni has been sought in parasites taken from treated, but uncured human patients, and in a laboratory isolate of S. mansoni subjected to successive passages under drug pressure. Patients from villages in Egypt and Senegal have yielded isolates that can tolerate higher dosages of PZQ than other ostensible control isolates when passaged and subjected to drug treatment in mice. In vitro tests on these and the laboratory-selected isolate support the conclusion that a degree of resistance to PZQ can occur in S. mansoni, but the levels of drug resistance found so far are low. Preliminary studies have begun on these isolates to identify genetic, physiological and morphological characteristics associated with PZQ resistance and some of these may find use as markers for monitoring whether or not resistance is developing in endemic areas where the drug is used. More intensive application of PZQ can be expected in future, particularly in other parts of Africa, and vigilance will be needed to ensure that it continues to be useful as a drug for treatment of schistosomiasis. Further work is needed to elucidate the mode of action of PZQ and there is already a need for alternative drugs to treat PZQ-resistant schistosomiasis, such as already exists in northern Senegal.

The potential of artemether for the control of schistosomiasis

Schistosomiasis continues to rank--following malaria--at the second position of the world's parasitic diseases in terms of the extent of endemic areas and the number of infected people. There is yet no vaccine available and the current mainstay of control is chemotherapy with praziquantel used as the drug of choice. In view of concern about the development of tolerance and/or resistance to praziquantel, there is a need for research and development of novel drugs for the prevention and cure of schistosomiasis. Interestingly, derivatives of artemisinin, which are already effectively used in the treatment of malaria, also exhibit antischistosomal properties. Significant advances have been made with artemether, the methyl ether derivative of artemisinin. We review the discovery of the antischistosomal activity of artemether by Chinese scientists two decades ago; the detailed laboratory studies of the susceptibility of, and effect on, the different developmental stages of Schistosoma japonicum, Schistosoma mansoni and Schistosoma haematobium to artemether; the possible mechanism of action and the potential long-term toxicity. Finally, we look at the effect of combined treatment with artemether and praziquantel; and clinical findings thus far obtained from randomised controlled trials with oral artemether for the prevention of patent infections and morbidity. The review intends to create a forum for strategic discussion of how these laboratory and clinical findings could be translated into public health actions. We conclude that artemether--as part of integrated current control measures and adapted to specific socio-ecological and epidemiological settings--has considerable potential to significantly reduce the current burden of schistosomiasis in many parts of the world.

Susceptibility of Schistosoma japonicum to praziquantel in China

To look for possible evidence of the development of resistance in Schistosoma japonicum to praziquantel, we conducted a field study in China. During the non-transmission period of schistosomiasis a random sample of 2860 individuals from six villages in three provinces of China were examined using a parasitological stool examination. Of the 372 stool-positive subjects, 363 subjects were treated with a single oral dose of 40 mg/kg of praziquantel. Six to Seven weeks after treatment, of 334 subjects examined using the same stool examination, stool-negative results were found in 319 patients which represents a 95.5% parasitologic cure rate. Fifteen subjects still excreting eggs were treated a second time with the same dose of praziquantel. All stool samples, including those from participants re-treated with praziquantel, were re-examined 12 weeks after the first treatment and no stool-positive subjects were found. The results indicate that there was no evidence for reduced susceptibility of S. japonicum to praziquantel despite its extensive use in the main endemic areas of China for more than 10 years. The in vitro responses to praziquantel of cercariae, miracidia and eggs of S. japonicum compared with S. mansoni demonstrate that the cercariae, miracidia and eggs of S. japonicum are more sensitive to praziquantel than those of S. mansoni. More sensitive worms would be less likely to develop resistance and this could explain why no evidence for resistance was found in S. japonicum in China.

In vitro responses of praziquantel-resistant and -susceptible Schistosoma mansoni to praziquantel

The resistance status of five praziquantel-susceptible and five praziquantel-resistant isolates was confirmed by chemotherapy in CD(1) mice with 3 x 200mg/kg micronised praziquantel. Micronised praziquantel had higher efficacy than two other praziquantel formulations (prepared without milling). The five resistant isolates were less responsive to praziquantel than the five susceptible isolates (59-74% reduction in worm burden in resistant isolates compared with 92-100% in susceptible isolates). Observations were made on the in vitro responses of different stages of 10 isolates to praziquantel. There were different in vitro responses to praziquantel at the egg, miracidial, cercarial and adult stages of Schistosoma mansoni between praziquantel-resistant and praziquantel-susceptible isolates. There were differences in the response of resistant and susceptible isolates following exposure of freshly hatched miracidia to 10(-6)M praziquantel for 1 min and observing the percent change in shape. Using this test it should be possible to determine whether failed therapy in patients infected with S. mansoni is due to the presence of praziquantel-resistant worms. Similarly, by exposing freshly shed cercariae to 4 x 10(-7)M praziquantel and observing the percent of tail shedding over 80 min it should be possible to monitor for the presence of praziquantel-resistant worms in snails collected in the field.

The biochemical basis of anthelmintic action and resistance

The most commonly used modern anthelmintics include the benzimidazoles, the nicotinic agonists. praziquantel, triclabendazole and the macrocyclic lactones. These drugs interfere with target sites that are either unique to the parasite or differ in their structural features from those of the homologous counterpart present in the vertebrate host. The benzimidazoles exert their effect by binding selectively and with high affinity to the beta-subunit of helminth microtubule protein. The target site of the nicotinic agonists (e.g. levamisole, tetrahydropyrimidines) is a pharmacologically distinct nicotinic acetylcholine receptor channel in nematodes. The macrocyclic lactones (e.g. ivermectin, moxidectin) act as agonists of a family of invertebrate-specific inhibitory chloride channels that are activated by glutamic acid. The primary mode of action of other important anthelmintics (e.g. praziquantel, triclabendazole) is unknown. Anthelmintic resistance is wide-spread and a serious threat to effective control of helminth infections, especially in the veterinary area. The biochemical and genetic mechanisms underlying anthelmintic resistance are not well understood, but appear to be complex and vary among different helminth species and even isolates. The major mechanisms helminths use to acquire drug resistance appear to be through receptor loss or decrease of the target site affinity for the drug. Knowledge on the mechanisms of drug action and resistance may be exploitable for the development of new drugs and may provide information on ways to overcome parasite resistance, respectively.

Praziquantel: is there real resistance and are there alternatives?

The low cure rates obtained with praziquantel in a Senegalese focus of schistosomiasis can best be interpreted on the basis of epidemiological factors, and are unlikely to be connected with any drug resistance in the parasite. Schistosome isolates obtained in Egypt from uncured patients present evidence of lower susceptibility to the drug, albeit to a rather limited extent. Similarly, laboratory schistosomes subjected to repeated passages under drug pressure are partly insensitive to the drug. Oxamniquine is at present the only available alternative to praziquantel. Research and development of new antischistosomal drugs is urgently needed.