Fasciola hepatica: ultrastructural effects of a combination of triclabendazole and clorsulon against mature fluke

Current Challenges for Fasciolicide Treatment in Ruminant Livestock

Pharmacological treatment remains essential to control fasciolosis in areas where infection is endemic. However, there are major constraints to treating food-producing animals. Of particular concern is the lack of flukicides for treating early Fasciola infections in ruminant livestock in some countries. In addition, the information provided in package leaflets, particularly regarding withdrawal periods, is often incomplete, confusing, and/or contradictory. International regulatory bodies should harmonize the use of flukicides in livestock in favor of fairer, safer international trade. In addition, monitoring the efficacy of fasciolicides on farms is also essential to minimize the spread of drug-resistant populations of Fasciola. The current situation regarding flukicide formulations in the European Union and other, non-European countries is analyzed in this review paper.

Drug resistance in liver flukes

Liver flukes include Fasciola hepatica, Fasciola gigantica, Clonorchis sinensis, Opisthorchis spp., Fascioloides magna, Gigantocotyle explanatum and Dicrocoelium spp. The two main species, F. hepatica and F. gigantica, are major parasites of livestock and infections result in huge economic losses. As with C. sinensis, Opisthorchis spp. and Dicrocoelium spp., they affect millions of people worldwide, causing severe health problems. Collectively, the group is referred to as the Food-Borne Trematodes and their true significance is now being more widely recognised. However, reports of resistance to triclabendazole (TCBZ), the most widely used anti-Fasciola drug, and to other current drugs are increasing. This is a worrying scenario. In this review, progress in understanding the mechanism(s) of resistance to TCBZ is discussed, focusing on tubulin mutations, altered drug uptake and changes in drug metabolism. There is much interest in the development of new drugs and drug combinations, the re-purposing of non-flukicidal drugs, and the development of new drug formulations and delivery systems; all this work will be reviewed. Sound farm management practices also need to be put in place, with effective treatment programmes, so that drugs can be used wisely and their efficacy conserved as much as is possible. This depends on reliable advice being given by veterinarians and other advisors. Accurate diagnosis and identification of drug-resistant fluke populations is central to effective control: to determine the actual extent of the problem and to determine how well or otherwise a treatment has worked; for research on establishing the mechanism of resistance (and identifying molecular markers of resistance); for informing treatment options; and for testing the efficacy of new drug candidates. Several diagnostic methods are available, but there are no recommended guidelines or standardised protocols in place and this is an issue that needs to be addressed.

A. Immature Fasciola gigantica: Time-dependent ultrastructural changes following in vivo treatment with triclabendazole

An in vivo study was carried out to investigate the ultrastructural effects of triclabendazole (TCBZ) on immature Fasciola gigantica in a goat model. Five goats were infected with an oral gavage of 150 metacercarial cysts of F. gigantica and anthelmintic treatment occurred at 4 weeks post infection with an oral dose of 10 mg/kg. They were euthanized at 0 (untreated), 24, 48, 72 and 96 h post treatment (h pt). Juvenile flukes were recovered from each of the goat's liver and processed for transmission electron microscopy (TEM). The untreated control flukes showed normal ultrastructure and no apparent changes were observed at 24 h pt. At 48 h pt, moderate levels of disruption were observed to the tegument and minor changes to the sub-tegument which included widespread blebbing and disruption of apical tegumental membrane, swollen mitochondria, reduced number of secretory bodies, swelling of basal infolds leading to severe vacuolation, and relatively mild disruption to the subtegumental muscle fibres, parenchyma and tegumental cells, whereas the gastrodermal cells appeared less affected. By 72 h pt, sloughing of the tegumental syncytium was evident leading to the exposure of the basal lamina and the disruption was severe in the subtegument too. At 96 h pt, the flukes were totally devoid of tegument and the disruption was extremely severe, distorting the ultrastructure of the entire fluke's body. The results of the present study revealed that the flukes showed time-dependent progressive disruption to the internal tissues which became increasingly severe over time pt. This is the first study to detail the time-scale and impacts on ultrastructural morphology of the in vivo TCBZ treatment of the immature tropical liver fluke, Fasciola gigantica.

Efficient copper-based DNA cleavers from carboxylate benzimidazole ligands

Four copper(II) coordination compounds from 2-benzimidazole propionic acid (Hbzpr) and 4-(benzimidazol-2-yl)-3-thiobutanoic acid (Hbztb) were synthesized and fully characterized by elemental analyses, electronic spectroscopy, FT-IR and mass spectrometry. The molecular structure for the four complexes was confirmed by single-crystal X-ray crystallography. The DNA-interacting properties of the two trinuclear and two mononuclear compounds were investigated using different spectroscopic techniques including absorption titration experiments, fluorescence spectroscopy and circular dichroism spectroscopy. Trinuclear [Cu₃(bzpr)₄(H₂O)₂](NO₃)₂·3H₂O·CH₃OH (2) and [Cu₃(bzpr)₄Cl₂]·3H₂O (3) bind to DNA through non-intercalative interactions, while for mononuclear [Cu(bzpr)₂(H₂O)]·2H₂O (1) and [Cu(bztb)₂]·2H₂O (4), at minor concentrations in relation to the DNA, a groove binding interaction is favored, while at higher concentrations an intercalative mode is preferred. The nuclease properties of all complexes were studied by gel electrophoresis, which showed that they were able to cleave supercoiled plasmid DNA (form I) to the nicked form (form II). Compound 4 is even capable of generating linear form III (resulting from double-strand cleavage). The proposed mechanism of action involves an oxidative pathway (Fenton-type reaction), which produces harmful reactive species, like hydroxyl radicals.

Identification of putative markers of triclabendazole resistance by a genome-wide analysis of genetically recombinant Fasciola hepatica

Despite years of investigation into triclabendazole (TCBZ) resistance in Fasciola hepatica, the genetic mechanisms responsible remain unknown. Extensive analysis of multiple triclabendazole-susceptible and -resistant isolates using a combination of experimental in vivo and in vitro approaches has been carried out, yet few, if any, genes have been demonstrated experimentally to be associated with resistance phenotypes in the field. In this review we summarize the current understanding of TCBZ resistance from the approaches employed to date. We report the current genomic and genetic resources for F. hepatica that are available to facilitate novel functional genomics and genetic experiments for this parasite in the future. Finally, we describe our own non-biased approach to mapping the major genetic loci involved in conferring TCBZ resistance in F. hepatica.

Update on trematode infections in sheep

Trematode parasites live in the liver, fore stomachs or blood vessels of a wide range of animals and humans. Most of them have a special economic and veterinary significance. Liver fluke disease of sheep and other animal species is caused by the common liver fluke Fasciola hepatica. Hepatic fasciolosis occurs throughout the world, where climatic conditions are suitable for the survival of aquatic intermediate host snails. Also of importance for ruminants, in some parts of the world, are Fasciola gigantica and Fascioloides magna. Other trematodes infecting ruminants include Dicrocoelium dendriticum; Eurytrema pancreaticum and Eurytrema coelomaticum. Among the Paramphistomidae, some species can infect sheep and other ruminants. Finally, Schistosoma spp. are found in the blood vessels of ruminants and are of minor importance in temperate regions. The manuscript concentrates on trematode species of veterinary importance for domestic sheep.

Erratum to: inhibition of triclabendazole metabolism in vitro by ketoconazole increases disruption to the tegument of a triclabendazole-resistant isolate of Fasciola hepatica

A study has been carried out to investigate whether the action of triclabendazole (TCBZ) against Fasciola hepatica is altered by inhibition of drug metabolism. The cytochrome P450 (CYP 450) enzyme pathway was inhibited using ketoconazole (KTZ) to see whether a TCBZ-resistant isolate could be made more sensitive to TCBZ action. The Oberon TCBZ-resistant and Cullompton TCBZ-susceptible isolates were used for these experiments. The CYP 450 system was inhibited by a 2-h pre-incubation in ketoconazole (40 μM), then incubated for a further 22 h in NCTC medium containing either KTZ, KTZ + nicotinamide adenine dinucleotide phosphate (NADPH) (1 nM), KTZ + NADPH + TCBZ (15 μg/ml), or KTZ + NADPH + triclabendazole sulphoxide (TCBZ.SO; 15 μg/ml). Changes to fluke ultrastructure following drug treatment and metabolic inhibition were assessed using transmission electron microscopy. After treatment with either TCBZ or TCBZ.SO on their own, there was greater disruption to the TCBZ-susceptible than TCBZ-resistant isolate. However, co-incubation with KTZ + TCBZ, but more particularly KTZ + TCBZ.SO, led to more severe changes to the TCBZ-resistant isolate than with each drug on its own: for example, there was severe swelling of the basal infolds and their associated mucopolysaccharide masses, accompanied by an accumulation of secretory bodies just below the apex. Golgi complexes were greatly reduced or absent in the tegumental cells and the synthesis, production, and transport of secretory bodies were badly disrupted. With the TCBZ-susceptible Cullompton isolate, there was limited potentiation of drug action. The results support the concept of altered drug metabolism in TCBZ-resistant flukes and this process may play a role in the development of drug resistance.

Inhibition of triclabendazole metabolism in vitro by ketoconazole increases disruption to the tegument of a triclabendazole-resistant isolate of Fasciola hepatica

A study has been carried out to investigate whether the action of triclabendazole (TCBZ) against Fasciola hepatica is altered by inhibition of drug metabolism. The cytochrome P450 (CYP 450) enzyme pathway was inhibited using ketoconazole (KTZ) to see whether a TCBZ-resistant isolate could be made more sensitive to TCBZ action. The Oberon TCBZ-resistant and Cullompton TCBZ-susceptible isolates were used for these experiments. The CYP 450 system was inhibited by a 2-h pre-incubation in ketoconazole (40 μM), then incubated for a further 22 h in NCTC medium containing either KTZ, KTZ + nicotinamide adenine dinucleotide phosphate (NADPH) (1 nM), KTZ + NADPH + TCBZ (15 μg/ml), or KTZ + NADPH + triclabendazole sulphoxide (TCBZ.SO; 15 μg/ml). Changes to fluke ultrastructure following drug treatment and metabolic inhibition were assessed using transmission electron microscopy. After treatment with either TCBZ or TCBZ.SO on their own, there was greater disruption to the TCBZ-susceptible than TCBZ-resistant isolate. However, co-incubation with KTZ + TCBZ, but more particularly KTZ + TCBZ.SO, led to more severe changes to the TCBZ-resistant isolate than with each drug on its own: in the syncytium, for example, there was severe swelling of the basal infolds and their associated mucopolysaccharide masses, accompanied by an accumulation of secretory bodies just below the apex. Golgi complexes were greatly reduced or absent in the tegumental cells and the synthesis, production, and transport of secretory bodies were badly disrupted. With the TCBZ-susceptible Cullompton isolate, there was limited potentiation of drug action. The results support the concept of altered drug metabolism in TCBZ-resistant flukes and this process may play a role in the development of drug resistance.

Liver fluke isolates: a question of provenance

A survey of literature on experimental infections with the liver fluke, Fasciola hepatica published between 2005 and 2009 has revealed a general lack of information on where fluke material (i.e. metacercariae) was sourced from. Even less information was given on the drug status of the fluke isolate used, which is a particular concern for those studies that involved anthelmintics. In these two respects, information on the liver fluke lags far behind that for nematodes, where such information is given almost as a matter of course. Of additional concern is that, at times, information about the source and drug history of fluke isolates was incorrect. The overall aim of the review is to demonstrate why it is important to provide as much information as possible on what fluke material is being used. It also attempts to correct some of the errors in the literature and gather together what information is available about the provenance of those isolates that have been used in recent experimental studies.

Piperonyl butoxide enhances triclabendazole action against triclabendazole-resistant Fasciola hepatica

A study has been carried out to determine whether the action of triclabendazole (TCBZ) against the liver fluke, Fasciola hepatica is altered by inhibition of the cytochrome P450 (CYP 450)-mediated drug metabolism pathway. The Oberon TCBZ-resistant and Cullompton TCBZ-susceptible fluke isolates were used for these experiments, the basic design of which is given in the paper by Devine et al. (2010a). Piperonyl butoxide (PB) was the CYP P450 inhibitor used. Morphological changes resulting from drug treatment and following metabolic inhibition were assessed by means of transmission electron microscopy. After treatment with either TCBZ or TCBZ.SO on their own, there was greater disruption to the TCBZ-susceptible than TCBZ-resistant isolate. However, co-incubation with PB+TCBZ, but more particularly PB+TCBZ.SO, led to greater changes to the TCBZ-resistant isolate than with each drug on its own, with blebbing of the apical plasma membrane, severe swelling of the basal infolds and their associated mucopolysaccharide masses in the syncytium and flooding in the internal tissues. Golgi complexes were greatly reduced or absent in the tegumental cells and the synthesis and production of secretory bodies were badly disrupted. The mitochondria were swollen throughout the tegumental system and the somatic muscle blocks were disrupted. With the TCBZ-susceptible Cullompton isolate, there was a limited increase in drug action following co-incubation with PB. The results provide evidence that the condition of a TCBZ-resistant fluke can be altered by inhibition of drug metabolism. Moreover, they support the concept that altered drug metabolism contributes to the mechanism of resistance to TCBZ.

Time-dependent changes to the tegumental system and gastrodermis of adult Fasciola hepatica following treatment in vivo with triclabendazole in the sheep host

Eight indoor-reared cross-bred sheep with no pre-exposure to Fasciola hepatica were infected by oral gavage with 200 metacercarial cysts of the triclabendazole (TCBZ)-susceptible Cullompton isolate of F. hepatica. At 12 weeks post-infection, sheep were dosed with 10mg/kg triclabendazole. Two sheep per time period were euthanized at 48 h, 72 h and 96 h post-treatment (pt). Two control sheep were euthanized alongside the 96 h triclabendazole-treated sheep. Flukes were recovered from each of the sheeps liver and, if present, from the gall bladder and they were processed for transmission electron microscopy (TEM). Disruption to the ultrastructure of the tegument became increasingly severe over time pt. Flukes recovered at 48 h pt showed widespread blebbing of the apical plasma membrane and swelling of the mucopolysaccharide masses surrounding the basal infolds. There was evidence of reduced secretory activity in the tegumental cells and spacing between the cells. Sloughing of the tegumental syncytium was observed at 72 h pt. The subtegumental musculature, parenchyma and tegumental cells were severely disrupted. At 96 h pt, all of the flukes were totally devoid of tegument. Disruption to the subtegumental tissue and somatic musculature was severe, and was so extreme in some specimens that the tegumental cells were barely discernible. Disruption to the gastrodermis was also progressive, though not as severe as disruption to the tegument. There was a general decline of secretory activity with time pt. Autophagic activity was apparent from 48 h pt and became more widespread with increasing time, culminating in breakdown of the gastrodermal cell cytoplasm. The mitochondria were swollen and electron-lucent and the cisternae of the granular endoplasmic reticulum were dilated and fragmented from 72 h pt.

Enhancement of the drug susceptibility of a triclabendazole-resistant isolate of Fasciola hepatica using the metabolic inhibitor ketoconazole

A study has been carried out to investigate whether the action of triclabendazole (TCBZ) is altered by using the metabolic inhibitor, ketoconazole (KTZ) to inhibit the cytochrome P450 (CYP 450) system within Fasciola hepatica. The Oberon TCBZ-resistant and Cullompton TCBZ-susceptible isolates were used for these experiments. The CYP 450 enzyme system was inhibited by a 2 h pre-incubation in KTZ (40 microM). Flukes were then incubated for a further 22 h in NCTC medium containing either KTZ; KTZ + nicotinamide adenine dinucleotide phosphate (NADPH; 1 nM); KTZ + NADPH + TCBZ (15 microg/ml); or KTZ + NADPH + triclabendazole sulphoxide (TCBZ.SO;15 microg/ml). Morphological changes resulting from drug treatment and following metabolic inhibition were assessed using scanning electron microscopy. After treatment with either TCBZ or TCBZ.SO alone, there was greater disruption to the TCBZ-susceptible isolate than the TCBZ-resistant isolate. However, co-incubation with KTZ and TCBZ/TCBZ.SO led to more severe surface changes to the TCBZ-resistant isolate than with each drug on its own, with greater swelling and blebbing of the tegument and even the loss of the apical plasma membrane in places. With the Cullompton isolate, there was limited potentiation of drug action in combination with KTZ, and only with TCBZ.SO. The results support the concept of altered drug metabolism within TCBZ-resistant isolates and indicate that this process may play a role in the development of drug resistance.

Potentiation of triclabendazole sulphoxide-induced tegumental disruption by methimazole in a triclabendazole-resistant isolate of Fasciola hepatica

A study has been carried out to investigate whether the action of triclabendazole (TCBZ) against Fasciola hepatica is altered by inhibition of drug metabolism. The flavin monooxygenase system (FMO) was inhibited using methimazole (MTZ) to see whether a TCBZ-resistant isolate could be made more sensitive to TCBZ action. The Oberon TCBZ-resistant and Cullompton TCBZ-susceptible isolates were used for these experiments. The FMO system was inhibited by a 2-h pre-incubation in methimazole (100 microM), then incubated for a further 22 h in NCTC medium containing either MTZ; MTZ+nicotinamide adenine dinucleotide phosphate (NADPH) (1 nM); MTZ+NADPH+TCBZ (15 microg/ml); or MTZ+NADPH+triclabendazole sulphoxide (TCBZ.SO) (15 microg/ml). Changes to fluke ultrastructure following drug treatment and metabolic inhibition were assessed using transmission electron microscopy. After treatment with either TCBZ or TCBZ.SO on their own, there was greater disruption to the TCBZ-susceptible than triclabedazole-resistant isolate. However, co-incubation with MTZ+TCBZ, but more particularly MTZ+TCBZ.SO, led to more severe changes to the TCBZ-resistant isolate than with each drug on its own, with severe swelling of the basal infolds and mucopolysaccharide masses in the syncytium, accompanied by a reduction in numbers of secretory bodies. The synthesis and production of secretory bodies in the tegumental cells was severely affected as well. With the TCBZ-susceptible Cullompton isolate, there was limited potentiation of drug action. The results support the concept of altered drug metabolism in TCBZ-resistant flukes, and this process may play a role in the development of drug resistance.

A transmission electron microscope study on the route of entry of triclabendazole into the liver fluke, Fasciola hepatica

Uptake of triclabendazole by the liver fluke, Fasciola hepatica has been studied by experiments designed to block either oral uptake of drug, by use of ligatures, or trans-tegumental diffusion, by allowing the drug to bind to an excess of bovine serum albumin (BSA) in the medium. Changes to the tegumental system, musculature and gut were assessed using transmission electron microscopy. Flukes were incubated in vitro for 24 h in TCBZ.SO (15 microg/ml). Disruption to the tegument and muscle was similar in ligatured and non-ligatured flukes, suggesting that closing the oral route did not affect drug uptake. The ultrastructure of the gastrodermal cells remained unchanged. Non-ligatured flukes were also incubated for 24 h in vitro in TCBZ.SO (15 microg/ml) in the presence of red blood cells (RBCs). Oral uptake of blood was demonstrated, but gut ultrastructure remained normal, whereas the tegument was severely disrupted. In separate experiments, ligatured and non-ligatured flukes were incubated in TCBZ.SO (15 microg/ml) in the presence of BSA (30 mg/ml) for 24 h in vitro. There was a marked decrease in the degree of tegumental disruption observed compared with TCBZ.SO action alone; again, the gut remained normal. The findings support previous morphological and pharmacological studies indicating that trans-tegumental uptake of triclabendazole predominates in the liver fluke.

Electron microscopical study to assess the in vitro effects of the synthetic trioxolane OZ78 against the liver fluke, Fasciola hepatica

Adult Fasciola hepatica were incubated for 48 h in vitro in the synthetic peroxide, OZ78 at a concentration of 100 microg/ml and then prepared for scanning and transmission electron microscopy. There was limited disruption to the external fluke surface, with only slight swelling and blebbing of the interspinal tegument in the midbody and ventral tail regions. By contrast, significant disruption was observed to the ultrastructure of the tegument and subtegumental tissues. There was severe swelling of the basal infolds in the tegumental syncytium and the flooding spread internally to affect the subtegumental tissues. In the tegumental system, there was swelling of the cisternae of granular endoplasmic reticulum and of the mitochondria, with the latter showing signs of breaking down. Autophagic vacuoles and lipid droplets were present and the synthesis of tegumental secretory bodies was much reduced. The gastrodermal cells were severely affected, with swelling and degeneration of the mitochondria and the presence of autophagic vacuoles and lipid droplets. The granular endoplasmic reticulum was swollen and vesiculated and the cells contained few secretory bodies. Both the vitelline and testis follicles showed evidence of extensive cellular disruption and degeneration. This study confirms previous data indicating the potential flukicidal activity of OZ78.

Physiological and morphological effects of genistein against the liver fluke, Fasciola hepatica

A study has been carried out to determine the activity of genistein against adult liver fluke, Fasciola hepatica. Flukes were incubated in vitro in genistein at a concentration of 0.27 mg/ml (=1 mM). They ceased to move after 3 h, at which point the experiment was terminated and the specimens prepared for examination by scanning and transmission electron microscopy. Surface changes to the flukes comprised swelling and blebbing, especially in the posterior region of the flukes, and there was particular disruption to the spines, accompanied by some spine loss. Fine structural changes to the tegumental syncytium indicated an accelerated release of secretory bodies at the surface, but a reduction in their production within the cell bodies. Autophagic activity was evident in the tegumental cells, a phenomenon that was also observed in the gastrodermal cells. Disruption to the testis and vitelline follicles was severe, with an apparent block in the normal developmental sequence of the spermatogenic and vitelline cells, respectively. Shell protein production by the vitelline cells was also disrupted. In separate experiments, somatic muscle strips were exposed to concentrations of genistein ranging from 1 microm to 1 mm. There were statistically significant increases in the frequency and/or amplitude of muscle contractions at concentrations of 10 microm, 100 microm and 1 mm. The results suggest that genistein is capable of causing severe morphological and neuromuscular disruption to adult flukes in vitro over a short time-span.

Triclabendazole progress report, 2005–2009: an advancement of learning?

Triclabendazole (TCBZ) remains the drug of choice for treating infections of the liver fluke,Fasciola hepaticain livestock and has become the main drug used to treat human cases of the disease as well. Cases of resistance in livestock continue to be reported, suggesting that the problem is increasing. In order to address the problem, there is a need for better understanding of drug action. A ‘state-of-play’ review on different aspects of TCBZ activity was published by the present author in 2005. The main purpose of the current review is to assess what progress has been made in the past four years towards understanding the main aspects of drug activity, including drug pharmacokinetics and pharmacodynamics and an understanding of the mechanism(s) of resistance. Also, what advances have been made in identifying alternative compounds and using drug combinations to enhance TCBZ activity. Stemming from a number ofin vivostudies, it has become evident that fluke isolates of differing sensitivity to TCBZ differ in some of their biological parameters, and information on this interesting phenomenon will be presented. An update on the use of TCBZ for human fascioliasis is also given. The review will indicate what progress has been made, but will also highlight areas that remain inadequately understood and require greater research focus.

An integrated transcriptomics and proteomics analysis of the secretome of the helminth pathogen Fasciola hepatica: proteins associated with invasion and infection of the mammalian host

To infect their mammalian hosts, Fasciola hepatica larvae must penetrate and traverse the intestinal wall of the duodenum, move through the peritoneum, and penetrate the liver. After migrating through and feeding on the liver, causing extensive tissue damage, the parasites move to their final niche in the bile ducts where they mature and produce eggs. Here we integrated a transcriptomics and proteomics approach to profile Fasciola secretory proteins that are involved in host-pathogen interactions and to correlate changes in their expression with the migration of the parasite. Prediction of F. hepatica secretory proteins from 14,031 expressed sequence tags (ESTs) available from the Wellcome Trust Sanger Centre using the semiautomated EST2Secretome pipeline showed that the major components of adult parasite secretions are proteolytic enzymes including cathepsin L, cathepsin B, and asparaginyl endopeptidase cysteine proteases as well as novel trypsin-like serine proteases and carboxypeptidases. Proteomics analysis of proteins secreted by infective larvae, immature flukes, and adult F. hepatica showed that these proteases are developmentally regulated and correlate with the passage of the parasite through host tissues and its encounters with different host macromolecules. Proteases such as FhCL3 and cathepsin B have specific functions in larvae activation and intestinal wall penetration, whereas FhCL1, FhCL2, and FhCL5 are required for liver penetration and tissue and blood feeding. Besides proteases, the parasites secrete an array of antioxidants that are also highly regulated according to their migration through host tissues. However, whereas the proteases of F. hepatica are secreted into the parasite gut via a classical endoplasmic reticulum/Golgi pathway, we speculate that the antioxidants, which all lack a signal sequence, are released via a non-classical trans-tegumental pathway.

Ultrastructural changes to the tegumental system and the gastrodermal cells in adult Fasciola hepatica following in vivo treatment with the experimental fasciolicide, compound alpha

Sheep infected with the triclabendazole-susceptible, Cullompton isolate of Fasciola hepatica were dosed with 15 mg/kg of compound alpha at 12 weeks post-infection. Adult flukes were recovered from the bile ducts at 24, 48 and 72 h post-treatment (p.t.). Ultrastructural changes to the flukes were assessed using transmission electron microscopy (TEM), with a view to gathering information on the mechanism(s) of action for compound alpha and on the possible route of its entry into F. hepatica. The tegumental syncytium was more severely affected than the gut at all time-points p.t. with compound alpha, suggesting a predominantly trans-tegumental route of uptake. Disruption to the tegumental system became increasingly severe over time. A stress response was observed at 24 h p.t. and took the form of blebbing and increases in the production and transport of secretory bodies. By 72 h p.t., extensive tegumental loss and degeneration of the tegumental cell bodies had occurred. Degeneration of subtegumental tissues and internal flooding were also observed. Changes in the gastrodermal cells were slow to develop: reduced secretory activity was evident at 72 h p.t.. There was progressive disruption to the somatic muscle layers, with disorganization of the muscle blocks and loss of muscle fibres.

Morphological response of triclabendazole-susceptible and triclabendazole-resistant isolates of Fasciola hepatica to treatment in vitro with nitroxynil (Trodax)

Adult liver flukes belonging to three isolates of differing sensitivity to triclabendazole were incubated for 24 h in vitro in nitroxynil at a concentration of 100 microg/ml. Fine structural changes to the tegument, sub-tegumental region and gut were assessed by transmission electron microscopy. Similar changes were observed in all three isolates. In the tegumental syncytium, the basal infoldings and mitochondria were swollen, and there was an accumulation and accelerated release of secretory bodies at the apex. The crystalline core of the spines was disrupted, and the tegumental covering sloughed off. Mitochondria in the tegumental cells were also swollen, the Golgi complexes were affected and reduced numbers of T1 secretory bodies were evident in the T1-type of tegumental cell. In the sub-tegumental region, large spaces were present between cells and tissues, indicative of severe internal flooding. Swelling of mitochondria and cisternae of the granular endoplasmic reticulum was seen in the gastrodermal cells, which contained few secretory bodies. The extent of disruption varied between the isolates: the triclabendazole-resistant Sligo isolate was the most severely affected, while the Fairhurst triclabendazole-susceptible isolate was the least affected. In all three isolates, the tegument was more severely affected than the gut.

Relative activity of triclabendazole metabolites against the liver fluke, Fasciola hepatica

A study has been carried out to determine the relative activity of triclabendazole (TCBZ) and its sulphoxide (TCBZSO) and sulphone (TCBZSO(2)) metabolites against the adult stage of the liver fluke, Fasciola hepatica. Flukes were incubated for 24h in vitro in 15mug/ml of each of the compounds and prepared for scanning and transmission electron microscopy. All three compounds induced changes to the surface morphology of the fluke, the changes comprising swelling and blebbing to a greater or lesser extent in different regions of the fluke. TCBZSO(2) was more disruptive anteriorly and TCBZSO posteriorly. Internal ultrastructural changes were evident following incubation with each of the compounds, with an order of severity TCBZSO(2)>TCBZSO>TCBZ. Swelling of the basal infolds and mitochondria were observed in the tegumental syncytium. In the tegumental cell bodies, there was a reduction in the number of secretory bodies, disruption of the Golgi complexes and swelling of the mitochondria. Severe flooding of the internal tissues was observed with TCBZSO(2) and, to a lesser extent, with TCBZSO and TCBZ. The results demonstrate that both TCBZ and TCBZSO(2) are capable of disrupting the fluke in vitro and are not the inactive compounds they were assumed to be previously. They may well contribute to drug action in vivo as well, indicating that drug action is due to the additive effects of several metabolites, rather than being due to a single active metabolite, namely, TCBZSO.

Surface and internal tegumental changes in juvenile Fasciola hepatica following treatment in vivo with the experimental fasciolicide, compound alpha

Eight indoor-reared, crossbred sheep with no pre-exposure to Fasciola hepatica were infected, by oral gavage, with 200 metacercarial cysts of the triclabendazole-susceptible, Cullompton isolate of F. hepatica. Anthelmintic dosing occurred at 4 weeks post-infection using 15mg/kg compound alpha. Two treated sheep per time period were euthanized at 24h, 48h and 72h post-treatment with compound alpha. The two sheep from the control group were euthanized alongside the 24h alpha-treated sheep. Juvenile flukes were recovered from each of the sheeps' liver and processed for examination by electron microscopy. The surface morphology of the flukes' tegument was assessed using scanning electron microscopy (SEM). The ultrastructure of the tegumental syncytium and underlying tegumental cells and connections and somatic musculature were investigated using transmission electron microscopy (TEM). Both the SEM and TEM results revealed a level of disruption that increased with time, culminating at 72h with extensive tegumental loss and substantial degeneration of the cell bodies. The effects of compound alpha on the surface morphology were not particularly apparent until 48h post-treatment, when disruption included swelling and blebbing of the tegument. At 72h post-treatment, SEM revealed loss of the entire syncytial layer over large areas of the flukes. In the areas where the syncytium was lost and the basal lamina exposed, lesions of varying sizes had developed, revealing underlying tissues. Though minor forms of disruption to the ultrastructure of the syncytium were observed using TEM 24h post-treatment, it was at 48h post-treatment that substantial stress responses occurred. They included the presence of autophagic vacuoles and 'open' bodies at the apex of the syncytium and swelling of the basal infolds. The mitochondria within the syncytium and tegumental cells became progressively more disrupted over the three time periods and, by 72h post-treatment, they were frequently distorted and swollen in appearance, and contained severely swollen cristae. By 72h, the number of secretory bodies, particularly T1 bodies, had become significantly depleted in their respective cell bodies, cytoplasmic processes and in the tegumental syncytium. Both the circular and longitudinal muscle bundles were severely disrupted 72h post-treatment. They frequently contained a reduced number of muscle fibres and, in more severe instances, there was an absence of fibres altogether.

Ultrastructural changes induced in the tegument and gut of Fasciola hepatica following in vivo and in vitro drug treatment with nitroxynil (Trodax)

Male Sprague-Dawley rats were dosed orally with nitroxynil at a concentration of 40 mg/kg, and adult Fasciola hepatica were recovered after 24, 48 and 72 h. Fine structural changes to the tegument and gut were monitored by transmission electron microscopy. Flukes were also incubated for 24 h in vitro in nitroxynil at a concentration of 100 microg/ml. Following treatment in vivo, there was an accumulation and accelerated release of secretory bodies at the apex of the tegumental syncytium. Some swelling of the mucopolysaccharide masses surrounding the basal infolds was evident after 48 and 72 h. There was an initial accumulation of T1 secretory bodies at the base of the syncytium, but this decreased at 72 h, coinciding with a decline in their production in the tegumental cells. The mitochondria were consistently swollen in the tegumental cells. At 72 h, large vacuolations were observed between the muscle layers and there was flooding around the underlying tissues. Some tegumental cells were seen to be degenerating and beginning to disintegrate. After 24 h treatment in vitro, the basal infolds were swollen and the crystalline structure of the spines was disrupted. Flooding of the internal tissues was evident and, in the tegumental cells, Golgi complexes and secretory bodies were absent. The mitochondria in the tegumental cells were swollen. In the gastrodermal cells, changes were evident at the earliest time period in vivo. The lamellae were disrupted, few secretory bodies were present, the mitochondria and cisternae of granular endoplasmic reticulum (ger) were swollen and there was an increased number of secretory bodies. These changes became progressively more severe with time. Similar changes were evident following treatment in vitro; vesiculation of the ger was also seen. The results indicate that oral uptake is the predominant route of entry of nitroxynil into the fluke.