Measurement of ivermectin concentrations in target worms and host gastrointestinal tissues: Influence of the route of administration on the activity against resistant Haemonchus contortus in lambs

Therapeutic advantages of the combined use of closantel and moxidectin in lambs parasitized with resistant gastrointestinal nematodes

The serious widespread development of nematode resistance has motivated the use of combined anthelmintic formulations. However, the advantages/disadvantages of the combined use of anthelmintics require further scientific characterization. The goals of the current trial were a) to characterize the pharmacokinetics of closantel (CLO) and moxidectin (MXD) administered both subcutaneously (sc) and orally either separately or co-administered (CLO + MXD) to lambs; b) to compare the nematodicidal activity of both molecules given individually or co-administered to lambs infected with resistant nematodes. Seventy (70) Corriedale lambs naturally infected with multiple resistant gastrointestinal nematodes were involved in the pharmacokinetic and efficacy trials. The animals were allocated into six groups (n = 10) and treated with either CLO, MXD, or with the CLO + MXD combined formulation by both the oral and sc routes. Additionally, an untreated control group (n = 10) was included for the efficacy trial. The efficacy was estimated by the faecal egg count reduction test (FECRT). Higher systemic exposure of both CLO and MXD was observed after the sc compared to the oral administration in lambs. The combined administration of CLO + MXD did not markedly alter their disposition kinetics. At 13 days post-treatment, the administration of both molecules as a single active principle reached efficacy levels ranging between 80% (MXDoral), 84% (CLOoral), 85% (CLOsc), and 92% (MXDsc). The combined oral and sc treatments reached 99% efficacy. No adverse effects were observed after the combined treatment of CLO + MXD, and their co-administration did not show any adverse pharmacokinetic interaction. The combined effect of CLO + MXD successfully restored the maximum efficacy levels, which were not reached by the individual active ingredients.

Phytochemicals in Gastrointestinal Nematode Control: Pharmacokinetic-Pharmacodynamic Evaluation of the Ivermectin plus Carvone Combination

A wide variety of plant-derived phytochemicals with anthelmintic effects have been described. Most of them have shown activity against parasites in vitro but have not been extensively explored in vivo. The aim of the current work was to study the pharmacokinetic-pharmacodynamic relationship of the combined administration of carvone (R-CNE) and ivermectin (IVM) to lambs. Three trials were conducted to evaluate the pharmacological interaction between R-CNE and IVM in lambs infected with resistant nematodes. Drug concentrations were measured in plasma, target tissues, and H. contortus by HPLC with fluorescent (IVM) and ultraviolet (R-CNE) detection. The effect of both compounds on parasites was estimated by the fecal egg count reduction. Coadministration with R-CNE significantly increased the plasma bioavailability of IVM. R-CNE showed a moderate anthelmintic effect, which was greater on the susceptible isolate of H. contortus. After the combination of R-CNE and IVM as an oral emulsion, both compounds were quantified in H. contortus recovered from infected lambs. However, R-CNE concentrations were much lower than those reported to achieve anthelmintic effects in the in vitro assays. Optimization of the pharmaceutical formulation, dose rate, and administration schedule is needed to take advantage of the intrinsic anthelmintic activity of phytochemicals.

Ivermectin systemic availability in adult volunteers treated with different oral pharmaceutical formulations

Ivermectin (IVM) is currently approved as an antiparasitic agent for human use in the treatment of onchocerciasis, lymphatic filariasis, strongyloidiasis, scabies, and pediculosis. Recent findings indicate that IVM may reach other pharmacological targets, which accounts for its proven anti-inflammatory/immunomodulatory, cytostatic, and antiviral effects. However, little is known about the assessment of alternative drug formulations for human use.

OBJECTIVE

To compare the systemic availability and disposition kinetics of IVM orally administered as different pharmaceutical formulations (tablet, solution, or capsule) to healthy adults.

EXPERIMENTAL DESIGN/MAIN FINDINGS

Volunteers were randomly assigned to 1 of 3 experimental groups and orally treated with IVM as either, a tablet, solution, or capsules at 0.4 mg/kg in a three-phase crossover design. Blood samples were taken as dried blood spots (DBS) between 2 and 48 h post-treatment and IVM was analyzed by HPLC with fluorescence detection. IVM Cmax value was higher (P < 0.05) after the administration of the oral solution compared to treatments with both solid preparations. The oral solution resulted in a significantly higher IVM systemic exposure (AUC: 1653 ng h/mL) compared to the tablet (1056 ng h/mL) and capsule (996 ng h/mL) formulations. The simulation of a 5-day repeated administration for each formulation did not show a significant systemic accumulation.

CONCLUSION

Beneficial effects against systemically located parasitic infections as well as in any other potential therapeutic field of IVM application would be expected from its use in the form of oral solution. This pharmacokinetic-based therapeutic advantage without the risk of excessive accumulation needs to be corroborated in clinical trials specifically designed for each purpose.

Confounding factors affecting faecal egg count reduction as a measure of anthelmintic efficacy

Increasing anthelmintic resistance (AR) in livestock has stimulated growing efforts to monitor anthelmintic effectiveness (AE) on livestock farms. On-farm assessment of AE relies on measuring the reduction in faecal egg count (FEC) following treatment; and if conducted rigorously, qualifies as a formal FEC reduction test (FECRT) for AR. Substantial research effort has been devoted to designing robust protocols for the FECRT and its statistical interpretation; however, a wide range of factors other than AR can affect FEC reduction on farms. These are not always possible to control, and can affect the outcome and repeatability of AE measurements and confound the on-farm classification of AR using FECRT. This review considers confounders of FEC reduction, focusing on gastrointestinal nematodes of ruminants, including host and parasite physiology and demography; pharmacokinetic variation between drugs, parasites and hosts; and technical performance. Drug formulation and delivery, host condition and diet, and seasonal variation in parasite species composition, can all affect AE and hence observed FEC reduction. Causes of variation in FEC reduction should be attenuated, but this is not always possible. Regular monitoring of AE can indicate a need to improve anthelmintic administration practices, and detect AR early in its progression. Careful interpretation of FEC reduction, however, taking into account possible confounders, is essential before attributing reduced FEC reduction to AR. Understanding of confounders of FEC reduction will complement advances in FECRT design and interpretation to provide measures of anthelmintic efficacy that are both rigorous and accessible.

Most common inappropriate drug usage factors in anthelmintic treatment on sheep farms in Latvia

Background and Aim:

There is little understanding about antiparasitic drug prescription trends and implementation to reduce possible drug overuse or misuse worldwide. This study aimed to review sheep parasite control strategies and antiparasitic drug use habits in Latvia. To the best of the author’s knowledge, this is the first study in the world that describes how antiparasitic drugs are used and what are the most common drug usage errors in a sheep farm.

Materials and Methods:

A semi-structured questionnaire was designed to collect relevant information from face-to-face interviews to assess 22 sheep farmers’ knowledge and management procedures in farms. We collected information about animal feeding, herding, parasite diagnostics, and antiparasitic drug usage. The questionnaire summary included information on pasture use, parasite control management, and anthelmintic drug choice/use.

Results:

Only 36% of farms regularly managed parasite control by analyzing fecal samples for parasites, but prophylactic dewormingwas employed in all farms. Ivermectin, albendazole, levamisole, and monepantel were used on the farms and most of the farms were multidrug users; 77.3% of the farms used albendazole and 72.7% used ivermectin.

Conclusion:

The results indicated a lack of parasitological examination and parasite control of the flock, mostly empiric drug selection, incorrect dosing, inaccurate drug administration, drug storage, and use errors. A proactive approach to herd health planning, regular parasitic control, and prophylactic measures may benefit farmers and veterinarians.

Case report: Assessment of a quarantine treatment for sheep on the Caribbean Island of St. Kitts

Quarantine treatments can be used to prevent bringing resistant parasites onto a farm. However, in the face of growing anthelmintic resistance and variations in farm practices, assessing efficacy of such treatments before full implementation is needed. In this case report, a combination treatment of ivermectin (injectable formulation) and fenbendazole (both approximately double doses) administered orally within minutes of each other was assessed in 19 sheep. The mean pretreatment Strongylid eggs per gram of feces, determined using a McMaster, was 2639. The mean post treatment count (at 14 days) was 2663 with 12 animals having a decrease in egg counts. There was an average increase of 0.9% using pre and post treatment egg count means. This combination treatment was not effective as a quarantine treatment.

Route of administration affects the efficacy of moxidectin against Ostertagiinae nematodes in farmed red deer (Cervus elaphus)

The influence of route of administration on the pharmacokinetics and efficacy of macrocyclic lactone anthelmintics has been a subject of interest due to its potential to influence the development of anthelmintic resistance. For most parasite species studied so far, oral administration results in the highest concentrations of drug in the parasites and the highest efficacy against resistant genotypes. However, a recent study in cattle measured the highest levels of ivermectin in the abomasal Ostertagia ostertagi following subcutaneous injection, but it was not possible to correlate these elevated levels with efficacy. Therefore, the current study was initiated to determine whether injectable delivery might be optimal for attaining high efficacy against this important group of parasites. Three on-farm trials were conducted to measure the efficacy of moxidectin administered by the oral, injectable, and pour-on routes against Ostertagiinae parasites in farmed red deer. Groups of rising 1-year old stags (red or red-wapiti crossbreds) in the 84-104 kg weight range were randomised on liveweight into treatment groups of 6 (1 farm) or 8 (2 farms). Animals were treated to individual liveweight with moxidectin oral (0.2 mg/kg), injectable (0.2 mg/kg), pour-on (0.5 mg/kg) or remained untreated. Twelve days later all animals were euthanised and abomasa recovered for worm count. Adult worms were counted in a 2% aliquot of abomasal washings, and adult and fourth stage larvae in a 10 % aliquot following mucosal incubation in physiological saline. In addition, blood was collected from the same 5 animals in each of the treatment groups on days 0, 1, 2, 3, 5, 7 and 12 after treatment and moxidectin levels in plasma were determined using a mass spectrometer. The number of Ostertagiinae surviving treatment was significantly different for each of the treatment groups with injectable administration being most effective, oral administration being the next most effective and pour-on administration the least effective. This applied to both adult worms and fourth stage larvae. A similar pattern was seen in the levels of moxidectin in plasma with both the peak value and area under the concentration curve being highest following injectable administration and lowest following pour-on treatment. Although undertaken in a different host species, the results support the proposition that injectable administration of macrocyclic lactone anthelmintics is likely to be optimal for efficacy against Ostertagiinae parasites and potentially useful in slowing the emergence of resistance in these parasites.

Nanotechnological Improvement of Veterinary Anthelmintics

Helminths infections are among the most important problems in animal health and husbandry. Moreover, zoonotic helminths endanger rural communities, particularly in developing countries. Helminthiasis are not only important in relation to the harmful effects of parasites; additional issues like anthelmintic resistance spread became more important over time. As new anthelmintic development takes many years and millions of dollars of investment, some strategies are currently focused on the modification of already available drugs, in order to improve their efficacy and overcome their limitations. In this field, nanotechnology has brought a novel approach, showing advantages like the regulation of the drug's delivery and kinetics, reaching of specific targets, and possibilities to avoid the systemic spread and side effects. Taking this into account, the present review aims to introduce some of the current knowledge in anthelmintic improvement based on nanotechnology, and how researchers could benefit from this technology in order to overcome the drugs limitations. Finally, some insights into potential field applications are discussed, based on the most important concerns of current anthelmintic therapy.

Pharyngeal Pumping and Tissue-Specific Transgenic P-Glycoprotein Expression Influence Macrocyclic Lactone Susceptibility in Caenorhabditis elegans

Macrocyclic lactones (MLs) are widely used drugs to treat and prevent parasitic nematode infections. In many nematode species including a major pathogen of foals, Parascaris univalens, resistance against MLs is widespread, but the underlying resistance mechanisms and ML penetration routes into nematodes remain unknown. Here, we examined how the P-glycoprotein efflux pumps, candidate genes for ML resistance, can modulate drug susceptibility and investigated the role of active drug ingestion for ML susceptibility in the model nematode Caenorhabditis elegans. Wildtype or transgenic worms, modified to overexpress P. univalens PGP-9 (Pun-PGP-9) at the intestine or epidermis, were incubated with ivermectin or moxidectin in the presence (bacteria or serotonin) or absence (no specific stimulus) of pharyngeal pumping (PP). Active drug ingestion by PP was identified as an important factor for ivermectin susceptibility, while moxidectin susceptibility was only moderately affected. Intestinal Pun-PGP-9 expression elicited a protective effect against ivermectin and moxidectin only in the presence of PP stimulation. Conversely, epidermal Pun-PGP-9 expression protected against moxidectin regardless of PP and against ivermectin only in the absence of active drug ingestion. Our results demonstrate the role of active drug ingestion by nematodes for susceptibility and provide functional evidence for the contribution of P-glycoproteins to ML resistance in a tissue-specific manner.

Combined moxidectin-levamisole treatment against multidrug-resistant gastrointestinal nematodes: A four-year efficacy monitoring in lambs

Nematicide combinations may be a valid strategy to achieve effective nematode control in the presence of drug resistance. The goal of the current trial was to evaluate the pharmaco-parasitological performance of the moxidectin (MOX) and levamisole (LEV) combination after four years of continuous use in lambs naturally parasitized with multi-resistant gastrointestinal nematodes. At the beginning of the trial, 40 lambs were divided into four groups (n = 10), which were untreated (control) or subcutaneously treated with MOX (0.2 mg/kg), LEV (8 mg/kg) or with the combination MOX + LEV (administered separately at 0.2 and 8 mg/kg, respectively). Blood samples were collected at different times post-treatment and LEV and MOX plasma concentrations were measured by HPLC. The clinical efficacy of the continuous use of MOX + LEV combination was assessed with the controlled efficacy test (CET), performed at the beginning and end of the study, and with the faecal egg count reduction (FECR) test, performed over the four-year study period. No significant adverse pharmacokinetic changes were observed either for MOX or LEV after their co-administration to infected lambs. The CET (first year) showed efficacies of 84.3 % (Haemonchus contortus), 100 % (Teladorsagia circumcincta and Trichostrongylus axei), and 97.4 % (T. colubriformis). After the repetitive use of the combined treatment for four years, those efficacies remained high (100 %) and only decreased to 58 % against T. colubriformis. The evaluation of the FECR over the study period showed fluctuations in the performance of the combined administration. The initial FECR (2014) was 99 % (MOX), 85 % (LEV) and 100 % (MOX + LEV). The co-administration of MOX + LEV during the four-year experimental period resulted in a significantly higher anthelmintic effect (87 %) than that of MOX (42 %) or LEV (69 %) given alone. The combined use of MOX + LEV to control resistant gastrointestinal nematodes appears to be a valid strategy under specific management conditions. A high initial therapeutic response to the combination would be a relevant feature for the success of this tool.

Route of administration influences the concentration of ivermectin reaching nematode parasites in the gastrointestinal tract of cattle

An animal trial was conducted to measure the concentrations of ivermectin occurring in abomasal and small intestinal contents and mucosa, and in the target parasites (Ostertagia ostertagi and Cooperia oncophora) following administration by subcutaneous, oral and pour-on routes. Twenty-five steers were infected with ivermectin-resistant isolates of O. ostertagi and C. oncophora and following patency randomly allocated to 3 treatment groups of 7 and 1 untreated control group of four. On day 0, animals in the treatment groups were administered ivermectin via the oral, injectable or pour-on routes. On days 1, 2, 3, 4, 5, 6 and 8, blood samples were collected from all live animals, one animal from each treatment group was euthanised and the abomasum and small intestine recovered. Control animals were euthanised on each of days 4, 5, 6 and 8. Samples of gastrointestinal tract organs, their contents, mucosa and parasites were collected and assayed for ivermectin concentration using HPLC. The highest plasma concentrations occurred following subcutaneous administration. In the gastrointestinal contents the highest levels occurred following oral administration, although one high value occurred following pour-on administration, which was attributed to self-licking by the treated animal. The lowest GI content levels followed subcutaneous injection. Ivermectin concentrations in the gastrointestinal mucosa were highest following subcutaneous injection. Drug levels in the abomasal parasite O. ostertagi were most closely correlated with levels in the abomasal mucosa whereas levels in the intestinal C. oncophora were most closely correlated with those in the intestinal contents. Thus, the maximun levels of drug reached C. oncophora in the small intestine following oral administration. In contrast, the highest levels of ivermectin in O. ostertagi followed subcutaneous injection. Therefore, route of administration is likely to influence the exposure to ivermectin for different parasite species.

Doramectin efficacy against Psoroptes ovis in sheep: Evaluation of pharmacological strategies

The aims of this study were to evaluate the efficacy of two injectable formulations of doramectin (DRM) against Psoroptes ovis in sheep infested under controlled experimental conditions and to characterize the DRM plasma disposition kinetics in the infested animals. To this end, sheep were experimentally infested with a P. ovis strain from a farm with a history of treatment failure, and then treated either with DRM 1% (traditional preparation) on days 0 and 7 or with DRM 3.15% (long-acting formulation) on day 0. The efficacy of each treatment was calculated by counting live mites in skin scrapings. Plasma samples were obtained from each animal and DRM concentrations were measured by HPLC. After the two doses of DRM 1%, the maximum efficacy (98.8%) was reached on day 28, whereas after the single dose of DRM 3.15%, the maximum efficacy (100%) was reached on day 35 and ratified on day 42. The long-acting formulation allowed obtaining higher exposure and more sustained concentrations of DRM than the traditional preparation. Although both DRM formulations studied were effective according to international protocols, they did not reach 100% effectiveness in the time required for approved pharmaceutical products against sheep scab, according to Argentine regulations.

Moxidectin: an oral treatment for human onchocerciasis

INTRODUCTION

Moxidectin is a milbemycin endectocide recently approved for the treatment of human onchocerciasis. Onchocerciasis, earmarked for elimination of transmission, is a filarial infection endemic in Africa, Yemen, and the Amazonian focus straddling Venezuela and Brazil. Concerns over whether the predominant treatment strategy (yearly mass drug administration (MDA) of ivermectin) is sufficient to achieve elimination in all endemic foci have refocussed attention upon alternative treatments. Moxidectin's stronger and longer microfilarial suppression compared to ivermectin in both phase II and III clinical trials indicates its potential as a novel powerful drug for onchocerciasis elimination.

AREAS COVERED

This work summarizes the chemistry and pharmacology of moxidectin, reviews the phase II and III clinical trials evidence on tolerability, safety, and efficacy of moxidectin versus ivermectin, and discusses the implications of moxidectin's current regulatory status.

EXPERT OPINION

Moxidectin's superior clinical performance has the potential to substantially reduce times to elimination compared to ivermectin. If donated, moxidectin could mitigate the additional programmatic costs of biannual ivermectin distribution because, unlike other alternatives, it can use the existing community-directed treatment infrastructure. A pediatric indication (for children <12 years) and determination of its usefulness in onchocerciasis-loiasis co-endemic areas will greatly help fulfill the potential of moxidectin for the treatment and elimination of onchocerciasis.

Combination of quercetin and ivermectin: In vitro and in vivo effects against Haemonchus contortus

The aim of the present study was to evaluate the in vitro effect of quercetin combined with ivermectin (IVM) on Haemonchus contortus larvae and adults with different resistance profiles and demonstrate the in vivo anthelmintic action of this combination when used in sheep naturally infected. The effect of combination was evaluated based on the analysis of the mean effective concentration (EC50) obtained for larvae using the larval migration inhibition test and for adults using the motility test on females. The tests with larvae and adults were conducted using isolates with different degrees of susceptibility to IVM (sensitive, intermediate and highly resistant). The in vivo effect was evaluated based on the reduction in the egg count (FEC) and reduction in the count of adult helminths recovered after parasitological necropsy. Using the combination of quercetin with IVM, it was observed that in larvae, quercetin did not significantly reduce the EC50 for IVM in the sensitive and highly resistant isolates, but led to a significant reduction in the EC50 for IVM in the intermediate isolate. In adults, quercetin did not significantly reduce the EC50 for IVM in any of the isolates. No significant effect of the combination was found regarding the reduction in FEC or total count of parasites. The results of the in vitro and in vivo tests performed in the present study on quercetin activity underscore the importance of evaluating resistance-reversing agents among different stages of parasite development as well as among isolates with different resistance profiles. The action of quercetin combined with IVM on the motility of H. contortus larvae and adults was influenced by the degree of resistance and development stage of the parasite. The combination was effective only on intermediate resistant larvae. No action of the combination against adults was found. Moreover, this combination, when administered through the intra-abomasal route, was not effective at reducing the FEC and parasite load of naturally infected sheep.

Review of the Eprinomectin effective doses required for dairy goats: Where do we go from here?

Eprinomectin (EPM) has been recently granted a marketing authorisation in the European Union for use in goats, with a zero-day milk withdrawal period. Considering the high prevalence of benzimidazole resistance worldwide and the economic implications of managing milk residues, EPM may today be considered the main (or even the only) affordable treatment option, at least in dairy goats in the EU. However, the chosen dose (1 mg/kg) seems to be suboptimal, especially for lactating goats, and the chosen route of administration (Pour-on) highly subject to inter-individual variability. Considering the scarcity of anthelmintic resources, such a dosage regimen might threat the sustainability of this crucial drug in goat milk production and needs to be urgently discussed and reassessed.

Perspectives on the utility of moxidectin for the control of parasitic nematodes in the face of developing anthelmintic resistance

Macrocyclic lactone (ML) anthelmintics are the most important class of anthelmintics because of our high dependence on them for the control of nematode parasites and some ectoparasites in livestock, companion animals and in humans. However, resistance to MLs is of increasing concern. Resistance is commonplace throughout the world in nematode parasites of small ruminants and is of increasing concern in horses, cattle, dogs and other animals. It is suspected in Onchocerca volvulus in humans. In most animals, resistance first arose to the avermectins, such as ivermectin (IVM), and subsequently to moxidectin (MOX). Usually when parasite populations are ML-resistant, MOX is more effective than avermectins. MOX may have higher intrinsic potency against some parasites, especially filarial nematodes, than the avermectins. However, it clearly has a significantly different pharmacokinetic profile. It is highly distributed to lipid tissues, less likely to be removed by ABC efflux transporters, is poorly metabolized and has a long half-life. This results in effective concentrations persisting for longer in target hosts. It also has a high safety index. Limited data suggest that anthelmintic resistance may be overcome, at least temporarily, if a high concentration can be maintained at the site of the parasites for a prolonged period of time. Because of the properties of MOX, there are reasonable prospects that strains of parasites that are resistant to avermectins at currently recommended doses will be controlled by MOX if it can be administered at sufficiently high doses and in formulations that enhance its persistence in the host. This review examines the properties of MOX that support this contention and compares them with the properties of other MLs. The case for using MOX to better control ML-resistant parasites is summarised and some outstanding research questions are presented.

Modelling the development of anthelmintic resistance in cyathostomin parasites: The importance of genetic and fitness parameters

Previously described models for the free-living and parasitic phases of the cyathostomin life-cycle were combined into a single model for the complete life-cycle. The model simulates a single free-living population on pasture utilising parasite egg output from the horses and localised temperature and rainfall data to estimate infective larval density on herbage. Multiple horses of different ages are possible, each with an individualised anthelmintic treatment programme. Genotypes for anthelmintic resistance are included allowing for up to three resistance genes with 2 alleles each. Because little is known of the genetics of resistance to anthelmintics in cyathostomins, the first use of this model was to compare the effect of different assumptions regarding the inheritance of resistance on model outputs. Comparisons were made between single and two-gene inheritance, where the heterozygote survival was dominant, intermediate or recessive under treatment, and with or without a fitness disadvantage associated with the resistance mechanism. Resistance developed fastest when the heterozygotes survived anthelmintic treatment (i.e., were dominant) and slowest when they did not (i.e., were recessive). Resistance was slower to develop when inheritance was poly-genic compared to a single gene, and when there was a fitness cost associated with the resistance mechanism, although the latter variable was the least influential. Importantly, while these genetic factors sometimes had a large influence on the rate at which resistant genotypes built up in the model populations, their order of ranking was always the same, when different anthelmintic use strategies were compared. Therefore, the described model is a useful tool for evaluating different treatment and management strategies on their potential to select for resistance.

Anthelmintic resistance of injectable macrocyclic lactones in cattle: A systematic review and meta-analysis

The purpose of this work was to identify, critically assess, and summarize available data from primary research about the anthelmintic resistance of injectable macrocyclic lactones in cattle. Meta-analysis was performed to estimate the pooled Odds Ratio and 95% Confidence Intervals. Of the 1504 abstracts screened for eligibility, 80 were deemed relevant for full publication review. Thirteen publications were included in the qualitative synthesis and assessed for systematic bias. Only five studies were included in the quantitative analysis because they showed a low risk of producing biased results in all the parameters. The forest plot indicated four studies that discuss anthelmintic resistance (P<0.05), while only one study did not discuss anthelmintic resistance (P<0.05). The pooled estimate showed 0.59 (95% Confidence intervals: 0.08, 0.47) times higher odds for studies that report anthelmintic resistance than for studies reporting efficacious anthelmintic treatment, with significant and substantially low heterogeneity (I2=25%). Anthelmintic resistance to injectable macrocyclic lactones is a reality. There are need to improve methodological reporting in studies, which is a problem for investigations that involves systematic review and meta-analysis (SR-MA).

Preparation and in vitro release kinetics of ivermectin sustained-release bolus optimized by response surface methodology

Sustained-release formulations of ivermectin (IVM) are useful for controlling parasitic diseases in animals. In this work, an IVM bolus made from microcrystalline cellulose (MCC), starch and low-substituted hydroxypropyl cellulose (LS-HPC) was optimized by response surface methodology. The bolus was dissolved in a cup containing 900 mL of dissolution medium at 39.5 °C, under with stirring at 100 rpm. A quadratic model was formulated using analysis of variance according to the dissolution time. The optimized formulation of the bolus contained 8% MCC, 0.5% starch, and 0.25% LS-HPC. The length, width, and height of the prepared IVM bolus were 28.12 ± 0.14, 16.1 ± 0.13, and 13.03 ± 0.05 mm, respectively. The bolus weighed 11.4842 ± 0.1675 g (with a density of 1.95 g/cm³) and contained 458.26 ± 6.68 mg of IVM. It exhibited in vitro sustained-release for over 60 days, with a cumulative amount and percentage of released IVM of 423.72 ± 5.48 mg and 92.52 ± 1.20%, respectively. The Korsmeyer–Peppas model provided the best fit to the dissolution release kinetics, exhibiting an R² value close to 1 and the lowest Akaike Information Criterion among different models. The parameter n (0.5180) of the Korsmeyer–Peppas model was between 0.45 and 0.89. It was demonstrated that the release mechanism of the IVM bolus followed a diffusive erosion style.

Field trial assessment of ivermectin pharmacokinetics and efficacy against susceptible and resistant nematode populations in cattle

The study compared the pharmacokinetic (PK) behaviour and anthelmintic efficacy against susceptible and resistant nematodes following subcutaneous (SC) and oral administration of ivermectin (IVM) to cattle. Six commercial farms were involved: Farms 1 and 2 (IVM-susceptible nematode population) and Farms 3, 4, 5 and 6 (IVM-resistant nematode population). On each farm, forty-five calves naturally infected with gastrointestinal (GI) nematodes were randomly allocated into three groups (n = 15): untreated control, IVM SC administration, and IVM oral administration (both at 0.2 mg/kg). PK assessment (plasma and faeces) was performed on Farm 1. Efficacy was determined by Faecal Egg Count Reduction Test. IVM systemic availability upon SC administration (421 ± 70.3 ng·d/mL) was higher (P < 0.05) compared to the oral treatment (132 ± 31.3 ng·d/mL). However, higher (P < 0.05) faecal IVM concentrations were observed following oral treatment (9896 ± 1931 ng·d/mL) compared to SC administration (4760 ± 924 ng·d/mL). Similar (91-93%) IVM efficacy was observed on Farms 1 and 2 by both routes. Efficacy against resistant nematodes was slightly higher on Farms 3 and 4 after the oral (63 and 82%, respectively) compared to the SC (36 and 68%, respectively) treatment. However, there was complete therapeutic failure (0% efficacy) on Farm 5 and a very low response on Farm 6 (40 and 41% for SC and oral administration, respectively). Although larger faecal concentrations following IVM oral administration may increase drug exposure of GI adult worms, this does not always improve efficacy against resistant nematodes. The potential therapeutic advantages of oral treatments should be cautiously assessed, especially in presence of anthelmintic resistance.

Exposure of Heligmosomoides polygyrus and Trichuris muris to albendazole, albendazole sulfoxide, mebendazole and oxantel pamoate in vitro and in vivo to elucidate the pathway of drug entry into these gastrointestinal nematodes

Millions of people are treated with anthelmintics to control soil-transmitted helminth infections; yet, drug distribution in the plasma and gastrointestinal tract compartments and the pathway of drug uptake into gastrointestinal nematodes responsible for the pharmacological effect are unknown. We assessed the distribution and uptake of albendazole, albendazole sulfoxide, albendazole sulfone in the hookworm Heligmosomoides polygyrus in vitro and in vivo as well as the distribution and uptake of albendazole, mebendazole, and oxantel pamoate in the whipworm Trichuris muris in vitro and in vivo. Oral and intraperitoneal treatments (100 mg/kg) were studied. Drug quantities in helminths and host compartments (stomach, the contents and mucosa of the small and large intestine, and the plasma) were determined using HPLC-UV/vis and anthelmintic activities were recorded using phenotypic readout. The influence of 1-aminobenzotriazole (ABT), an irreversible and unspecific cytochrome P450 inhibitor, on albendazole disposition in mice harboring H. polygyrus was evaluated. In vivo, albendazole was found in quantities up to 10 nmol per ten H. polygyrus and up to 31 nmol per ten T. muris. ABT did not change the levels of albendazole or its metabolites in the plasma of mice harboring H. polygyrus or in H. polygyrus, whereas drug levels in the gastrointestinal tract of host mice doubled. Mebendazole and oxantel pamoate quantities per ten T. muris were as high as 21 nmol and 34 nmol, respectively. Albendazole revealed a very dynamic distribution and high rate of metabolism, hence, H. polygyrus and T. muris are exposed to albendazole and both metabolites via multiple pathways. Diffusion through the cuticle seems to be the crucial pathway of oxantel pamoate uptake into T. muris, and likely also for mebendazole. No relationship between concentrations measured in helminths and concentrations in plasma, intestinal content and mucosa of mice, or drug efficacy was noted for any of the drugs studied.

Managing anthelmintic resistance-Variability in the dose of drug reaching the target worms influences selection for resistance?

The concentration profile of anthelmintic reaching the target worms in the host can vary between animals even when administered doses are tailored to individual liveweight at the manufacturer's recommended rate. Factors contributing to variation in drug concentration include weather, breed of animal, formulation and the route by which drugs are administered. The implications of this variability for the development of anthelmintic resistance was investigated using Monte-Carlo simulation. A model framework was established where 100 animals each received a single drug treatment. The 'dose' of drug allocated to each animal (i.e. the concentration-time profile of drug reaching the target worms) was sampled at random from a distribution of doses with mean m and standard deviation s. For each animal the dose of drug was used in conjunction with pre-determined dose-response relationships, representing single and poly-genetic inheritance, to calculate efficacy against susceptible and resistant genotypes. These data were then used to calculate the overall change in resistance gene frequency for the worm population as a result of the treatment. Values for m and s were varied to reflect differences in both mean dose and the variability in dose, and for each combination of these 100,000 simulations were run. The resistance gene frequency in the population after treatment increased as m decreased and as s increased. This occurred for both single and poly-gene models and for different levels of dominance (survival under treatment) of the heterozygote genotype(s). The results indicate that factors which result in lower and/or more variable concentrations of active reaching the target worms are more likely to select for resistance. The potential of different routes of anthelmintic administration to play a role in the development of anthelmintic resistance is discussed.

Host pharmacokinetics and drug accumulation of anthelmintics within target helminth parasites of ruminants

Anthelmintic drugs require effective concentrations to be attained at the site of parasite location for a certain period to assure their efficacy. The processes of absorption, distribution, metabolism and excretion (pharmacokinetic phase) directly influence drug concentrations attained at the site of action and the resultant pharmacological effect. The aim of the current review article was to provide an overview of the relationship between the pharmacokinetic features of different anthelmintic drugs, their availability in host tissues, accumulation within target helminths and resulting therapeutic efficacy. It focuses on the anthelmintics used in cattle and sheep for which published information on the overall topic is available; benzimidazoles, macrocyclic lactones and monepantel. Physicochemical properties, such as water solubility and dissolution rate, determine the ability of anthelmintic compounds to accumulate in the target parasites and consequently final clinical efficacy. The transcuticular absorption process is the main route of penetration for different drugs in nematodes and cestodes. However, oral ingestion is a main route of drug entry into adult liver flukes. Among other factors, the route of administration may substantially affect the pharmacokinetic behaviour of anthelmintic molecules and modify their efficacy. Oral administration improves drug efficacy against nematodes located in the gastroinestinal tract especially if parasites have a reduced susceptibility. Partitioning of the drug between gastrointestinal contents, mucosal tissue and the target parasite is important to enhance the drug exposure of the nematodes located in the lumen of the abomasum and/or small intestine. On the other hand, large inter-animal variability in drug exposure and subsequent high variability in efficacy is observed after topical administration of anthelmintic compounds. As it has been extensively demonstrated under experimental and field conditions, understanding pharmacokinetic behaviour and identification of different factors affecting drug activity is important for achieving optimal parasite control and avoiding selection for drug resistance. The search for novel alternatives to deliver enhanced drug concentrations within target helminth parasites may contribute to avoiding misuse, and prolong the lifespan of existing and novel anthelmintic compounds in the veterinary pharmaceutical market.

Anthelmintic resistance in grazing beef cattle in central and northeastern areas of Argentina - An update

The presence of anthelmintic resistance in Argentina has experienced a marked increase in cattle, with numerous reports showing levels of resistance of different parasite genera to different chemical groups. The aim of this study is to update comprehensively the situation of anthelmintic resistance to the different chemical groups in the most important areas of cattle production in Argentina. The study involved the determination of anthelmintic resistance in 62 cattle farms in 7 provinces using the faecal egg count reductions test. The results showed a marked increase of anthelmintic resistance compared to previous reports; the main resistant genera were Cooperia and Haemonchus to ivermectin, Ostertagia and Cooperia to ricobendazole, and Haemonchus to fenbendazole. There was also a distinct difference in clinical efficacies between subcutaneous ricobendazole and oral fenbendazole in favour of the latter, probably attributed to the administration route. Levamisole has showed high efficacy and broad antiparasitic spectrum. Anthelmintic resistance is widely and firmly established in grazing cattle production systems in the country; the diagnosis of resistance must be done in every particular farm in order to design a sustainable parasite control based on anthelmintics use.

Confirmation of ivermectin resistance in Ostertagia ostertagi in cattle in New Zealand

Six suspected cases of ivermectin resistance in Ostertagia spp. in cattle were investigated after routine anthelmintic efficacy testing on commercial farms. On four farms a comprehensive faecal egg count reduction test (FECRT) was undertaken using oral formulations of ivermectin (0.2mg/kg), albendazole (10mg/kg) and levamisole (7.5mg/kg) while on two farms only ivermectin was tested. The proportions of Ostertagia spp. in the untreated control and post-treatment larval cultures were used to apportion egg counts to genera and determine efficacy against this genus. Isolates of Ostertagia spp. recovered from three of the farms were each used to infect 18 six month old calves. The efficacy of oral formulations of ivermectin and moxidectin, both at 0.2mg/kg, was determined against each isolate by slaughter and worm count. The efficacy of ivermectin against Ostertagia spp., based on differentiated FECRT for each of the farms varied from 0% to 88%. The efficacy of ivermectin based on worm counts in the slaughter trial varied from 13% to 75% but moxidectin was >99% effective against all isolates. In addition, in the FECRT albendazole, at a dose rate of 10mg/kg, failed to achieve 95% efficacy against Ostertagia spp. on two farms (82% and 85%). Levamisole consistently failed to achieve 95% efficacy against Ostertagia spp. which is consistent with its known lesser efficacy against this parasite. These results confirm the presence of macrocyclic lactone resistant O. ostertagi in cattle in New Zealand and the likely presence of dual resistance, to macrocyclic lactones and albendazole, in some isolates. Resistant populations of this highly pathogenic parasite are probably not uncommon in New Zealand and pose a significant threat to animal production and welfare in the future.

Gaining Insights Into the Pharmacology of Anthelmintics Using Haemonchus contortus as a Model Nematode

Progress made in understanding pharmacokinetic behaviour and pharmacodynamic mechanisms of drug action/resistance has allowed deep insights into the pharmacology of the main chemical classes, including some of the few recently discovered anthelmintics. The integration of pharmaco-parasitological research approaches has contributed considerably to the optimization of drug activity, which is relevant to preserve existing and novel active compounds for parasite control in livestock. A remarkable amount of pharmacology-based knowledge has been generated using the sheep abomasal nematode Haemonchus contortus as a model. Relevant fundamental information on the relationship among drug influx/efflux balance (accumulation), biotransformation/detoxification and pharmacological effects in parasitic nematodes for the most traditional anthelmintic chemical families has been obtained by exploiting the advantages of working with H. contortus under in vitro, ex vivo and in vivo experimental conditions. The scientific contributions to the pharmacology of anthelmintic drugs based on the use of H. contortus as a model nematode are summarized in the present chapter.

Pharmacokinetic comparison of different flubendazole formulations in pigs: A further contribution to its development as a macrofilaricide molecule

Despite the well established ivermectin activity against microfilaria, the success of human filariasis control programmes requires the use of a macrofilaricide compound. Different in vivo trials suggest that flubendazole (FLBZ), an anthelmintic benzimidazole compound, is a highly efficacious and potent macrofilaricide. However, since serious injection site reactions were reported in humans after the subcutaneous FLBZ administration, the search for alternative pharmaceutical strategies to improve the systemic availability of FLBZ has acquired special relevance both in human and veterinary medicine. The goal of the current experimental work was to compare the pharmacokinetic plasma behavior of FLBZ, and its metabolites, formulated as either an aqueous hydroxypropyl- β -cyclodextrin-solution (HPBCD), an aqueous carboxymethyl cellulose-suspension (CMC) or a Tween 80-based formulation, in pigs. Animals were allocated into three groups and treated (2 mg/kg) with FLBZ formulated as either a HPBCD-solution (oral), CMC-suspension (oral) or Tween 80-based formulation (subcutaneous). Only trace amounts of FLBZ parent drug and its reduced metabolite were measured after administration of the different FLBZ formulations in pigs. The hydrolyzed FLBZ (H-FLBZ) metabolite was the main analyte recovered in the bloodstream in pigs treated with the three experimental FLBZ formulations. The oral administration of the HPBCD-solution accounted for significantly higher (P < 0.05) Cmax and AUC (23.1 ± 4.4 μg h/mL) values for the main metabolite (H-FLBZ), compared with those observed for the oral CMC-suspension (AUC = 3.5 ± 1.0 μg h/mL) and injectable Tween 80-based formulation (AUC: 7.5 ± 1.7 μg h/mL). The oral administration of the HPBCD-solution significantly improved the poor absorption pattern (indirectly assessed as the H-FLBZ plasma concentrations) observed after the oral administration of the FLBZ-CMC suspension or the subcutaneous injection of the Tween 80 FLBZ formulation to pigs. Overall, the work reported here indicates that FLBZ pharmacokinetic behavior can be markedly changed by the pharmaceutical formulation.

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The pharmacokinetics of three different FLBZ formulations was assessed in pigs.

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Hydrolyzed-FLBZ was the main metabolite detected in pigs given the formulations.

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Traces of FLBZ and reduced-FLBZ were measured after administration of FLBZ.

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Oral administration of FLBZ-HPBCD solution resulted in a high systemic H-FLBZ exposure.

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Similar FLBZ plasma exposure was observed after parenteral FLBZ-Tween 80 and HPBCD administration.

Comparative pharmacokinetic and pharmacodynamic response of single and double intraruminal doses of ivermectin and moxidectin in nematode-infected lambs

AIMS

To compare the pharmacokinetics, distribution and efficacy (pharmacodynamic response) of intraruminal ivermectin (IVM) and moxidectin (MXD) administered at 0.2 and 0.4 mg/kg to naturally nematode-infected lambs, and to determine the ex vivo accumulation of these anthelmintics by Haemonchus contortus.

METHODS

Romney Marsh lambs, naturally infected with IVM-resistant H. contortus, were allocated to treatment groups based on faecal nematode egg counts. They received 0.2 or 0.4 mg/kg IVM or MXD (n=10 per group), or no treatment (Control; n=6), on Day 0. Samples from four animals from each treatment group, including abomasal parasites, were obtained on Day 1. Plasma samples were also collected from Day 0 to 14, and a faecal egg count reduction test (FECRT) and a controlled efficacy trial were carried out on Day 14. Concentrations of IVM and MXD in plasma, in abomasal and intestinal tissues and in H. contortus were evaluated by high-performance liquid chromatography. Additionally, the ex vivo drug accumulation of IVM and MXD by H. contortus was determined.

RESULTS

Peak plasma concentrations and the area under the concentration vs. time curve for both IVM and MXD were higher for 0.4 than 0.2 mg/kg treatments (p<0.05), but there were no differences for other parameters. Concentrations of IVM and MXD in the gastrointestinal target tissues and in H. contortus were higher compared to those measured in plasma. Concentrations of both drugs in H. contortus were correlated with those observed in the abomasal content (r=0.86; p<0.0001). The exposure of H. contortus to IVM and MXD was related to the administered dose. Mean FECRT and efficacy for removal of adult H. contortus was 0% for IVM at 0.2 and 0.4 mg/kg. For MXD, FECRT were >95% for both treatments, and efficacy against H. contortus was 85.1% and 98.1% for 0.2 and 0.4 mg/kg, respectively. The ex vivo accumulation of IVM and MXD in H. contortus was directly related to the drug concentration present in the environment and was influenced by the duration of exposure.

CONCLUSION

Administration of IVM and MXD at 0.4 compared with 0.2 mg/kg accounted for enhanced drug exposure in the target tissues, as well as higher drug concentrations within resistant nematodes. The current work is a further contribution to the evaluation of the relationship between drug efficacy and basic pharmacological issues in the presence of resistant parasite populations.

Integrated assessment of ivermectin pharmacokinetics, efficacy against resistant Haemonchus contortus and P-glycoprotein expression in lambs treated at three different dosage levels

The main goals of the current work were: (a) to assess the ivermectin (IVM) systemic exposure and plasma disposition kinetics after its administration at the recommended dose, x5 and x10 doses to lambs, (b) to compare the clinical efficacy of the same IVM dosages in lambs infected with an IVM-resistant isolate of Haemonchus contortus, and (c) to assess the expression of the transporter protein P-glycoprotein (P-gp) in H. contortus recovered at 14 days after administration of the IVM dose regimens. There were two separated trials where IVM was administered either subcutaneously (SC, Experiment I) or intraruminally (IR, Experiment II). Each experiment involved twenty-four (24) lambs artificially infected with a highly resistant H. contortus isolate. Animals were allocated into 4 groups (n=6) and treated with IVM at either 0.2 (IVM x1), 1 (IVM x5) or 2mg/kg (IVM x10). Plasma samples were collected up to 12 days post-treatment and analysed by HPLC. An untreated-control Group was included to assess the comparative anthelmintic efficacy of the different treatments. The level of expression of Pgp in H. contortus specimens obtained from lambs both untreated and IR treated with the different IVM doses was quantified by real time PCR. Parametric and non-parametric tests were used to compare the statistical significance of the results (P<0.05). After the SC treatment, the IVM plasma area under the concentration-time curve (AUC0-LOQ) increased from 41.9 (IVM SCx1) up to 221 (IVM SCx5) and 287 (IVM SCx10)ng.day/mL and after the IR treatment from 20.8 (IVM IRx1) up to 121 (IVM IRx5) and 323 (IVM IRx10)ng.day/mL. Dose-adjusted AUC0-LOQ and Cmax were similar among doses, demonstrating dose proportionality for IVM after both SC and IR administration at the three different doses. The efficacies against resistant H. contortus after the SC treatment were 42% (IVM SC1), 75% (IVM SCx5) and 75% (IVM SCx10). However, the IR IVM treatment reached clinical efficacies ranging from 48% (IVM IRx1) up to 96% (IVM IRx5) and 98% (IVM IRx10). None of the IR IVM treatments increased the expression of P-gp in adult H. contortus at 14 days post-treatment compared to samples collected from the untreated control group. An enhanced parasite exposure of the drug at the abomasum may explain the improved efficacy against this recalcitrant H. contortus isolate observed only after the IR administration at 5- and 10-fold the IVM therapeutic dosage.

The relative plasma availabilities of ivermectin in reindeer (Rangifer tarandus tarandus) following subcutaneous and two different oral formulation applications

Background

Overwintering (breeding) reindeer (Rangifer tarandus tarandus) are commonly treated with ivermectin against parasitic infestations once yearly in autumn-winter roundups. The only preparations registered to reindeer are those for subcutaneous injection. However, also oral extra-label ivermectin administration is used. Twenty-six, 8-month-old reindeer calves were randomly allocated into three groups. Group 1 (n = 9) received oral ivermectin mixture (Ivomec® vet mixt. 0.8 mg/ml, oral ovine liquid drench formulation), Group 2 (n = 9) oral ivermectin paste (Ivomec® vet 18.7 mg/g equine paste), and Group 3 (n = 8) subcutaneous injection of ivermectin (Ivomec® 10 mg/ml vet inj.), each group at a dose of 200 μg/kg body weight. Blood samples were collected at treatment and at days 1, 2, 3, 6, 9 and 16 post treatment. Plasma concentrations of ivermectin were determined by high-pressure liquid chromatography (HPLC) with fluorescence detection.

Results

The peak plasma concentration (C_max) was reached by 2 days after each treatment. The C_max and Area Under Curve (AUC) differed significantly between the groups: C_max was 30.2 ± 3.9, 14.9 ± 5.7 and 63.1 ± 13.1 ng/ml, and AUC_∞ was 2881 ± 462, 1299 ± 342 and 6718 ± 1620 ng*h/ml for groups 1, 2 and 3, respectively (mean ± standard deviation).

Conclusions

The differences in plasma concentrations of ivermectin are concomitant with earlier observed differences in antiparasitic efficacy, which discounts the use of the equine paste in reindeer in favour of the oral ovine liquid drench formulation, or preferably, the reindeer-registered subcutaneous injection formulation.

Ivermectin exposure leads to up-regulation of detoxification genes in vitro and in vivo in mice

The biodisposition of the antiparasitic drug ivermectin in host and parasite is decisive for its efficacy and strongly depends on the efflux by ATP-Binding Cassette (ABC) transporters and on its biotransformation by cytochromes P450. The purpose of this study was to evaluate, in vitro and in vivo, the ivermectin ability in modulating the expression of the most important genes involved in drug detoxification. Gene expression of ABC transporters and cytochromes was evaluated by RT-qPCR in murine hepatic and intestinal cell lines exposed to increasing ivermectin doses, and in liver and intestine of mice orally administered with single or repeated therapeutic doses of ivermectin (0.2 mg/kg). Plasma, brain, liver and intestinal concentrations of ivermectin and its main metabolite were measured by HPLC in ivermectin-treated mice. In hepatocyte cell line, ivermectin up-regulated expression of Abcb1a, Abcb1b, Abcc2, Cyp1a1, Cyp1a2, Cyp2b10; while Abcb1a, Abcb1b, Abcg2, Cyp1a1, Cyp1a2, Cyp2b10 and Cyp3a11 levels were induced in intestinal cell line. In mice, repeated administration of ivermectin induced the expression of Abcb1a, Abcc2, Cyp1a1 and Cyp2b10 in intestine while only Cyp3a11 was induced in liver. Compared with single administration, repeated ivermectin administration lowered plasma, liver and intestine drug concentration, while increasing main metabolite content in plasma and intestine. These findings can be regarded as a warning that repeated ivermectin exposure is able to induce detoxification systems in mammals that may lead to subtherapeutic drug concentration. This may also be an important consideration in the assessment of drug-drug interaction and toxicity for other ABC transporters and CYP450s substrates.

Caenorhabditis elegans neuromuscular junction: GABA receptors and ivermectin action

The prevalence of human and animal helminth infections remains staggeringly high, thus urging the need for concerted efforts towards this area of research. GABA receptors, encoded by the unc-49 gene, mediate body muscle inhibition in Caenorhabditis elegans and parasitic nematodes and are targets of anthelmintic drugs. Thus, the characterization of nematode GABA receptors provides a foundation for rational anti-parasitic drug design. We therefore explored UNC-49 channels from C. elegans muscle cultured cells of the first larval stage at the electrophysiological and behavioral levels. Whole-cell recordings reveal that GABA, muscimol and the anthelmintic piperazine elicit macroscopic currents from UNC-49 receptors that decay in their sustained presence, indicating full desensitization. Single-channel recordings show that all drugs elicit openings of ∼2.5 pA (+100 mV), which appear either as brief isolated events or in short bursts. The comparison of the lowest concentration required for detectable channel opening, the frequency of openings and the amplitude of macroscopic currents suggest that piperazine is the least efficacious of the three drugs. Macroscopic and single-channel GABA-activated currents are profoundly and apparently irreversibly inhibited by ivermectin. To gain further insight into ivermectin action at C. elegans muscle, we analyzed its effect on single-channel activity of the levamisol-sensitive nicotinic receptor (L-AChR), the excitatory receptor involved in neuromuscular transmission. Ivermectin produces a profound inhibition of the frequency of channel opening without significant changes in channel properties. By revealing that ivermectin inhibits C. elegans muscle GABA and L-AChR receptors, our study adds two receptors to the already known ivermectin targets, thus contributing to the elucidation of its pleiotropic effects. Behavioral assays in worms show that ivermectin potentiates piperazine-induced paralysis, thus suggesting that their combination is a good strategy to overcome the increasing resistance of parasites, an issue of global concern for human and animal health.

The management of anthelmintic resistance in grazing ruminants in Australasia--strategies and experiences

In many countries the presence of anthelmintic resistance in nematodes of small ruminants, and in some cases also in those infecting cattle and horses, has become the status quo rather than the exception. It is clear that consideration of anthelmintic resistance, and its management, should be an integral component of anthelmintic use regardless of country or host species. Many years of research into understanding the development and management of anthelmintic resistance in nematodes of small ruminants has resulted in an array of strategies for minimising selection for resistance and for dealing with it once it has developed. Importantly, many of these strategies are now supported by empirical science and some have been assessed and evaluated on commercial farms. In sheep the cost of resistance has been measured at about 10% of the value of the lamb at sale which means that losses due to undetected resistance far outweigh the cost of testing anthelmintic efficacy. Despite this many farmers still do not test for anthelmintic resistance on their farm. Many resistance management strategies have been developed and some of these have been tailored for specific environments and/or nematode species. However, in general, most strategies can be categorised as either; identify and mitigate high risk management practices, maintain an anthelmintic-susceptible population in refugia, choose the optimal anthelmintic (combinations and formulations), or prevent the introduction of resistant nematodes. Experiences with sheep farmers in both New Zealand and Australia indicate that acceptance and implementation of resistance management practices is relatively easy as long as the need to do so is clear and the recommended practices meet the farmer's criteria for practicality. A major difference between Australasia and many other countries is the availability and widespread acceptance of combination anthelmintics as a resistance management tool. The current situation in cattle and horses in many countries indicates a failure to learn the lessons from resistance development in small ruminants. The cattle and equine industries have, until quite recently, remained generally oblivious to the issue of anthelmintic resistance and the need to take pre-emptive action. In Australasia, as in other countries, a perception was held that resistance in cattle parasites would develop very slowly, if it developed at all. Such preconceptions are clearly incorrect and the challenge ahead for the cattle and equine industries will be to maximise the advantages for resistance management from the extensive body of research and experience gained in small ruminants.

Pharmacological knowledge and sustainable anthelmintic therapy in ruminants

Considering the increasing concern for the development of anthelmintic resistance, the use of pharmacology-based information is critical to design successful strategies for the future of parasite control in livestock. Integrated evaluation of the available knowledge on pharmacological features is required to optimize the activity and to achieve sustainable use of the existing anthelmintic drugs. The assessment of the drug disposition in the host and the comprehension of the mechanisms of drug influx/efflux/detoxification in different target helminths, has signified a relevant progress on the understanding of the pharmacology of anthelmintic drugs in ruminant species. However, additional scientific knowledge on how to improve the use of available and novel molecules is required to avoid/delay resistance development. Different pharmacokinetic-based approaches to enhance parasite exposure and the use of mixtures of drugs from different chemical families have been proposed as valid strategies to delay the development of anthelmintic resistance. The rationale behind using drug combinations is based on the fact that individual worms may have a lower degree of resistance to a multiple component formulation (each chemical with different mode of action/resistance) compared to that observed when a single anthelmintic is used. However, the limited available information is unclear on the potential additive or synergistic effects occurring after co-administration of two (or more) drugs with different mode of action. This review article contributes to the topic with some pharmacology-based data emerging from the assessment of combined anthelmintic preparations. The activity against multi-drug-resistant isolates based on novel modes of action is a highly favorable element to judge the future of some of the recently developed anthelmintic compounds. More specific knowledge on the basic host-parasite kinetic behavior as well as a highly responsible use of those novel compounds will be necessary to secure their maximum lifespans. Overall, the outcome from integrated pharmaco-parasitological research approaches has greatly contributed to optimize drug activity, which seems relevant to preserve existing and particularly novel active ingredients as useful tools for parasite control in livestock animals.

Time-course and accumulation of triclabendazole and its metabolites in bile, liver tissues and flukes collected from treated sheep

The flukicidal compound triclabendazole (TCBZ) has a complex metabolic pattern that includes the systemic presence of its sulphoxide (TCBZ.SO) and sulphone (TCBZ.SO2) metabolites, usually recovered from the bile of treated animals. The aim of the current work was to evaluate the time-course and pattern of in vivo accumulation of TCBZ/metabolites into adult Fasciola hepatica specimens recovered from infected sheep. Twelve (12) healthy Corriedale sheep were orally infected with one hundred (100) metacercariae of the TCBZ-susceptible Cullomptom isolate of F. hepatica. Sixteen weeks after infection, animals were intraruminally treated with TCBZ (10mg/kg). At 3, 24, 48 and 60h post-treatment (pt), animals were sacrificed (n=3/time period) and samples of blood, bile, liver tissue and adult F. hepatica specimens were collected. The concentrations of TCBZ/metabolites were measured by HPLC. TCBZ.SO and TCBZ.SO2 were the only molecules recovered in the bloodstream, with peak plasma concentrations of 10.8μg/mL (TCBZ.SO) and 12.6μg/mL (TCBZ.SO2). The same metabolites were also the main analytes accumulated within the adult flukes, reaching peak concentrations between 6.35μg/g (TCBZ.SO) and 13.9μg/g (TCBZ.SO2) at 24h pt, which was coincident with the time when the maximum plasma concentration was attained. Low levels of TCBZ parent drug (0.14μg/g at 24h pt) were measured within collected flukes. TCBZ parent drug and its sulpho- and hydroxy-derivatives were recovered in bile collected from treated sheep between 3 and 60h pt. Although relatively high concentrations of hydroxy-TCBZ (ranging from 0.86 to 10.1μg/mL) were measured in bile, this metabolite was not recovered within the flukes at any time pt. Finally, TCBZ parent drug was the main compound accumulated in liver tissue over the 60h pt period. The time-course and drug concentration patterns within the adult liver fluke after TCBZ treatment followed a similar trend to those observed in plasma. Overall, the data reported here confirm that oral ingestion is a main route of drug entry into the trematode in vivo exposed to TCBZ/metabolites. However, the presence of TCBZ within the adult fluke (despite being absent in the systemic circulation) may be related to some degree of trans-tegumental diffusion from bile or by a direct oral ingestion from portal blood.

Relative bioavailability and comparative clinical efficacy of different ivermectin oral formulations in lambs

Background

Several oral ivermectin (IVM) formulations for use in sheep are available in the pharmaceutical veterinary market in different countries. All of them are indicated at the same dose rate to treat the gastrointestinal nematodes. However, there is a lack of information on the relative systemic exposure (plasma bioavailability) and clinical efficacy among oral formulations routinely used in sheep. The main goal of the work reported here was to perform a pharmaco-parasitological assessment of three different IVM oral formulations in lambs infected with multiple resistant gastrointestinal nematodes. The comparative drug systemic exposure (IVM plasma concentrations) and nematodicidal efficacies (clinical efficacy) in lambs were determined for a reference (RF) and two different test (T1, T2) IVM oral formulations. One hundred and fifty six (n= 156) healthy Corriedale lambs, naturally infected with multiple resistant gastrointestinal nematodes were allocated into four experimental groups (n=39). Animals in each group received treatment (200 μg/kg) with either the RF, one of the test IVM formulations or were kept as untreated control. Blood samples were collected over 15 days post-treatment (n=8). The IVM plasma concentrations were measured by high performance liquid chromatography with fluorescence detection. The faecal nematode egg count reduction test (FECRT) (n=39) and evaluation of the clinical efficacy were performed at day 14 post-treatment (n=6), where a predominance of IVM highly resistant nematodes was observed.

Results and conclusions

Neither the overall kinetic behaviour nor the IVM systemic exposure differed among all the tested oral formulations. Equivalent efficacy results were obtained for the different preparations, with an evident therapeutic failure to control Haemonchus spp. and Teladorsagia circumcincta, which correlates with a high degree of nematode resistance to IVM.

Comparative tissue pharmacokinetics and efficacy of moxidectin, abamectin and ivermectin in lambs infected with resistant nematodes: Impact of drug treatments on parasite P-glycoprotein expression

The high level of resistance to the macrocyclic lactones has encouraged the search for strategies to optimize their potential as antiparasitic agents. There is a need for pharmaco-parasitological studies addressing the kinetic-dynamic differences between various macrocyclic lactones under standardized in vivo conditions. The current work evaluated the relationship among systemic drug exposure, target tissue availabilities and the pattern of drug accumulation within resistant Haemonchus contortus for moxidectin, abamectin and ivermectin. Drug concentrations in plasma, target tissues and parasites were measured by high performance liquid chromatography. Additionally, the efficacy of the three molecules was evaluated in lambs infected with resistant nematodes by classical parasitological methods. Furthermore, the comparative determination of the level of expression of P-glycoprotein (P-gp2) in H. contortus recovered from lambs treated with each drug was performed by real time PCR. A longer persistence of moxidectin (P < 0.05) concentrations in plasma was observed. The concentrations of the three compounds in the mucosal tissue and digestive contents were significant higher than those measured in plasma. Drug concentrations were in a range between 452 ng/g (0.5 day post-treatment) and 32 ng/g (2 days post-treatment) in the gastrointestinal (GI) contents (abomasal and intestinal). Concentrations of the three compounds in H. contortus were in a similar range to those observed in the abomasal contents (positive correlation P = 0.0002). Lower moxidectin concentrations were recovered within adult H. contortus compared to abamectin and ivermectin at day 2 post-treatment. However, the efficacy against H. contortus was 20.1% (ivermectin), 39.7% (abamectin) and 89.6% (moxidectin). Only the ivermectin treatment induced an enhancement on the expression of P-gp2 in the recovered adult H. contortus, reaching higher values at 12 and 24 h post-administration compared to control (untreated) worms. This comparative pharmacological evaluation of three of the most used macrocyclic lactones compounds provides new insights into the action of these drugs.