The efficacy of fenbendazole and albendazole against immature and adult stages of benzimidazole-resistant sheep trichostrongylids

The efficacy of two recently introduced benzimidazole anthelmintics, albendazole and fenbendazole, was determined for six-day, 10-day and adult stages of resistant strains of Haemonchus contortus and Trichostrongylus colubriformis. Albendazole, at 3.8 mg/kg reduced H contortus worm counts by 92.4, 70.8 and 67.1 per cent while fenbendazole, at 5.0 mg/kg, reduced worm burdens by 51.7, 95.5 and 93.4 per cent against six-, 10- and 25-day-old parasites respectively. For T colubriformis, the corresponding reductions with albendazole were 97.7, 95.8 and 64.9 per cent and for fenbendazole 29.0, 66.3 and 33.4 per cent. Compared with susceptible strains of H contortus and T colubriformis, for which therapeutic doses of benzimidazole anthelmintics are generally highly active against all stages of development, the present results show that these drugs do not have a uniform level of activity against all developmental stages of resistant strains.     

THE ANTHELMINTIC EFFICACY OF NON-BENZIMIDAZOLE ANTHELMINTICS AGAINST BENZIMIDAZOLE RESISTANT STRAINS OF HAEMONCHUS CONTORTUS AND TRICHOSTRONGYLUS COLUBRIFORMIS IN SHEEP

The anthelmintic efficacy of 6 non-benzimidazole compounds and thiabendazole against standardised, benzimidazole resistant strains of H. contortus and T. colubriformis in sheep was determined using a controlled test. All compounds were administered intraruminally at their recommended therapeutic dose rates and 4 of the compounds were also assessed at half their recommended therapeutic dose rates. Levimisole at a dose of 6.4 mg/kg body weight, morantel at 4.4 mg/kg and 8.8 mg/kg, naphthalophos at 6.25 mg/kg and 12.5 mg/kg, rafoxanide at 7.5 mg/kg and phenothiazine at 530 mg/kg were 98% or more effective in removing adult infections of benzimidazole resistant H. contortus. Rafoxanide at 3.75 mg/kg, phenothiazine at 265 mg/kg and carbon tetrachloride at 0.05ml/kg were less effective, removing 80%, 73% and 72% respectively of the benzimidazole resistant H. contortus worm burdens. Against the benzimidazole resistant T. colubriformis levamisole at 6.4 mg/kg, and morantel at 4.4 mg/kg and 8.8 mg/kg removed 99%, 68% and 86% respectively of the adult infections. All other anthelmintics had little or no efficacy at the dose rates tested. Thiabendazole at 44 mg/kg had no significant effect against these strains of H. contortus and T. colubriformis. Several non-benzimidazole anthelmintics have shown high efficacy in this experiment and should be suitable for treating infections with benzimidazole resistant H. contortus. Of the anthelmintics tested, only levamisole and morantel show high efficacy against benzimidazole resistant T. colubriformis.     

The dose response of several benzimidazole anthelmintics against resistant strains of Haemonchus contortus and Trichostrongylus colubriformis selected with thiabendazole

Dose response lines for eight benzimidazole anthelmintics and thiophanate were determined, using standardised strains of thiabendazole selected and resistant Haemonchus contortus and Trichostrongylus colubriformis. Against H contortus, thiophanate, thiabendazole, parbendazole and oxibendazole were inactive. Mebendazole was inactive at dose rates of 6.26 and 12.5 mg/kg, although significant activity occurred at 25 mg/kg. Fenbendazole, cambendazole, oxfendazole and albendazole demonstrated significant activity at dose rates equal to or greater than the recommended therapeutic level. Thiophanate was inactive against resistant T colubriformis. The remaining compounds only showed significant activity when used at dose rates in excess of the recommended therapeutic level. These results show that a side resistance exists among the benzimidazole anthelmintics and suggests that changes in dose response lines could be expected to occur if resistant strains are selected with benzimidazoles other than thiabendazole.     

The efficacy of triclabendazole and other anthelmintics against Fasciola hepatica in controlled studies in cattle

In eight controlled tests 274 cattle were used to assess the efficacies of triclabendazole, albendazole, clorsulon, nitroxynil, oxyclozanide and rafoxanide against Fasciola hepatica. Against one-, two- and four-week-old early immature fluke the mean efficacies of triclabendazole given orally at 12 mg/kg were 88.1, 95.3 and 90.7 per cent, respectively. Clorsulon, nitroxynil and rafoxanide administered at recommended dose rates showed negligible activity against these stages of the parasite. Against six- and eight-week-old infections the mean efficacies of triclabendazole at 12 mg/kg were 87.5 per cent and 95.7 per cent, respectively. Against F hepatica aged six weeks, albendazole and oxyclozanide showed no activity and clorsulon, nitroxynil and rafoxanide had only slight to moderate activity. The efficacies of triclabendazole, clorsulon, nitroxynil and rafoxanide against 10- or 12-week-old parasites were 100, 99.0, 99.1 and 90.1 per cent, respectively. Albendazole and oxyclozanide showed poor efficacy against 12-week-old infections.     

Laboratory investigations in sheep with a new anthelmintic.

The efficacy of fenbendazole against the following gastrointestinal nematodes was tested in experimentally infected lambs: O circumcincta, H contortus, T colubriformis and N filicollis. As low a dose as 0.5 mg per kg reduced the egg output of a patent infection with H contortus and T colubriformis by 85 per cent to 100 per cent. A dose of 3.5 mg per kg has a 100 per cent effect on three or 10-day-old stages of H contortus and one of more than 99 per cent or 100 per cent T colubriformis. The same dose reduces seven-day-old stages of O circumcincta and N filicollis by more than 94 per cent or 100 per cent, while 10 mg per kg produces a 100 per cent elimination of seven-day-old stages of O circumcincta. In field trials and efficacy of more than 99 per cent was determined after oral dosage with 5.0 mg per kg against Ostertagia, Haemonchus, Trichostrongylus, Cooperia, Nematodirus and Chabertia. Fenbendazole has a wide therapeutic-toxic dose ratio. Discoloration of the wool does not occur. Fenbendazole can be administered as a drench or with the feed.     

The effect of single and divided dose administration on the efficacy of fenbendazole against adult stages of benzimidazole resistant sheep trichostrongylids.

Sheep infected with benzimidazole resistant strains of Haemonchus contortus and Trichostrongylus colubriformis were used to compare the anthelmintic efficacy of fenbendazole given as a single dose or administered in a divided dose regime over five days. Statistical analysis showed no significant difference between the two methods of administration for H contortus. On the other hand, divided dose fenbendazole was significantly less effective than single doses against adult T colubriformis at dose rates of 5 and 7.5 mg/kg. In the case of H contortus a highly significant correlation coefficient between post treatment egg counts and worm counts (r = 0.789) was obtained. This suggests that reduction in faecal egg output following drug treatment would provide a useful field indication of anthelmintic performance of fenbendazole (and possibly related compounds) against benzimidazole resistant strains of this parasite.     

Efficacy of levamisole against immature and mature nematodes in goats with induced infections

Anthelmintic efficacy of levamisole against induced infections with 7- and 21-day-old Haemonchus contortus, Ostertagia circumcincta, Trichostrongylus axei, and T colubriformis was evaluated as an oral drench in goats. Group 1 (n = 8) was not treated, group 2 (n = 8) was given 3.96 mg of levamisole/kg of body weight, group 3 (n = 8) was given 7.92 mg of levamisole/kg, and group 3 (n = 7) was given 11.88 mg of levamisole/kg. Efficacy against all worms was low in goats given 3.96 mg of levamisole/kg, but was high against adult H contortus (99%) and adult T colubriformis (99.7%) in goats given 7.92 mg of levamisole/kg. Although efficacy against adults of all species was high in goats given 11.88 mg of levamisole/kg, some immature worms of all species remained in the abomasa of goats.     

Efficacy of ivermectin against induced gastrointestinal nematode infections in goats

The efficacy of ivermectin (0.08 per cent w/v oral solution) at different dose levels was evaluated against induced infections of adult Haemonchus contortus (21 days old) and Trichostrongylus colubriformis (21 days old) and fourth stage larvae of Oesophagostomum columbianum (17 days old), Ostertagia circumcincta (five days old) and Strongyloides papillosus (five days old). Twenty-five Boergoats (mutton goats) were randomly allocated by bodyweight within each sex to an untreated control group and four ivermectin treatment groups; ivermectin was administered at either 25, 50, 100 or 200 micrograms/kg orally, once. The goats were killed and processed for worm recovery 25 to 27 days after treatment. At 25 micrograms/kg the efficacy of ivermectin varied from 43 per cent for adult T colubriformis to more than 99 per cent for fourth larval stage O columbianum. Ivermectin at 50 micrograms/kg or higher was 99 per cent or more effective against all induced parasite infections with the exception of ivermectin at 50 micrograms/kg against S papillosus (97 per cent). For all parasites there was a statistically significant (P less than 0.05) difference between the control group and the pooled treated groups. No adverse reactions to ivermectin treatment were observed in the goats.     

Anthelmintic efficacy of albendazole in cattle: comparison of critical and controlled tests.

Twelve calves were experimentally inoculated with 6 species of gastrointestinal nematodes and the liver fluke, Fasciola hepatica. Six calves were administered albendazole at 10 mg/kg of body weight, and the drug efficacy was determined by critical and controlled tests. Albendazole was only 74% effective against Haemonchus contortus (cambendazole-resistant strain), 99 to 100% effective against Ostertagia ostertagi, Trichostrongylus colubriformis, Cooperia oncophora, and Oesophagostomum radiatum, and 54% effective against the liver fluke. Except for H contortus, the critical test provided as accurate a measurement of drug activity against gastrointestinal nematodes as the controlled test provided.     

The effect of fenbendazole on nematode parasites in experimentally infected lambs.

Fenbendazole given orally to experimentally infected lambs at a dose rate of 5 mg per kg was found to be 100 per cent effective against three and 10-day larave and also against 20-day adults of H contortus, O circumcincta, N battus and T colubriformis. The same dose was also 100 per cent effective against 10-day, 17-day larvae and 27-day adult D filaria.     

Levels of benzimidazole resistance in Haemonchus contortus and Trichostrongylus colubriformis recorded from an egg hatch test procedure

A method of using commercially available benzimidazole anthelmintics in an egg hatch test, to record levels of resistance is described. The test shows that a side-resistance is present to all the benzimidazole anthelmintics tested against both Haemonchus contortus and Trichostrongylus colubriformis after selection with thiabendazole. Additional selection on the field resistant strain of H contortus with thiabendazole showed increased levels of resistance to all the compounds tested while similar selection of T colubriformis failed to change the level of resistance after 20 generations. The level of resistance recorded by the egg hatch test in vitro is greater than that recorded for adults in vivo for the CF strain of H contortus and T colubriformis. Problems associated with reading the test are mentioned as are the different levels for the threshold of activity.     

Efficacy of ivermectin against benzimidazole-resistant nematodes of sheep

Two trials involving a total of 36 Dorset horn lambs were conducted to assess the anthelmintic efficacy of ivermectin against experimental infections of benzimidazole-resistant strains of Haemonchus contortus and Ostertagia circumcincta. Two resistant strains of each of the two species were used and in each trial the lambs were allocated to three groups. One group was given 200 micrograms ivermectin/kg bodyweight orally, the second group was given 5 mg oxfendazole/kg bodyweight orally and the third group remained untreated as controls. Fourteen days after treatment the lambs were necropsied. Ivermectin was found to be more than 99 per cent to 100 per cent effective against all four benzimidazole-resistant strains, whereas oxfendazole was 78.6 per cent and 83.8 per cent effective against the H contortus strains, and 25.6 per cent and 39.8 per cent effective against the O circumcincta strains.     

Comparison of the activity of some fasciolicides against immature liver fluke in calves

The anthelmintic activities of nitroxynil, closantel and rafoxanide were tested in cattle against six- and eight-week-old infections of Fasciola hepatica. Administered by subcutaneous injection, nitroxynil and rafoxanide at dose rates of 10 and 3 mg/kg bodyweight respectively were 88.5 and 36.1 per cent effective in eliminating a six-week-old infection of F hepatica and 85.8 and 60.4 per cent effective against an eight-week-old infection. Closantel administered by intramuscular injection at a dose rate of 2.5 mg/kg bodyweight was 60.0 per cent effective against an eight-week-old F hepatica infection.     

The anthelmintic efficacy of fenbendazole against a mixed nematode infection in sheep.

Fenbendazole (methyl-5-(phenylthio)-2-benzimidazole carbamate) at dose rates of 5 mg/kg and above was 100 per cent effective in eliminating a naturally acquired Dictyocaulus filaria infection in sheep. The drug was 100 per cent effective in eliminating concurrent infections of adult Trichostrongylus axei, Haemonchus contortus, Haemonchus placei, Ostertagia circumcincta, Ostertagia ostertagii, Cooperia oncophora, Cooperia mcmasterii, Nematodirus spathiger, Neumatodirus filcollis, Oesophagostomum venulosum and Chabertia ovina. Fenbendazole was 93 per cent and 97 per cent effective at doses of 5 and 10 mg/kg respectively in removing infection with adult T colubriformis, and post-treatment worm-egg production was completely suppressed in surviving female worms. No adverse side-effects were observed in treated sheep at either of the two dose rates used.     

Anthelmintic activity of ivermectin against immature gastrointestinal nematodes of sheep

Twenty-four lambs of mixed breeding with mixed experimental infections of Haemonchus contortus, Ostertagia circumcincta, Trichostrongylus axei, and T colubriformis were allotted to 4 groups. One group (control) was given the vehicle propylene glycol, and the others were given 100, 200, or 300 micrograms of ivermectin/kg of body weight by mouth. Twelve days after treatment, the sheep were necropsied. The compound was greater than 99% effective against immature stages of 4 nematode species at all dosages, except at the 100 micrograms/kg dosage, where efficacy was 96% against H contortus.     

Efficacy of moxidectin, nemadectin and ivermectin against an ivermectin-resistant strain of Haemonchus contortus in sheep.

The efficacies of ivermectin, nemadectin and moxidectin were evaluated when administered orally to lambs infected with either a susceptible laboratory strain of Haemonchus contortus or a strain reported to be resistant to ivermectin. Groups of 24 Dorset cross Cheviot cross Suffolk lambs were infected with either the susceptible or resistant strain of H contortus and allocated to treatment groups according to their faecal egg counts 27 days after infection. One day later the lambs were dosed orally with one of the three anthelmintics at 0.2 mg/kg bodyweight, and they were killed and surviving worms were recovered 13 or 14 days after treatment. Against the ivermectin resistant strain, ivermectin did not significantly reduce the egg count or the numbers of adult H contortus; however, both nemadectin and moxidectin reduced the nematode egg counts and the numbers of H contortus by 99 and 100 per cent, respectively. Against the susceptible strain, all the anthelmintics reduced the egg counts by 100 per cent as early as four days after treatment and reduced the numbers of susceptible H contortus by 100 per cent. No adverse reactions to any of the drugs were observed.     

Changes in resistance in mixed infections of susceptible and benzimidazole resistant strains of Haemonchus contortus and Trichostrongylus colubriformis passaged in sheep

Benzimidazole resistant strains of Haemonchus contortus and Trichostrongylus colubriformis were each diluted with equal numbers of their respective susceptible genotypes and passaged in separate worm-free sheep. The progeny of the mixed susceptible and resistant infections were diluted with equal numbers of susceptible genotypes before passaging on two further consecutive occasions in worm-free sheep. In H contortus the amount of thiabendazole required to prevent a 50 per cent egg hatching (LC50) was reduced at each generation, but the reduction was significant (P less than 0.05) only at the third passage. In T colubriformis the LC50 for thiabendazole was reduced only at the first passage and thereafter remained constant. The reduction was not significant. In another experiment, the dilution with susceptible genotypes was made only at the first passage. Thereafter for two further generations the progeny produced from the mixed infection were passaged in worm-free sheep. In H contortus a reduced amount of thiabendazole was required to prevent a 50 per cent egg hatching at each generation, but did not reach a level of significance. The result for T colubriformis was identical to the continuous dilution experiment.     

Efficacy of eprinomectin pour-on against gastrointestinal nematodes and the nasal bot fly (Oestrus ovis) in sheep

The efficacy of the pour-on formulation of eprinomectin, at a dose rate of 0.5 mg/kg bodyweight, was assessed in sheep against three main species of gastrointestinal nematodes and against the nasal bot fly, Oestrus ovis, and some pharmacokinetic parameters were determined for 21 days after the treatment. By comparison with untreated control sheep, infected experimentally with Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis, eprinomectin was 100 per cent effective against the two abomasal species and 99.5 per cent effective against T. colubriformis. In ewes naturally infected with the nasal bot fly, the efficacy of the drug against O. ovis was 97.7 per cent. The mean (se) systemic area under the curve (AUC) was 56.0 (26.2) ng/day/ml and the mean residence time was 5.3 (1.0) days, but there were wide variations between individual sheep.     

The effect of route of administration on the anthelmintic efficacy of benzimidazole anthelmintics in sheep infected with strains of Haemonchus contortus and Trichostrongylus colubriformis resistant or susceptible to thiabendazole

Observations of erratic anthelmintic activity of fenbendazole against known standardised thiabendazole-resistant strains of Haemonchus contortus and Trichostrongylus colubriformis in sheep were investigated. Fenbendazole at a dose rate of 10 mg/kg body weight was administered by oral, intra-ruminal or intra-abomasal routes, and was most effective against both resistant strains following intra-ruminal administration. In addition thiabendazole, oxibendazole, fenbendazole, parbendazole and mebendazole plus two unrelated compounds, levamisole and morantel tartrate, were used at one and a half times their suggested or recommended therapeutic dose rate against thiabendazole-resistant strains of H contortus and T colubriformis in sheep; each drug being administered by the intra-ruminal or intra-abomasal routes. Fenbendazole was more effective against both strains following intra-ruminal administration. Parbendazole was more effective against the resistant strain of T colubriformis following intra-ruminal administration. At the dose rate chosen for the other benzimidazoles used against these resistant strains, there was no difference in anthelmintic efficacy due to route of administration. Levamisole was highly effective against both resistant strains, irrespective of the route of administration. In the groups treated with morantel tartrate, the results obtained were difficult to interpret due to mortalities and a highly variable response in the surviving sheep. Fenbendazole, thiabendazole and mebendazole when used at their suggested or recommended therapeutic dose rate in sheep, were highly effective against known thiabendazole-susceptible strains of H contortus and T colubriformis following both intra-ruminal or intra-abomasal administration.     

In vitro and in vivo characterization of p-amino-phenethyl-m-trifluoromethylphenyl piperazine (PAPP), a novel serotonergic agonist with anthelmintic activity against Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis

The neurotransmitter serotonin (5-hydroxy tryptamine or 5HT) regulates key physiological processes in nematodes such as locomotion and feeding. PAPP (p-amino-phenethyl-m-trifluoromethylphenyl piperazine) is a known agonist of the 5-HT(1Hc) receptor of the barber pole worm, Haemonchus contortus. In this study, PAPP was highly active against L3-stage larvae of H. contortus and Trichostrongylus colubriformis in an in vitro larval migration assay, with EC50 values of 9.36 and 11.8 microM, respectively, that were comparable to levamisole (10.2 microM) and superior to pyrantel (55.39 microM). When administered orally or subcutaneously to nematode infected gerbils, PAPP provided >99% efficacy against H. contortus and >98% efficacy against Teladorsagia circumcincta at 100 mg/kg, comparable to levamisole at 10 mg/kg. Drug titration revealed significant activity down to 50 mg/kg against these two species. Spectrum was limited, however, with somewhat lower efficacy (83%) in T. colubriformis infected gerbils at 100 mg/kg. Oral delivery of hydrochloride, acetate and phosphate salts of PAPP to nematode infected gerbils did not result in an increase in either potency or spectrum. The finding that PAPP exhibits significant anthelmintic activity suggests that the nematode-specific serotonergic system is a viable target for future anthelmintic discovery.