Fasciola hepatica: histological changes in the reproductive structures of triclabendazole (TCBZ)-sensitive and TCBZ-resistant flukes after treatment in vivo with TCBZ and the related benzimidazole derivative, Compound Alpha

Twenty-four shed-reared lambs were each infected orally with 250 metacercariae of Fasciola hepatica, using either the triclabendazole (TCBZ)-sensitive Cullompton isolate or the TCBZ-resistant Sligo isolate. Twelve weeks after infection the lambs were treated with TCBZ (10mg/kg) or with the experimental fasciolicide, Compound Alpha (Cpd alpha), a benzimidazole derivative of TCBZ (15mg/kg). The lambs were euthanised 48, 72 and 96h after TCBZ treatment, or 24, 48 and 72h after Cpd alpha treatment, and flukes were collected from the liver and/or gall bladder of each animal. Untreated animals harbouring 12-week infections were euthanized 24h after administration of anthelmintic to the treatment groups, and the untreated flukes provided control material. A semi-quantitative assessment of the degree of histological change induced by the two drugs after different times of exposure was achieved by scoring the intensity of three well-defined lesions that developed in the testes and uteri of a representative sample of flukes from each lamb. In general, it was found that in those tissues where active meiosis and/or mitosis occurred (testis, ovary, and vitelline follicles), there was progressive loss of cell content due to apparent failure of cell division to keep pace with expulsion of the mature or effete products. Further, actively dividing cell types tended to become individualised, rounded and condensed, characteristic of apoptotic cell death. Protein synthetic activity was apparently inhibited in the Mehlis' secretory cells. In the uterus, where successful formation of shelled eggs represents the culmination of a complex sequence of cytokinetic, cytological and synthetic activity involving the vitelline follicles, the ovary and the Mehlis' gland, histological evidence indicating failure of ovigenesis was evident from 24h post-treatment onwards. The development of these lesions may be related to the known anti-tubulin activity of the benzimidazole class of anthelmintics, to the induction of apoptosis in cells where mitosis or meiosis has aborted due to failure of spindle formation, and to drug-induced inhibition of protein synthesis. The semi-quantitative findings indicated that Cpd alpha is slightly less efficacious than TCBZ itself in causing histological damage to the reproductive structures of TCBZ-sensitive flukes, and that, like TCBZ, it caused no histological damage in flukes of the TCBZ-resistant isolate. This study illustrates the potential utility of histological techniques for conveniently screening representative samples of flukes in field trials designed to validate instances of drug resistance or to test the efficacy of new products against known drug-resistant and drug-susceptible fluke isolates. It also provides reference criteria for drug-induced histopathological changes in fluke reproductive structures which may aid interpretation of TEM findings.

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.

Resistance-induced changes in triclabendazole transport in Fasciola hepatica: ivermectin reversal effect

Triclabendazole (TCBZ) and albendazole (ABZ) are flukicidal benzimidazole compounds extensively used in veterinary medicine. Although TCBZ has excellent activity against mature and immature stages of the liver fluke, Fasciola hepatica, ABZ action is restricted to flukes older than 12 wk. The intensive use of TCBZ has resulted in the development of resistance. To gain insight into the mechanisms of resistance to TCBZ, the ex vivo diffusion of TCBZ, TCBZ sulfoxide (TCBZSO, the active metabolite of TCBZ), and ABZ into TCBZ-susceptible and -resistant adult flukes was compared. TCBZ-susceptible (Cullompton) and -resistant (Sligo) flukes were incubated in Krebs-Ringer Tris buffer with either TCBZ, TCBZSO, or ABZ (5 nmol/ ml) for 90 min. Drug/metabolite concentrations were quantified by high-performance liquid chromatography. All the assayed molecules penetrated through the tegument of both susceptible and resistant flukes. However, significantly lower concentrations of TCBZ and TCBZSO were recovered within the TCBZ-resistant flukes. In contrast, ABZ entrance into the susceptible and resistant flukes was equivalent. The influx/efflux balance for TCBZ, TCBZSO, and ABZ in susceptible and resistant flukes in the presence or absence of a substrate (ivermectin) of the drug transporter P-glycoprotein was assessed. The ivermectin-induced modulation of P-glycoprotein activity decreased TCBZ efflux from the resistant flukes. Higher concentrations of TCBZ and TCBZSO were recovered from the resistant liver flukes in the presence of ivermectin. Thus, an altered influx/efflux mechanism may account for the development of resistance to TCBZ in F. hepatica.

Understanding triclabendazole resistance

Triclabendazole (TCBZ) has been the drug of choice to treat liver fluke infections in livestock for >20 years, due to its high activity against both adult and juvenile flukes. More recently, it has been used successfully to treat human cases of fascioliasis. Resistance to TCBZ first appeared in the field in Australia in the mid-1990s. Since then, resistance has been reported from a number of countries throughout Europe: Ireland, Scotland, Wales, Spain and The Netherlands. The heavy reliance on a single drug puts treatment strategies for fascioliasis at risk. Should resistance develop further, the prospect is an alarming one. This review will present an overview of progress in understanding the mechanism of resistance to TCBZ, examining possible changes in the target molecule, in drug influx/efflux mechanisms and in the metabolism of TCBZ by the fluke. The review will also consider ways to deal with resistance, covering drug-oriented options such as: the use of alternative drugs, drug combinations and the search for new compounds.

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

A study has been carried out to investigate the ultrastructural effects of triclabendazole (TCBZ) at half-normal concentration, clorsulon at half-normal concentration, and a combination of these two drugs against mature Fasciola hepatica. The Cullompton TCBZ-susceptible isolate was used for these experiments. Flukes were incubated for 24 h in vitro in TCBZ sulphoxide (7.5 microg/ml), clorsulon (5 microg/ml), or a combination of the two drugs. For the in vivo experiment, rats were dosed with TCBZ (5 mg/kg body weight), clorsulon (5 mg/kg body weight), or a combination of the two drugs, and flukes recovered after 48 h. Fine structural changes within the tegumental syncytium and tegumental cells were assessed by transmission electron microscopy. Treatment with the combination of drugs produced greater disruption to the flukes than the individual drugs at half-normal concentrations, both in vivo and in vitro; also than TCBZ.SO at normal concentration in vitro. The changes observed aid in the understanding of the gross changes to the tegumental surface described previously (Meaney M, Allister J, McKinstry B, McLaughlin K, Brennan GP, Forbes AB, Fairweather I. Parasitol Res 99:609-621, 2006). The results indicate that there are additive effects between TCBZ and clorsulon and suggest that the use of drug combinations would be of value in the treatment of TCBZ-resistant fluke.

Immature triclabendazole-resistant Fasciola hepatica: tegumental responses to in vitro treatment with the sulphoxide metabolite of the experimental fasciolicide compound alpha

Juvenile triclabendazole-resistant liver flukes, Fasciola hepatica, were incubated in vitro with 10 microg/ml of the sulphoxide metabolite of the experimental fasciolicide, compound alpha [5-chloro-2-methylthio-6-(1-naphthyloxy)-1H-benzimidazole], for 6 and 18 h. Following treatment, the specimens were examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and tubulin immunocytochemistry. The SEM results revealed a posterior-directed disruption comprised predominantly of swelling and blebbing of the tegument; these changes were more severe and extensive after the longer 18-h incubation. Along with swelling of the tegument and blebbing, the TEM results also revealed swelling of the mitochondria and basal infolds. A decrease in the number of both T1 and T2 secretory bodies was observed in the syncytium and cytoplasmic connections after the 18-h treatment. The circular muscle bundles were also disrupted, in that the organisation of the muscle fibres was irregular and the total number of muscle fibres was reduced. The immunocytochemical studies revealed no significant disruption to the distribution of tubulin immunoreactivity within the tegumental syncytium, the cytoplasmic connections or the associated tegumental cells. The results indicate that alpha.SO is capable of disrupting the tegument of 4-week-old triclabendazole-resistant liver flukes, though the morphological changes were not associated with any significant differences in tubulin immunostaining.

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

A study has been carried out to investigate the morphological effects of half-strength triclabendazole (TCBZ), half-strength clorsulon, and a combination of these two drugs against mature Fasciola hepatica. The Cullompton TCBZ-susceptible isolate was used for these experiments. Flukes were incubated for 24 h in vitro in TCBZ sulphoxide (7.5 microg/ml), clorsulon (5 microg/ml), or a combination of the two drugs. For the in vivo experiment, rats were dosed with TCBZ (6.25 mg/kg body weight), clorsulon (5 mg/kg body weight), or a combination of the two drugs and flukes recovered after 48 h. Surface changes to the flukes were assessed by scanning electron microscopy. Treatment with the combination of drugs produced greater disruption to the flukes than the individual drugs at half-strength, both in vivo and in vitro. Disruption to the tegument of the flukes induced by the individual drugs at half-strength was relatively minor and less than that caused by the drugs at full-strength. The results suggest that there are additive effects between TCBZ and clorsulon, which may be indicative of synergy: the use of drug combinations would be of value in the treatment of triclabendazole-resistant fluke.

Adult triclabendazole-resistant Fasciola hepatica: surface and subsurface tegumental responses to in vitro treatment with the sulphoxide metabolite of the experimental fasciolicide compound alpha

Mature Fasciola hepatica of the triclabendazole-resistant Sligo isolate were incubated in vitro with 10 microg/ml of the sulphoxide metabolite of compound alpha [5-chloro-2-methylthio-6-(1-naphthyloxy)-H-benzimidazole]; the metabolite will be referred to as alpha.SO. Changes resulting from drug treatment were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and tubulin immunocytochemistry (ICC). SEM revealed that disruption to the tegumental surface mainly took the form of swelling and blebbing. Extensive spine loss occurred on the ventral surface of the oral cone, and sloughing of the tegument was observed along the lateral margins of the fluke. Examination of sections from the anterior mid-body region at the TEM level revealed that treatment with alpha.SO led to swelling of the basal infolds and mitochondria within the tegumental syncytium; also, accumulations of secretory bodies beneath the apical plasma membrane. The tegumental cell bodies contained swollen mitochondria and cisternae of granular endoplasmic reticulum, but few Golgi complexes were observed. An increase in T2 secretory bodies was observed, whilst in the T1 tegumental cells, the T1 secretory bodies had decreased in number. Immunocytochemical (ICC) studies showed that incubation with alpha.SO, ABZ.SO and TCBZ.SO did not cause significant changes to the distribution of tubulin within the tegumental syncytium of the Sligo isolate. In contrast, alpha.SO, ABZ.SO and TCBZ.SO caused severe disruption to tubulin organization within the syncytial layer of the TCBZ-susceptible Cullompton isolate. The EM results confirm that compound alpha is a fasciolicide capable of disrupting the tegument of mature TCBZ-resistant F. hepatica; however, this was not accompanied by any change in tubulin immunoreactivity.

Stage-specific differences in fecundity over the life-cycle of two characterized isolates of the liver fluke, Fasciola hepatica

The variability inherent in different isolates of Fasciola hepatica has been evident from reports in the literature but to date there has been no systematic examination of the relationship between these differences and the fecundity of the parasite. In this study we have attempted to remedy this situation by comparing the relative efficiencies with which 2 well-characterized isolates of the liver fluke (Oberon and Fairhurst) progress through both their definitive and intermediate hosts. We did not observe a reduction in fitness in the Oberon isolate which has been reported to be triclabendazole-resistant, compared to the triclabendazole-susceptible Fairhurst isolate, but considerable inter- and intra-isolate variability at different life-cycle stages was recorded. Thus the Oberon isolate gave 4-fold the number of cercariae when 100 snails were each challenged with a single miracidium and was more successful in establishing productive infections in rats. Fairhurst metacercariae excysted at a higher rate than those from the Oberon isolate and Fairhurst flukes produced 4-fold more eggs. The extent of the intra- and inter-isolate variability revealed in this work will provide a basis for the development of models of population dynamics aimed at predicting the response of the liver fluke to changing environmental conditions such as the use of anthelmintics or climatic change.

Altered drug influx/efflux and enhanced metabolic activity in triclabendazole-resistant liver flukes

Triclabendazole (TCBZ) is a halogenated benzimidazole compound that possesses high activity against immature and adult stages of the liver fluke, Fasciola hepatica. The intensive use of TCBZ in endemic areas of fascioliasis has resulted in the development of liver flukes resistant to this compound. TCBZ sulphoxide (TCBZSO) and TCBZ sulphone (TCBZSO2) are the main molecules recovered in the bloodstream of TCBZ-treated animals. In order to gain some insight into the possible mechanisms of resistance to TCBZ, the goals of the work described here were: to compare the ex vivo transtegumental diffusion of TCBZ parent drug and its sulpho-metabolites (TCBZSO and TCBZSO2) into TCBZ-susceptible and -resistant liver flukes; and to assess the comparative pattern of TCBZ biotransformation by TCBZ-susceptible and -resistant F. hepatica. For the tegumental diffusion studies, TCBZ-susceptible (Cullompton) and -resistant (Sligo) adult flukes collected from untreated infected sheep were incubated (15-180 min) in KRT buffer containing either TCBZ, TCBZSO or TCBZSO2 (5 nmol.ml-1). For the metabolism studies, microsomal fractions obtained from TCBZ-susceptible and -resistant flukes were incubated for 60 min with TCBZ (40 microM), and the amount of the formed metabolic product (TCBZSO) was measured. Drug/metabolite concentrations were quantified by HPLC. All the assayed TCBZ-related molecules penetrated through the tegument of both TCBZ-susceptible and -resistant flukes. However, significantly lower (approximately 50%) concentrations of TCBZ and TCBZSO were recovered within the TCBZ-resistant flukes compared to the TCBZ-susceptible ones over the 180 min incubation period. The rate of TCBZ sulphoxidative metabolism into TCBZSO was significantly higher (39%) in TCBZ-resistant flukes. The flavin-monooxigenase (FMO) enzyme system appears to be the main metabolic pathway involved in the formation of TCBZSO in both TCBZ-susceptible and -resistant flukes. The altered drug influx/efflux and enhanced metabolic capacity identified in TCBZ-resistant liver flukes may account for the development of resistance to TCBZ.

Triclabendazole: new skills to unravel an old(ish) enigma

Triclabendazole was introduced in the early 1980s for the treatment of Fasciola hepatica infections in livestock. Due to its high activity against immature flukes, it has become established as the principal anti-fluke drug on the market. More recently, triclabendazole has been used to treat human cases of fascioliasis and is now the drug of choice for this infection, too. Resistance to triclabendazole was first reported in 1995 in a field population in Australia and, since that time, resistant populations have been identified in several countries in Europe. Parallel to the spread of resistance has been a sharp increase in the prevalence of fascioliasis, which has been attributed largely to climate changes. Consequently, farmers are faced with an alarming scenario, as none of the other fasciolicides on the market possess such high activity against the damaging immature stages of fluke. The main aim of this review is to assess current understanding of the mechanism of action of triclabendazole against the fluke and the mechanism by which the fluke has become resistant to it. The use of triclabendazole against animal and human infections is summarized and suggestions are given on ways to deal with resistance. Gaps in the knowledge of various aspects of its use are highlighted and this may serve to open up future research areas.

Nematode neuropeptides: Localization, isolation and functions

Historically, peptidergic substances (in the form of neurosecretions) were linked to moulting in nematodes. More recently, there has been a renewal of interest in nematode neurobiology, initially triggered by studies demonstrating the localization of peptide immunoreactivities to the nervous system. Here, David Brownlee, Ian Fairweather, Lindy Holden-Dye and Robert Walker will review progress on the isolation of nematode neuropeptides and efforts to unravel their physiological actions and inactivation mechanisms. Future avenues for research are suggested and the potential exploitation of peptidergic pathways in future therapeutic strategies highlighted.

Ultrastructural observations on oral ingestion and trans-tegumental uptake of clorsulon by the liver fluke, Fasciola hepatica

Three experiments have been carried out in vitro to determine the effect of oral and trans-tegumental uptake of clorsulon on the fine structure of the tegument and gut of Fasciola hepatica. Changes were assessed by transmission electron microscopy. In the first experiment, the flukes were ligatured to prevent the oral ingestion of drug and treated for 24 h in clorsulon (10 microg/ml). Limited swelling of the basal infolds was observed in the tegumental syncytium. Swollen mitochondria were present in the syncytium, the underlying tegumental cells and in the gastrodermal cells. Swelling and vesiculation of the cisternae of the granular endoplasmic reticulum (ger) was evident in the gastrodermal cells, together with a reduction in secretory activity. In the second experiment, flukes were fed for 24 h on red blood cells isolated from rats dosed with clorsulon at 12.5 mg/kg body weight; this experiment was designed to prevent the exposure of the tegumental surface to the drug. There was severe swelling of the basal infolds in the tegumental syncytium and swelling of mitochondria in the syncytium, tegumental cells and gastrodermal cells. In the tegumental cells there was a decrease in the number of Golgi complexes as well. A number of changes were evident in the gastrodermal cells: swelling of the ger cisternae, an increase in the number of autophagic vacuoles, a reduction in the number of secretory bodies and disruption of the lamellae projecting from the surface of the cells. In the third experiment, flukes were incubated for 24 h in clorsulon (10 microg/ml), with both absorptive surfaces being available for drug uptake. There was severe swelling of the basal infolds in the tegumental syncytium and large autophagic vacuoles were present. Swollen mitochondria were a feature of the tegument, tegumental cells and gastrodermal cells, as were swollen cisternae of ger in the tegumental and gastrodermal cells. Fewer Golgi complexes were observed in the tegumental cells and in the gastrodermal cells there were fewer secretory bodies and an increased number of autophagic vacuoles. Overall, the gastrodermal cells were more severely affected than the tegument. Greater disruption of the tegument occurred when the oral route of uptake was available. The results support those of previous studies which point to oral uptake of clorsulon being the major route of entry into the fluke.

A scanning electron microscope study on the route of entry of clorsulon into the liver fluke, Fasciola hepatica

Three experiments were carried out in vitro to determine the roles of the tegument and gut of Fasciola hepatica in the uptake of the flukicidal drug, clorsulon. Changes to the two surfaces were assessed by scanning electron microscopy. In the first experiment, the flukes were ligatured to prevent the oral ingestion of drug and treated for 24 h in clorsulon (10 microg/ml). The gastrodermal surface remained normal and few changes to the tegumental surface were observed. In the second experiment, flukes were fed for 24 h on red blood cells isolated from rats dosed with clorsulon at 12.5 mg/kg body weight; this experiment was designed to prevent the exposure of the tegumental surface to the drug. The gastrodermal surface was severely disrupted and the gut lamellae were disorganised and necrotic. Swelling of the tegument and blebbing on the tegumental surface were evident, but the changes were not severe. More severe swelling of the tegument was observed in the third experiment, in which flukes were incubated for 24 h in clorsulon (10 microg/ml), with both absorptive surfaces being available for drug uptake. The gastrodermal surface was badly disrupted and the gut lamellae were disorganised and necrotic. Taking the results of the three experiments together, the gastrodermal surface was more affected than the tegument and the greatest disruption to the two surfaces was seen when both routes of entry were available to the fluke. The data support a previous study which indicated that entry of clorsulon into the fluke in vivo is principally by the oral ingestion of drug bound to the red blood cells.

Electrophysiological and pharmacological characterization of K+-currents in muscle fibres isolated from the ventral sucker ofFasciola hepatica

Fibres isolated from the ventral sucker ofFasciola hepaticawere identified as muscle on the basis of their contractility, and their actin and myosin staining. They were voltage-clamped at a holding potential of −40 mV and depolarization-activated outward currents were characterized both electrophysiologically and pharmacologically. Activation was well fitted by a Boltzmann equation with a half-maximal potential of +9 mV and a slope factor of −14·3 mV, and the kinetics of activation and deactivation were voltage-sensitive. Tail current analysis showed that the reversal potential was shifted by +46±3 mV when EKwas increased by 52 mV, confirming that this was a K+-current with electrophysiological characteristics similar to delayed rectifier and Ca2+-activated K+-currents in other tissues. The peak current at +60 mV was inhibited by 76±6% by tetrapentylammonium chloride (1 mM) and by 84±7% by Ba2+(3 mM), but was completely resistant to block by tetraethylammonium (30 mM), 3,4-diaminopyridine (100 μM) and 4-aminopyridine (10 mM). Penitrem A, a blocker of high-conductance Ca2+-activated K+-channels reduced the current at +60 mV by 23±5%. When the effects of Ca2+-channel blocking agents were tested, the peak outward current at +60 mV was reduced by 71±7% by verapamil (30 μM) and by 59±4% by nimodipine (30 μM). Superfusion with BAPTA-AM (50 μM), which is hydrolysed intracellularly to release the Ca2+-buffer BAPTA, also decreased the current by 44±16%. We conclude that voltage-and Ca2+-sensitive K+-channels are expressed in this tissue, but that their pharmacology differs considerably from equivalent channels in other phyla.

Response of two isolates of Fasciola hepatica to treatment with triclabendazole in vivo and in vitro

In this study, the susceptibility of two isolates of Fasciola hepatica--the Fairhurst and Oberon isolates--to treatment with triclabendazole was investigated, both in vivo and in vitro. The Fairhurst isolate originated in England, but has since been maintained in Australia; the Oberon isolate originated in Australia. Triclabendazole had a very high efficacy against the Fairhurst isolate. In sheep (dose: 10 mg/kg), the efficacy ranged from 78.4% at 2 weeks post-infection to 98.5% at 6 weeks post-infection. In cattle (dose: 12 mg/kg) efficacy was 89% at 2 weeks post-infection and 100% at 12 weeks. In contrast, against the Oberon isolate, triclabendazole had 0% efficacy against 2-week-old flukes in sheep (dose: 10 mg/kg) and 5% against 4-week-old flukes. Surface changes to flukes of the two isolates were assessed by scanning electron microscopy following treatment in vitro for 24 h in triclabendazole sulphoxide (15 and 50 microg/ml). Disruption took the form of blebbing, swelling and furrowing of the tegument and was greater in the Fairhurst than the Oberon isolate. Surface changes generally were more severe in the anterior than posterior region of the fluke and the dorsal surface was also consistently more severely affected than the ventral surface. Disruption was more severe at the higher drug concentration for both isolates. The morphological data is consistent with the efficacy data, which indicates that the Fairhurst isolate of F. hepatica is susceptible to triclabendazole treatment, whilst the Oberon isolate is refractory.

The occurrence and significance of triploidy in the liver fluke, Fasciola hepatica

Karyotyping of Fasciola hepatica samples from Britain and Ireland has identified a triploid isolate which is effectively aspermic, rendering it necessarily asexually reproducing. Considering the extensive presence of asexually reproducing diploid and triploid Fasciola in Asia it is suggested that facultative gynogenesis is widespread in this parasite. This has important implications for the population genetics and evolution of Fasciola, especially in relation to the development and spread of drug resistance, and must be considered in the mathematical modelling of this process.

Observations on the musculature and isolated muscle fibres of the liver fluke, Fasciola hepatica

The liver fluke, Fasciola hepatica relies on a well-developed muscular system, not only for attachment, but for many aspects of its biology. Despite this, little is known about the system beyond the gross organization of the main somatic muscle layers. In the present study, a range of techniques have been applied to F. hepatica in order to understand more about various aspects of muscle organization, biochemistry (in terms of muscle proteins) and identity of isolated muscle fibres. Scanning electron microscopy has provided a direct visualization in situ of the somatic muscle layers and the organization of the muscle fibres within the ventral sucker. The muscle bundles contributing to the main somatic muscle layers are made up of up to 10 individual muscle fibres. Phalloidin staining for actin, in conjunction with confocal microscopy, confirmed the presence of 2 main somatic muscle layers (outer circular, inner longitudinal), beneath which lies a third layer of oblique muscle fibres. The use of propidium iodide in combination with phalloidin staining for actin demonstrated that the cell bodies associated with the 2 main somatic muscle layers are situated beneath the longitudinal muscle layer and are connected to their respective muscle fibres by short cytoplasmic processes. Myosin immunoreactivity was demonstrated in the somatic muscle layers and in the muscle layers surrounding various organ systems within the fluke. Double labelling for actin and myosin confirmed the co-localization of the 2 muscle proteins in the muscle fibres of the ventral sucker. Muscle fibres from the somatic muscle layers and the ventral sucker have been isolated and images obtained with phase-contrast microscopy and scanning electron microscopy. The muscle fibres contain actin and myosin, but lack a nucleus, the connection with the cell body having been broken during the isolation procedure.

Controlled dermal cell damage as human in vivo model for localised pain and inflammation

OBJECTIVE AND DESIGN

An unspecific human in vivo model of dermal pain and inflammation was developed by means of limited, localised and controlled cell damage.

SUBJECTS

Twelve participants were recruited.

TREATMENT

Dermal microdialysis was used to deliver randomised and single blinded aqueous sodium dodecyl sulphate (SDS) at concentrations of 0.01%, 0.1% and 0.5% w/v to the volar forearm.

METHODS

Nociceptive responses were recorded on a numerical scale, vasodilatation was assessed by laser Doppler scanning and sampled tissue fluid was analysed for PGE2 by ELISA.

RESULTS

Saline control and 0.01% SDS did not differ in their ability to cause vasodilatation, flare reaction or pain. In contrast, SDS (0.1 and 0.5%) evoked a significant increase of blood flow (p<0.005), a widespread reddening (p<0.01), and stinging-burning pain (p<0.005). PGE2 concentration in the dialysate did not change during 0.01% SDS perfusion (p>0.9), but increased significantly following the stimulation with 0.1% and 0.5% SDS (20 to 30-fold). No significant differences of released PGE2 levels were determined between 0.1% and 0.5% SDS stimulation (p>0.05).

CONCLUSIONS

We demonstrated that localised intradermal administration of SDS induces a limited pain and inflammatory response in humans. Excitation of nociceptors was accompanied by a massive PGE2 release. Employing this experimental model, the relative contribution of endogenous mediators to induce, maintain or facilitate pain and vasodilatation can be investigated.