Anthelmintic efficacy of low-dose phenothiazine against strains of sheep nematodes susceptible or resistant to thiabendazole, levamisole and morantel tartrate: effect on patent infections

The anthelmintic efficacy of low-dose phenothiazine therapy, administered as a 1:40 phenothiazine: molasses mixture, was tested against patent infections of strains of Haemonchus contortus, Trichostrongylus colubriformis and Ostertagia spp susceptible or resistant to thiabendazole (an other benzimidazoles), levamisole and morantel tartrate. Phenothiazine reduced faecal egg output for both susceptible and resistant strains of all three nematodes. In daily doses of 0.25 g per sheep per day and above it completely inhibited larval production in susceptible strains. Against resistant strains, there was a reduced efficiency with 82.3 per cent inhibition of egg hatch at the 0.25 g per sheep per day level. Phenothiazine treatment had no significant effect on worm numbers for either susceptible or resistant worms. It is suggested that benzimidazole-resistant strains may be cross-resistant to phenothiazine.

Changes in response of a benzimidazole resistant strain of Haemonchus contortus from sheep after passing through calves

A benzimidazole resistant strain of Haemonchus contortus was passaged through lambs only or from lambs to calves and back into lambs. Changes in response to thiabendazole were monitored by using an egg hatch test at each animal passage and by a controlled experiment on adult worms at the final passage in lambs. An increased level of resistance was shown for the isolate during its passage through calves by the egg hatch test, although this was not supported on the adult worms in sheep using a single dose rate of 66 mg/kg of thiabendazole.

The isolation of a field strain of Haemonchus contortus in Queensland showing multiple anthelmintic resistance

Following the apparent failure of levamisole to control infections of Haemonchus contortus in sheep at Lawes in south eastern Queensland, a strain of this parasite was isolated at the Animal Research Institute, Yeerongpilly. This strain was used to infect sheep at Yeerongpilly and the Merrindale Research Station, Victoria where four experiments to classify the resistance pattern of the parasite were carried out. Resistance to thiabendazole was first suspected in 1969, and these experiments confirmed that resistance to this drug was still present. They also showed that a strong degree of resistance had been developed to both levamisole and morantel tartrate. Other benzimidazole anthelmintics and also the organophosphorus compound naphthalophos were only moderately effective against the original isolate but rafoxanide, nitroxynil and phenothiazine were almost 100% effective. Other highly effective chemicals were disophenol and closantel. After passaging the strain for four generations with both levamisole and albendazole, resistance to both naphthalophos and the newer benzimidazole anthelmintics increased dramatically. This is the first report of a field strain of H. contortus exhibiting resistance to benzimidazole, non-benzimidazole and organophosphorus anthelmintics.

Relationship among particle size distribution, dissolution profile, plasma values, and anthelmintic efficacy of oxfendazole

Three mean particle sizes of oxfendazole raw material (1.65 micron, lot A; 3.2 micron, 10t B; 12.0 micron, lot C) were prepared and identically formulated as corresponding (A, B, and C) suspensions at 2.26% (W/V) concentration. Studies involving microscopic examination, scanning electron microscope analysis, particle size distribution, and surface area measurement were carried out on raw materials. In vitro dissolution profiles were obtained for the suspensions. A comparative bioavailability study of these 3 suspensions was performed in 12 sheep with each sheep given each formulation in a Latin square crossover study design; oxfendazole was dosed at rate of 5 mg/kg of body weight. Plasma-value measurements were made followed by an analysis of various bioavailability studies. Plasma area values indicated that suspension C (dw = 12.0 micron) was significantly (P less than 0.05) less bioavailable than was suspension A (dw = 1.65 micron); there was no difference between suspension A and suspension B. Significant differences were not seen in biological half-life and maximum plasma concentrations. The term dw refers to that particle diameter (determined by Coulter counting) at which 50% of the oxfendazole mass was in the form of particles having a lesser diameter and 50% was in the form of particles having a greater diameter. In a separate study involving 20 Merino weaner sheep infected with benzimidazole-resistant Haemonchus contortus larvae, oxfendazole's anthelmintic efficacy was demonstrated in the 2.26% suspension dosage form (90% particles less than 10 micron) at a dosing rate of 5 mg/kg. A correlation was found between its anthelmintic activity and plasma area values when compared in individual sheep. Data demonstrated that substantial differences in particle size distribution of oxfendazole could influence its dissolution rate, plasma concentrations, and absorption characteristics, thus indicating that oxfendazole's absorption could be dissolution-rate limited.

Resistance of Haemonchus contortus to thiophanate

The occurrence of a field strain of Haemonchus contortus which was resistant to thiophanate and susceptible to thiabendazole and levamisole is reported. The LD95 of thiophanate for the strain was found to be 156.1 mg/kg and its resistance factor was 6.7. In possessing specific thiophanate resistance it differed from previously reported thiophanate resistant strains of H contortus which have been benzimidazole resistant worms that have possessed cross resistance to thiophanate.

Use of disophenol for the control of Haemonchus contortus in sheep in West Java, Indonesia

The result of a field trial investigating the anthelmintic effect of disophenol against H. contortus in sheep raised under Asian small farming practice is reported. it was clearly shown that a single injection of disophenol suppressed the numbers of H. contortus to very low levels for periods up to 3 months despite the presence of large populations of H. contortus infective larvae available on pasture and herbage grasses.

Thiabendazole-resistant strains of Haemonchus and Ostertagia in California lambs

Ten parasite-free experimental lambs were inoculated intraruminally with a mixed-larvae inoculum which contained approximately 5,000 Haemonchus contortus infective larvae. Five lambs were treated with thiabendazole (66 mg/kg of body weight) and 5 were untreated controls. All lambs were killed 35 days after inoculations were done, and parasites present were recovered. Reduction in numbers of recovered Haemonchus contortus (25.7%) and Ostertagia spp (26.8%) were not significant (P greater than 0.05).

Economic value and course of infection after treatment of cattle having a low level of nematode parasitism

To determine whether it is economically advantageous to treat calves having inapparent parasitism, we conducted experiments from 1971 to 1976, involving more than 1,800 calves from 30 pens or lots, using formulations of thiabendazole, levamisole, and crufomate (ruelene). Differential egg counts, cultured larvae, and cultured parasitic stages were used to estimate the kind and degree of nematode parasitism. Differentiation of infective larvae consistently established Cooperia as the predominating (%) genus in all fecal samplings. Bunostomum, when initially present, decreased or disappeared, whereas Trichostrongylus increased; other genera fluctuated less consistently. These qualitative generic fluctuations were not primarily the result of treatment, but more likely were seasonal variation. Judged by average daily gain (ADG), anthelmintic treatment was statistically advantageous at one or more points during the observation periods in 10 of 13 treated groups. In seven treated groups, the observation periods were concluded with statistical advantage in ADG, whereas in three groups, compensatory gain by corresponding controls had neutralized earlier advantages. The comparative influence of the various anthelmintics was not consistent from year to year. When total cost/kilogram gain was calculated from feed efficiency measuremnts and other costs, economic treatment advantage was evident in seven of 11 tests (7 of 10 treatment groups) from 1973 through 1976. This financial advantage, due primarily to feed efficiency and noted after 28 to 51 days, justified anthelmintic treatment. This advantage was not likely lost by the animals in subsequent periods (to 218 days) on pasture or in lots, since ADG indicated the treated calves performed either as well as, or better than, the nontreated controls. Considering all aspects of the study, the results indicate calves coming into Kansas from southern states and weighing 184 to 267 kg may possess a level of subclinical (symptomless) nematode parasitism that when treated will result in a response justifying expense for using anthelmintics.

Efficacy of albendazole and levamisole in sheep against Thysanosoma actinioides and Haemonchus contortus from the Edwards Plateau, Texas

Lambs naturally infected with Thysanosoma actinioides and naturally and experimentally infected with Haemonchus contortus were used in a controlled experiment to determine the anthelmintic activity of albendazole as compared with levamisole. The lambs were given 10,000 H contortus larvae 25 to 35 days before treatment, were slaughtered 7 to 9 days after treatment, and were housed on concrete from the time of treatment until slaughter. In lambs given doses of 7.5 mg or 3.8 mg of albendazole/kg or 7.5 mg of levamisole/kg, reductions in numbers of H contortus were 99%, 85%, and 98%, respectively. Reductions in numbers of T actinioides were 98%, 45%, and 0% respectively, as compared with untreated controls.

Efficacy of oxfendazole against Nematodirus battus and inhibited stages of sheep nematodes

Oxfendazole administered at a dose rate of 5 mg/kg was 100 per cent effective against artificially acquired seven-day-old and 23-day-old infections of Nematodirus battus in lambs. At the same dose rate it was 100 per cent effective against naturally acquired infections of adult Trichostrongylus spp, adult and arrested early fourth stage larvae of Haemonchus contortus and Nematodirus spp. Efficacy against Ostertagia spp was 99 to 100 per cent against adults and 100 per cent against inhibited larvae, the predominant species of this genus being O circumcincta.

The incidence of thiabendazole resistance in field populations of Haemonchus contortus on the northern tablelands of New South Wales

A survey was undertaken to assess the incidence of thiabendazole resistance in field populations of H. contortus in sheep flocks on 40 farms distributed throughout the sheep grazing area of the northern tablelands of New South Wales. Treatment with thiabendazole at 66 mg/kg reduced H. contortus ova output by 96-100% in 18 flocks while in the remaining 22 there was a range of responses down to levels where the anthelmintic was without apparent effect. Factors which may contribute to the high incidence of anthelmintic resistance are discussed and the possibility of widespread resistance occurring in other sheep raising areas is considered.

The efficacy of fenbendazole at a dosage rate of 7,5 mg/kg against nematode infestations in cattle

Fenbedazole, dosed to artificially infested cattle at 7,5 mg/kg live mass, was more than 80% effective in more than 80% of the treated animals against immature and adult Haemonchus placei, Ostertagia ostertagi; Cooperia spp., Bunostomum phlebotum and Oesophagostomum radiatum.

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 oxfendazole and haloxon against arrested Haemonchus contortus larvae in sheep

In a controlled trial with natural infection of Haemonchus contortus, oxfendazole at 4.53 mg/kg body weight was highly effective against arrested larvae, developmental stages and adult helminths. Haloxon at 40 mg/kg significantly reduced helminth burdens.

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 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 febantel in sheep experimentally infected with five species of gastrointestinal nematodes

The anthelmintic activity of the guanidine derivative febantel was tested in a total of 76 experimentally infected lambs in two control tests. Doses of 5.0 and 7.5 mg per kg body-weight were administered orally against fourth and pre-adult fifth or adult stages of Haemonchus contortus, Ostertagia circumcincta, Trichostrongylus colubriformis, B unostomum trigonocephalum and Oesophagostomum columbianum. The efficacy was between 98.5 and 100 per cent. The drug was well tolerated.

Anthelmintic activity of albendazole in white-tailed deer

Albendazole at dose rates of 11 to 54 mg/kg (mean of 25 +/- 11 SD) of body weight was highly effective (greater than 99%) against Haemonchus contortus, Ostertagia odocoilei, O mossi, Trichostrongylus askivali and Nematodirus odocoilei in 22 white tailed deer (Odocoileus virginianus) in southern Texas. It was 38% effective against mature and immature deer liver flukes (Fascioloides magna). Toxicoses associated with treatment were not observed. Large numbers of abomasal nematodes (mean of 2,949 +/- 3.147 SD) were recovered from 16 untreated control deer, indicating these may have had a potential impact on herd health and indicating the importance of treatment or other management alternatives.

The efficacy of febantel on gastrointestinal nematodes in sheep

Febantel (Bay h 5757, Rintal) is a broad-spectrum anthelmintic of a new type. A single oral dose of 5 mg/kg is fully effective against larval and adult stages of Haemonchus contortus, Trichostrongylus colubriformis, Oesophagostomum columbianum, Ostertagia circumcincta and Bunostomum trigonocephalum in sheep.

In vitro and in vivo studies on the ovicidal activity of fenbendazole

The in vitro ovicidal activity of fenbendazole against eggs of Ostertagia ostertagi, Haemonchus contortus and Trichostrongylus colubriformis was observed using different concentrations of active principle. The optimum concentration for ovicidal activity is evidently 0.5 ppm fenbendazole. A further increase in concentration results in a decrease in activity which is probably due to physical causes. Ovicidal activity of the product is assessed by the proportion of non embryonated eggs as well as embryonated eggs which are not capable of hatching. By this method of evaluation a higher ovicidal activity is recorded. Ten h after treating sheep with fenbendazole (dose 5 mg/kg) a large number of eggs whose development was inhibited, as well as atypical eggs were detectable in the faeces. Twelve h after treatment, the capacity for development of the eggs was largely suppressed.

Efficacy of oxfendazole against an ovine isolate of benzimidazole resistant Haemonchus contortus

Eighteen parasite-free sheep were each infected with 6000 +/- 25 infective larvae of the AH-2, benzimidazole-resistant strain of Haemonchus contortus on each of experimental days -27, -15, -8 and -1. The sheep were subsequently randomized into 3 groups of 6 sheep each. Group 1 served as untreated controls; Groups 2 and 3 were treated orally with oxfendazole at dose levels of 5 mg/kg and 10 mg/kg body weight, respectively. At both dosage levels, oxfendazole was 100% effective against third, fourth, early fifth, and adult stages of the worms.

Thiophanate as a low daily dosage anthelmintic in sheep

Thiophanate administered daily at low dosages reduced nematode faecal egg output, egg hatchability and parasitic worm burdens in treated lambs and ewes. Six daily doses of 1 or 3 mg per kg thiophanate (approximately 1/25th to 1/75th of the median therapeutic dose), given to lambs experimently infected with Trichostrongylus colubriformis, were partially effective in suppressing faecal egg output and egg hatchability. Six doses of 5 mg per kg per day were effective in lambs infected with Haemonchus contortus and Nematodirus spathiger. Daily doses of thiophanate (50 or 200 mg per head) given over 14 weeks to lambs grazing contaminated pasture resulted in improved productivity (the higher dosage) and suppression of output of viable eggs and reduced worm burdens (both dosages). Reduced output of viable eggs was also obtained in housed, lactating ewes receiving 5 or 7 mg per kg thiophanate dispersed daily in the feed for 11 or nine weeks respectively after lambing.

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.

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.

Levamisole: anthelmintic activity in calves following dermal application

A series of seven experiments designed to evaluate the anthelmintic efficacy of levamisole (1-tetramisole) by dermal application is described. This work involved use of 181 artificially infested calves. The drug was formulated at a concentration of 10% m/v levamisole base in a solvent system promoting dermal absorption and was applied to both sides of the spine in the lumber region at a dose rate of 10 mg levamisole base per kg livemass. A consistent performance was achieved in these investigations, the results from all of which were based upon necropsy findings. Efficacy of treatment was assessed against the third and fourth larval stages and fifth stage larvae/adult worms of six nematode species; the respective efficacies obtained were Haemonchus placei 72,0%; 99,3%; and 100%: Ostertagia ostertagi 85,5%; 38,1% and 74,5%: Cooperia spp. 98,9%; 99,9% and 100%: Bunostomum phlebotomum 83,0%; 100% and 98,5%: Oesophagostomum radiatum 47,4%; 94,9% and 99,6% and Dictyocaulus viviparus 79,5%; 94,1%; 90.9% (fifth stage larvae) and 93,8% (adult worms). The anthelmintic efficacy of levamisole in these experiments is of the same order as that achieved by orthodox methods of administration at dose rates between 7,5 and 10,0 mg levamisole HC1 per kg livemass. These findings add a new dimension to the use of levamisole and to anthelmintic therapy in general.

Trials with rafoxanide. 8. Efficacy of an injectable solution against trematodes and nematodes in cattle

Four experiments are described in which the efficacy of an experimental 5% injectable solution of rafoxanide was evaluated against various adult and immature helminths in cattle. Subcutaneous injection at a dosage of 3 mg/kg live mass resulted in the following reductions in mean worm burdens: adult Fasciola hepatica, 82,6%; adult Fasciola gigantica, 99,8% immature Paramphistomum microbothrium, 10,1% adult Haemonchus placei, 99,6%, third stage H. placei, 73,7%; adult Bunostomum phlebotomum, 99,8%; adult Oesophagostomum radiatum, 99,9%; and fourth stage O. radiatum, 76,9%. At 5 mg/kg live mass, rafoxanide solution was 97,5% and 99,2% effective against 8-week old F. gigantica and third stage H. placei respectively and at 7,5 mg/kg, 92,4% against 6-week old F. gigantica.

Further controlled evaluations of fenbendazole as a bovine anthelmintic

The efficacy of fenbendazole (methyl-5-(phenythio)-2-benzimidazole-carbamate) in cattle parasitized by the lungworm Dictyocaulus viviparus and in other cattle parasitized by Haemonchus, Ostertagia, Trichostrongylus, Cooperia, and Oesophagostomum was investigated in 3 controlled critical experiments. A single treatment with 5.0 mg/kg of body weight administered in the feed or in a 10% suspension exceeded 99% efficacy against these helminths. Dosages of 1, 0.5, 0.35, and 0.25 mg/kg of body weight administered for 5 days to individual calves in their feed were 99% efficacious against all helminths, except Haemonchus. The 1.0 mg/kg dose removed 99.5% of the Haemonchus, and the other 3 dosages had efficacies of 91.0, 93.7, and 95.6%, respectively. The small daily dosages were not evaluated against Dictyocaulus.

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.

Efficacy of albendazole against Haemonchus, Nematodirus, Dictyocaulus, and Monieza of sheep

Albendazole was highly efficacious in the removal of monospecific and mixed infection of Haemonchus contortus, Nematodirus spathiger, and Dictyocaulus filaria from sheep. A dose level of 5 mg/kg removed nearly all gastrointestinal nematodes, and 10 mg/kg removed all lungworms. Tapeworms of the genus Moniezia were completely removed by a dose level of 10 hg/kg.

Efficacy of oxibendazole against gastrointestinal nematodes of cattle

In 3 separate studies, oxibendazole in drench and premix formulation was shown efficacious against larvae and adults of the genera Haemonchus, Ostertagia, Trichostrongylus, Strongyloides, Nematodirus, Cooperia, Bunostomum, Capillaria, Oesophagostomum, and Trichuris. In the first study, artificially infected calves were cleared of virtually all histotrophic larvae and adult parasites after medication with oxibendazole drench at a dose level of 10 mg/kg of body weight. Smaller doses (7.5 and 5.0 mg/kg) expelled 84 to 100% of the parasites. Oxibendazole at a dose level of 15 mg/kg in premix form was given to artificially infected calves 3, 7, or 42 days after infection in a 2nd study. Third stage and 4th stage Cooperia oncophora larvae were, respectively, 92 and 98% susceptible to the drug at day 3 and day 7 after infection; neither larval stages of Ostertagia ostertagi and of Oesophagostomum radiatum were susceptible. Premix given on day 42 after infection removed 83 to 100% of adult O ostertagi, Nematodirus spp, C oncophora, O radiatum, and Trichuris spp. In a 3rd study, calves harboring a mixture of parasitic stages from artificial and pasture-acquired infections were medicated with oxibendazole at a dose of 15 mg/kg in premix form. The burdens of larvae from the abomasum and small intestines were reduced 93 to 95%, respectively; the burdens of adults of 11 species of worms were reduced 87 to 100%.

The anthelmintic efficacy of fenbendazole against thiabendazole-resistant strains of Haemonchus contortus and Trichostrongylus colubriformis in sheep

The anthelmintic efficacy of fenbendazole (methyl 5-(phenylthio)-2-benzimidazole carbamate) was tested in sheep against standardised strains of Hcaemonchus contortus and Trichostrongylus colubriformis, known to be resistant to thiabendazole (LD90 for thiabendazole against H ontortus was 200 mg/kg bodyweight and against T colubriformis was 150 mg/kg). Fenbendazole at dose rates of 5, 10 and 20 mg/kg per os reduced total worm counts in H contortus infected sheep by 66, 90 and 100 per cent respectively, with similar reductions recorded for worm egg outputs. For the thiabendazole resistant strain of T colubriformis, fenbendazole reduced total worm counts in infected sheep by 4, 44 (40-48), 79 (75-83), 96 and 100 per cent at dose rates of 5, 10, 20, 40 and 80 mg/kg per os. Significant suppression of worm-egg production by thiabendazole resistant T colubriformis was obtained with fenbendazole at dose levels of 5 mg/kg and above. The implications of these results are discussed in the light of the increasing occurrence of strains of trichostrongylid nematodes resistant to currently available benzimidazole anthelmintics.