Efficacy Of Moxidectin Research Articles

Investigation of Anthelmintic Resistance and Deworming Regimens in Horses

AbstractThree studies were conducted to examine parasite resistance and anthelmintic efficacy in the horse. In Experiment 1, 3 fenbendazole (FBZ) treatments (5 mg/kg BW, 10 mg/kg BW, or a larvicidal, 50 mg/kg BW) were administered to horses treated only with FBZ for 18 mo prior. Resistance to FBZ was documented by larval hatch assay and fecal egg count reduction (FECR) tests. In all treatment groups, FECR was not greater than 90%. In Experiment 2, the efficacy of moxidectin (MOX), ivermectin (IVM), and the larvicidal FBZ dose were studied. Mean FECR was 99.91%, 98.73%, and 84.3%, respectively. Animals treated with MOX and IVM had higher (P < 0.05) FECR than the FBZ group. When FECR was analyzed by age, there were no differences among treatment groups in the mature horses, whereas the MOX and IVM groups were higher (P < 0.05) than the FBZ group in the young horses. Experiment 3 determined the efficacy of a year-long quarterly rotation. A farmwide study, including the animals from Experiments 1 and 2, showed a quarterly rotation of pyrantel pamoate (June), IVM (September), larvicidal FBZ (December), and MOX (March) effectively reduced or broke resistance to FBZ, based on FECR tests. The mean FECR for the treatments were 95.86, 100, 97.84, and 100, respectively. These studies indicate that neither frequent use nor larger doses of FBZ will alleviate a FBZ resistance problem in the horse. However, Experiment 3 demonstrated that a rotational regimen can effectively break or reduce FBZ resistance and that FBZ can be successfully implemented in a rotational scheme subsequent to prior resistance.

Comparison of efficacy of moxidectin and ivermectin in the treatment of Strongyloides fulleborni infection in rhesus macaques

Strongyloides infection may result in clinical disease or confound experimental protocols that utilize non-human primates. There is presently a Strongyloides fulleborni infection rate of approximately 27% in the Tulane National Primate Research Center's breeding colonies despite the routine therapeutic and prophylactic use of ivermectin. A study was conducted to determine if moxidectin treatment offers advantages to the intestinal parasite control program. A total of 150 rhesus macaques (Macaca mulatta) that were removed from the breeding colonies due to illness were selected for the study. The animals were randomly assigned to treatment groups with 75 receiving ivermectin and 75 receiving moxidectin. Egg counts were performed on fecal samples collected pre- and post-treatment. Both treatments resulted in decreases in the number of eggs/g in the post-treatment sample as compared with the pre-treatment sample; however, no significant difference was found between treatment groups. With the data demonstrating a similar efficacy in both ivermectin and moxidectin treated macaques, the benefit of moxidectin treatment relates to biosafety and topical application.

Comparative efficacy of Moxidectin and Ivermectin in cattle naturally infected with old world screw worm larvae

Moxidectin is a second generation semi-synthetic Macrocyclic Lactone of the Milbemycins family, which have a broad spectrum of activity against external and internal parasite including Oestrus ovis and hypodermatosis. A trial was conducted to identify the efficacy of moxidectin against the natural infection of cattle with larvae of screw worm fly Chrysomya bezziana. A dose of 0.2 mg/ kg b.wt. (1 ml /50kg b. wt.) S.C . which was found to be very effective for more than 40 days? The trial shows the effect of Moxidectin is over that of Ivermectin in re-infection of the animal. No side effects & no abnormal behaviors, was noticed on the animals treated with Moxidectin during the trial.

Efficacy of moxidectin 0.5% pour-on against naturally acquired nematode infections in cattle in the Mexican tropics

The efficacy of 0.5% moxidectin pour-on in cows with naturally acquired nematode infections was evaluated. The study was carried out in a ranch in Veracruz, Mexico. Four groups of 15 cows were randomly allocated. Animals in the treated group received 0.5% moxidectin pour-on at a dose of 0.5 mg/kg body weight on a single occasion. The other two groups remained as untreated controls. Fecal samples from all cattle were taken on days 0 (pre-treatment), 7, 14, 28 and 60 (post-treatment, PT). Fecal egg-counts were determined using a modified McMaster technique and fecal cultures were performed to identify gastrointestinal nematodes infected larvae (L 3). Treatment with moxidectin was associated with a significant reduction in fecal trichostrongyle egg-counts compared with the controls; efficacy was 100% at 28 days PT. Haemonchus spp. and Strongyloides spp. were the two genera identified from coprocultures.

Evaluation of the efficacy of moxidectin in a herd of goats after long-standing consistent use

Two trials were carried out in a herd of goats, where moxidectin had been consistently used for a long-standing period, in order to monitor its field efficacy. Trial I was carried out after the drug had been used for 4 years/13 times and trial II after it had been used for 7 years/23 times. In each trial, 90 goats naturally infested with gastrointestinal nematodes were divided into three groups ( n = 30) and treated as follows: groups I-A and II-A with moxidectin 1% injectable solution at a dose rate of 0.2 mg kg −1 bodyweight, groups I-B and II-B with moxidectin 0.1% oral drench at a dose rate of 0.2 mg kg −1 bodyweight, groups I-C and II-C were kept as untreated controls. Before treatment, mean faecal epg counts were >350.0 for all groups, whilst 14 days later, mean faecal epg counts were 0.0 for groups I-A, II-A, I-B and II-B (percentage reduction after moxidectin administration 100%); 56 days after treatment, mean faecal epg counts were <30.0 for groups I-A, II-A, I-B and II-B (percentage reduction after moxidectin administration >93.5%). Coprocultures revealed the following genera of nematodes in faecal samples Trichostrongylus spp., Teladorsagia spp., Haemonchus contortus, Chabertia ovina, Oesophagostomum spp. and Bunostomum spp. It is concluded that injectable and oral moxidectin are effective against gastrointestinal nematodes in goats at the dose rate of 0.2 mg kg −1 bodyweight.

Therapeutic and persistent efficacy of moxidectin 1% nonaqueous injectable formulation against natural and experimentally induced lung and gastrointestinal nematodes in cattle

Four controlled trials were conducted to evaluate the therapeutic and persistent efficacy of a new moxidectin formulation (moxidectin 1% nonaqueous injectable) against nematode parasites in cattle. This injectable moxidectin formulation, given as a single subcutaneous injection at a dose rate of 0.02 ml/kg BW to provide 0.2 mg moxidectin/kg BW, was highly efficacious (>90–100%) against larval and/or adult stages of many species of nematodes in cattle including, Dictyocaulus viviparus, Ostertagia spp., Trichostrongylus axei, Haemonchus placei, Trichostrongylus colubriformis, Cooperia spp., Nematodirus helvetianus, Strongyloides papillosus, Oesophagostomum radiatum and Trichuris spp. This formulation had persistent efficacy of >90% against D. viviparus for at least 6 weeks post-treatment, H. placei and Oe. radiatum for 5 weeks post-treatment, and Ostertagia spp. and T. axei for 2 weeks post-treatment.

Dose confirmation of moxidectin 0.5% pour-on against adults and fourth-stage larvae of various Cooperia spp. and Trichostrongylus colubriformis in Louisiana

Thirty-five castrate or intact Holstein male calves, ranging in weight from 122 to 210 kg, were used in the study. On study Days −15 and −14, all study calves were treated with fenbendazole 10% paste at 10 mg/kg to remove existing nematode infections. All cattle had zero egg counts on Day −1. Experimental infections of a mixed species inoculum were administered to all calves on Day 0. The inoculum consisted of Cooperia spp. (primarily C. punctata, but also C. pectinata, C. spatulata, C. oncophora and C. surnabada—total 40,961); Ostertagia ostertagi—1550; Trichostrongylus colubriformis—4996; and Oesophagostomum radiatum—38. Necropsy results from two of three monitor calves slaughtered on Day 6 after infection indicated that Day 6 was an appropriate time to evaluate efficacy of moxidectin against fourth larval stages (L 4). The remaining 32 calves were randomly allotted to four groups of eight based on body weights. Eight calves (Group 1) were treated with moxidectin 0.5% pour-on at 0.5 mg/kg on Day 6 to evaluate efficacy against nematode larval stages; eight control calves (Group 2) were matched with these principals. Eight calves (Group 3) were also treated with moxidectin pour-on at the same dosage on Day 23 in order to determine efficacy against adult nematodes; eight control calves (Group 4) were matched with these principals. In both cases, principals and controls of the Days 6 and 23 treatments were necropsied at 14–15 days after treatment. C. punctata was the only species found in a sufficient number of controls to evaluate moxidectin efficacy against the L 4. Moxidectin pour-on was not effective ( P<0.05) against C. punctata L 4 by arithmetic means, but was highly effective based on geometric means. Regardless of whether arithmetic or geometric means were used for percent efficacy calculations, moxidectin pour-on was demonstrated to be highly effective (>99%) against the adult stages of C. oncophora, C. punctata, C. spatulata, C. surnabada, Cooperia spp. adult females and T. colubriformis adults.

Comparative studies of early season moxidectin treatment and conventional ivermectin/ benzimidazole treatments in the control of cyathostomes in horses

Moxidectin administered in January or February at a single dose was tested for efficacy in horses on two farms for 12 and 11 months, respectively. Horses were infected with cyathostomes naturally in the previous grazing period. Forty horses of farm 1 and 20 horses of farm 2 were used in controlled tests to evaluate the efficacy of moxidectin 2% gel formulation at the dosage 0.4 mg moxidectin per kg of live weight, ivermectin commercial paste formulation at the dosage 0.2 mg ivermectin per kg of live weight, mebendazole and fenbendazole commercial paste formulation at the dosage both 7.5 mg mebendazole and fenbendazole per kg of live weight, all applied orally. Three control groups of 10 horses each (farm 1) were treated twice a year with ivermectin and benzimidazoles, respectively. Individual faecal egg counts, faecal cultures and larval differentiation were performed. Moxidectin had more prolonged and greater suppressive effects on the post-treatment reappearance and magnitude of strongyle egg counts than did ivermectin or benzimidazoles. In the moxidectin treated group (M1) strongyle eggs were seen for the first time in April and a slight increase in the mean count of eggs per gram of faeces (EPG) was observed during the rest of the season. Litter larval counts significantly reflected levels of exposure during the tested season. Twenty animals of farm 2 were allocated into two groups of ten horses each based on pre-treatment eggs per gram (EPG) counts (moxidectin treated group and control group). In the moxidectin treated group mean egg counts remained very low throughout the study. A plateau was reached by autumn, with egg counts ranging from 74 to 145 EPG. The faecal egg counts of moxidectin treated group (M2) were significantly higher in March, April, May and June.

Efficacy of moxidectin 1% injectable and 0.2% oral drench against natural infection by Dictyocaulus filaria in sheep

Two separate trials (I and II) with 34 and 32 Churra ewes, respectively, and distributed into two groups, have been carried out to evaluate the efficacy of two different formulations of moxidectin at a dose rate of 0.2 mg/kg body weight (b.w.) against natural infection by Dictyocaulus filaria in sheep. Trial I was designed to evaluate a 1% moxidectin injectable formulation, whereas in trial II a 0.2% moxidectin oral drench formulation was used. The efficacy was measured on the basis of the reduction of the faecal larval counts and of adult worm recoveries at slaughter. In each trial, a group of animals was treated on day 0 with moxidectin 1% injectable or moxidectin 0.2% oral drench and the other group acted as untreated control. When the faecal larval counts was compared within the treated groups, the efficacy was over 95% until day +13, and 100% at the remainder of the sampling dates after the application of injectable moxidectin, whereas in trial II, the larvae per gram (lpg) of faeces increased until the first sampling time post treatment (p.t.), day +6, and zero counts were recorded for all animals by the following days. On the basis of adult worm recoveries at necropsy, the efficacy of the treatment was 100% in both trials, however, adult worms were detected at slaughter for all control sheep. These results indicate that moxidectin 1% injectable and moxidectin 0.2% oral drench, administered at 0.2 mg/kg b.w., were 100% effective against D. filaria infection in sheep. No adverse reactions to the treatments were observed in the animals.

Efficacy of moxidectin 1% injectable against natural infection of Sarcoptes scabiei in sheep

Thirty ewes naturally infected with Sarcoptes scabiei var. ovis, were allocated into three groups of 10 animals each. Animals in groups B and C were treated on day 0 and on days 0 and +10, respectively, with moxidectin 1% injectable at a dose of 0.2 mg moxidectin/kg body weight (BW). Group A remained untreated. Seven days before treatment, the geometric mean of Sarcoptes scabiei var. ovis per square centimeter of skin in groups A, B and C were not significantly different. From the day of treatment to the end of the trial, the average number of mites/cm 2 increased in untreated animals and decreased in groups B and C, but these values were higher for group C. Active lesions produced by S. scabiei var. ovis consistently increased during the trial in the untreated animals; in group B the minimum count occurred on day +56 this reduction being more evident in group C (no lesions on days +49 and +56). Also in this group, the number of cured animals was 100%, therefore, the application of two treatments with moxidectin (group C) showed higher efficacy than a single treatment (group B). Body condition score decreased in the three experimental groups along the trial. All animals were individually weighed on days −1, +28 and at the end of the trial. No adverse reactions were observed in the animals treated with 0.2 mg moxidectin/kg BW.

Effect of multidrug resistance modulators on the activity of ivermectin and moxidectin against selected strains of Haemonchus contortus infective larvae

Nematode parasites have shown resistance to the anthelmintics, ivermectin and moxidectin, and there is evidence that the over-expression of parasite P-glycoprotein (P-gp) may account, at least in part, for resistance to ivermectin. The objective of this study was to evaluate whether the multidrug resistance (MDR) modulators, verapamil, CL 347.099 (an analog of verapamil) and cyclosporin A, would enhance the efficacy of ivermectin and moxidectin against selected strains of Haemonchus contortus using an in vitro larval migration assay. The modulators had no effects on the number of migrating larvae when used alone. Ivermectin and moxidectin showed a significant (P&lt;0.05) increase in its efficacy by 52.8 and 58.5% respectively, when used in association with verapamil against a moxidectin-selected strain. CL 347,099 also increased significantly (P&lt;0.05) the ivermectin and moxidectin efficacy by 24.2 and 40.0% respectively, against an ivermectin-selected strain and by 40.0 and 75.6% respectively, against an moxidectin-selected strain. At the concentrations tested cyclosporin A showed a variable effect on increasing the efficacy of the anthelmintics against the susceptible and resistant strains.

Resistance to macrocyclic lactone anthelmintics by Haemonchus contortus and Ostertagia circumcincta in sheep in New Zealand

AIM: To establish the efficacy of oral formulations of ivermectin and moxidectin against naturally acquired abomasal nematode infections on a North Island sheep farm. METHODS: Two controlled slaughter trials were undertaken. In the first, 30 sheep on pasture were randomly allocated on the basis of faecal egg count to 1 of 3 groups, comprising an untreated control group and 2 treatment groups. One treatment group was given a single oral dose of ivermectin and the other a single oral dose of moxidectin, both at the manufacturer's recommended dose rates of 0.2 mg/kg liveweight. Six days after treatment, all animals were slaughtered and their abomasa recovered for worm counting. The second trial, which involved 47 animals, was essentially the same as the first except that, as well as involving the slaughter of 30 sheep from all 3 groups, 6 days after treatment, it also included a further 8 untreated control animals and 9 moxidectin treated animals which were slaughtered 27 days after treatment. RESULTS: At 6 days after treatment, moxidectin was highly effective against all 3 of the abomasal nematodes present. While ivermectin was similarly effective against Trichostrongylus axei 6 days after treatment, it was not effective against either Ostertagia circumcinta or Haemonchus contortus, against which average efficacies of only 63.6% and 61.6%, respectively, were recorded. At 27 days after treatment, moxidectin, was also highly effective against T. axei (97.3% reduction) but not against either H. contortus (71.4% reduction) or O. circumcinta (61.0% reduction). CONCLUSIONS: These results provide the first record of macrocyclic lactone resistance in H. contortus in sheep or in any other host in New Zealand, and the first case where such resistance has been exhibited in more than one parasite species at a time. Although the therapeutic efficacy of moxidectin was high against these resistant H. contortus and O. circumcincta strains, resistance to moxidectin was indicated by its diminished prophylactic activity against them. It is suggested that this reduction in the prophylactic activity of moxidectin is also likely to reduce its apparent current high therapeutic efficacy. CLINICAL RELEVANCE: As well as providing further evidence that it can no longer be automatically assumed that macrocylic lactone anthelmintics will be effective on sheep farms in this country, these findings also present a warning that increasingly complex parasite control options may have to be faced in the future.

Efficac of moxidectin 2 per cent gel against naturally acquired strongyle infections in horses, with particular reference to larval cyathostomes

The efficacy of moxidectin 2 per cent equine gel against naturally acquired strongyle infections was assessed in 18 ponies which had grazed on contaminated pasture before being housed for eight...

Field efficacy of moxidectin in dogs and rabbits naturally infested with Sarcoptes spp., Demodex spp. and Psoroptes spp. mites

The efficacy of moxidectin 1% injectable for cattle was evaluated in dogs and rabbits with naturally acquired sarcoptic, demodectic or psoroptic mites. Twenty-two dogs with generalised demodicosis were orally treated with 0.4 mg/kg moxidectin daily. Forty-one dogs suffering from sarcoptic mange were treated with 0.2–0.25 mg/kg moxidectin either orally or subcutaneously every week for three to six times. Seven rabbits were treated orally with 0.2 mg/kg moxidectin twice 10 days apart. Of the 22 dogs with demodicosis, 14% were stopped treatment because of side effects, 14% were lost and of the remaining 72% all were cured (mean therapy duration 2.4 months). Thirty-seven of the sarcoptic mange-infected dogs finished treatment and were cured. In 17% of dogs, side effects were noted. All seven rabbits treated for psoroptic mange were cured and did not show any side effect. Our results indicate that moxidectin is effective and a good alternative for the treatment of demodicosis and scabies in dogs and psoroptic mange in rabbits. Side effects seem to occur more frequently if applied subcutaneously, therefore the oral route should be preferred.

Ivermectin-resistant Ostertagia circumcincta from sheep in the lower North Island and their susceptibility to other macrocyclic lactone anthelmintics

Aim: To confirm the ivermectin resistance status of a strain of Ostertagia circumcincta which was isolated from a sheep farm in the lower North Island of New Zealand and to assess the susceptibility of this strain to other macrocycliclactone anthelmintics. Methods: Twenty-five lambs housed indoors were each infected with 12,000 L3 larvae of the above parasite strain. Approximately 3 weeks after infection the lambs were allocated to 1 of 4 treatment groups (3 groups of 6, and 1 group of 7 lambs), one of which remained untreated while the others were drenched orally with ivermectin, moxidectin or abamectin at 0.2 mg/kg liveweight. Faecal egg counts (FECs) before and after treatment, and post-mortem worm burdens 10 days after treatment were examined to assess efficacies of each anthelmintic. Results: Treatment with ivermectin reduced the mean FEC by only 18% and the mean worm burden by only 42%, whereas moxidectin and abamectin reduced FECs by > 92% and worm burdens by >95%. Conclusion: These results, together with a similar case described recently from the South Island, confirm the emergence of ivermectin resistance in nematode parasites of sheep in New Zealand. The superior efficacy of moxidectin and abamectin in this case indicates that, following the emergence of resistance to ivermectin, some short-term practical use may still be made of these other anthelmintics. However, their continued use will undoubtedly result in increased levels of resistance and eventual therapeutic failure of these products also.

Persistent efficacy of moxidectin against Sarcoptes scabiei in sheep.

Persistent efficacy of moxidectin against Sarcoptes scabiei in sheep.

Efficacy of moxidectin equine oral gel against endoscopically-confirmed Gasterophilus nasalis and Gasterophilus intestinalis (Diptera: Oestridae) infections in horses

A 3 m, video gastroscope was used to screen 47 horses suspected of being naturally infected with equine bot larvae. 17 of 47 (36.2%) candidate horses harbored Gasterophilus nasalis larvae in the proximal duodenum and 46 of 47 (97.9%) had G. intestinalis larvae in the stomach. All horses infected with G. nasalis had concurrent infections with G. intestinalis. 14 horses with dual infections were allocated randomly to two treatment groups. Seven horses in Group 1 received 2% moxidectin oral gel once at a dosage of 0.4 mg/kg bodyweight (BW), and seven horses in Group 2 were untreated controls. 14 days after treatment, all horses were necropsied and the stomach and proximal duodenum harvested from each. Bot larvae were recovered, identified to species and instar, and counted. At the label dosage, moxidectin oral gel was 100 and 97.6% effective ( P < 0.05) against third-instar G. nasalis and G. intestinalis, respectively. In addition to demonstrating the boticidal efficacy of moxidectin, this trial illustrated that gastroscopy/duodenoscopy is a feasible method for confirming infections with different species of bot larvae in the horse.

Efficacy of moxidectin against sarcoptic mange and effects on milk yield of ewes and growth of lambs

Forty-eight dairy ewes and 45 lambs naturally infested with sarcoptic mange, were divided into three equal groups and treated with moxidectin 1% inj. sol. at a dose rate of 0.2 mg moxidectin kg −1 bodyweight once or twice ten days apart. During the study, the skin lesions were scored and the number of mites recovered in facial skin scrapings from the animals were recorded. Moxidectin was found to be effective against ovine sarcoptic mange, when given twice; the percentage reduction of mites was 100% ( p < 0.0001) and the skin lesions disappeared completely ( p = 0.0001). In sheep treated once only, the percentage reduction of mites varied from 75% to 92%. Treated ewes produced up to 22.4% more milk than the infested ones ( p = 0.0001). Treated lambs had up to 15% greater final weight and up to 23.7% more growth than infested controls ( p = 0.0001). It is concluded that two injections of moxidectin 1% inj. sol. at a dose rate of 0.2 mg kg −1 bodyweight, given ten days apart, were effective against ovine sarcoptic mange. Furthermore, it is considered that sarcoptic mange adversely affects the production of the infested animals.

Efficacy of moxidectin 0.5% pour-on against swine nematodes

Forty pigs with induced infections of Ascaris suum, Trichuris suis, Metastrongylus spp., Oesophagostomum dentatum and O. quadrispinulatum were assigned to five-dose groups of moxidectin 0.5% pour-on with eight pigs per dose group. The doses were: moxidectin, 0 (vehicle control), 0.75, 1.00, 1.25, and 1.50 mg/kg −1 body weight. Worm egg counts (EPG) were made from fecal samples collected on Day 2 pretreatment and on Day 14 or 15 post-treatment. Animals were ranked according to the descending order of A. suum egg counts made on Day 2 and blocked in groups of five. Pigs in blocked groups were assigned randomly to each of the five dose groups. Treatment doses were calculated on the basis of weights taken on Day 1 and were administered topically from the neck to the base of the tail. Pigs were housed by pairs in individual pens provided with self-feeders and automatic waterers. Necropsies were performed on equal numbers of pigs from each treatment group on days 14 and 15 post-treatment. Adult and larval worms were collected, identified and counted by standard parasitological techniques. All counts were transformed by Y = log 10 (count +1) transformation prior to analysis. A two-way analysis of variance was conducted and treatment effect was tested for significance at the 5% level. Efficacies based on geometric means and optimal doses were as follows: Ascaris suum, 98.3% at 1.25; Metastrongylus spp., 100% at 0.75; Oesophagostomum quadrispinulatum, 100% at 1.50; and Trichuris suis, 93.5% at 0.75 . Efficacy for O. dentatum was from 81.3% to 100%; however, the average number of O. dentatum (30) was too small for significance. Two species of lungworms were present, Metastrongylus apri and M. pudendotectus but they were not speciated at necropsy. As reported for several anthelmintics, the efficacy of moxidectin was variable for Trichuris. The highest efficacy was in the 0.75 dose group with six pigs harboring a few or no worms. The lowest efficacy was in the 1.25 group with only two pigs harboring a few or no worms.

Moxidectin: persistence and efficacy against drug-resistant Ostertagia circumcincta.

In order to determine whether the efficacy of moxidectin against Ostertagia circumcincta is enhanced by its persistency, therapeutic efficacy was compared at intervals after treatment and with that of ivermectin, a closely related but more transient endectocide. Groups of 7-month-old New Zealand Romney lambs were infected with a strain of O. circumcincta known to be resistant to moxidectin. At patency of the infections, groups of lambs were treated with either moxidectin or ivermectin at the manufacturer's recommended dosages, or left untreated. At 3, 6 and 10 days post-treatment, faecal egg count was measured and groups of lambs were slaughtered for estimation of adult worm burden. Drug-resistant worm burdens were significantly reduced in those animals treated with moxidectin but not in those treated with ivermectin. No effect of time of slaughter on worm burden was observed with either drug, demonstrating that the higher therapeutic efficacy of moxidectin against this parasite was not due to an increased period of drug exposure. Faecal egg counts in the moxidectin treated animals increased with time after treatment indicating a temporary suppression of egg output by surviving worms. The implications of these findings on selection for anthelmintic resistance are discussed.