Efficacy of abamectin against ivermectin-resistant strain of Trichostrongylus colubriformis in sheep

Pharmacokinetic-pharmacodynamic assessment of the ivermectin and abamectin nematodicidal interaction in cattle

In a context of nematodicidal resistance, anthelmintic combinations have emerged as a reliable pharmacological strategy to control gastrointestinal nematodes in grazing systems of livestock production. The current work evaluated the potential drug-drug interactions following the coadministration of two macrocyclic lactones (ML) ivermectin (IVM) and abamectin (ABM) to parasitized cattle using a pharmacokinetic/pharmacodynamic (PK/PD) approach. The kinetic behavior of both compounds administered either separately or coadministered was assessed and the therapeutic response of the combination was evaluated under different resistance scenarios. In the pharmacological trial, calves received a single subcutaneous (s.c.) injection of IVM (100 μg/Kg); a single s.c. injection of ABM (100 μg/Kg) or IVM + ABM (50 μg/Kg each) administered in different injection sites to reach a final ML dose of 100 μg/Kg (Farm 1). Plasma samples were taken from those animals up to 20 days post-treatment. IVM and ABM plasma concentrations were quantified by HPLC. A parasitological trial was carried out in three farms with different status of nematodes resistance to IVM. Experimental animals received IVM (200 μg/Kg), ABM (200 μg/Kg) or IVM + ABM (100 μg/Kg each) in Farm 2, and IVM + ABM (200 μg/Kg each) in Farms 3 and 4. The anthelmintic efficacy was determined by fecal egg count reduction test (FECRT). PK analysis showed similar trends for IVM kinetic behavior after coadministration with ABM. Conversely, the ABM elimination half-life was prolonged and the systemic exposure during the elimination phase was increased in the presence of IVM. Although IVM alone failed to control Cooperia spp., the combination IVM + ABM was the only treatment that achieved an efficacy higher than 95% against resistant Cooperia spp. in all farms. In fact, when Cooperia spp. was the main genus within the nematode population and Haemonchus spp. was susceptible or slightly resistant to ML (Farms 2 and 4), the total FECR for the combination IVM + ABM was higher than 90%. Instead, when the predominant nematode genus was a highly resistant Haemonchus spp. (Farm 3), the total FECR after the combined treatment was as low as the single treatments. Therefore, the rational use of these pharmacological tools should be mainly based on the knowledge of the epidemiology and the nematode susceptibility status in each cattle farm.

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.

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.

Eco-toxicological effects of the avermectin family with a focus on abamectin and ivermectin

Avermectin family members are categorised as highly effective but toxic natural products that are used as pharmaceuticals in both humans and animals and for crop protection. Abamectin and ivermectin are the two most commonly used compounds from this family with abamectin the only compound to be used for both crop protection and pharmaceutical purposes. Avermectins are produced by the soil dwelling actinomycetes Streptomyces avermitilis and despite having complex chemical structures, they are manufactured via synthesis in large scales for commercial use. Although the extent of the eco-toxicological effects of avermectins is not well documented, reports of eco-toxicity exist. Avermectins have short half-lives and their residues can be eliminated through different food processing methods. However, avermectins can persist in water, sediment, soil and food products and therefore management practices that reduce the potential risks associated with eco-toxicity of these highly toxic compounds need to be further developed. This manuscript provides a critical review of the eco-toxicological risks and the potential for food contamination associated with avermectin use.

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.

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.

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.

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.

Pharmacokinetics of a new long acting endectocide formulation containing 2.25% ivermectin and 1.25% abamectin in cattle

The objective of this study was to determine the kinetic parameters of a new formulation that contained 2.25% ivermectin combined with 1.25% abamectin in bovine plasma. The results for 2.25% ivermectin: Cmax (37.11 ng/mL +/- 7.42), Tmax (16 days +/- 5.29), T(1/2) (44.62 days +/- 53.89), AUC (928.2 ng x day/mL +/- 153.83) and MRT (36.73 days +/- 33.64), and for 1.25% abamectin: Cmax (28.70 ng/mL +/- 9.54), Tmax (14 days +/- 4.04), T(1/2) (15.40 days +/- 11.43), AUC (618.05 ng x day/mL +/- 80.27) and MRT (20.79 days +/- 8.43) suggest that this combination of 2.25% ivermectin + 1.25% abamectin possesses properties that give this pharmaceutical formula a longer activity time than two of the commercial products tested (1% ivermectin and 1% abamectin), and showed similarity to 3.15% ivermectin.

Suppressive treatment of abamectin against Dictyocaulus viviparus and the occurrence of resistance in first-grazing-season calves

Dictyocaulus viviparus cause severe lung infections and are endemic in some areas of temperate and tropical countries. Treatment is based on strategic nematode control programs using potent compounds with no reports of drug failure. Macrocyclic lactones (MLs) are available at different concentrations and combinations and have being used heavily by producers. The objective of this work was to determine the efficacy of the MLs ivermectin, moxidectin, doramectin, and abamectin, and the combination ivermectin plus abamectin in naturally infected calves (n=70). Initial infection was determined by necropsy of tracer animals. Faecal larval counts determined that none of the compounds used was able to eliminate D. viviparus for up to 28 days after treatment. The "Area Nova" strain was isolated for future work.

Tolerance of therapeutic doses of abamectin and doramectin in Istrian Pramenka sheep

Avermectin endectocides are very effective and safe veterinary drugs, when used at recommended doses. Adverse reactions are described in some species and breeds of animals. In this study, the effects of therapeutic doses of abamectin and doramectin on some haematological and biochemical parameters in Istrian Pramenka sheep are discussed. In the pilot trial, we compared selected haematological and blood biochemical parameters of an experimental sheep flock (40 sheep) with the reference values. Then, two groups of 12 sheep (and their suckling lambs) were chosen from the experimental sheep flock. Each group was subdivided into a control (six animals) and treated (six sheep and their six suckling lambs) groups. We compared haematological and biochemical parameters between control and treated group before subcutaneous administration of abamectin or doramectin (0.2 mg/kg b.w.) and on days 15 and 42 after treatment. In addition, animals were observed for neurological signs. We detected some significant differences (P < 0.05) in some haematological and biochemical parameters between control and treated animals, but none of them appeared to be of clinical importance. No neurological symptoms were observed. Therefore, abamectin and doramectin might be well tolerated in Istrian Pramenka sheep.

Influence of the route of administration on efficacy and tissue distribution of ivermectin in goat

The tissue concentration and efficacy of ivermectin after per os and subcutaneous administration were compared in goats experimentally infected with Trichostrongylus colubriformis (ivermectin-susceptible strain, INRA). Infected goats (n = 24) were treated per os (n = 9) or subcutaneously (n = 9) with ivermectin, 0.2 mg/kg, or kept as not treated controls. The faecal egg counts and small intestine worm counts were determined. Ivermectin concentration was measured in the plasma, gastrointestinal tract, lung, skin or hair, liver and adipose tissues at 0, 2, 7 and 17 days post-treatment. The efficacy of ivermectin against T. colubriformis infection in goat was 98.7 and 99.9% for subcutaneous and oral administration, respectively. Ivermectin concentration declined with time and only residual concentration was measured at 17 days post-treatment in plasma and gastrointestinal tract. Ivermectin concentration was higher after subcutaneous compared to per os injection in most of the tissue examined. In skin, hair and subcutaneous adipose tissue ivermectin persisted at significant concentrations 17 days post-treatment for both routes of administration. In our experimental conditions, ivermectin provides similar efficacy against T. colubriformis after subcutaneous or per os administration in goat. However, the lower ivermectin levels in tissues after per os administration suggest that the lasting of efficacy may be shortened after per os compared to subcutaneous administration especially in animals with poor body condition in pasture where re-infection occurs quickly after anthelmintic treatment.

Structure of the major cytosolic glutathione S-transferase from the parasitic nematode Onchocerca volvulus

Onchocerciasis is a debilitating parasitic disease caused by the filarial worm Onchocerca volvulus. Similar to other helminth parasites, O. volvulus is capable of evading the host's immune responses by a variety of defense mechanisms, including the detoxification activities of the glutathione S-transferases (GSTs). Additionally, in response to drug treatment, helminth GSTs are highly up-regulated, making them tempting targets both for chemotherapy and for vaccine development. We analyzed the three-dimensional x-ray structure of the major cytosolic GST from O. volvulus (Ov-GST2) in complex with its natural substrate glutathione and its competitive inhibitor S-hexylglutathione at 1.5 and 1.8 angstrom resolution, respectively. From the perspective of the biochemical classification, the Ov-GST2 seems to be related to pi-class GSTs. However, in comparison to other pi-class GSTs, in particular to the host's counterpart, the Ov-GST2 reveals significant and unusual differences in the sequence and overall structure. Major differences can be found in helix alpha-2, an important region for substrate recognition. Moreover, the binding site for the electrophilic co-substrate is spatially increased and more solvent-accessible. These structural alterations are responsible for different substrate specificities and will form the basis of parasite-specific structure-based drug design investigations.