The anthelmintic efficacy of albendazole against Fasciola hepatica and benzimidazole resistant strains of Haemonchus contortus and Trichostrongylus colubriformis in sheep.

The anthelmintic efficacy of albendazole (methyl [5-(propylthio) - 1H - benzimidazole -2 -yl] carbamate) against immature and adult Fasciola hepatica and against standardised strains of benzimidazole resistant Haemonchus contortus and Trichostrongylus colubriformis was evaluated in experimentally infected sheep. A single intrarumenal treatment of dose rates of 3.8 and 7.6 mg/kg was ineffective against immature (six weeks old) F hepatica. Dose rates of 5.7 and 7.6 mg/kg reduced the number of mature (12 weeks old) F hepatica by 70 and 91 per cent respectively. Dose rates of 5.7 and 7.6 mg/kg removed 92 and 99 per cent of four-week-old, benzimidazole resistant H contortus and 89 and 99 per cent of four-week-old, benzimidazole resistant T colubriformis.     

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 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.     

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.     

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.     

Resistance to benzimidazole anthelmintics in field strains of Ostertagia and Nematodirus in sheep

SUMMARY: Resistance to a range of benzimidazole anthelmintics was investigated in 2 strains of Ostertagia spp. One strain (SRBO) had been exposed to fenbendazole, oxfendazole and thiabendazole, the other strain (KR79) only to thiabendazole. Both strains showed a high degree of resistance to albendazole, fenbendazole, oxfendazole and thiabendazole, which had efficiencies of 11–38% and 0–60% against all developmental stages of SR80 and KR79, respectively. There was no significant reduction in the KR79 worm count by thiabendazole at 132 mg kg-1 given either as a single dose or divided into 12 equal portions, one administered every 6h. Nematodirus spp were also found to be resistant to all benzimidazoles tested. Levamisole at 7 mg kg^-1 and naphthalophos at 30 mg kg-1 had efficiences of 89 and 66%, respectively, against SR80 Ostertagia and 99 and 19% against Nematodirus, all but the last of these being significant reductions in worm burdens. The resistant SR80 Ostertagia occurred on a research station, but possibly originated from a property which 3 years earlier had supplied ewes to the station. The use of benzimidazole anthelmintics and subsequent grazing on worm-free pasture may have enhanced the level of resistance. A relationship was established between egg counts and adult worm counts 10 days post-treatment, which suggested that for Ostertagia the worm count could be predicted from the geometric mean egg count from about 10 animals. Thus, where an Ostertagia population is suspected of being resistant, an anthelmintic efficiency assay using pre-and post-treatment faecal egg counts should provide a satisfactory diagnostic procedure.     

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.     

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.     

Characterization of moxidectin resistant Trichostrongylus colubriformis and Haemonchus contortus

The development of moxidectin resistance (MOX-R) in sheep parasitic gastrointestinal nematodes already carrying multiple resistances to other anthelmintic groups has made control of these strains very difficult. The anthelmintic resistance patterns of MOX-R strains of Trichostrongylus colubriformis and Haemonchus contortus were characterized to provide an insight into the remaining role of anthelmintics in the control of such strains. Homozygous MOX-R individuals of both genera were unaffected by moxidectin. For MOX-R heterozygotes a dose rate of 200 microg/kg abamectin (ABA) given orally removed 25% of H. contortus while 200 microg/kg MOX given orally achieved a 72% reduction. Doubling the dose rate of ABA improved the mean efficacy to 37%. Consequently, in H. contortus, the degree of dominance differs markedly between the two anthelmintics. A dose rate of 8 mg/kg levamisole and 185 mg/kg napthalophos achieved >95% reduction in worm count of the MOX-R homozygous H. contortus but only 85 and 7%, respectively against the MOX-R homozygous T. colubriformis.     

Isolation of benzimidazole resistant strains of Ostertagia circumcincta from British sheep

Two strains of Ostertagia circumcincta were isolated from sheep in Great Britain; one (CVL strain) from a breeding flock maintained at the Central Veterinary Laboratory, the other (H2 strain) from a commercial flock in southern England. Their resistance to benzimidazole anthelmintics was assessed by means of in vitro egg hatch assays and slaughter trials. In vitro egg hatch assays gave calculated ED50 estimates of 0.799 micrograms thiabendazole/ml for the CVL strain and 0.794 micrograms thiabendazole/ml for the H2 strain, compared with ED50 estimates of 0.038 micrograms thiabendazole/m and 0.036 micrograms thiabendazole/ml for two known susceptible strains of O circumcincta. There was a 40.7, 28.4 and 66.9 per cent reduction in the group mean worm burdens of lambs infected with the CVL strain following treatment with thiabendazole, fenbendazole and oxfendazole, respectively, and 23.8, 0.0, 79.6, 52.7, 99.9 and 100 per cent reduction in the group mean worm burdens of lambs infected with the H2 strain following treatment with thiabendazole, fenbendazole, oxfendazole, albendazole, levamisole and ivermectin, respectively. Detailed field histories for both strains are given.     

Efficacy of three broad-spectrum anthelmintics against gastrointestinal nematode infections of goats

Groups of 10 goats, harbouring both naturally acquired and experimental infections of gastrointestinal nematodes, were drenched with either levamisole (5 mg kg-1), albendazole (3.8 mg kg-1) or parbendazole (15 mg kg-1), or remained untreated. Haemonchus contortus was the numerically dominant infection, with Strongyloides papillosus, Trichostrongylus colubriformis and Oesophagostomum columbianum also present. At 5-6 days post-treatment, goats were killed and necropsied. Post-mortem worm counts showed that the reduction in mean total worm burdens was 57.4% in levamisole-treated animals, 71.1% in the albendazole group and 85.1% in the parbendazole group. Reductions for H. contortus were 80.2, 87.9 and 83.9% in the levamisole-, albendazole- and parbendazole-treated groups, respectively. These data indicate that the anthelmintics in question are not being applied at an adequate dose rate for goats, and/or resistance to anthelmintics is occurring in the field in Pernambuco State, northeast Brazil.     

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.     

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.     

Removal of Haemonchus contortus, Ostertagia circumcincta and Trichostrongylus spp. from goats, by morantel citrate, levamisole hydrochloride, fenbendazole, and oxfendazole

Young feral goats harbouring 3, 7 and 36 day old experimental infections of strains of gastrointestinal nematodes, known to be susceptible to anthelmintics in sheep, were dosed orally with commercially available anthelmintics at the manufacturers' recommended rates for sheep. The drugs and overall removals of Ostertagia circumcincta and Trichostrongylus spp. (T. colubriformis and T. vitrinus) respectively were: morantel citrate 69% and 68%; levamisole hydrochloride 55% and >99%; fenbendazole 98.5% and >99%; oxfendazole >99% and >99%. Results for Haemonchus contortus, while not inconsistent with high efficacy for all four drugs, must be interpreted with caution, in view of the erratic establishment of this worm in the controls.     

The comparative efficacy of four broad spectrum anthelmintics against some experimentally induced trichostrongylid infections in sheep and goats

A controlled slaughter trial was undertaken to compare the efficacies of oxfendazole (5 mg/kg), morantel citrate (10 mg/kg), levamisole (8 mg/kg) and ivermectin (0.2 mg/kg) against experimentally induced infections of adult Haemonchus contortus, Ostertagia spp., Trichostrongylus colubriformis and Cooperiacurticei in sheep and goats. Ivermectin and oxfendazole achieved similar levels of efficacy in both hosts against all four worm genera as did levamisole and mortantel against H. contortus and C. curticei. Against Ostertagia spp. and T. colubriformis, however, the latter two drugs were less effective in goats than sheep. Neither the numbers of Ostertagia spp. removed from goats by levamisole (81% reduction) nor the numbers of T. colubriformis removed from goats by morantel (56% reduction) were statistically significant (P>0.05).     

Multiple species of nematodes resistant to ivermectin and a benzimidazole-levamisole combination on a sheep farm in New Zealand

AIM: To confirm the presence of multiple anthelmintic resistance on a sheep farm in New Zealand. METHODS: Three groups of 10 weaned Romney-cross lambs were treated either with an oral dose of ivermectin (0.2 mg/kg), or a benzimidazole/levamisole (BZ/LEV) combination (4.75 albendazole and 7.5 mg/kg levamisole), or were left untreated. Ten days later, animals were necropsied, and adult worms recovered and identified from the abomasa and small intestines. Pre- and post-treatment faecal nematode egg counts (FEC) were recorded, and larval cultures were performed. RESULTS: In a faecal egg count reduction test (FECRT), adjusted to reflect pre- and post-treatment larval culture results, ivermectin resistance was detected in Teladorsagia (Ostertagia), Trichostrongylus and Haemonchus spp, while BZ/LEV combination- resistant Teladorsagia and Trichostrongylus spp were also present. Adult worm counts confirmed these results, and identified the species involved as Teladorsagia circumcincta, Trichostrongylus colubriformis and H. contortus. CONCLUSION: Multiple, multi-generic anthelmintic resistance was confirmed on a sheep property in New Zealand. This included the first confirmed case of ivermectin resistance in T. colubriformis from sheep in New Zealand.     

The efficacy of benzimidazole anthelmintics against late fourth stage larvae of Trichostrongylus colubriformis in gerbils and Nippostrongylus brasiliensis in rats

The efficacy of eight benzimidazole anthelmintics has been examined against Trichostrongylus colubriformis in gerbils and Nippostrongylus brasiliensis in rats, when the parasites were entering into the fourth and final moult. The dose--response slopes obtained from the data for both host--parasite systems did not deviate significantly from a non-parallel model, and were used for relative potency determinations. The T. colubriformis assay ranked the benzimidazoles in order of potency as follows: albendazole, oxfendazole, fenbendazole, cambendazole, mebendazole, oxibendazole, parbendazole and thiabendazole. This compares favourably with the expected relative efficacies against trichostrongyles in sheep. N. brasiliensis was found to be far less useful in this respect. All compounds tested, with the exception of thiabendazole, were highly effective against both parasites. The T. colubriformis/gerbil assay could be a very useful tool for preliminary in vivo evaluation and possibly in the early selective optimisation of chemical series.     

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.     

MULTIPLE AND MULTIGENERIC ANTHELMINTIC RESISTANCE ON A SHEEP FARM IN KENYA

The anthelmintic efficacy of benzimidazoles (albendazole, fenbendazole and oxfendazole), levamisole, oral ivermectin and closantel was evaluated on a farm in Kenya using faecal egg count reduction test, larval cultures and a controlled slaughter trial. The results of this study indicated simultaneous resistance of Haemonchus contortus against benzimidazoles, levamisole and ivermectin, and of Trichostrongylus colubriformis and Oesophagostomum spp. against levamisole on the same farm. Ivermectin resistance developed to 47% within 15 months of first use. Closantel was effective against the benzimidazoles, levamisole and ivermectin resistant H. contortus.     

Colorimetric assay for the detection of benzimidazole resistance in trichostrongyles

A modified version of the aphid tile-test, which is used to detect insecticide resistance in single adult aphids that are resistant or susceptible to organophosphate or carbamate insecticides, was used to compare the levels of non-specific esterases in strains of the trichostrongyle nematodes Haemonchus contortus, Ostertagia circumcincta and Trichostrongylus colubriformis which were known to be resistant or susceptible to benzimidazole (BZ) anthelmintics. This colorimetric assay has shown that there is significantly more non-specific esterase in the infective-stage larvae of BZ-resistant strains than in susceptible strains and this may prove to be of use in the detection of resistance to benzimidazole anthelmintics.