Tolerance of Kenyan Schistosoma mansoni to oxamniquine

A review of the genetic determinants of praziquantel resistance in Schistosoma mansoni: Is praziquantel and intestinal schistosomiasis a perfect match?

Schistosomiasis is a neglected tropical disease (NTD) caused by parasitic trematodes belonging to the Schistosoma genus. The mainstay of schistosomiasis control is the delivery of a single dose of praziquantel (PZQ) through mass drug administration (MDA) programs. These programs have been successful in reducing the prevalence and intensity of infections. Due to the success of MDA programs, the disease has recently been targeted for elimination as a public health problem in some endemic settings. The new World Health Organization (WHO) treatment guidelines aim to provide equitable access to PZQ for individuals above two years old in targeted areas. The scale up of MDA programs may heighten the drug selection pressures on Schistosoma parasites, which could lead to the emergence of PZQ resistant schistosomes. The reliance on a single drug to treat a disease of this magnitude is worrying should drug resistance develop. Therefore, there is a need to detect and track resistant schistosomes to counteract the threat of drug resistance to the WHO 2030 NTD roadmap targets. Until recently, drug resistance studies have been hindered by the lack of molecular markers associated with PZQ resistance. This review discusses recent significant advances in understanding the molecular basis of PZQ action in S. mansoni and proposes additional genetic determinants associated with PZQ resistance. PZQ resistance will also be analyzed in the context of alternative factors that may decrease efficacy within endemic field settings, and the most recent treatment guidelines recommended by the WHO.

A Schistosoma mansoni tri- and tetramer microsatellite catalog for genetic population diversity and differentiation

All Schistosoma mansoni tri- and tetranucleotide repeat microsatellites published as of December 2018 were identified. All 52 were evaluated for autosomal location, strength of amplification, scorability and behavior as single-copy loci by polyacrylamide and capillary gel electrophoresis. Of these, 27 were unique, autosomal, polymorphic, easily scored and single copy as assessed on pooled adult worm DNA from two different continental origins and adult worm clones. These microsatellites were distributed across all seven autosomal chromosomes. On laboratory strains their heterozygosity ranged from 0.22 to 0.77. Individual markers had 5-13 alleles, allelic richness of 2-10 and an effective allele number of 1.3-8.14. Those infected by Schistosoma mansoni carry many genetically distinct, sexually reproducing parasites, therefore, for an individual infection the complete allele frequency profile of their progeny consists of a pool of DNA from multiple diploid eggs. Using a set of 25 microsatellites, we calculated allele frequency profiles of eggs in fecal samples from people in two Brazilian communities separated by 6 km: Jenipapo (n = 80) and Volta do Rio (n = 38). There were no a priori characteristics that could predict the performance of markers in natural infections based on their performance with laboratory strains. Increasing marker number did not change accuracy for differentiation and diversity but did improve precision. Our data suggest that using a random set of 10-20 microsatellites appears to result in values that exhibit low standard deviations for diversity and differentiation indices. All identified microsatellites as well as PCR conditions, allele size, primer sequences and references for all tri- and tetramer microsatellites markers presented in this work are available at: https://sites.google.com/case.edu/cwru-and-fiocruz-wdrc/home.

Oxamniquine resistance alleles are widespread in Old World Schistosoma mansoni and predate drug deployment

Do mutations required for adaptation occur de novo, or are they segregating within populations as standing genetic variation? This question is key to understanding adaptive change in nature, and has important practical consequences for the evolution of drug resistance. We provide evidence that alleles conferring resistance to oxamniquine (OXA), an antischistosomal drug, are widespread in natural parasite populations under minimal drug pressure and predate OXA deployment. OXA has been used since the 1970s to treat Schistosoma mansoni infections in the New World where S. mansoni established during the slave trade. Recessive loss-of-function mutations within a parasite sulfotransferase (SmSULT-OR) underlie resistance, and several verified resistance mutations, including a deletion (p.E142del), have been identified in the New World. Here we investigate sequence variation in SmSULT-OR in S. mansoni from the Old World, where OXA has seen minimal usage. We sequenced exomes of 204 S. mansoni parasites from West Africa, East Africa and the Middle East, and scored variants in SmSULT-OR and flanking regions. We identified 39 non-synonymous SNPs, 4 deletions, 1 duplication and 1 premature stop codon in the SmSULT-OR coding sequence, including one confirmed resistance deletion (p.E142del). We expressed recombinant proteins and used an in vitro OXA activation assay to functionally validate the OXA-resistance phenotype for four predicted OXA-resistance mutations. Three aspects of the data are of particular interest: (i) segregating OXA-resistance alleles are widespread in Old World populations (4.29–14.91% frequency), despite minimal OXA usage, (ii) two OXA-resistance mutations (p.W120R, p.N171IfsX28) are particularly common (>5%) in East African and Middle-Eastern populations, (iii) the p.E142del allele has identical flanking SNPs in both West Africa and Puerto Rico, suggesting that parasites bearing this allele colonized the New World during the slave trade and therefore predate OXA deployment. We conclude that standing variation for OXA resistance is widespread in S. mansoni.

It has been argued that drug resistance is unlikely to spread rapidly in helminth parasites infecting humans. This is based, at least in part, on the premise that resistance mutations are rare or absent within populations prior to treatment, and take a long time to reach appreciable frequencies because helminth parasite generation time is long. This argument is critically dependent on the starting frequency of resistance alleles–if high levels of “standing variation” for resistance are present prior to deployment of treatment, resistance may spread rapidly. We examined frequencies of oxamniquine resistance alleles present in Schistosoma mansoni from Africa and the Middle East where oxamniquine has seen minimal use. We found that oxamniquine resistance alleles are widespread in the Old World, ranging from 4.29% in the Middle East to 14.91% in East African parasite populations. Furthermore, we show that resistance alleles from West African and the Caribbean schistosomes share a common origin, suggesting that these alleles travelled to the New World with S. mansoni during the transatlantic slave trade. Together, these results demonstrate extensive standing variation for oxamniquine resistance. Our results have important implications for both drug treatment policies and drug development efforts, and demonstrate the power of molecular surveillance approaches for guiding helminth control.

Exposure to hycanthone alters chromatin structure around specific gene functions and specific repeats in Schistosoma mansoni

Schistosoma mansoni is a parasitic plathyhelminth responsible for intestinal schistosomiasis (or bilharzia), a disease affecting 67 million people worldwide and causing an important economic burden. The schistosomicides hycanthone, and its later proxy oxamniquine, were widely used for treatments in endemic areas during the twentieth century. Recently, the mechanism of action, as well as the genetic origin of a stably and Mendelian inherited resistance for both drugs was elucidated in two strains. However, several observations suggested early on that alternative mechanisms might exist, by which resistance could be induced for these two drugs in sensitive lines of schistosomes. This induced resistance appeared rapidly, within the first generation, but was metastable (not stably inherited). Epigenetic inheritance could explain such a phenomenon and we therefore re-analyzed the historical data with our current knowledge of epigenetics. In addition, we performed new experiments such as ChIP-seq on hycanthone treated worms. We found distinct chromatin structure changes between sensitive worms and induced resistant worms from the same strain. No specific pathway was discovered, but genes in which chromatin structure modifications were observed are mostly associated with transport and catabolism, which makes sense in the context of the elimination of the drug. Specific differences were observed in the repetitive compartment of the genome. We finally describe what types of experiments are needed to understand the complexity of heritability that can be based on genetic and/or epigenetic mechanisms for drug resistance in schistosomes.

Praziquantel: its use in control of schistosomiasis in sub-Saharan Africa and current research needs

Treatment with praziquantel (PZQ) has become virtually the sole basis of schistosomiasis control in sub-Saharan Africa and elsewhere, and the drug is reviewed here in the context of the increasing rate that it is being used for this purpose. Attention is drawn to our relative lack of knowledge about the mechanisms of action of PZQ at the molecular level, the need for more work to be done on schistosome isolates that have been collected recently from endemic areas rather than those maintained in laboratory conditions for long periods, and our reliance for experimental work mainly on Schistosoma mansoni, little work having been done on S. haematobium. There is no evidence that resistance to PZQ has been induced in African schistosomes as a result of its large-scale use on that continent to date, but there is also no assurance that PZQ and/or schistosomes are in any way unique and that resistant organisms will not be selected as a result of widespread drug usage. The failure of PZQ to produce complete cures in populations given a routine treatment should therefore solicit considerable concern. With few alternatives to PZQ currently available and/or on the horizon, methods to monitor drug-susceptibility in African schistosomes need to be devised and used to help ensure that this drug remains effective for as long a time as possible.

Praziquantel: mechanisms of action, resistance and new derivatives for schistosomiasis

PURPOSE OF REVIEW

Praziquantel (PZQ) is the only drug being used to treat human schistosomiasis on a large scale. This review focuses on current knowledge about the mechanisms of action of PZQ, prospects for PZQ resistance, possible future alternative drugs and on exhortations that control of schistosomiasis and other so-called neglected tropical diseases becomes more integrated.

RECENT FINDINGS

Schistosome calcium ion (Ca2+) channels are the only moiety so far identified as the molecular target of PZQ, but the evidence remains indirect. In the presence of cytochalasin D worms survive high concentrations of PZQ and experiments with cytochalasin D also indicated that PZQ induced worm death and Ca2+ influx are not correlated. Despite PZQ being widely used, there is no clinically relevant evidence for resistance to date, but worryingly low-cure rates have been recorded in some studies in Africa. Artemisinins and the related 1,2,4-trioxolanes are new promising antischistosomal compounds, as are inhibitors of a schistosome-specific bifunctional enzyme, thioredoxin-glutathione reductase.

SUMMARY

Use of PZQ will increase in the foreseeable future, whether given alone or coadministered with other anthelminthics in integrated control programmes. PZQ resistance remains a threat and its prevention requires adequate monitoring of current mass drug administration programmes and development of new schistosomicides.

Response of Schistosoma mansoni isolates having different drug sensitivity to praziquantel over several life cycle passages with and without therapeutic pressure

The stability of praziquantel (PZQ)-insusceptible S. mansoni isolates and the possible selection of PZQ-insusceptible parasites upon applying therapeutic pressure were examined over several life cycle passages (snails to mice). To test isolate stability, 3 PZQ-susceptible and 7 PZQ-insusceptible isolates were used to establish infection in mice, and they were passaged each for 2-5 life cycles. After each passage, 6 groups of mice were used to assess the PZQ dose at which the worm burden was decreased by 50% (ED50). Five of them were treated with doses of PZQ (12.5, 25, 50, 100, and 200 mg/kg for 5 days) 7 wk after infection; the last group represented infected, but untreated, controls. Possible selection of PZQ-insusceptible parasites under therapeutic pressure was examined by subjecting 1 PZQ-susceptible and 1 PZQ-insusceptible S. mansoni isolate to therapeutic pressure by PZQ for 8 passages. After the final passage, PZQ ED50 was estimated. All PZQ-susceptible S. mansoni isolates showed stable susceptibility to PZQ (mean PZQ ED50 = 85 mg/kg) over all passages. Two of the 7 PZQ-insusceptible S. mansoni isolates (847 and ER5) showed normal sensitivity to PZQ in 1-2 passages (although not the last passage, and without a declining ED50 profile), whereas the remaining passages kept a sustained insusceptibility to the drug (mean PZQ ED50 = 217 mg/kg). Worm maturity and sex were irrelevant to variability in drug ED50 within an individual isolate over different passages, revealing the heterogeneous nature of the parasite. Therapeutic pressure for limited life cycle passages did not result in a significant increase in drug ED50. The fact that reversion of some of the PZQ-insusceptible S. mansoni isolates to normal drug-sensitive state is not long lasting and that the therapeutic pressure by PZQ in the field is not comparable with that in the laboratory (unlimited), make monitoring the response of patients to the drug in the field an integral part of schistosomiasis control measures.

Microsatellite typing reveals strong genetic structure of Schistosoma mansoni from localities in Kenya

Genetic diversity and population structure of seven populations of Schistosoma mansoni sampled in Kenya were assessed using five microsatellite markers. The mean number of alleles per locus, expected heterozygosity in Hardy-Weinberg equilibrium and pairwise F(ST) values ranged from 5.2 to 10.7, 0.5-0.8 and 3.6-27.3%, respectively. These data reveal that S. mansoni populations in Kenyan have relatively high levels of genetic diversity and is significantly differentiated. Our data combined with information on biogeography support the hypothesis that the strong genetic structure in Kenyan schistosomes is as a result of limited gene flow and large population sizes. Resistance to anthelminthics has not been reported among the Kenyan schistosomes, we hypothesize that this is probably due to the very little gene flow among populations, thereby limiting opportunities for the spread of rare alleles that might confer resistance to the drugs.

Drug resistance in schistosomes

Drug resistance in schistosomes is confined essentially to compounds of the hyconthone/oxamniquine family, since no documented case of resistance has so far been reported for the widely used drug praziquantel. The availability of strains of Schistosoma mansoni that are resistant to hyconthone and oxomniquine has permitted a detailed genetic and biochemical study of the mechanism of action of these compounds. Drugs must be activated by enzymatic esterification and this ultimately results in the production of an electrophilic moiety capable of alkylating DNA and other parasite macromolecules. As reviewed here by Donato Cioli, Livia Pica-Mattoccia and Sydney Archer, resistance is due to the loss of a drug-activating enzyme that is present in sensitive schistosomes and absent in resistant worms and in the mammalian hosts. Further study of this enzyme may yield valuable clues for drug design and for a basic understanding of parasite metabolism.

Anthelmintic resistance – looking to the future: a UK perspective

The full extent of anthelmintic resistance in nematodes of farm animals is not known. Resistance can be detected with a faecal egg count reduction test and two in vitro tests, the egg hatch and larval development tests. The sensitivity of these two in vitro tests can be increased by using discriminating doses rather than calculating LD50 values. Only benzimidazole resistance can be detected with PCR based tests because the molecular mechanisms of resistance to levamisole and the macrocyclic lactones remain unknown. Resistance detection is important because it enables the appropriate management strategies to be put in place. The development of resistance is delayed by keeping sufficient parasites in refugia (not exposed to anthelmintic), but the necessary management details have not yet been validated in the field. It is probably too late to use combination products to delay the development of resistance, except in cattle but quarantining animals to prevent introduction of resistant helminths onto a farm is important. Dilution of resistant worms with susceptible ones is only at the preliminary research stage and the application of non-chemical methods of control to delay resistance is not yet a practical option. Extensive research is required to manage resistance, especially in the control of resistance in Fasciola hepatica.

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.

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.

Study of Schistosoma mansoni isolates from patients with failure of treatment with oxamniquine

After three successive treatments with oxamniquine the continuing elimination of Schistosoma mansoni eggs was observed in patients, who came from various regions of Brazil, with different clinical forms of schistosomiasis. The objective of the present study was to determine the experimental behaviour of five different S. mansoni isolates in Swiss Webster mice that were submitted to treatment with the same drug. The experimental group with failure of treatment showed higher mean number of surviving male worms when it was compared to the group without failure of treatment. These date suggest the possibility of resistance to oxamniquine.

Resistance to oxamniquine of a Schistosoma mansoni strain isolated from patient submitted to repeated treatments

A strain of Schistosoma mansoni (R1) was isolated from patient previously submitted to four treatments with oxamniquine, and to another one with praziquantel. The results obtained with chemotherapeutic test, by using oxamniquine in mice infected with the strains R1 and LE (standard), showed an evident resistance to the drug in worms of the strain R1. Thus, at the dose of 250 mg/kg oxamniquine, all mice (17) infected with the LE strain did not show surviving worms, whereas 12 out of 17 mice infected with the R1 strain presented surviving worms. At the dose of 200 mg/kg, the LE strain showed recovery rates of 1.06% and 20.58%, whereas the R1 strain presented 18.57% and 61.14%, for male and female worms, respectively. At the dose of 100 mg/kg, the recovery of male worms was 2.6% for the LE strain, and 29.9% for the R1 strain. At the same dose, the recovery of females did not show statistically significant differences between the two strains (LE = 76.38%, R1 = 79.12%). Praziquantel showed similar antischistosomal activity against both studied strains, when administered at the dose of 500 mg/kg.

Antischistosomal drugs: past, present ... and future?

The major antischistosomal drugs that have been or still are in use against infections with schistosomes are considered here together with some compounds that have not been in clinical use, but show interesting characteristics. Each individual compound presents aspects that may be enlightening about parasite biochemistry, parasite biology, and host-parasite relationships. Special attention is given to the mechanisms of action, an understanding of which is seen here as a major factor of progress in chemotherapy. Three compounds are currently in use, i.e., metrifonate, oxamniquine, and praziquantel, and all three are included in the World Health Organization list of essential drugs. They are analyzed in some detail, as each one presents advantages and disadvantages in antischistosomal therapy. The reported occurrence of drug-resistant schistosomes after treatment with oxamniquine and praziquantel suggests strict monitoring of such phenomena and encourages renewed efforts toward the development of multiple drugs against this human parasite.

Correlation between cell-mediated immunity and degree of infection in subjects living in an endemic area of schistosomiasis

Cell-mediated immunity to Schistosoma mansoni antigens, unrelated antigens and mitogens was evaluated in 50 subjects with the same degree of exposure to infection living in an endemic area of schistosomiasis. The degree of infection, assessed by the number of eggs/g of stool, was variable in this population (0-5604), suggesting differences in susceptibility to infection. Absence of lymphoproliferative response was observed in 56% of this group, despite having a response to purified protein derivative of tuberculin (PPD) and tetanus toxoid (TT) antigens and to pokeweed mitogen. The 50 subjects were divided into two groups, according to their degree of infection. The lymphoproliferative responses to schistosomula and adult worm antigens in the group with a low degree of infection (< 400 eggs/g of stool) were higher than the ones documented in patients with a high degree of infection (> 400 eggs/g of stool), and these differences were statistically significant (p < 0.001). An inverse correlation between the lymphocyte proliferation in response to S. mansoni antigens and the degree of infection was also observed (p = 0.02), indicating that subjects with a lower degree of infection have a higher lymphoproliferative response to schistosomula and adult worm antigens. No differences in the lymphocyte reactivity to other antigens (PPD and TT) were detected in these groups. An impairment of interferon-gamma in vitro production was observed when the lymphocytes from these subjects were stimulated with S. mansoni adult worm antigen, although they produced gamma interferon in response to phytohemagglutinin.

Hycanthone resistance in schistosomes correlates with the lack of an enzymatic activity which produces the covalent binding of hycanthone to parasite macromolecules

Crude extracts of hycanthone sensitive Schistosoma mansoni incubated at 37 degrees C in the presence of ATP and Mg2+ induced the covalent binding of tritiated hycanthone (HC) to macromolecules. The same behavior was shown by the HC sensitive species, Schistosoma rodhaini, whereas two independently isolated HC resistant S. mansoni strains had no detectable activity. Sensitive male schistosomes had more activity than females or immature worms. Virtually no activity was present in mouse liver, in human liver, in HeLa cells or in the naturally resistant species Schistosoma japonicum. The activity was destroyed by boiling or by Proteinase K treatment. Covalent binding of tritiated HC to macromolecules could be inhibited by cold HC, oxamniquine or IA-4, while none of the in vitro ineffective analogs, like lucanthone, UK-3883 or 4-desmethyl lucanthone, were inhibitory. These results strongly support the previously advanced suggestion that HC is activated by enzymatic mechanisms which are present only in drug sensitive schistosomes.

Experimental chemotherapy of Schistosoma mansoni with praziquantel and oxamniquine: differential effect of single or combined formulations of drugs on various strains and on both sexes of the parasite

The susceptibility of two Venezuelan (YT and SM) and one Brazilian (BH) strain of Schistosoma mansoni to single oral doses of praziquantel (Pz; 250 or 500 mg/kg), oxamniquine (Ox; 40, 60, or 100 mg/kg) or to low-dose combinations of both drugs (33 mg/kg Pz and 25 mg/kg Ox; 66 mg/kg Pz and 12.5 mg/kg Ox; 250 mg/kg Pz and 40 mg/kg Ox) was experimentally evaluated in mice. At lower doses of either drug, adult worms of the SM isolate were less susceptible than those of the BH and YT isolates. However, no difference in liver or intestinal egg counts (IECs) could be detected among the isolates after this treatment. At such doses, Pz was better than Ox at reducing IECs. In spite of lowered IECs, eggs continued to accumulate in the liver after Ox treatment. At higher individual doses or following treatment with low-dose combinations of both drugs, no difference in susceptibility could be detected among the parasite isolates. Under such conditions, oviposition was drastically reduced in all three isolates. We confirm that Ox preferentially kills male parasites and present for the first time evidence for the preferential killing of female worms by Pz. We propose that the synergistic effect obtained in the present study and in other investigations using low-dose combinations of both drugs may be due to the preferential cytotoxicity of each drug against a different parasite sex.

Schistosoma mansoni: biochemical characteristics of the antischistosomal effects of Ro 15-5458

We evaluated a variety of biochemical parameters in Schistosoma mansoni isolated from mice up to 4 days after dosing with 15 mg/kg Ro 15-5458. While no drug effect could be demonstrated in the utilization of media glucose, glycogen content, gut pigment, or ATP levels of the parasites, a significant reduction (P less than or equal to 0.05) in parasite weight and protein content was observed. Possible drug actions that may contribute to the loss in parasite protein and perhaps ultimately result in parasite death have been investigated. We noted significant reduction in the incorporation of leucine and thymidine into acid-insoluble fractions of the parasites. The reduction in the incorporation of leucine into parasite proteins was nonspecific and preceded the effect of the drug on the uptake of the amino acid. Parasite and host liver RNA isolated after dosing were translated in vitro in a rabbit reticulocyte system. Drug-treated parasite mRNA, but not that of the host, was less effective than control mRNA in directing the incorporation of [35S]methionine. We propose a hypothesis that attributes the loss in protein content to a defect in the biosynthesis of parasite proteins as a result of a drug-induced reduction in the quantity of mRNA in the parasites This effect of Ro 15-5458 on the parasite may provide the basis for its schistosomicidal action.

Effects of praziquantel and oxamniquine on a Saudi Arabian strain of Schistosoma mansoni in mice

The prophylactic and curative effects of praziquantel and oxamniquine on a Saudi Arabian strain of Schistosoma mansoni in MF-1 mice were assessed. The drugs were administered orally. At 240 mg/kg praziquantel, there was a reduction of 89.1% in adult worm recovery and a marked reduction in tissue deposited eggs. The reduction in adult worm recovery after dosing with 50 mg/kg oxamniquine was 89.2%. At low doses (40 mg/kg praziquantel and 30 mg/kg oxamniquine) administered at 11 days, 5 days and 3 h before and 5, 21 and 49 days after infection, the reduction in adult worm recovery was 0.0%, 65.1%, 58.8%, 33.6%, 0.0% and 76.0% for praziquantel and 0.0%, 66.0%, 60.0%, 41.3%, 10.8% and 79.0% for oxamniquine. Numbers of lung schistosomula and the size of hepatic granulomata were also reduced.

Recent advances in laboratory models for evaluation of helminth chemotherapy

Extensive research on nematodes of veterinary importance has resulted in new tests based on incubation of parasitic nematodes in culture media, with a variety of detection systems including visual observation, inhibition of aggregation, reduced secretion of acetylcholinesterase and electronic motility measurements. There has been a proliferation of tests to detect benzimidazole resistant nematodes including egg embryonation, egg hatch, larval motility, differences in esterases, and tubulin binding by labelled benzimidazoles. A simple larval development test, which detects resistance to all types of anthelmintics, is likely to become the preferred test for evaluation of resistance in nematodes of grazing animals. By contrast the relative unimportance of trematodes and cestodes from a commercial point of view has resulted in few recent developments in models for evaluating chemotherapy. But the emergence of drug resistant strains of both schistosomes and liver fluke indicates the need for simple tests to detect resistance.

Effect of combined low dose praziquantel and oxamniquine on different stages of schistosome maturity

Combined low doses of praziquantel and oxamniquine were tested against different stages of Schistosoma mansoni in infected Swiss albino mice. The effect of combination therapy (1/3 the curative dose of praziquantel plus 1/3 the curative dose of oxamniquine) was compared with the effect of each drug alone, in reduced or full dose. Comparison with infected untreated controls was also made. Drug effects were evaluated on different growth stages of schistosomes by administering the drugs 24 h before infection and 4 h, 1, 2, 3, 4 and 5 weeks after infection. Animals were killed 8 weeks after infection. Worm burden, distribution, tissue egg load and oogram pattern were used in assessing drug efficacy. A potentiating effect was observed in animals receiving combination therapy. The combination regimen was most effective 4 h after infection, producing 96% worm reduction; eggs were not detected in the liver or intestine. Five weeks after infection the same regimen resulted in 98% reduction in the tissue egg load.

Drug resistance in ostertagiasis

Benzimidazole- and levamisole-resistant Ostertagia circumcincta from sheep have been reported from several countries, but there are no reports of anthelmintic-resistant O. ostertagi in cattle, where resistance has been confirmed in controlled trials. The reasons for this may include (1) the biology of the worm, (2) avoidance of drug action by inhibition, (3) host metabolism of drugs, (4) less anthelmintic used in cattle than in sheep, (5) high costs of controlled trials, (6) lack of adequate in vitro tests to detect resistant worms and (7) lack of reporting of anthelmintic failures. Reduced efficacy of the anthelmintics levamisole and thiabendazole to adult O. ostertagi and of modern benzimidazoles to the inhibited L4 larvae has been reported in cattle. This latter effect might, in part, represent a stage-specific expression of resistance. Although anthelmintic resistance in O. ostertagi is not known to be a problem at present, it could potentially become a serious issue.