Effects of a preventive and suppressive control scheme on the development of thiabendazole-resistance in Ostertagia spp

Three years after the start of an experiment to assess the merits of thiabendazole (TBZ) treatment of trichostrongylid parasites in weaner sheep, field isolates of Ostertagia spp and Trichostrongylus spp were made from weaner sheep treated under one of three treatment schemes. Treatment frequencies were "nil", "planned" (5 or 6/year) and "regular" (every 3 weeks). In addition an isolate was taken from a group of "tracer" sheep drenched with TBZ every 10 days. Resistance to TBZ was assessed using an in vitro egg hatch assay, pre- and post-treatment faecal egg counts and a controlled anthelmintic efficiency test. Pre- and post-treatment egg counts revealed the presence of TBZ-resistance in field isolates of mixed species. Egg hatch assays indicated a level of resistance for Ostertagia spp which was proportional to the frequency of TBZ treatment. The "planned", "regular" and "tracer" strains of Ostertagia spp had resistance ratios for eggs of 4, 13 and 15 respectively when compared to the "nil" strain. In the anthelmintic efficiency assay treatment with 44 mg kg-1 and 88 mg kg-1 of TBZ removed 82 and 96% respectively of the total Ostertagia burden (adults and larvae) from the "nil" strain and 30 and 75% respectively from the "planned" strain. The same dose rates against the "regular" and "tracer" strains and additional rates of 132 or 176 mg kg-1 against the "tracer" strain failed to reduce the Ostertagia burden significantly. Intestinal Trichostrongylus spp from all isolates were fully susceptible to TBZ at 44 mg kg-1. Levamisole at 7.0 mg kg-1 was highly effective (99% reduction) against the "tracer" strain of Ostertagia.

Comparison of changes in resistance of Haemonchus contortus eggs following withdrawal of thiabendazole selection

Two strains of Haemonchus contortus with different levels of thiabendazole resistance were developed from the same parent strains by being subjected to different selection intensities. These two strains were each subdivided into two; one selected by thiabendazole in each generation, the other unselected for four generations. Levels of thiabendazole resistance were monitored in each generation by an in vitro egg hatch technique. During this time the unselected strain with the higher level of resistance maintained an LC50 similar to its selected control. The LC50 of the unselected strain with the lower level of resistance actually increased until at the end of the fourth generation it had an LC50 similar to the thiabendazole selected strains.

Comparative efficacy of standard antihookworm drugs against various test nematodes

The chemotherapeutic responses of three test nematodes, Nippostrongylus brasiliensis, Nematospiroides dubius and Ancylostoma ceylanicum to standard antihookworm drugs were assessed in order to select a suitable host-parasite system for the primary screening of potential antihookworm compounds. N. dubius behaved inconsistently and, with some infections, required more drug to achieve 100% clearance. Nippostrongylus brasiliensis was found to be sensitive to thiabendazole, tetramisole and levamisole but the broad spectrum anthelmintic mebendazole was ineffective. A. ceylanicum was very sensitive to mebendazole, sensitive to tetramisole and levamisole and refractory to thiabendazole. In vitro, none of the compounds had any lethal effect against any of the nematodes, except mebendazole against A. ceylanicum. A. ceylanicum does occur in man and its chemotherapeutic reactions are similar to those of target hookworm infections of economic importance. As such, although not equally sensitive to standard anthelmintics, it is recommended for routine primary screening.

Efficacy of ivermectin against inhibited larvae of Ostertagia ostertagi

The efficacy of ivermectin against inhibited early 4th-stage larvae of ostertagia ostertagi and other nematodes of the abomasum and intestinal tract was determined in naturally infected yearling beef cattle. The time when large numbers of inhibited larvae were acquired was determined by monthly slaughter of monitor cattle, beginning in January. In April, 12 animals were removed from pasture and maintained free of further helminth exposure until slaughter (21 days). At 9 days after the cattle were removed from pasture, ivermectin was administered to the principals by subcutaneous injection (200 micrograms/kg); the other 6 animals were given subcutaneous injections of the ivermectin vehicle. both groups were klled and necropsied at 12 days after treatment. Mean numbers of O ostertagi in the 6 controls were: adults, 41,906; developing 4th stage, 73,813; and early 4th stage, 334,965. The mean proportion of early 4th-stage larvae was 73.7%. In the 6 principals (treated with ivermectin), the following reductions were observed: O ostertagi adults, 100%; developing 4th stage, 99.8%; and early 4th stage, 99,9%. Small numbers of dead and degenerated O ostertagi of all developmental stages were recovered from abomasal washings before fixation; few viable worms were recovered.

Reduced activity of levamisole against Ostertagia ostertagi in dairy calves

Levamisole bolus formulation was administered in single doses, at rates of 5.8 to 8.1 mg/kg body weight, in 3 controlled tests in male dairy calves (n = 22) in Kentucky. Removals of natural infections of adult Ostertagia ostertagi were 70% and 76% for calves from a hear with a history of regular treatments with levamisole and 85% for calves from a herd receiving levamisole occasionally. Activity against other abomasal nematodes and lungworms was also evaluated.

Efficiency of oxfendazole administered as a single dose or in a controlled release capsule against benzimidazole-resistant haemonchus contortus, Ostertagia circumcincta and Trichostrongylus colubriformis

Laboratory strains of Haemonchus contortus and Ostertagia circumcincta selected for thiabendazole resistance were found to have a strong side resistance to a single dose of oxfendazole. The LD50 and LD95 in mg of drug per host liveweight were respectively 4.28 and 18.46 mg/kg for H contortus and 3.61 and 11.20 mg/kg for O circumcincta. A field strain of Trichostrongylus colubriformis that had not been selected with thiabendazole for seven years also had a strong side resistance to oxfendazole with approximately 50 per cent of its population resistant to the recommended dose rate of 5 mg/kg. Prolonged administration of oxfendazole by intraruminal controlled release capsules was found to be effective against both susceptible and resistant strains of the above parasites. The first observed effect of oxfendazole, from controlled release capsules, on resistant worms was a decrease in the percentage of eggs developing to third stage larvae. This was followed by a decrease in egg count and in worm numbers.

Efficacy of levamisole against abomasal nematodes and lungworms in dairy calves: preliminary tests indicating reduced activity for Ostertagia ostertagi

Dairy calves (n = 18) were used in controlled tests to evaluate activity of levamisole against abomasal nematodes and lungworms. Two formulations of levamisole, injectable (experiment A) and bolus (experiment B), were given in single doses at rates of 6 mg/kg of body weight and 5.4 to 10.2 mg/kg (5.4, 7.6, 8.0, 8.8, or 10.2 mg/kg), respectively. Experiment A included 4 treated and 4 nontreated calves and experiment B consisted of 5 treated and 5 nontreated calves. Aggregate removals of Ostertagia spp (4th stage), Ostertagi (adult), Trichostrongylus spp (4th stage), and T axei (adult) were 0%, 54%, 86%, and 95%, respectively, for injectable treated calves and were 0%, 23%, no data, and 85%, respectively, for bolus-treated calves. Haemonchus spp, present in small numbers, were completely removed from treated calves in experiment A. Several possible explanations are discussed regarding poor removal activity of levamisole by both formulations against O ostertagi. Removal activity was 95% and 100% against Dictyocaulus viviparus for injectable-treated and bolus-treated calves, respectively.

Further studies on the efficacy of fenbendazole against inhibited larvae of Ostertagia ostertagi

Fenbendazole, at low dosage levels, was evaluated for its efficacy against inhibited early fourth stage larvae of Ostertagia ostertagi in naturally infected, yearling beef cattle in April. The mean number of inhibited larvae in 10 untreated control cattle was 51,500. At a dosage level of 1 mg per kg (pelleted fenbendazole) per day in feed over five days in 10 cattle, the percentage reduction in comparison to controls was: adults, 85.9 per cent; developing stages, 79.0 per cent; inhibited early fourth stage, 47.6 per cent; other abomasal genera (Haemonchus species and Trichostrongylus axei, 100 per cent). At a dosage level of 5 mg per kg (10 per cent liquid suspension) administered as an oral drench to 10 cattle, the percentage reduction in comparison to controls was: adults, 96.9 per cent; developing stages, 82.9 per cent; inhibited early fourth stage, 74.7 per cent; other genera, 98.6 per cent. Efficacy against parasite genera of the intestinal tract (primarily Cooperia species) was in excess of 94 per cent at both dosage levels. Some variability of efficacy was observed, in that large numbers of surviving inhibited O ostertagi larvae were recovered from two or three animals in treated groups.

Use of guniea pigs to assay anthelmintic resistance in ovine isolates of Trichostrongylus colubriformis

Guinea pigs infected with Trichostrongylus colubriformis were used to develop an assay for anthelmintic resistance by determination of worm burdens following treatment with test anthelmintics. To achieve comparable efficacy with the recommended dose of thiabendazole and levamisole in sheep, dose rates in guinea pigs had to be increased two to four fold. For example, thiabendazole at 100 mg/kg in guinea pigs was 96.6 per cent effective against a thiabendazole susceptible (GS) strain of T colubriformis, but had no effect against a thiabendazole-resistant (VRSG) strain. In sheep 50 mg/kg of thiabendazole would have a similar efficacy against each strain respectively. Morantel tartrate at 10 mg/kg in guinea pigs was 99 to 100 per cent effective against the GS strain but only 54 per cent effective against a morantel resistant (PF4) strain. A slope ratio assay was used to calculate the relative potency of anthelmintics by comparing efficacies against resistant strains with efficacy against the GS strain. Resistance of the VRSG strain to thiabendazole was confirmed with a relative potency for this drug of 0.047 in guinea pigs. The PF4 strain was resistant to both thiabendazole and levamisole which had relative potencies of 0.168 and 0.255 respectively. The advantages of this statistical treatment together with the cost and time savings of the guinea pig model over a conventional critical anthelmintic assay in sheep are discussed.

Anthelmintic efficacy of low-dose phenothiazine against strains of sheep nematodes susceptible or resistant to thiabendazole, levamisole and morantel tartrate: efficiency against sequentially administered infections

Low-dose daily phenothiazine therapy, administered as a 1:40 phenothiazine:molasses mixture, was tested against sequential infections of strains of Haemonchus contortus. Trichostrongylus colubriformis and Ostertagia spp susceptible or resistant to thiabendazole, levamisole and morantel tartrate. Phenothiazine at a dose rate of 0.5 g per sheep per day significantly reduced the numbers of worm eggs per gram of faeces in both susceptible and resistant strains by an average of 48 per cent and 60 per cent respectively. Phenothiazine completely inhibited larval production in susceptible strains. With resistant worms, larval production was depressed by up to 85.7 per cent. Treatment with phenothiazine : molasses mixture reduced the numbers of worms which became established in resistant Ostertagia spp and susceptible T colubriformis but had no effect on the establishment of susceptible H contortus and Ostertagia spp or resistant H contortus and T colubriformis.

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.

Changes in the adenylate energy charge of Nematospiroides dubius and Trichostrongylus colubriformis paralysed by levamisole in vivo

The adenine nucleotide content and adenylate energy charge of Nematospiroides dubius from laboratory mice and of Trichostrongylus colubriformis from lambs has been measured. Administration of the anthelmintic, levamisole, to infected hosts resulted in only a slight fall in the adenylate energy charge of N. dubius over a 3-h period but there was a greater fall in the adenylate energy charge of T. colubriformis during this period. In neither case did the energy charge fall quickly, nor did it fall to the low levels which would be expected if the levamisole were inhibiting synthesis of ATP. The changes in energy charge of the nematodes which occurred following administration of levamisole to their hosts was of the order which can be satisfactorily explained by changes in the environment of the nematodes, such as reduced oxygen tension. It is concluded that the maintenance of levamisole-induced paralysis of these two species of trichostrongyle in vivo does not rely on the inhibition of fumarate reductase.

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.

Benzimidazole-resistant Haemonchus contortus--the effect of cryopreservation on the resistance of successive generations

Infective larvae (L3) of a strain of benzimidazole-resistant Haemonchus contortus were stored in the gas phase of liquid nitrogen, thawed and used to infest worm-free sheep from which, in turn, larvae were cultured, frozen and thawed. This cycle was repeated 5 times. Thereafter, the progeny of the cryopreserved larvae were compared with the progeny of the original untreated larvae for susceptibility to benzimidazole anthelmintics. Repeated freezing of consecutive generations of L3 in liquid nitrogen did not appear to affect their relative resistance to benzimidazoles and, although other strains must also be tested, it would seem that cryopreservation may be used for storing resistant strains.

A field strain of Haemonchus contortus showing slight resistance to rafoxanide

A field strain of H. contortus, already resistant to benzimidazole anthelmintics, was also found to be slightly resistant to rafoxanide. This is apparently the first report of resistance to rafoxanide in a field strain of H. contortus.

Trichostrongylus colubriformis and Ostertagia sp resistant to levamisole, morantel tartrate and thiabendazole: infectivity, pathogenicity and drug efficacy in two breeds of sheep

Merino and Border Leicester cross Merino sheep, nine months old, were infected with 10,000 third stage larvae of both Trichostrongylus colubriformis (PF4) and Ostertagia circumcincta/O trifurcata (PF5), known to have varying degrees of resistance to levamisole, morantel tartrate and thiabendazole. Crossbred sheep carried heavier Ostertagia sp worm burdens but there was no difference in susceptibility between the two breeds of sheep to infection with T colubriformis. The anthelmintic efficacy of thiabendazole, at 50 mg/kg, against T colubriformis was 81.8 per cent and 92.4 per cent for Merinos and crossbreds respectively while levamisole, at 6.75 mg/kg, was 12.3 per cent and 18 per cent effective. Thiabendazole removed 92.3 per cent and 83.8 per cent of Ostertagia sp in Merinos and crossbreds respectively. However, worm burdens in levamisole treated sheep were not significantly different from controls. No significant breed differences were found in 24 h faecal egg outputs. It is suggested that breed differences previously described in four-month-old sheep may have been due to differences in the rate of development of immune responsiveness. This disparity was no longer detectable in the immunologically mature sheep used in this study.

Trichostrongylus colubriformis and Ostertagia sp resistant to levamisole, morantel tartrate and thiabendazole: isolation into pure strain and anthelmintic titration

Strains of Trichostrongylus colubriformis (designated PF4) and Ostertagia sp (O circumcincta/O trifurcata, designated PF5), with varying degrees of resistance to levamisole, morantel tartrate and thiabendazole were isolated into pure culture. Detailed dose response studies showed that both T colubriformis and Ostertagia sp were highly resistant to levamisole and morantel with low level resistance to thiabendazole. The effective dose required to remove 80 per cent worm burdens (ED80) was calculated for each anthelmintic. For T colubriformis the ED80 for levamisole and thiabendazole were 12.6 and 40.1 mg/kg respectively. For Ostertagia sp, the ED80 for levamisole, thiabendazole and morantel were 20.4, 45.2 and 35.8 mg/kg respectively. The implications of these results are discussed with reference to alternative means of chemical control.

Oxfendazole: anthelmintic activity in sheep artificially infected with nematodes. Results of trials against nine species including benzimidazole-resistant Haemonchus contortus

In a series of 11 trials, in which 281 lambs were artificially infected with 9 species of nematodes, larval anthelmintic tests were carried out to assess the activity of a dose of 5 mg/kg oxfendazole against all 3 parasitic stages of each species. An efficacy of over 99,4% was obtained against all stages of Haemonchus contortus, Ostertagia circumcincta, Trichostrongylus colubriformis, Nematodirus spathiger, Gaigeria pachyscelis, Chabertia ovina and Oesophagostomum columbianum with the exception of the third larval stage of O. Columbianum for which an efficacy of 93,5% was recorded. Against Dictyocaulus filaria efficacies against the fifth stages and adults, fourth larval and third larval stages were 99,8%, 92,0% and 37,5% respectively, and in two additional tests against the combined third and fourth larval stages, efficacies of 86,4% and 85,3% were recorded. Efficacy against Strongyloides papillosus was poor. In 2 supplementary experiments oxfendazole exhibited a 92,2--94,8% efficacy against adults of the Boshof benzimidazole-resistant strain of H. contortus, the highest activity so far recorded against the strain of any benzimidazole tested.

The activity of albendazole against adult and larval gastrointestinal nematodes in naturally infected calves in the Netherlands

The activity of albendazole against gastrointestinal helminths in naturally infected calves in the Netherlands was tested. The calves were in their first grazing season and kept in two groups of ten. One of these groups was grazed alternately with sheep. Five out of each group were drenched with albendazole (7.5 mg/kg) on the day they were housed (November 1). Before and 2, 14, and 28 days after treatment individual faecal samples were taken from all calves and larval cultures were made. Ten calves, six treated and four untreated, were killed for post mortem studies 14 days after treatment. The remaining calves were slaughtered 14 days later. The drug was highly effective in reducing the egg output, measured as the number of larvae cultured per gram of faeces. Compared with the untreated calves, the reduction was more than 99% two days after treatment, 100% at 14 days, and 99% after 28 days. It was shown that egg output 28 days after treatment came from worms which had developed from arrested larvae of Ostertagia ostertagi that had survived treatment. Post mortem results showed an efficacy of 100% against adult O. ostertagi, of almost 100% against Trichostrongylus axei, and 100% against adult and larval Cooperia oncophora. Twenty-eight days after treatment, the reduction of arrested early fourth stages of O. ostertagi was 85% in comparison with the untreated calves. Apparently less effect was found against Trichuris ovis at the given dose rate.

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.

Activity of fenbendazole against inhibited early fourth-stage larvae of Ostertagia ostertagi

The efficacy of fenbendazole (Panacur, Hoechst-Roussel) against inhibited early fourth-stage larvae of Ostertagia ostertagi and other nematodes of the abomasum and intestinal tract was investigated in naturally infected, yearling cattle in April 1978. The time when peak levels of inhibited larvae occurred was determined by epizootiologic study which began in November 1977. All animals were removed from pasture and maintained free from further helminth infection until slaughter (19 to 21 days). The fenbendazole liquid suspension was administered as an oral drench at dose level of 10 mg/kg to 10 animals and then at dose level of 15 mg/kg to an additional 10 animals at 10 days after removal from pasture. Eleven animals were maintained as untreated controls. In cattle given the dose of 10 mg/kg, the following reductions were observed: O ostertagi adults--100%, developing stages--80%, and inhibited larvae--97%; other worm genera in the abomasum and nematodes of the intestinal tract--100%. In the cattle given the larger dose, the following reductions were observed: O ostertagi adults--100%, developing stages--98%, and inhibited larvae--99%; other worm genera in the abomasum and nematodes of the intestinal tract--100%.

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.

Trichostrongylus colubriformis and Ostertagia circumcincta resistant to levamisole, morantel tartrate and thiabendazole: occurrence of field strains

Field strains of Trichostrongylus colubriformis and Ostertagia circumcincta, designated PF4 and PF5 respectively, were recovered from a farm on which the sole use of levamisole over a preceding 12 year period led to the development of anthelmintic resistance. The results of field observations and preliminary critical trials in both Merino and crossbred sheep showed that both species have varying degrees of resistance to three major anthelmintics; levamisole, morantel tartrate and thiabendazole. Mean worm count reductions for adult T colubriformis (PF4) for therapeutic doses of morantel tartrate, thiabendazole and levamisole in crossbreds were 45.7 per cent, 97.3 per cent and zero respectively, and for Merinos 80.7 per cent, 88.3 per cent and 92.0 per cent respectively. Against O circumcincta the corresponding reductions for crossbreds were 51.4 percent, 95.4 per cent and 20.3 per cent and for Merinos, 52.5 per cent, 73.1 per cent and 29.8 per cent. There was no statistically significant difference in the responses of both parasite species to either levamisole or morantel. This result suggests that resistance to the two chemically unrelated drugs may be co-inherited.

Activity against Ostertagia ostertagi of low doses of oxfendazole continuously released from intraruminal capsules in cattle

Slow release from intraruminal capsules of 0.48 mg/kg oxfendazole per day for 5.5 days gave percentage efficiencies of 99 +/- 0.6, 86.6 +/- 6.3 and 93.1 +/- 4.5 for the removal of adult worms, developing 4th stages and inhibited larvae of Ostertagia ostertagi respectively. These efficiencies were not significantly different from those obtained from a single oral dose of 2.5 mg/kg. Peak plasma levels of oxfendazole were similar for the 2 types of administration but high levels were maintained longer in cattle given capsules.

Resistance to thiabendazole in a field population of Ostertagia circumcincta from sheep

A population of Ostertagia circumcincta from sheep has been found which, in field and laboratory trials, showed a lack of response to the recommended dose (44 mg/kg) of thiabendazole, suggesting that a degree of resistance exists to this anthelmintic. Preliminary experiments indicated a less than expected response to levamisole. This was not confirmed in a laboratory trial and therefore resistance to this anthelmintic has not been shown conclusively.

Efficacy of oxfendazole against inhibited Ostertagia ostertagi in naturally infected cattle

Oxfendazole was administered to pregnant cows at 2.5 and 5 mg/kg body weight to determine the anthelmintic efficacy against naturally acquired larvae which became inhibited at the early 4th stage. The experimental design included three groups of orally-treated cows, that is, 10 placebo treated control cows, 11 cows treated with 2.5 mg/kg of oxfendazole and 10 cows treated with 5.0 mg/kg of oxfendazole. Oxfendazole at 2.5 mg/kg body weight was 82 and 94% effective against EL-4 and adult O. ostertagi, respectively. At 5 mg/kg, Oxfendazole was 95 and 99% effective against EL-4 And adult O. ostertagi, respectively. The results suggested the use of a field dosage level of 5 mg/kg body weight oxfendazole where inhibited larvae may be encountered.

Thiabendazole resistance in field populations of Haemonchus contortus

Six populations of H. contortus were selected for a study of thiabendazole resistance from a collection of 40 populations made during a survey of the efficiency of thiabendazole on the Northern Tablelands of New South Wales. Based on survey results, 3 of these populations were considered susceptible and the remaining 3 were considered resistant. However, when these populations were compared with a known susceptible strain on the ability of their eggs to hatch in a solution containing thiabendazole and 0.1% NaCl all 6 had significantly greater LC50's. The resistance ratios of the LC50 for each of the 6 populations to that of the known susceptible strain were 5.1, 4.3, 3.2, 3.1, 2.3, and 1.6 respectively. Following dosing of their host with 44 mg/kg thiabendazole the resistance ratios of the survivors increased to 5.4, 5.1, 4.7, 4.4, 3.1 and 2.4. Eggs produced by the F1 generation of the worms surviving 44 mg/kg thiabendazole did not revert back to the lower LC50's of the unselected parents. Rather the LC50 remained at a level near that of a known highly resistant strain of H. contortus.

Effectiveness of anthelmintic treatments against levamisole-resistant Ostertagia

Adult worms from a population of Ostertagia consisting of 88% O. circumcincta and 12% O. trifurcata were found to survive a double drench (14 mg/kg) of levamisole. An anthelmintic assay on this population found that against levamisole the LD50 was 11.95 mg/kg and the LD99 was 67.75 mg/kg. Three benzimidazoles were also tested against this population and the LD50 and LD99 were respectively thiabendazole 11.8 and 144.9 mg/kg, oxfendazole 0.17 and 6.10 mg/kg and albendazole 0.08 and 2.55 mg/kg. The percentage of worms killed by the recommended dose rate was levamisole 24%; thiabendazole 91%; oxfendazole 99% and albendazole 99%.

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.

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.

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.

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.

Physiological characteristics of free-living and parasitic stages of strains of Haemonchus contortus, susceptible or resistant to benzimidazole anthelmintics

The effect of changes in genetic constitution (associated with developing benzimidazole resistance) on the physiological characteristics of Haemonchus contortus was investigated. Benzimidazole resistant H contortus were significantly more infective for sheep (20 per cent) than benzimidazole susceptible strains. Pathological changes were more severe in sheep infected with resistant H contortus as measured by packed cell volume, plasma protein concentration and haemoglobin levels. Faecal egg output was significantly higher for resistant strains as was the development and survival of eggs and free-living stages on pasture. Infective third stage larvae of benzimidazole resistant H contortus had a significantly faster exsheathment rate in rumenal fluid than did susceptible strains. These adaptive changes in response to continuing anthelmintic selection of resistant worm populations are discussed and the implications of these findings for worm control programmes indicated.