Use of a long acting injectable formulation of moxidectin to control the periparturient rise in faecal Teladorsagia circumcincta egg output of ewes

A field study was conducted in a sheep flock in the south east of Scotland with a history of ivermectin resistance in Teladorsagia circumcincta. The objective of the study was to compare the effects of single anthelmintic treatments in ewes before turn-out onto pasture that was contaminated with a moderate level of overwintered, ivermectin resistant, T. circumcincta infective larvae. The ewes were treated according to label directions with either a long acting injectable formulation of moxidectin (1mg/kg; affording up to 14weeks persistent action against macrocyclic lactone (ML)-susceptible T. circumcincta) or an oral formulation of moxidectin (0.2mg/kg; affording up to 5weeks persistent action against ML-susceptible T. circumcincta). The lambs were enrolled in the normal management of the farm, and received a total of three oral ivermectin treatments during the 16week study. The efficacy of both treatment strategies in controlling the periparturient rise in faecal nematode worm egg counts and subsequent pasture contamination was assessed from the faecal worm egg counts of the ewes and their lambs between lambing and weaning. Ewes that were treated with the oral formulation of moxidectin shed approximately 3.5 times more T. circumcincta eggs between lambing and weaning than ewes that were treated with the long acting formulation of moxidectin. This difference was reflected in the faecal worm egg counts of the lambs that were grazed alongside the different treatment groups of ewes. The results of the current study demonstrate persistent efficacy of the long acting formulation of moxidectin against an ivermectin resistant T. circumcincta population. The decreased pasture contamination after treatment could lead to improved lamb growth and a need for fewer anthelmintic treatments, thus potentially reducing one possible selection pressure for anthelmintic resistance. However, treatment with the long acting formulation of moxidectin would give rise to fewer susceptible nematodes being present in refugia, which could increase another possible selection pressure for anthelmintic resistance, depending on the subsequent grazing management of that pasture. The rationale for use of a persistent anthelmintic drug to control the periparturient rise in faecal ML-resistant T. circumcincta egg output of the ewes is discussed and potential differences in selection for macrocyclic lactone anthelmintic resistance using the different formulations of moxidectin are acknowledged.     

Ivermectin-resistant Ostertagia circumcincta from sheep in the lower North Island and their susceptibility to other macrocyclic lactone anthelmintics

AIM: To confirm the ivermectin resistance status of a strain of Ostertagia circumcincta which was isolated from a sheep farm in the lower North Island of New Zealand and to assess the susceptibility of this strain to other macrocycliclactone anthelmintics. METHODS: Twenty-five lambs housed indoors were each infected with 12,000 L3 larvae of the above parasite strain. Approximately 3 weeks after infection the lambs were allocated to 1 of 4 treatment groups (3 groups of 6, and 1 group of 7 lambs), one of which remained untreated while the others were drenched orally with ivermectin, moxidectin or abamectin at 0.2 mg/kg liveweight. Faecal egg counts (FECs) before and after treatment, and post-mortem worm burdens 10 days after treatment were examined to assess efficacies of each anthelmintic. RESULTS: Treatment with ivermectin reduced the mean FEC by only 18% and the mean worm burden by only 42%, whereas moxidectin and abamectin reduced FECs by 92% and worm burdens by 95%. CONCLUSION: These results, together with a similar case described recently from the South Island , confirm the emergence of ivermectin resistance in nematode parasites of sheep in New Zealand. The superior efficacy of moxidectin and abamectin in this case indicates that, following the emergence of resistance to ivermectin, some short-term practical use may still be made of these other anthelmintics. However, their continued use will undoubtedly result in increased levels of resistance and eventual therapeutic failure of these products also.     

The difference in efficacy of ivermectin oral, moxidectin oral and moxidectin injectable formulations against an ivermectin-resistant strain of Trichostrongylus colubriformis in sheep

AIM: To evaluate the efficacy of ivermectin oral, moxidectin oral and moxidectin injectable formulations against an ivermectin-resistant strain of Trichostrongylus colubriformis in sheep. METHODS: Twenty-four mixed breed lambs were infected with 15,000 infective third-stage larvae of an ivermectin-resistant strain of T. colubriformis which had originally been isolated from a goat farm in Northland in 1997. Twenty-six days post infection, the lambs were divided into 3 treatment groups and a control group (n=6 lambs/group). Treatment consisted of either ivermectin oral formulation (0.2 mg/kg), moxidectin oral formulation (0.2 mg/kg), or moxidectin injectable formulation (0.2 mg/kg). Faecal egg counts (FECs) were determined at 0, 3, 5, 7 and 10 days after treatment. All animals were necropsied 12 days after treatment and worm counts were performed. Larval development assays were conducted 24 days post infection. A further 3 lambs were infected with 15,000 infective third-stage larvae of a fully susceptible strain of T. colubriformis for comparative purposes in the larval development assay. The efficacy of the moxidectin injectable formulation was also confirmed in these 3 lambs. RESULTS: The FEC reduction test at day 10 after treatment revealed 62%, 100% and 0% reductions in arithmetic-mean FECs for ivermectin oral, moxidectin oral and moxidectin injectable groups, respectively. The ivermectin oral, moxidectin oral and moxidectin injectable formulations achieved 62%, 98% and 4% reductions in arithmetic-mean worm burdens, respectively. Larval development assays showed resistance ratios for ivermectin of 4:1, avermectin B2 of 2.7:1, ivermectin aglycone of 37:1, moxidectin of 1.4:1, thiabendazole of 14.6:1 and levamisole of 1.8:1. CONCLUSIONS: The moxidectin oral formulation provided a high degree of control against ivermectin-resistant T. colubriformis whereas the moxidectin injectable formulation had very low efficacy. Ivermectin aglycone was the analogue of choice for diagnosis of ivermectin resistance in T. colubriformis in the larval development assay.     

Resistance to macrocyclic lactone anthelmintics by Haemonchus contortus and Ostertagia circumcincta in sheep in New Zealand

AIM: To establish the efficacy of oral formulations of ivermectin and moxidectin against naturally acquired abomasal nematode infections on a North Island sheep farm. METHODS: Two controlled slaughter trials were undertaken. In the first, 30 sheep on pasture were randomly allocated on the basis of faecal egg count to 1 of 3 groups, comprising an untreated control group and 2 treatment groups. One treatment group was given a single oral dose of ivermectin and the other a single oral dose of moxidectin, both at the manufacturer's recommended dose rates of 0.2 mg/kg liveweight. Six days after treatment, all animals were slaughtered and their abomasa recovered for worm counting. The second trial, which involved 47 animals, was essentially the same as the first except that, as well as involving the slaughter of 30 sheep from all 3 groups, 6 days after treatment, it also included a further 8 untreated control animals and 9 moxidectin treated animals which were slaughtered 27 days after treatment. RESULTS: At 6 days after treatment, moxidectin was highly effective against all 3 of the abomasal nematodes present. While ivermectin was similarly effective against Trichostrongylus axei 6 days after treatment, it was not effective against either Ostertagia circumcinta or Haemonchus contortus, against which average efficacies of only 63.6% and 61.6%, respectively, were recorded. At 27 days after treatment, moxidectin, was also highly effective against T. axei (97.3% reduction) but not against either H. contortus (71.4% reduction) or O. circumcinta (61.0% reduction). CONCLUSIONS: These results provide the first record of macrocyclic lactone resistance in H. contortus in sheep or in any other host in New Zealand, and the first case where such resistance has been exhibited in more than one parasite species at a time. Although the therapeutic efficacy of moxidectin was high against these resistant H. contortus and O. circumcincta strains, resistance to moxidectin was indicated by its diminished prophylactic activity against them. It is suggested that this reduction in the prophylactic activity of moxidectin is also likely to reduce its apparent current high therapeutic efficacy. CLINICAL RELEVANCE: As well as providing further evidence that it can no longer be automatically assumed that macrocylic lactone anthelmintics will be effective on sheep farms in this country, these findings also present a warning that increasingly complex parasite control options may have to be faced in the future.     

Ivermectin-resistant Ostertagia circumcincta from sheep in the lower North Island and their susceptibility to other macrocyclic lactone anthelmintics

Aim: To confirm the ivermectin resistance status of a strain of Ostertagia circumcincta which was isolated from a sheep farm in the lower North Island of New Zealand and to assess the susceptibility of this strain to other macrocycliclactone anthelmintics.

Methods: Twenty-five lambs housed indoors were each infected with 12,000 L3 larvae of the above parasite strain. Approximately 3 weeks after infection the lambs were allocated to 1 of 4 treatment groups (3 groups of 6, and 1 group of 7 lambs), one of which remained untreated while the others were drenched orally with ivermectin, moxidectin or abamectin at 0.2 mg/kg liveweight. Faecal egg counts (FECs) before and after treatment, and post-mortem worm burdens 10 days after treatment were examined to assess efficacies of each anthelmintic.

Results: Treatment with ivermectin reduced the mean FEC by only 18% and the mean worm burden by only 42%, whereas moxidectin and abamectin reduced FECs by > 92% and worm burdens by >95%.

Conclusion: These results, together with a similar case described recently from the South Island, confirm the emergence of ivermectin resistance in nematode parasites of sheep in New Zealand. The superior efficacy of moxidectin and abamectin in this case indicates that, following the emergence of resistance to ivermectin, some short-term practical use may still be made of these other anthelmintics. However, their continued use will undoubtedly result in increased levels of resistance and eventual therapeutic failure of these products also.     

The effects of ivermectin and moxidectin on egg viability and larval development of ivermectin-resistant Haemonchus contortus

The in vivo effects of ivermectin and moxidectin on egg viability and larval development of ivermectin-resistant Haemonchus contortus were examined over time after anthelmintic treatment of sheep. Twenty merino sheep, (12 months old) were allocated to five treatment groups and infected with ivermectin-resistant H. contortus. Thirty one days later, the sheep were treated with intraruminal ivermectin capsules, oral ivermectin, oral moxidectin or injectable moxidectin at the manufacturer's recommended dosages, or left untreated. At various times up to 112 days after treatment, faecal egg counts (FEC) were determined and development rates of infective larvae (L3) cultured in faeces or on agar were measured. Eggs in faecal cultures from ivermectin capsule treated sheep showed reduced L3 development percentages in comparison to faecal cultures from untreated sheep. Eggs from ivermectin capsule treated sheep, isolated from faeces, and cultured on agar showed similar L3 development to eggs from control sheep. These results demonstrate an inhibitory effect of excreted ivermectin in faeces on larval development of ivermectin-resistant H. contortus. L3 development in faecal culture from animals receiving oral ivermectin were reduced for only 3 days after treatment. Faecal egg counts and development of L3 larvae in both culture systems from moxidectin treated sheep were low, due to the high efficacy of the drug. Egg counts in moxidectin treated sheep were reduced by approximately 90% 24h after treatment, before decreasing to almost 100% at 48h, suggesting that the current quarantine recommendation of holding sheep off pasture for 24h after treatment may still lead to some subsequent pasture contamination with worm eggs.     

The efficacy of a combined oral formulation of derquantel-abamectin against anthelmintic resistant gastro-intestinal nematodes of sheep in the UK

The objective of the present studies was to evaluate the efficacy of a combined formulation (Startect(?) Dual Active Oral Solution for Sheep, Pfizer Animal Health) of derquantel (DQL) and abamectin (ABA) for the treatment of: (1) sheep experimentally infected with a moxidectin (MOX)-resistant isolate of Teladorsagia circumcincta, and (2) multi-drug resistant gastrointestinal nematode parasites under UK field conditions. In the first study, a total of 40 animals were allocated into 4 treatment groups, and were either left untreated or treated with DQL+ABA, MOX or ABA. Faecal samples were collected on days 1-5 and on day 7 after treatment to examine the reduction in faecal egg excretion and to evaluate the egg viability. On day 14 post treatment all animals were euthanised for abomasal worm counts. There was a 100% reduction in geometric mean worm counts for the DQL+ABA treated animals compared to the untreated control animals (P<0.0001), whereas the percentage reduction in worm counts for the MOX- (P>0.05) and ABA-treated (P=0.0004) animals was 12.4% and 71.8%, respectively. The data from the egg hatch assay (EHA) indicated that in the MOX-treated and the ABA-treated animals, the majority of the eggs hatched after treatment. In the field study, performed on four farms, animals were allocated into 6 groups of 11-15 animals each in order to conduct a faecal egg count reduction test (FECRT), based on arithmetic mean egg counts. One group of animals remained untreated, whereas the other animals were treated with DQL+ABA, MOX, fenbendazole (FBZ), levamisole (LV) or ivermectin (IVM). On each of the farms the reduction in egg excretion after treatment with FBZ, LV or IVM was below 95.0%, indicating anthelmintic resistance. The efficacy of DQL+ABA ranged from 99.1 to 100%, yielding significantly lower egg counts compared to the untreated control group (P≤0.003). For MOX the egg counts were significantly (P≤0.003) lower compared to the untreated group at each farm, with reductions varying from 98.2 to 100%. The post-treatment copro-cultures for larva identification indicated that T. circumcincta was the most abundant worm species after treatment (52-99% of the larvae). The results of these studies confirm the high efficacy of the DQL+ABA combination formulation against anthelmintic resistant nematodes in the UK.     

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

The efficacy of moxidectin was determined against ivermectin-susceptible and resistant strains of Haemonchus contortus. At the onset of the trial, 40 lambs were each infected with 5000 third stage larvae of one of two strains of Haemonchus contortus. The lambs were randomly sorted into eight treatment groups 28 days post-infection and were treated as follows: Group 1, susceptible strain with no treatment; Group 2, resistant strain with no treatment; Group 3, susceptible strain treated with 0.2 mg moxidectin kg-1 body weight; Group 4, resistant strain treated with 0.2 mg moxidectin kg-1; Group 5, resistant strain treated with 0.4 mg moxidectin kg-1; Group 6, susceptible strain treated with 0.2 mg ivermectin kg-1; Group 7, resistant strain treated with 0.4 mg ivermectin kg-1; Group 8, resistant strain treated with 0.8 mg ivermectin kg-1. The lambs were killed 1 week post-treatment. Comparisons were made among groups based on the number of eggs per gram of feces on the day of treatment and the numbers of worms recovered from each lamb. Both moxidectin and ivermectin were effective in removing susceptible Haemonchus with efficacies of 100% and 99.7%, respectively. The efficacy of moxidectin against the resistant strain was 99.9% and 100% at 0.2 mg kg-1 and 0.4 mg kg-1, respectively, whereas there were only 38.8% and 53.1% efficacies in the lambs treated with 0.4 mg ivermectin kg-1 and 0.8 mg kg-1 body weight, respectively.     

How widespread is resistance to invermectin among gastrointestinal nematodes in sheep in The Netherlands?

In Autumn 2009, a faecal egg count reduction test (FERCT) was carried out on three sheep farms. Groups of 8-11 lambs were treated with ivermectin or moxidectin, with a 14-day interval between treatment and sampling. Ivermectin resistance was present on all three farms. Treatment with ivermectin resulted in a reduction in faecal egg numbers of 94.6%, 63%, and 59%. On two farms, 14 days after treatment pooled faecal samples yielded predominantly larvae of Hamonchus contortus (100% and 98%, respectively). On the third farm, H. contortus and (probably) Teladorsagia circumcincta were resistant to ivermectin (64% and 36% of the larvae, respectively). Treatment with moxidectin resulted in a 100% reduction in egg output in sheep on all three farms. More sensitive culture techniques failed to detect any larvae in samples taken from two farms, but a few Ostertagia-type larvae, probably of T. circumcincta, were detected in samples from the third farm. It can be concluded that gastrointestinal nematodes in sheep from these three farms were resistant to ivermectin, whereas resistance to moxidectin was not detected.     

An evaluation of an oral formulation of moxidectin against selected anthelmintic-resistant and -susceptible strains of nematodes in lambs

The efficacy of an oral formulation of the newly developed parasiticide, moxidectin, was tested against benzimidazole-resistant Haemonchus contortus, Trichostrongylus colubriformis, and Nematodirus spathiger, levamisole-resistant Ostertagia circumcincta, and susceptible Cooperia curticei infections in weaned lambs. Thirty-two lambs were experimentally infected with mixed doses of the above strains of nematodes. They were allocated into four treatment groups by stratified randomisation using liveweights and faecal egg counts 28 days later. One group received moxidectin at 0.2 mg/kg liveweight, one group oxfendazole at 4.5 mg/kg liveweight, one group levamisole at 7.5 mg/kg liveweight and the last group remained untreated as the control. Worm burdens in the lambs at slaughter 10 days after oral treatment confirmed the resistance status of the nematode strains used, and showed that moxidectin had a greater than 99.9% efficacy (p<0.01) against all of them. No adverse effects due to treatment with moxidectin were observed in any of the animals.     

Resistance to therapeutic treatment with macrocyclic lactone anthelmintics in Ostertagia circumcincta

Eighty-eight lambs were allocated to one of four groups which were then dosed with 10,000 infective-stage larvae (L3) of one of four populations of Ostertagia circumcincta; the first (S) was an isolate known to be anthelmintic-susceptible; the second (OR) was a multiple anthelmintic-resistant isolate which had been recovered from the field following therapeutic failure of both ivermectin and moxidectin and subsequently maintained in the laboratory without further anthelmintic selection. The third (R) was derived from OR but had been passaged for five generations in the laboratory with each generation being screened with all three broad-spectrum drench families; the fourth (R x S) was an F1 cross between the S and R isolates. On patency, each of the four infection groups was sub-divided into five treatment groups, one of which received no anthelmintic while the others were administered either oral ivermectin (IVM-oral), controlled-release capsules containing ivermectin (IVM-CRCs), oral moxidectin (MOX-oral) or injectable MOX (MOX-inj). Neither formulation of IVM reduced FEC in the R, R x S and OR infected lambs compared to their untreated controls, but significant reductions were observed in all cases following MOX-oral or MOX-inj treatment. Similarly, neither IVM formulation significantly reduced the numbers of R or R x S worms compared to their untreated controls, although the numbers of OR worms were reduced in both cases (P<0.05). Direct comparisons of efficacy across the isolates, however, indicated that neither formulation was any more effective against R x S or OR worms than against the more highly selected R worms. In contrast, both MOX formulations significantly reduced worm numbers of all the resistant isolates compared to their respective untreated controls; furthermore, worm burdens of R x S were reduced significantly more than burdens of R (P<0.05). Reductions in OR burdens, which were intermediate between the two, did not differ significantly from either. The results are consistent with published work on Haemonchus contortus, which suggests that macrocyclic lactone (ML) resistance is expressed as a dominant trait under treatment with IVM. However, these data differ from the H. contortus studies in suggesting that ML resistance in O. circumcincta may effectively be rendered incompletely dominant or recessive by treatment with MOX.     

Efficacy of abamectin against ivermectin-resistant strain of Trichostrongylus colubriformis in sheep

The efficacy of two formulations of abamectin, i.e. oral and injectable was determined against ivermectin-resistant strain of T. colubriformis in sheep. Twenty-four lambs were infected with 10,000 third stage larvae of ivermectin-resistant strain of T. colubriformis. Twenty-four days post-infection, the lambs were divided randomly into four groups of six animals each according to egg counts. The first group was left untreated and kept as a control. The second group was treated with ivermectin (oral) at 0.2mg kg(-1) body weight. The third group was treated with oral formulation of abamectin at 0.2mg kg(-1) body weight. The fourth group was treated with injectable formulation of abamectin at 0.2mg kg(-1) body weight. Fecal egg count and controlled slaughter tests were employed to determine the efficacy of abamectin (oral and injection) against ivermectin-resistant strain of T. colubriformis in sheep. Reduction in arithmetic mean fecal egg counts achieved by ivermectin (oral), abamectin (oral) and abamectin (injection) was 66, 98 and 76%, respectively 10 days after treatment. Ivermectin (oral), abamectin (oral) and abamectin (injection) reduced arithmetic mean worm burden by 63, 97 and 74%, respectively. The findings demonstrated that abamectin oral formulation was more effective than abamectin injection against ivermectin-resistant strain of T. colubriformis in sheep.     

Efficacy of oral,injectable and pour-on formulations of moxidectin against gastrointestinal nematodes in cattle in New Zealand

The efficacy of moxidectin administered by different routes, against naturally acquired infections of gastrointestinal nematode parasites of cattle, was compared using faecal egg count reduction tests on 14 commercial farms throughout New Zealand. On each farm, groups of 15 calves were sampled for faecal nematode egg count and then treated with ivermectin administered orally, or with moxidectin administered either by the oral, subcutaneous injection or topical (pour-on) route. Samples were again collected 14 days after treatment and efficacy was calculated as the percentage reduction in-group mean egg count between the pre- and post-treatment samples. In addition, efficacy was calculated for individual animals, in order to compare the variability of the different treatments. On four farms untreated control groups were run and five animals from each of the control and all of the moxidectin-treated groups were bled over time to estimate plasma–moxidectin concentrations.Averaged across all tests, the reduction in faecal egg count was significantly greater after treatment with moxidectin oral (91.1%) than following treatment with moxidectin injection (55.5%) or with moxidectin pour-on (51.3%). Low efficacies were invariably against Cooperia oncophora. The oral treatments were significantly less variable in efficacy than the injection and pour-on treatments. Moxidectin concentrations in plasma were highest following subcutaneous injection and lowest following pour-on administration. Plasma levels following oral administration were intermediate, being significantly lower than post-injection and significantly higher than post-pour-on. There was no evidence of transfer of moxidectin to untreated animals through licking. Based on these results, along with those of other studies, it is proposed that oral administration of macrocyclic lactone anthelmintics results in higher concentrations of active reaching the target worms in the gastrointestinal tract than following either administration by injection or by pour-on.     

Resistance to moxidectin and abamectin in naturally acquired Ostertagia circumcincta infections in sheep

AIM: To investigate the occurrence of resistance to macrocyclic lactone (ML) anthelmintics by Ostertagia circumcincta in lambs on a sheep and cattle property in the North Island of New Zealand. METHODS: Thirty lambs were randomly allocated to one of five equal-sized groups, consisting of an untreated control and four treatment groups. The treatments, which were administered at the manufacturer's recommended dose rates, included oral moxidectin, oral abamectin (both at 0.2 mg/kg), an albendazole-levamisole combination, and an albendazole-levamisole-ivermectin combination. Post mortem worm counts were undertaken 7 days after treatment to determine the efficacy of each anthelmintic. RESULTS: The albendazole-levamisole and albendazole-levamisole-ivermectin combinations both reduced O. circumcincta burdens to zero whereas for moxidectin and abamectin efficacies of only 72% and 29%, respectively, were recorded. CONCLUSIONS: These results clearly demonstrated the occurrence of resistance to MLs by O. circumcincta. Although this is not the first occasion where resistance to this anthelmintic family has been detected in this parasite in sheep in New Zealand, it is the first instance that resistance to either moxidectin or abamectin has been reported.     

Effectiveness of copper oxide wire particles for Haemonchus contortus control in sheep

OBJECTIVE: To assess the efficacy of copper oxide wire particles (COWP) for the control of H contortus infections in grazing sheep. PROCEDURE: In experiment 1, 40 worm-free Merino hoggets (11 to 12 months of age) were divided into four equal groups and allocated to separate 0.8 ha pasture plots. Two groups then received 2.5 g COWP whereas the other two groups were untreated. From 1 week after COWP treatment all lambs received a weekly infection of 2000 H contortus larvae. At week 8, six sheep from the untreated group were then allocated to two groups and treated with either 2.5 or 5.0 g of COWP to establish therapeutic efficacy of treatment. Experiment 2 followed a similar protocol but was conducted with 40 worm-free Merino lambs (3 to 4 months of age) and no assessment of therapeutic efficacy was made. Results: In experiment 1 no significant difference in faecal worm egg counts was observed between treatments and faecal worm egg counts remained less than 3000 epg in all animals. Total worm counts were reduced by 37% by COWP treatment (P = 0.055). Both 2.5 g and 5.0 g doses of COWP at 8 weeks of infection reduced faecal worm egg counts by > 85% with the higher dose giving an earlier response to treatment. In experiment 2, faecal worm egg counts at 4 and 6 weeks were reduced by more than 90% in the COWP treated lambs and worm numbers were 54% lower after 6 weeks when all remaining untreated lambs had to be treated for haemonchosis. Mean faecal worm egg counts in the COWP lambs remained below 3500 epg and clinical disease did not develop in the majority of lambs before the end of the experiment at 10 weeks. CONCLUSION: Treatment with COWPs appears to have the potential to reduce establishment and worm fecundity of Haemonchus spp for an extended period and may offer livestock producers a supplementary means of reducing larval contamination of pasture particularly in areas where anthelmintic resistance is a problem and copper supplementation is likely to be beneficial.     

Transmission of Babesia spp by the cattle tick (Boophilus microplus) to cattle treated with injectable or pour-on formulations of ivermectin and moxidectin

OBJECTIVE: To assess the efficacy of ivermectin and moxidectin to prevent transmission of Babesia bovis and Babesia bigemina by Boophilus microplus to cattle under conditions of relatively intense experimental challenge. DESIGN: Naive Bos taurus calves were treated with either pour-on or injectable formulations of either ivermectin or moxidectin and then exposed to larvae of B microplus infected with B bovis or larvae or adults of B microplus infected with B bigemina. One calf was used for each combination of haemoparasite, B microplus life stage, drug and application route. PROCEDURE: Groups of calves were treated with the test drugs in either pour-on or injectable formulation and then infested with B microplus larvae infected with B bovis or B bigemina. B bigemina infected adult male ticks grown on an untreated calf were later transferred to a fourth group of animals. Infections were monitored via peripheral blood smears to determine haemoparasite transmission. RESULTS: Cattle treated with either pour-on or injectable formulations of ivermectin and moxidectin became infected with B bovis after infestation with infected larvae. Similarly, larvae infected with B bigemina survived to the nymphal stage to transmit the haemoparasite to animals treated with each drug preparation. Cattle treated with pour-on formulations of ivermectin and moxidectin then infested with adult male ticks infected with B bigemina did not become infected with B bigemina whereas those treated with the injectable formulations of ivermectin and moxidectin did show a parasitaemia. CONCLUSIONS: Injectable or pour-on formulations of ivermectin and moxidectin do not prevent transmission of Babesia to cattle by B microplus. Use of these drugs can therefore not be recommended as a primary means of protecting susceptible cattle from the risk of Babesia infection.     

Further attempts to immunise lambs using metabolites and in vitro grown larvae of Ostertagia circumcincta

Attempts were made to immunise lambs by the intramuscular injection of either lyophilised or concentrated metabolites produced by Ostertagia circumcincta when grown in vitro, or macerated fourth and fifth stage worms. The lyophilised metabolites did not stimulate protection against the establishment of worms in nine-months-old lambs but the macerated worms and concentrated metabolites elicited protection in six-months-old and three-months-old lambs respectively. The worm burdens and worm egg counts of the immunised lambs were less than those of control lambs.     

Long-acting moxidectin for the control of trichostrongylid infections of sheep in south-eastern Australia

Objective   To compare the effectiveness of a single long-acting injection of moxidectin, given in either summer or winter, with a two-summer ('standard') treatment strategy for controlling gastrointestinal nematodes in spring-lambing Merino ewes.
Design   A replicated study over 2 years in the high winter rainfall environment of western Victoria.
Methods   Measures of worm infection were worm egg counts of ewes and total worm counts of tracer lambs. Measures of production were body weight, proportion of pregnant ewes and number of weaned lambs.
Results   The egg counts of ewes given the standard strategy followed a characteristic pattern, rising to 300 to 600 eggs per gram before the second summer treatment in February. During this time, ewes given long-acting moxidectin in December had zero or negligible counts. There were no consistent differences between tracer worm counts or production measures for the two groups treated in summer, but when data from both years were pooled, the total egg output from December to May was significantly lower for ewes treated with long-acting moxidectin in December. Ewes not treated during summer had lower body weights, and higher worm egg counts and tracer worm counts, showing that this was not a suitable strategy at that time of the year. However, following treatment with long-acting moxidectin in winter, ewes had rapid compensatory weight gain and lower egg output than ewes treated in summer.
Conclusion   A single injection of long-acting moxidectin in December is an effective treatment strategy for Merino ewes lambing in spring.     

Confirmation of resistance to ivermectin by Cooperia curticei in sheep

AIM: To investigate the occurrence of resistance to a full dose of oral ivermectin by Cooperia curticei in sheep. METHODS: Twelve lambs on a sheep and cattle property in the North Island of New Zealand were randomly allocated to one of two equal-sized groups. One group was treated orally with a single dose of ivermectin at the manufacturer's recommended dose rate of 0.2 mg/kg, while the other remained as an untreated control. Worm counts were carried out post mortem on the abomasa and small intestines of all animals in both groups 7 days after treatment. RESULTS: While treatment with ivermectin reduced the numbers of all other worm genera to almost zero, those of Ostertagia(= Telodorsagia) circumcincta and C. curticei were reduced by only 37% and 19%, respectively. CONCLUSIONS: These results provide clear evidence of resistance to ivermectin by O. circumcincta and C. curticei. They also appear to represent the first record of macrocylic lactone (ML) resistance in C. curticei in sheep in New Zealand or elsewhere.     

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

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