Efficacy of pyrantel tartrate against experimental infections with Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis in goats

The efficacy of pyrantel tartrate was evaluated in goats against induced infections with Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis. All the strains were of sheep origin and tested for susceptibility to pyrantel tartrate in sheep at the standard dose rate (20 mg kg-1) prior to the infection of goats. Fifteen French Alpine female goats were inoculated with the three nematode species. On Day 25 post-infection, goats were randomized into an untreated control group and two pyrantel treatment groups (20 mg kg-1 bodyweight once, and 40 mg kg-1 bodyweight as two doses 24 h apart). The goats were killed and processed for worm recovery 10 days after treatment. The two dose rates achieved high and similar levels of efficacy (> 96%) against Haemonchus contortus and Teladorsagia circumcincta. Against Trichostrongylus colubriformis, however, pyrantel tartrate was less effective at both dose rates as worm reductions ranged from 55 to 62%.

Trichostrongylus colubriformis infection in rabbits: persistence of the distal adaptive response to parasitism after anthelmintic treatment

Mucosal changes associated with anthelmintic treatment in Trichostrongylus colubriformis-infected rabbits were assessed along the entire length of the small intestine. The following groups, each of five rabbits, were compared: infected (group I); infected and treated on day 21 with fenbendazole (group IT); uninfected but given fenbendazole on day 21 (group C, controls). All animals were killed on day 28. In the proximal part of the small intestine of group I rabbits, the worms were associated with shortening of the villi and a significant depletion in alkaline phosphatase activity, which differed significantly from the findings in groups C and IT. In the same region, no difference was found between groups C and IT. In the distal small intestine, hypertrophy of villi and crypts (an adaptive response to the infection), coupled with an increase in enzymic activity, were present in both groups I and IT, in contrast to group C. These results suggest that a complete mucosal restoration occurred within 7 days of anthelmintic treatment in the parasitized part of the intestine. In contrast, the adaptive response observed beyond the main site of parasitism was not abolished by treatment. The functional significance of these findings is discussed in relation to the compensatory growth commonly observed after anthelmintic treatment in ruminants.

The influence of a heavy infection with sensitive and resistant strains of Ostertagia circumcincta and with Trichostrongylus colubriformis on the pharmacokinetics of febantel in lambs

Plasma concentrations of febantel and its major metabolites, fenbendazole, oxfendazole and fenbendazole sulphone, were determined after oral administration of 7.5 mg/kg febantel in lambs before and 28 days after infection with 100,000 L3 larvae of a benzimidazole (BZ)-sensitive or BZ-resistant strain of Ostertagia circumcincta or with 75,000 L3 larvae of a BZ-sensitive Trichostrongylus colubriformis strain. The febantel concentrations were always low, and in only a few samples were higher than the limit of detection. A mean decrease in the area under the curve (AUC) for the three metabolites of 10.2%, 16.4% and 4.9% in lambs infected, respectively, with BZ-sensitive O. circumcincta, BZ-resistant O. circumcincta and T. colubriformis was observed. The Cmax for all the metabolites was higher in the BZ-sensitive O. circumcincta group than in the naive sheep, while the Tmax occurred earlier. The Cmax and the Tmax values for all the metabolites were lower in the BZ-resistant O. circumcincta group than in their own naive controls. In the T. colubriformis group the Cmax values of the metabolites were lower and the Tmax occurred much later.

Immunization of sheep against parasitic nematodes leads to elevated levels of globule leukocytes in the small intestine lumen

In sheep that had been given three immunizing infections with Trichostrongylus colubriformis and Ostertagia circumcincta infective (L3) larvae, drenched after the last infection and challenged with larvae of the same species, there was a significant increase in numbers of small intestine mucosal tissue globule leukocytes (TGLs) and lumenal globule leukocytes (LuGLs) compared with sheep that had only been drenched and challenged. There was a positive correlation between the numbers of LuGLs and TGLs in the small intestine but the ratio of these two cell types was lower in non-immunized than immunized sheep. In immunized sheep positive correlations were observed between LuGLs and levels of arylsulphatase and peroxidase in the intestinal mucus and between arylsulphatase and larval migration inhibition (LMI) activity in mucus. Lumen eosinophils correlated with blood eosinophils, serum antibody against T. colubriformis correlated with peroxidase in the mucus and blood eosinophils correlated with nematode specific IgM levels in the intestinal mucus. In the abomasum, TGLs were present but not LuGLs. Sheep repeatedly infected with T. axei also had significantly more LuGLs in the small intestine than control animals. Two sheep that had a surgically prepared isolated small intestinal loop, after oral infection with T. colubriformis had TGLs and LuGLs in the intact intestine, but not in the isolated loop. Significantly more LuGLs were produced in sheep by allowing repeated T. colubriformis L3 infections to develop to adult stages compared to sheep treated with the same number of larvae, but where the infections were terminated by drenching at various intervals.

Efficacy of oral moxidectin against benzimidazole-resistant isolates of gastrointestinal nematodes in sheep

The efficacy of orally administered moxidectin was determined against four benzimidazole-resistant nematode isolates. At the start of the trial, 30 lambs were each infected experimentally with 20,000 third stage larvae (5000 Haemonchus contortus, 7000 Teladorsagia circumcincta, 3000 Trichostrongylus colubriformis and 5000 Cooperia curticei); 28 days later they were allocated randomly to three groups of 10: one untreated group, one group treated orally with fenbendazole (5 mg/kg bodyweight) and one group treated orally with moxidectin (0.2 mg/kg). Samples of faeces were taken five and 10 days after treatment and the lambs were killed 10 days after treatment. Fenbendazole reduced the average number of nematode eggs in faeces by 95 per cent and the average number of worms by 25 to 45 per cent according to the species. The efficacy of moxidectin against these benzimidazole-resistant isolates was 100 per cent. No adverse reactions to either of the drugs were observed.

The effect of reduced feed intake on the efficacy of oxfendazole against benzimidazole resistant Haemonchus contortus and Trichostrongylus colubriformis in sheep

Within 12 h of reducing the daily intake of a 50:50 lucerne and wheaten hay ration from 800 to 400 g, there was an increase in digesta marker concentration indicating a reduction in the flow rate of digesta fluid and particulates through the abomasum of sheep. The efficacy of the recommended dose of oxfendazole (OFZ) was then tested against benzimidazole resistant Trichostrongylus colubriformis and Haemonchus contortus in sheep whose feed intake was halved 36 h before and after drug treatment. In animals fed the reduced ration there was a 60% reduction of T. colubriformis and 94% reduction of H. contortus compared with a 19% and 60% reduction respectively on the higher feed intake. It is therefore suggested that greater anthelmintic efficacy can be achieved in sheep by temporary feed reduction.

Prolonged suppression of trichostrongyle egg output of N'Dama cattle by a single larvicidal treatment

Gambian village cattle herds were treated with a single dose of ivermectine (Ivomec, MSD-AGVET Inc. 1 ml/50 kg body weight) during the dry season. This treatment suppressed the trichostrongyle egg rise prior to the rains and led to delayed egg production for at least 6 months after the onset of the rainy season, compared to untreated animals from neighbouring villages. However, the results clearly indicate that a single treatment with ivermectine during the dry season does not lead to complete suppression of the gastrointestinal strongyle infections, since a worm population still gradually built up. During the second year the ranked level of the egg excretion was significantly lower than that of the control group throughout the year until December, except in August. The results further support the hypothesis that trichostrongyle reinfection is unimportant during the dry season in this climatic zone.

Multiple and multigeneric anthelmintic resistance on a sheep farm in Malaysia

The anthelmintic efficacy of benzimidazoles, levamisole, closantel, ivermectin and moxidectin was evaluated on an institutional farm in Malaysia using faecal egg count reduction tests, controlled slaughter trials and an in vitro egg hatch assay. The results of this study indicated simultaneous resistance of Haemonchus contortus against benzimidazoles and ivermectin and of Trichostrongylus colubriformis against benzimidazoles and levaminsole on the same farm. Moxidectin was effective against the ivermectin resistant H. contortus.

Mutual resistance to avermectins and milbemycins: oral activity of ivermectin and moxidectin against ivermectin-resistant and susceptible nematodes

To determine whether there is mutual resistance to avermectin and milbemycin anthelmintics, ivermectin and moxidectin sheep drenches were tested against ivermectin-resistant and susceptible isolates of Ostertagia circumcincta and Trichostrongylus colubriformis in sheep. None of the isolates had been exposed to moxidectin previously. The dosage of ivermectin required to remove 95 per cent of the ivermectin-resistant O circumcincta and T colubriformis were 23 times and six times larger, respectively, than the dosages required to remove the same percentage of susceptible isolates. The dosages of moxidectin required to remove 95 per cent of the ivermectin-resistant O circumcincta and T colubriformis were 31 times and nine times larger, respectively, than the dosages required to remove the same percentage of susceptible isolates. It is concluded that the worms resistant to ivermectin were also resistant to moxidectin.

The efficacy and persistent anthelmintic effect of ivermectin in sheep

The direct efficacy and the long-term persistent anthelmintic effect of an oral suspension and an injectable formulation of ivermectin at a dose rate of 0.2 mg kg-1 was studied in sheep. Lambs were infected experimentally with Haemonchus contortus, Trichostrongylus vitrinus and Cooperia curticei. After 3 weeks they were treated with an oral suspension or with an injection, and reinfected with the same dose of larvae 3, 6 or 10 days after treatment. Post-mortem worm counts showed no persistent effect of the oral suspension. The injectable formulation showed an excellent persistent effect for up to 10 days against H. contortus. Reinfection with C. curticei 3 days after treatment resulted in a 64% reduction of the worm burden, but reinfection after 6 and 10 days was 100% reduced. The reduction of T. vitrinus was 46% after 3 days and 92% after 6 and 10 days.

Doramectin--a potent novel endectocide

Doramectin, 25-cyclohexyl-5-O-demethyl-25-de(l-methylpropyl)avermectin A1a, was selected as the best of a series of novel avermectins prepared by mutational biosynthesis. The primary evaluation of its in vivo antiparasitic activity was carried out using a rat Trichostrongylus colubriformis model and a rabbit Psoroptes cuniculi model. In each case the new avermectin performed favourably relative to dihydroavermectin B1a (DHAVM), the major component of ivermectin. Doramectin was extensively evaluated in cattle using an experimental micelle formulation, proving highly effective in cattle infected with Ostertagia ostertagi, Cooperia oncophora and Dictyocaulus viviparus when administered subcutaneously at 200 micrograms kg-1. The plasma pharmacokinetic characteristics of doramectin in cattle following intravenous administration revealed a plasma half-life of approximately 89 h. In the micelle formulation, doramectin administered subcutaneously at 400 micrograms kg-1 provided persistent activity against infection of cattle with C. oncophora and O. ostertagi for at least 8 and 12 days respectively.

Efficacy of moxidectin against gastrointestinal nematodes of cattle

Three groups of 11 naturally infected crossbred beef calves were injected subcutaneously with moxidectin 1 per cent injectable at 0.2 or 0.3 mg moxidectin/kg bodyweight or with the unmedicated vehicle. Nematode infections had been acquired during grazing from December to April. Based on the faecal egg counts and total worm counts of the control calves at necropsy (11 to 13 days after treatment) most of the calves had heavy parasitic burdens. Ostertagia ostertagi was predominant and the mean numbers of adults, developing fourth stage larvae (L4) and inhibited early L4 were 45,906, 10,061 and 68,918, respectively. Haemonchus placei and Trichostrongylus axei were also present in the abomasa. Three species of Cooperia, Oesophagostomum radiatum L4 and T colubriformis adults were found in the intestinal tract. Both dosages of moxidectin were equally effective (P < 0.05) against all the abomasal nematodes (99.9 to 100 per cent) and the intestinal tract nematodes (99.4 to 100 per cent). No adverse reactions to the moxidectin treatment were observed. Abomasal pathology characteristic of heavy O ostertagi infection was observed in the control calves, but not in the treated calves.

A new anthelmintic assay using rats infected with Trichostrongylus colubriformis

A new anthelmintic assay is described which uses immunosuppressed (60 ppm hydrocortisone acetate in diet) rats infected with the nematode Trichostrongylus colubriformis. Immunosuppressed rats were infected with 1500 T. colubriformis larvae, treated either orally or subcutaneously on Day 14 post-infection and necropsied 4 days after treatment. The worm counts in immunosuppressed control animals averaged 775 worms per rat. A range of benzimidazoles, levamisole hydrochloride, morantel tartrate, 22,23-dihydroavermectin B1a and alpha-milbemycin have been evaluated in the assay. The ED95 values obtained indicate that rats infected with T. colubriformis provide a highly predictive model for assaying the activity of experimental drugs in vivo prior to studies in ruminants.

Evaluation of ivermectin against experimental infections of Haemonchus contortus and Trichostrongylus colubriformis in goats

Fourteen nematode parasite-naïve kids were given a mixed larval inoculum (approximately 17,250 infective larvae) via stomach tube. Twenty-two days after infection, the kids were randomly assigned to control and treatment groups of seven animals each. Mean eggs per gram for control and treatment groups were 4304 and 5315, respectively. Kids in the treatment group were given ivermectin (0.2 mg kg-1) administered by subcutaneous injection. At necropsy, the mean numbers of Haemonchus contortus and Trichostrongylus colubriformis for control and treatment groups were 2259 and 0, and 3033 and 773, respectively. This reduction was significant (P less than 0.05) for both species, resulting in an efficacy of 100.0% and 74.5%, respectively. The lower efficacy against T. colubriformis may be a sign of resistance, a reduced effectiveness due to route of administration, or a higher dose may be required with subcutaneous administration, as has been observed previously.

Effects of dietary molybdenum on nematode and host during Trichostrongylus vitrinus infection in lambs

The addition of molybdenum (0.05 mmol kg-1 dry matter) to the diet of lambs exposed for four weeks to a trickle (2500 third stage larvae per day) infection with Trichostrongylus vitrinus reduced the number and length of adult worms retrieved from the small intestine 11 days later: both effects were particularly marked in female worms from female lambs (P less than 0.01). Worms from lambs given molybdenum contained less proteinase enzyme activity and secreted less proteinases in culture irrespective of the sex of the host. Pathogenicity was not attenuated by molybdenum. Damage to the intestinal mucosa was severe in both dietary groups but infected females given molybdenum developed lower plasma albumin concentrations and lighter dressed carcases than those not given molybdenum. Neither the effects on the parasite nor those on the host could be attributed simply to molybdenum-induced copper depletion, using conventional measures of copper status. Molybdenum may be toxic to T vitrinus but may also facilitate or enhance the inflammatory process limiting larval establishment or increasing parasite rejection.

Effect of a controlled-release albendazole capsule on parasitism and productivity of sheep

The efficacy of intraruminal albendazole (ABZ) capsules (Profitril-Captec) and the effect of treatment on productivity were studied in 300 ewes infected with gastrointestinal nematodes and the trematode Dicrocoelium dendriticum. Coprological tests revealed that treated animals remained negative for 10 weeks after the administration of capsules. Contamination of pasture with nematode larvae was significantly reduced during the whole experiment. Necropsy of 14 animals (seven treated and seven untreated) showed 96.9-99.2% efficacy against the nematodes Nematodirus spp., Oesophagostomum spp., Cooperia spp., Trichostrongylus spp. and Trichuris ovis, while efficacy was 88.5% against D. dendriticum. During the 6 month pasture season (May-October 1989), treated ewes produced on average 2.56 kg cheese and 0.6 kg wool per ewe more than untreated controls. Our study confirms the reliability of the ABZ slow-release capsules over 90 days and the positive effect of treatment on nematode contamination of pasture and ewe productivity.

Disposition of oxfendazole in goats and efficacy compared with sheep

The disposition of intraruminally administered oxfendazole (OFZ) in goats was studied at 5, 10 and 20 mg kg-1. The area under the plasma concentration with time curve (AUC) increased with increasing dose but at a declining rate. AUC was lower after intra-abomasal compared with intraruminal administration. OFZ was less effective against drug resistant Trichostrongylus colubriformis in goats than in sheep but was of similar efficacy against drug resistant Haemonchus contortus in both host species. In the same experiment peak plasma levels of OFZ in goats were about half those in sheep given the same dose. Of 70 goats tested in the field, total rumen bypass occurred in 12 per cent and partial bypass in 67 per cent. Lower systemic availability due to bypass would be expected to reduce further anthelmintic efficacy in goats. From the results of these experiments a dose rate of 10 mg kg-1 is recommended for goats. When given at this rate as a divided dose at 12 hourly intervals over 24 hours, OFZ was significantly more effective than a single dose in reducing egg counts.

Resistance to thiabendazole, fenbendazole and levamisole in Haemonchus and Trichostrongylus species in sheep on a Kenyan farm

Resistance to thiabendazole (TBZ), fenbendazole (FBZ) and levamisole (LVM) in naturally acquired gastrointestinal nematode parasites in sheep was investigated on a farm where anthelmintic resistance was suspected. This was measured by both the in vitro egg hatch assay, and reductions in faecal egg and worm counts in treated animals. In the egg hatch assay, nematode eggs were incubated in various concentrations of either TBZ or LVM. The level of resistance was expressed as the drug concentration inhibiting 50% of the eggs from hatching (LC50). The nematode population had LC50 values of 0.26 microgram ml-1 TBZ and 3.12 micrograms ml-1 LVM. In the faecal egg and worm count reduction test, naturally infected sheep were treated with either TBZ (88 mg kg-1), FBZ (10 mg kg-1) or LVM (15 mg kg-1). Faecal egg and total worm counts from these sheep were then compared with counts from untreated sheep. TBZ, FBZ and LVM failed to reduce the faecal egg counts and total worm counts by more than 90%. Based on the identification of larvae from faecal cultures, the most predominant nematode species in the resistant population were Haemonchus (62%) and Trichostrongylus (28%). TBZ reduced faecal egg counts for both species by less than 90%. FBZ and LVM also reduced Haemonchus spp. eggs by less than 90%. Other nematode species numbers did not satisfy criteria for the determination of efficacy.

Correlation between gut hypersensitivity and resistance to Trichostrongylus colubriformis infection of outbred and inbred lines of guinea pigs

The relationship between gut sensitivity and immunity to challenge infection was examined in outbred and inbred guinea pig lines. Primary infections terminated at 3,6,9 or 13 days and multiple infections of 3 days' duration confirmed the importance of direct gut stimulation and the period of exposure in the induction of immunity and gut hypersensitivity. The studies with the multiple 3-day infections confirmed that the third-stage larvae alone are capable of inducing strong protective immunity and showed that this is accompanied by pronounced gut sensitivity to parasite extracts and secretions. Finally, two inbred guinea pig lines selected for enhanced resistance or susceptibility to T. colubriformis infection displayed corresponding high or low capacities to mount hypersensitivity reactions following a single truncated 3-day primary infection.

Field evaluation of a mixture of albendazole sulphoxide and levamisole against Ostertagia and Trichostrongylus spp in sheep

The efficacy of a mixture of albendazole sulphoxide and levamisole, 3.6 and 8.25 mg/kg respectively, at single and double dose rates, was compared with the recommended dose rate of each anthelmintic alone. The comparison was conducted on groups of 6 to 14-week-old lambs on 22 farms, 16 of which had evidence of multiple resistance to benzimidazole and levamisole. A single dose of the mixture reduced mean egg counts by 95% on half the farms with multiple resistance and on all the remaining farms. Consequently, the mixture should be included in an assessment of effective anthelmintics on farms to determine its value for nematode control. A double dose rate of mixture was effective on all but 4 farms. Reductions caused by the mixture were due to the additive actions of the drugs on 18 of 22 farms; synergistic action was noted on only 3 farms. It was concluded that the mixture of albendazole sulphoxide and levamisole offered many farmers an effective anthelmintic for use in preventive control programs. Recommendations for such programs include annual rotation of effective anthelmintics as a means of delaying selection for drug resistance.

The efficacy of mixtures of albendazole sulphoxide and levamisole against sheep nematodes resistant to benzimidazole and levamisole

Faecal egg count reduction tests and an anthelmintic efficiency assay were used to assess the efficacy of combinations of albendazole sulphoxide and levamisole against populations of Ostertagia and Trichostrongylus sp. which contained different proportions of worms resistant to both benzimidazole and levamisole anthelmintics. Compared to the effects of either drug alone, significantly greater efficacy was obtained using combinations which included dose rates similar to those recommended for the separate components. At these dose rates, the mixtures reduced mean faecal egg counts by 95% or more, and caused a reduction of 68% in adult Ostertagia sp. and more than 95% for 4th stage Ostertagia and T colubriformis. The increased efficacy of the mixtures could be accounted for by actions of the drugs acting independently.

Effects of ivermectin and fenbendazole in strategic treatment of gastrointestinal nematode infections in cattle

Four groups of 18 beef calves each were used to evaluate effects of different treatments on parasite control and weight gains. The investigation extended from November 1986 (weaning) to October 1987. Group-1 calves were treated with ivermectin (200 micrograms/kg of body weight, SC) at approximately 6-week intervals for a total of 8 treatments; group-2 calves were given the same dosage of ivermectin by the same route of administration as group-1 calves in November, March, and July; group-3 calves were given fenbendazole paste (5 mg/kg, PO) at the same times as group-2 calves; and group-4 calves served as untreated controls with provision for ivermectin salvage treatment. All groups grazed on individual pairs of larval-contaminated, 1.6-ha pastures. Highest (P less than 0.05) initial worm counts in fall tracer calves were found in group 3 (Ostertagia ostertagi and Trichostrongylus axei adults) and group 4 (O ostertagi and Haemonchus adults). Fecal egg counts of group-1 calves were low throughout the experiment and pasture larval counts remained negligible after July. Egg counts and larval counts of other groups remained higher into summer. Worm counts, including O ostertagi inhibited early fourth-stage larvae (EL4), were highest (P less than 0.05) in groups-3 and -4 spring tracer calves; numbers of O ostertagi EL4 were similarly high in groups 2, 3, and 4; and T axei counts were highest (P less than 0.05) in groups-3 and -4 yearlings slaughtered in spring. Liveweights of group-1 calves were greater (P less than 0.05) than in other groups from March 2 to October, and by July 2, group-2 calves had a liveweight advantage over group-4 calves. Group-3 calves had the lowest rate of gain from March to July and mean liveweight of the group was less (P less than 0.05) than in all other groups from April to October. Only minimal worm numbers were recovered from groups-1 or -2 calves in October. Large numbers of O ostertagi and T axei were recovered from group-4 calves and O ostertagi from group-3 calves. A few calves in groups 3 and 4, but particularly in group 4, were affected by type-II disease (chronic to acute gastritis caused by maturation and emergence of previously inhibited larvae) from August to October. Final mean liveweights in descending order were 365 kg in group 1, 328 kg in group 2, 316 kg in group 4, and 281 kg in group 3.

A field strain of Trichostrongylus colubriformis resistant to levamisole and morantel in South Africa

A strain of Trichostrongylus colubriformis from Nottingham Road, in Natal, was found to be solidly resistant to levamisole and morantel at the recommended dose levels. Untreated control sheep in fact harboured fewer worms at slaughter than either of the 2 treated groups. In contrast, the benzimidazoles and ivermectin were more than 99.9% effective against this worm strain. The possible implications of escalating resistance to anthelmintics in the gastrointestinal nematodes of sheep are discussed.

Laboratory selection of a benzimidazole-resistant isolate of Trichostrongylus colubriformis for ivermectin resistance

In vivo ivermectin resistance was selected in an isolate of Trichostrongylus colubriformis (TcR) already known to be benzimidazole resistant. This was accomplished in sheep by using levels of ivermectin calculated to reduce the fecal egg output from each generation of T. colubriformis by congruent to 95%. The first indication of ivermectin resistance was observed with the F10. A dosage-titration trial comparing the parent TcR with the ivermectin-selected F21 demonstrated that the latter was congruent to 20 times more resistant to oral ivermectin therapy in experimentally infected sheep than was the parent isolate. Treatment of the F16 generation with 50 mg/kg of thiabendazole resulted in only 54% egg reduction and confirmed that benzimidazole resistance was stable.

Efficacy of paraherquamide against immature Trichostrongylus colubriformis in the gerbil (Meriones unguiculatus)

Paraherquamide was 98 to 100 per cent effective against six-day-old Trichostrongylus colubriformis infections in gerbils when given as single oral doses of 1.56 mg kg-1 and above. Doses of 0.78 or 0.39 mg kg-1 were 96 and 66 per cent effective, respectively. A single oral dose of 200 mg kg-1 was well tolerated.

A novel series of milbemycin antibiotics from Streptomyces strain E225. I. Discovery, fermentation and anthelmintic activity

A novel series of milbemycin antibiotics were produced by soil isolate, strain E225 which was shown to be a Streptomyces species. The antibiotics displayed anthelmintic activity against Trichostrongylus colubriformis in the gerbil. Two of the compounds, VM 44857 and VM 44866 were shown to be potent anthelmintics against mixed nematode infections in sheep.

Detecting benzimidazole resistance with faecal egg count reduction tests and in vitro assays

Composite strains of Trichostrongylus colubriformis and Ostertagia spp consisting of 0, 1, 10, 25, 50, 75, 90, and 100% of known resistant strains were prepared and tested for benzimidazole resistance using faecal egg count reduction tests, in vitro egg hatch assays and tubulin binding assays. All tests detected resistance where the proportion of the resistant strain in the composite was 50% or more, whereas none of the tests unequivocally detected resistance below 25%. Egg count reduction tests were no less sensitive than the in vitro tests in detecting low levels of resistance but the egg hatch and tubulin binding assays provided a better quantitative estimate of moderate to high levels of resistance. Faecal egg count reduction therefore, provides a suitable means of detecting resistance in the field but tests, more sensitive to low levels of resistance are required. Results indicate that the use of post-treatment counts alone provides an adequate indication of anthelmintic efficiency.

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

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

Directed biosynthesis of avermectins

Avermectin homologs are produced by Streptomyces avermitilis when externally supplied with sodium 2-methylpentanoate and sodium 2-methylhexanoate. The homologs carry 2-pentyl and 2-hexyl groups, respectively, at C-25 of the aglycone moiety as opposed to the 2-butyl group of "a" components and the isopropyl group of "b" components of natural avermectins. The new homologs designated as avermectin "c" and "d" components, respectively, possess potent anthelmintic and insecticidal activity.

Anthelmintic resistance in the field: changes in resistance status of parasitic populations in response to anthelmintic treatment

Changes in anthelmintic resistance in nematode parasites were monitored in sheep grazing on 2 separate farms, but with the same anthelmintic treatment program, over 16 years. High levels of benzimidazole resistance emerged in Ostertagia and Trichostrongylus spp populations on both farms following 9 years of continuous use of this class of drug. Subsequently, variations in the levels of resistance occurred for the same species between farms and between species on the same farm. A change to levamisole for 2 years resulted in a significant reversion towards benzimidazole susceptibility, but a concomitant rise in levamisole resistance, in Ostertagia on one farm. However, benzimidazole resistance increased rapidly following the re-introduction of oxfendazole into the anthelmintic treatment program. Results from both farms illustrate the pitfalls of using one anthelmintic class for an extended period and provide indirect support for the alternation of anthelmintic classes at approximately yearly intervals.

Selection of ivermectin-resistant Trichostrongylus colubriformis in lambs

A mixed population of Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis was passed for four generations through lambs. In successive passages and on Day 6 (D6), lambs were dosed subcutaneously with ivermectin at 100, 200, 250 and 225 micrograms kg-1 body weight, respectively. No larvae of H. contortus were recovered after the first passage and following the third drug exposure only T. colubriformis was present. Larvae of the selected strain of T. colubriformis developed to third stage, sheathed larvae in vitro in concentrations of ivermectin that inhibited development of larvae from the original strain of T. colubriformis. A definitive control test was then run with 45 lambs, to compare this selected strain to the original strain. The treatments were 150 micrograms kg-1 subcutaneous (SC) D6, 200 micrograms kg-1 SC D6, 200 micrograms kg-1 intraruminal (IR) D6 and 200 micrograms kg-1 SC D21; all treatments were compared to non-treated controls, selected or original strain as applicable. The D6 IR and D6 SC 150 micrograms kg-1 treatments were greater than 99% effective in both strains, although the D6 SC 200 micrograms kg-1 treatment was 85 and 48% effective in the original and selected strain, respectively. The D21 200 micrograms kg-1 was only 58% effective against the selected strain. There was significant (P less than 0.001) differences between the original and selected strains of T. colubriformis for both fecal egg output and worm counts. The exposure of four generations of immature T. colubriformis to ivermectin selected for partial resistance.

A field evaluation of benzimidazole and nonbenzimidazole drugs in a herd of dairy goats

A study was conducted to investigate the efficacy of six anthelmintics in a herd of dairy goats. Pretreatment larval cultures indicated that the goats were infected with Haemonchus contortus and Trichostrongylus colubriformis. Three separate treatment regimens were administered. In each trial, mature nonlactating goats were allocated into two treatment groups and a control group. Treatment groups received thiabendazole (TBZ) or levamisole (LEV), mebendazole (MBZ) or fenbendazole (FBZ), and morantel tartrate (MOR) or ivermectin (IVR). LEV, MOR, and IVR reduced fecal strongyle egg counts by 99% to 100% of pretreatment values. The benzimidazole (BZD) drugs changed pretreatment fecal egg counts by +2% to -32%. Results of posttreatment larval culture demonstrated the presence of H contortus larvae following the administration of BZD drugs.

Anthelmintic activity of continuous low doses of fenbendazole into the rumen of sheep

Fenbendazole (FBZ) was continuously infused for 30 days into the rumen of 103 lambs which had mature or developing benzimidazole-susceptible or thiabendazole-resistant Haemonchus contortus and susceptible Trichostrongylus colubriformis infections. Ovicidal, larvicidal and adulticidal activities were exhibited against benzimidazole-susceptible and benzimidazole-resistant H. contortus worms by FBZ at a dose level of greater than or equal to 0.2 mg kg-1 body weight day-1. Reasonably consistent high level efficacy against H. contortus was obtained with dose levels greater than 0.4 mg kg-1. Excellent control of susceptible T. colubriformis worms was achieved with the lowest dose tested of 0.4 mg kg-1 day-1. The intraruminal infusion critical study method is a tool to determine the feasibility of incorporating a candidate anthelmintic compound in a continuous sustained-release rumen device formulation. The anthelmintic profile of FBZ obtained by low-level intraruminal administration suggests that it would be a potential candidate.

Prevalence of benzimidazole resistance of nematodes in sheep in The Netherlands

A survey was carried out in The Netherlands to determine the prevalence of benzimidazole resistance in sheep nematodes. It was shown that 47 per cent of the 59 investigated flocks harboured trichostrongylid populations with resistance to benzimidazoles, as demonstrated by an LD50 value of 0.12 micrograms ml-1 thiabendazole or higher in an in vitro egg hatch assay. On these farms Haemonchus contortus was found to be the dominant species. In two flocks resistant Ostertagia and, or, Trichostrongylus species populations were found. Farms with resistance were revisited to determine the anthelmintic efficacy in vivo by means of treatment and egg count. In only four of the 19 flocks which showed resistance in vitro was an efficacy of less than 95 per cent in vivo achieved. This means that there is a discrepancy between the in vivo and in vitro results. Resistance was found more frequently on pure sheep farms than on mixed farms. It is suggested that mixed farms offer more refuge for susceptible H contortus populations.

The effect of Trichostrongylus tenuis on the caecal mucosa of young, old and anthelmintic-treated wild red grouse, Lagopus lagopus scoticus

The caecal mucosa of wild young and adult grouse infected naturally with Trichostrongylus tenuis was examined by means of scanning electron microscopy and compared with adult grouse which had been treated with an anthelmintic. The caecal mucosa of young red grouse with low worm burdens possessed longitudinal plicae and exhibited little damage. The caeca from adult grouse, most of which carried high worm burdens, showed a localized depression of plicae and atrophy and cell disruption in areas of nematode aggregation. Caeca from adult birds treated with an anthelmintic showed a similar caecal structure to lightly infected caeca from young birds. It is concluded that the normal functioning of the caeca is probably affected by heavy infections of T. tenuis.

A genetic model describing the evolution of levamisole resistance in Trichostrongylus colubriformis, a nematode parasite of sheep

Data from 21 generations of selection on a levamisole-resistant strain of Trichostrongylus colubriformis, either exposed to selection with the anthelmintics levamisole (LEV) or thiabendazole (TBZ), or unexposed, were used to fit a genetic model describing the evolution of LEV resistance in this parasite species. A statistical model describing the dose-response relationship for a mixed population of susceptible and resistant parasite eggs exposed to anthelmintic was fitted to egg-hatch assay data for each generation and for each selection regimen. Estimated parameters from the statistical model provided the input for the genetic model from which were obtained estimates of the relative fitness of susceptible and resistant genotypes under each selection regimen. The experimental data and the genetic models both indicated that, in this parasite strain, LEV resistance was determined by a single dominant gene, and that TBZ selects for LEV susceptibility. A variety of drug alternation programmes was simulated for this genetic system. The programme that minimized the development of LEV resistance involved alternating the drugs (LEV and TBZ) between each worm generation.

Population dynamics of Trichostrongylus colubriformis in sheep following anthelmintic treatment

Three groups of five, three-and-a-half to four-month-old lambs were put to graze on three plots contaminated by Trichostrongylus colubriformis. One group (control) was free of parasites, while the other two were infected just before turning out with 3 X 700 third stage larvae. Depending on whether or not the lambs were treated with 50 mg kg-1 thiabendazole nine days after the first infection, the worm burden and the egg output increased or decreased during the grazing period when compared with the control group. In tracer animals put to graze on the same plots afterwards, it was found that the smaller the number of eggs excreted by the first lambs, the higher were the worm burdens of the tracer lambs.

Resistance of Trichostrongylus colubriformis to levamisole and morantel: differences in relation to selection history

Two field strains of Trichostrongylus colubriformis were tested by in vitro and in vivo methods for resistance to morantel, levamisole and thiabendazole and compared with an anthelmintic-naive laboratory-passaged strain (McM). One field strain (TAS) was isolated from a dairy goat herd which had experienced severe helminthiasis despite intensive anthelmintic usage. The other (BCK) was isolated from sheep which had been treated solely with levamisole over a 6-year period. The BCK strain had very high levels of both levamisole and morantel resistance. In contrast the TAS strain was resistant to morantel but highly susceptible to levamisole. This finding is contrary to the expectation that selection with morantel automatically confers resistance to levamisole, the converse of which was shown to apply in the BCK strain. Although the TAS strain was exposed to levamisole prior to isolation, examination of the drug's pharmacokinetics in goats indicated that it exerted little if any anthelmintic effect, and therefore selection pressure, on the parasite population. This study suggests that the mechanism of levamisole resistance covers a wide spectrum, and embraces that for morantel. It also suggests that in order to conserve the effectiveness of the levamisole/morantel group of broad spectrum anthelmintics, morantel should be used to the exclusion of levamisole until resistance is detected, at which time levamisole may be introduced to re-establish high levels of control.