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.

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.

The influence of the administration route on the relationship between efficacy and ivermectin concentration profiles achieved in the bloodstream, the gastrointestinal mucosal tissues/fluid contents and within a target abomasal parasite (Haemonchus contortus) was evaluated in lambs. Twenty-six (26) parasitized lambs were assigned into three experimental groups: untreated (control) and ivermectin treated by the subcutaneous and intraruminal route at 0.2mg/kg. Blood samples were collected between 0 and 15 days post-treatment (plasma disposition study). Four animals from each group were sacrificed at day 3 post-treatment. Mucosa and content samples from abomasum and small intestine and adult specimens of H. contortus were collected. Drug concentrations were measured by HPLC. Individual fecal egg counts were evaluated at -1, 3 and 15 days post treatment. Post-mortem examination was done at day 15 post-treatment. Adult nematodes recovered from the digestive tract were counted and identified by species. Ivermectin plasma availability was higher (P<0.05) after the subcutaneous administration (129 ng.d/ml) compared to the intraruminal treatment (58.4 ng.d/ml). However, ivermectin concentrations measured in the gastrointestinal contents were higher in lambs treated by the intraruminal route. The mean ivermectin concentrations achieved (3 days post-treatment) in the abomasal content were 143 ng/g (intraruminal) and 2.53 ng/g (subcutaneous). Ivermectin concentrations were 15-fold higher in H. contortus recovered from intraruminally treated lambs. Whereas the subcutaneous administration reduced the number of adult nematodes from 4376 to 1300, the number of adult nematodes after the treatment with ivermectin given by the intraruminal route was 206 (P<0.05). The higher ivermectin concentrations achieved in the digestive tract shortly after the intraruminal treatment may account for the observed enhanced efficacy compared to the parenteral administration against parasites of reduced susceptibility.

Albendazole failure to control resistant nematodes in lambs: lack of effect of fasting-induced improvement on drug absorption.

Enhanced plasma availability of albendazole sulphoxide (ABZSO), the active metabolite of albendazole (ABZ), has been described in feed-restricted sheep. The aim of the present work was to determine if the absorption-related pharmacokinetic changes derived from fasting animals prior to drug treatment would modify the clinical efficacy of ABZ against resistant gastrointestinal nematodes in lambs. Forty Corriedale lambs, naturally infected with resistant gastrointestinal nematodes, were divided into 4 groups. Controls were fed ad libitum and did not receive any drug treatment. Treated animals were fed ad libitum up to 30 min prior to treatment with ABZ (3.8 mg/kg) by the intraruminal route. The control (fasted) animals were not fed during the 24-hr period prior to the start of the experiment and did not receive any drug treatment. A second treated group of animals were fasted 24 hr prior to the treatment with ABZ, as previously described for the fed-treated group. Blood samples were collected over a period of 72 hr post-treatment from 6 animals in each treated group. Plasma samples were analyzed by high performance liquid chromatography. The pharmacokinetic parameters were statistically compared using parametric statistical tests. The estimation of the efficacy of the different treatments was performed by the fecal egg count reduction test (FECRT). Additionally, 4 animals randomly chosen from the control-fed and treated groups were killed 13 days post-treatment to evaluate the efficacy against different adult nematode parasites. The results were statistically compared by parametric and non-parametric tests. Significantly (P < 0.05) higher Cmax and AUC values were observed for both the ABZSO and ABZ-sulphone (ABZSO(2)) metabolites in the fasted compared to the fed animals. These kinetic results may be due to a fasting-induced delay in the GI transit time which increases ABZ dissolution and GI absorption. However, a poor ABZ efficacy (measured as FECRT), compatible with a high degree of nematode resistance, was obtained in both fed (48%) and fasted (49%) animals. Haemonchus contortus and Trichostrongylus colubriformis appeared as the more reluctant species with respect to ABZ treatment. The efficacy against H. contortus ranged between 37 (fed) and 54% (fasted) and against T. colubriformis between 0% (fed) and 16% (fasted). Under these experimental conditions, the fasting-induced improvement on ABZ systemic availability (>60%) did not improve its activity against nematodes with a high degree of resistance. However, the data described here for a highly resistant nematode population should not discourage the use of fasting as a practical and well-proven management tool for parasite control in ruminants.

Interference with P-glycoprotein improves ivermectin activity against adult resistant nematodes in sheep.

The in vivo co-administration of ivermectin (IVM) with P-glycoprotein (P-gp) modulator agents has been shown to enhance its systemic availability. However, there is no sufficient evidence on the impact that this type of drug-drug interaction may have on the in vivo efficacy against resistant nematodes in ruminant species. The current work reports on the effects of loperamide (LPM), a P-gp modulating agent, on both IVM kinetic behaviour and anthelmintic activity in infected lambs. Eighteen (18) lambs naturally infected with IVM-resistant gastrointestinal nematodes were allocated into three (3) experimental groups. Group A remained as untreated control. Animals in Groups B and C received IVM (200mug/kg, subcutaneously) either alone or co-administered with LPM (0.2 mg/kg, twice every 12h), respectively. Individual faecal samples were collected from experimental animals at days -1 and 14 post-treatment to perform the faecal eggs count reduction test (FECRT). Blood samples were collected between 0 and 14 days post-treatment and IVM plasma concentrations were determined by HPLC. Additionally, at day 14 post-treatment, lambs from all experimental groups were sacrificed and adult gastrointestinal nematode counts were performed. FECRT values increased from 78.6 (IVM alone) to 96% (IVM+LPM). Haemonchus contortus was highly resistant to IVM. The IVM alone treatment was completely ineffective (0% efficacy) against adult H. contortus. This efficacy value increased up to 72.5% in the presence of LPM. The efficacy against Trichostrongylus colubriformis increased from 77.9% (IVM alone) to 96.3% (IVM+LPM). The described favorable tendency towards improved anthelmintic efficacy was in agreement with the enhanced IVM plasma availability (P<0.05) and prolonged elimination half-life (P<0.05) induced by LPM in infected lambs. A LPM-induced P-gp modulation increases IVM systemic exposure in the host but also it may reduce P-gp efflux transport over-expressed in target resistant nematodes.

Clinical efficacy assessment of the albendazole-ivermectin combination in lambs parasitized with resistant nematodes.

Combination of anthelmintic drugs from different chemical groups has been proposed as alternative parasite control strategies where failure of individual drugs is documented. The main goal of the current trial was to compare the clinical anthelmintic efficacy of albendazole (ABZ) and ivermectin (IVM) given either separately or co-administered to lambs naturally infected with gastrointestinal nematodes resistant to both molecules. Seventy (70) Corriedale lambs naturally infected with multiple resistant gastrointestinal nematodes were involved in the efficacy trial: the animals were allocated into 7 experimental groups (n=10) and treated with either ABZ intravenously (iv) (ABZ(IV)), IVM(IV), ABZ(IV)+IVM(IV), ABZ intraruminally (ir) (ABZ(IR)), IVM subcutaneously (sc) (IVM(SC)) and ABZ(IR)+IVM(SC) or kept as untreated controls. The indirect estimation of the efficacy of the different treatments was performed by the faecal egg count reduction test (FECRT). Additionally, four animals randomly chosen from the untreated control and ABZ(IV,) IVM(IV) and ABZ(IV)+IVM(IV) experimental groups were sacrificed 15 days post-treatment to evaluate the efficacy against different adult resistant nematode parasites. The results were statistically compared by a non-parametric ANOVA (Kruskal-Wallis test). The following egg output reduction values were obtained: 73.4% (ABZ(IV)), 79.0% (IVM(IV)), 91.9% (ABZ(IV)+IVM(IV)), 43.5% (ABZ(IR)), 79.8% (IVM(SC)) and 70.8% (ABZ(IR)+IVM(SC)). The efficacy against Haemonchus spp. was 95.1 (ABZ(IV)), 99.3 (IVM(IV)) and 99.9% (ABZ(IV)+IVM(IV)), while the efficacy against Trichostrongylus colubriformis for the same treatment groups was 79.6, 100 and 99.9%. The data obtained on the assessment of the ABZ-IVM combination indicates that no potentiation synergism is observed. This work is complementary to a parallel study that demonstrated the lack of negative pharmacokinetic interactions between the two anthelmintics acting by different mode of action. Thus, an additive effect may be achieved against nematodes resistant to both compounds. Further work is required to understand the implications of potential pharmacokinetic/pharmacodynamic interactions between anthelmintics before drug combined formulations are developed to be introduced into the pharmaceutical market.

Evaluation of the interaction between ivermectin and albendazole following their combined use in lambs.

Mixtures of drugs from different chemical families have been proposed as a valid strategy to delay the development of anthelmintic resistance. The current work summarizes the outcome of the evaluation of the plasma disposition kinetics of albendazole (ABZ) and ivermectin (IVM) administered either alone or co-administered to lambs infected with gastrointestinal (GI) nematodes resistant to both anthelmintic molecules. Thirty six (36) Corriedale lambs naturally infected with multiple resistant GI nematodes were allocated into six treatment groups: (a) ABZ intravenous (ABZ(IV)); (b) IVM(IV); (c) ABZ(IV) + IVM(IV); (d) ABZ intraruminal (IR); (e) IVM subcutaneous (SC) and (f) ABZ(IR) + IVM(SC). Plasma samples were collected over 15 days post-treatment and analysed by HPLC. The estimated pharmacokinetic (PK) parameters were statistically compared using parametric and non-parametric statistical tests. The presence of IVM did not affect the plasma disposition kinetics of ABZ and its metabolites after the i.v. administration. However, the ABZ sulphoxide (ABZSO) area under the concentration vs. time curve (AUC) was significantly lower (P < 0.01) after the intraruminal (i.r.) administration of ABZ alone compared to that obtained for the combined treatment with IVM [subcutaneous (s.c.) injection]. The IVM plasma AUC obtained after its i.v. co-administration with ABZ was 88% higher (P < 0.05) compared to the treatment with IVM alone. Any marked difference on IVM PK parameters was observed between the treatments ABZ + IVM and IVM alone injected subcutaneously. The data obtained here indicate that the co-administration of ABZ and IVM does not induce an adverse kinetic interaction. This type of pharmacology-based evaluation of drug interactions is becoming highly relevant as drug combinations are now widely used as an alternative to control resistant helminth parasites in livestock.

Epidemiology and control of bovine ostertagiasis in South America.

Gastrointestinal parasitism has been recognized by practitioners as the most common disease in beef cattle, mainly in weaning calves and fattening steers. Among the different genera, Ostertagia ostertagi is the predominant parasite in the temperate climate, in which the major beef and dairy cattle area of South America is situated. Outbreaks of Type I ostertagiasis are usually seen after weaning time (autumn-winter) when larvae counts are high and food availability is low. The development of the disease is rapidly established and 15-30 kg are lost in 30-50 days. Epidemiological studies have demonstrated a fast evolution of parasite eggs to larvae (L3) in summer (1 week or less), evolution being 30-45 days during winter. Inhibition O. ostertagi occurs during spring (September-December) and development resumes in late summer and early autumn. The production effect is seen as a significant reduction in body weight gain and occasionally clinical Type II ostertagiasis appears. A similar epidemiological pattern of inhibition of Ostertagia sp. has been recorded in Uruguay and temperate areas in southern Brazil.

The sequential development of type I and type II ostertagiasis in young cattle with special reference to biochemical and serological changes.

The sequential development of Type I and Type II ostertagiasis over a 2-year period in the same naturally infected cattle is described for the first time. Particular reference is made to biochemical and serological changes. Positive relationships were demonstrated between the clinical signs of both Type I and Type II disease, and marked increases in the levels of plasma pepsinogen, plasma gastrin and antibody titres to adult Ostertagia antigen. At necropsy, there were significant relationships between the combined total of adult and developing 5th stage larvae of Ostertagia spp. and the levels of both plasma pepsinogen and gastrin. By the end of the second grazing season the cattle had acquired an immunity to infection with Ostertagia spp. and had very low burdens of this parasite at necropsy. However some of these cattle maintained elevated plasma pepsinogen levels when under natural challenge by Ostertagia spp. larvae and the aetiology of these changes and the problems of diagnosis using this parameter are discussed. Similar trends of infection were observed for Cooperia oncophora, although resistance to the parasite developed more rapidly.

Metabolism and growth in housed calves given a morantel sustained release bolus and exposed to natural trichostrongyle infection.

Three groups of calves, previously grazed on permanent pastures contaminated with bovine trichostrongyle (Ostertagia and Cooperia species) infective larvae, were housed from October to May. During the grazing season one group had received fenbendazole at fortnightly intervals to suppress trichostrongyle infections, one received a morantel sustained release bolus before grazing to limit trichostrongyle contamination of the pasture, and the control group was only medicated when heavy infections caused clinical type 1 ostertagiasis. Digestibility of the whole diet was poorer in the control cattle, particularly for dry matter, crude protein and energy fractions. Balance studies conducted throughout the winter housed period showed that both increased faecal and urinary nitrogen outputs in the control animals contributed to a significantly reduced overall nitrogen retention. The effects were most apparent during clinical type 2 ostertagiasis, which occurred in March in the control group.

Production, parasitological and carcase evaluation studies in steers exposed to trichostrongyle infection and treated with a morantel bolus or fenbendazole in two consecutive grazing seasons.

Friesian steers were exposed to natural trichostrongyle infection and left untreated (controls) or treated with either a morantel sustained release bolus or fenbendazole at two week intervals (clean) in each of two consecutive grazing seasons. In the second grazing season the mean liveweight gains of the clean, morantel treated and control groups of cattle over 152 days were 105, 131 and 109 kg respectively (morantel treated greater than clean, control P less than 0.02). The cattle were slaughtered following an indoor fattening period. The dressed carcase weight, killing out percentage and related carcase measurements were superior in the treated groups. Eighty per cent of the clean and morantel treated cattle graded satisfactorily and qualified for subsidy payment whereas less than 30 per cent of the control cattle were successful. Rib joint analyses showed significantly lower total weight, eye muscle weight and area together with a higher bone content in control cattle.