Efficacy of moxidectin and other anthelmintics against small strongyles in horses

Objective To compare the efficacy of moxidectin to ivermectin, oxibendazole and morantel against some gastrointestinal nematodes in horses.
Design Faecal egg count reduction after treatment.
Procedure A farm was selected where the population of small strongyles in horses was known to be resistant to oxibendazole. Horses were allocated to treatment groups based on faecal egg counts. After treatment, faecal samples were taken up to 109 days after treatment and faecal egg counts estimated. Faecal cultures were used to estimate the contribution of small and large strongyles to the faecal egg counts at each sampling.
Results Moxidectin (0.4 mg/kg) suppressed faecal egg counts for 109 days after treatment in most horses compared to 40 days with ivermectin (0.2 mg/kg), 13 days with morantel (9.4 mg/kg) and less than 13 days with oxibendazole (10 mg/kg). Most of the faecal egg count was attributable to small strongyles based on faecal culture, although Strongylus vulgaris was present in some samples in low numbers. Oxibendazole resistance in small strongyles was confirmed and a less than expected efficacy of morantel was also seen.
Conclusion Moxidectin was highly effective in reducing faecal egg counts after treatment for at least 12 weeks and up to 16 weeks in most horses. These horses were infected with a population of small strongyles known to be resistant to oxibendazole and possibly morantel. The duration of the reduction in faecal egg counts after treatment with moxidectin (0.4 mg/kg) was at least twice that of ivermectin (0.2 mg/kg) and greater than four times that for morantel and oxibendazole.     

Searching for ivermectin resistance in Dutch horses

A study was conducted to evaluate the occurrence of resistance against, in particular, ivermectin in cyathostomins in the Netherlands. Seventy horse farms were visited between October 2007 and November 2009. In initial screening, faecal samples were collected 2 weeks after deworming with either ivermectin, moxidectin or doramectin. Pooled faecal samples from a maximum of 10 horses were examined for worm eggs using a modified McMaster technique and for worm larvae after faecal larval cultures. In total 931 horses were involved. On 15 of 70 farms eggs and/or larvae were found. On 8 of these 15 farms a FECRT with ivermectin was performed on 43 horses. Efficacy of ivermectin against cyathostomins of 93% was found in one animal on one farm. Additionally, the strategies and efforts of the horse owners to control cyathostomins, as well as risk factors for the development of macrocyclic lactone resistance were evaluated with a questionnaire. Strikingly, many responders indicated that the control of cyathostomins in horses is achieved through very frequent deworming. Fourteen percent of these owners deworm seven times per year or more. On 34% of the 70 farms treatment was repeated within the Egg Reappearance Period of a product.     

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.     

Faecal excretion profile of moxidectin and ivermectin after oral administration in horses

A study was undertaken to evaluate and compare faecal excretion of moxidectin and ivermectin in horses after oral administration of commercially available preparations. Ten clinically healthy adult horses, weighing 390-446 kg body weight (b.w.), were allocated to two experimental groups. Group I was treated with an oral gel formulation of moxidectin at the manufacturer's recommended therapeutic dose of 0.4 mg/kg b.w. Group II was treated with an oral paste formulation of ivermectin at the recommended dose of 0.2 mg/kg b.w. Faecal samples were collected at different times between 1 and 75 days post-treatment. After faecal drug extraction and derivatization, samples were analysed by High Performance Liquid Chromatography using fluorescence detection and computerized kinetic analysis.For both drugs the maximum concentration level was reached at 2.5 days post administration. The ivermectin treatment groups' faecal concentrations remained above the detectable level for 40 days (0.6 +/- 0.3 ng/g), whereas the moxidectin treatment group remained above the detectable level for 75 days (4.3 +/- 2.8 ng/g). Ivermectin presented a faster elimination rate than moxidectin, reaching 90% of the total drug excreted in faeces at four days post-treatment, whereas moxidectin reached similar levels at eight days post-treatment. No significant differences were observed for the values of maximum faecal concentration (C(max)) and time of C(max)(T(max)) between both groups of horses, demonstrating similar patterns of drug transference from plasma to the gastrointestinal tract. The values of the area under the faecal concentration time curve were slightly higher in the moxidectin treatment group (7104 +/- 2277 ng.day/g) but were not significantly different from those obtained in the ivermectin treatment group (5642 +/- 1122 ng.day/g). The results demonstrate that although a 100% higher dose level of moxidectin was used, attaining higher plasma concentration levels and more prolonged excretion and gut secretion than ivermectin, the concentration in faeces only represented 44.3+/- 18.0% of the total parental drug administered compared to 74.3 +/- 20.2% for ivermectin. This suggests a higher level of metabolization for moxidectin in the horse.     

The persistence of benzimidazole-resistant cyathostomes on horse farms in Ontario over 10 years and the effectiveness of ivermectin and moxidectin against these resistant strains

Three clinical trials with fecal egg count reduction tests and coproculture were conducted on 2 standardbred farms in Ontario. On Farm A, the treatment groups were mebendazole and ivermectin in trial 1, and fenbendazole and moxidectin in another. On Farm B, treatment groups were mebendazole and ivermectin. All horses treated with mebendazole or fenbendazole were subsequently treated with ivermectin or moxidectin. Strongyle eggs/g feces were estimated pre- and post-treatment using the Cornell-McMaster dilution and Cornell-Wisconsin centrifugal flotation techniques. After treatment, there was no change in the arithmetic mean eggs/g feces for horses given mebendazole, and a reduction of only 49.1% for those given fenbendazole. All horses receiving ivermectin or moxidectin had their egg counts reduced to 0. Only cyathostomes were found on culture. On both farms the benzimidazole resistant strains appeared to have persisted for at least 10 years. Development of and monitoring for anthelmintic resistance are briefly discussed.     

Prevalence and control of benzimidazole-resistant small strongyles on German thoroughbred studs

The prevalence of benzimidazole-resistant small strongyles was determined in a survey, conducted on 14 thoroughbred studs, which compared the faecal egg counts of groups of horses before and after treatment with the recommended doses of cambendazole (20 mg kg-1 b.w.) or febantel (6 mg kg-1 b.w.). Benzimidazole-resistant cyathostomes were found on all farms examined. Pyrantel pamoate (19 mg kg-1 b.w.), oxibendazole (10 mg kg-1 b.w.) and ivermectin (0.2 mg kg-1 b.w.) reduced the strongyle egg counts on these studs by 97-100% at 2 weeks post-treatment. However, 6 weeks after dosing the reduction of the strongyle egg output had decreased to an average of 67.8% (8.7-97.1%) with pyrantel pamoate and 51.2% (0-95.8%) with oxibendazole, whereas ivermectin still suppressed the egg counts by 98.2% (95-100%).     

Reduced efficacy of anthelmintics in young compared with adult horses

Studies on a Thoroughbred breeding farm in Ohio from 1982 to 1988 demonstrated the value of three anthelmintic pastes (ivermectin, oxibendazole, pyrantel pamoate) in controlling benzimidazole resistant cyathostomes (small strongyles) in adult horses. However, a comparison of drug efficacy in suppressing faecal egg counts for the full period between treatments showed a significant reduction in efficacy of all drugs in yearling horses compared with adults. Mean faecal egg counts of adult horses were generally kept below 100 eggs per gram (epg) of faeces when using oxibendazole or pyrantel pamoate at four to five week intervals and ivermectin at eight week intervals. By contrast, mean counts of young horses rose as high as 655 epg (oxibendazole), 729 epg (pyrantel pamoate) and 852 epg (ivermectin) within the same time period after treatment. Individual counts of treated yearlings sometimes exceeded 3,000 epg. Three distinct mechanisms appeared to be involved in the poor results in young horses. These were 1) anthelmintic refuge, 2) anthelmintic resistance, and 3) anthelmintic avoidance.     

Prevalence of benzimidazole resistance on horse farms in Germany

Faecal egg counts (FECs) were made on samples from 1383 horses on 64 farms in northern Germany between August 2000 and November 2001. There were significant differences between the mean FECs in the two years; in 2000, 59.6 per cent of 369 samples were positive and in 2001, 32.6 per cent of 1014 samples were positive for strongyle eggs. The results of a FEC reduction test indicated that resistance to fenbendazole was present on all 10 farms where it had been used, including in 33 of 60 horses tested. In contrast, treatment with ivermectin resulted in the complete elimination of nematode eggs in all the 77 horses tested. The mean LD50 values of the egg hatch test for thiabendazole indicated resistance on all 20 farms investigated and in 94 of 134 samples (70 per cent).     

Anthelmintic treatment in horses: the extra-label use of products and the danger of under-dosing

Anthelmintic products form the basis of helminth control practices on horse stud farms at present. Regular evaluation of the efficacy of these products is advisable, as it will provide information on the worm egg reappearance period and the resistance status in the worm population. The aim of this study was to evaluate the efficacy of doramectin, pyrantel pamoate, ivermectin and moxidectin on a Thoroughbred stud farm in the Western Cape Province, South Africa. The study also compared the anthelmintic efficacy of two moxidectin formulations administered at their recommended dosages (an injectable, at 0.2 mg/kg, not registered for horses, and an oral gel at 0.4 mg/kg, registered for horses). Two mixed-sex groups of 30 yearlings and 40 weaners were tested in 2001 and 2002, respectively, divided into 3 and 4 groups of equal size. In 2001, moxidectin was one of 3 drugs administered orally and at a dose rate of 0.4 mg/kg. In 2002, pyrantel pamoate and ivermectin were orally administered at 19 and 0.2 mg/kg. Moxidectin and doramectin (the latter not registered for horses) were administered by intramuscular injection at a dose of 0.2 mg/kg, the dosage registered for other host species. The faecal egg count reduction test was used to determine the anthelmintic efficacies in both years. Each animal acted as its own control and the arithmetic mean faecal egg count and lower 95% confidence limit was calculated for each of the groups. A 100% reduction in the faecal egg counts and a 100% lower 95% confidence limit was recorded for moxidectin (0.4 mg/kg) in 2001. In 2002, a 99% and 96% reduction was recorded for pyrantel pamoate and ivermectin, respectively. In the same year doramectin and moxidectin (both injectable and given at 0.2 mg/kg) did not have any effect on worm egg counts. Of the 4 drugs tested in 2002, only pyrantel pamoate recorded lower 95% confidence limits above 90%.     

A survey on anthelmintic resistance in nematode parasites of sheep in the Slovak Republic

The prevalence of anthelmintic resistance on 27 sheep farms in Slovakia was investigated in 2003 and 2004 using the faecal egg count reduction test (FECRT) according to the WAAVP guidelines. Resistance to albendazole was detected on one farm (3.7%) and suspected on two farms (7.4%) out of 27 sheep flocks. Resistance to ivermectin was tested on 26 farms. On six (23.1%) farms, results indicated the presence of ivermectin resistance. Resistance to ivermectin was suspected on eight farms (30.8%). However, it is also possible that generic ivermectin anthelmintics used in survey have a lower efficacy against sheep nematodes.     

Impaired efficacy of ivermectin against Parascaris equorum, and both ivermectin and pyrantel against strongyle infections in trotter foals in Finland

In order to assess the resistance situation against macrocyclic lactones in Parascaris equorum and against tetrahydropyrimidine derivatives in strongyles in Finnish trotter horses, 112 foals on 18 farms, mostly 1 year old, were examined for these parasites with a modified McMaster faecal flotation method. P. equorum positive foals (n=24) were given ivermectin orally at a dose of 200 μg/kg b.w., while strongyle positive but P. equorum negative foals (n=38) received pyrantel embonate orally at a dose of 19 mg/kg. Sixteen P. equorum infected foals, treated with ivermectin, also harboured strongyles. During the anthelmintic treatment visit to the farm, Faecal Egg Count Reduction Test (FECRT) reference (first) samples were collected. Fourteen days later, the second sampling (reduction samples) was done. The FECR was calculated for each foal/parasite combination. The reduction efficacies of ivermectin against P. equorum (mean 52%, calculated from the individual egg count reductions) and pyrantel against strongyles (43%) were strongly indicative of widespread resistance. Also indication of ivermectin resistance among strongyles was seen. The widespread use of anthelmintics for Finnish horses obviously has resulted in resistance, as has happened elsewhere, too.     

Efficacy of major anthelmintics against horse cyathostomins in France

This paper reports a survey conducted in France during 2011 to evaluate the efficacy of commonly used anthelmintics against horse cyathostomins. A total of 40 farms and 1089 horses were screened for the presence of cyathostomins. All farms but one were positive, with an overall animal infection rate of 53.7%, ranging from 9% to 83% on individual farms. On 445 horses from 30 of these farms, a faecal egg count reduction test (FECRT) was performed to evaluate the efficacy of oral formulations of fenbendazole (FBZ), pyrantel embonate (PYR), ivermectin (IVM) and moxidectin (MOX). Calculation of the mean FECR and 95% confidence intervals (95% CI) around the mean was performed using bootstrap analysis. Resistance to FBZ was found on 17 of 18 farms investigated, with a mean reduction of 57% (95% CI: 38.5-71.2%). Suspected resistance for PYR was found on 6 of 30 farms, and confirmed on another 3 of 30 farms, with a mean reduction for PYR of 94.7% (95% CI: 88.9-98.5%). Reduced efficacy simultaneously of FBZ and PYR was found in 7 farms. Reduced efficacy of IVM was found in one animal on one farm and of MOX in one animal on another farm, and was combined with resistance against FBZ and/or PYR. These results indicate that single and multiple drug resistance and reduced efficacy in equine cyathostomins is present in France. Macrocylic lactones proved to be highly effective compounds against cyathostomins, with reduced efficacy for IVM and MOX in two farms only. These results extend present knowledge on the occurrence of drug resistant cyathostomins in Europe, and illustrate the necessity to use anthelmintics in appropriate worm control programmes.     

A survey of the prevalence of emerging macrocyclic lactone resistance and of benzimidazole resistance in sheep nematodes in the lower North Island of New Zealand

AIM: To investigate the occurrence of emerging macrocyclic lactone (ML) resistance and of resistance to benzimidazole anthelmintics on a number of sheep farms in the North Island of New Zealand. METHODS: On commercial sheep farms (n=30) in the Taihape district in the North Island of New Zealand, 30 animals were randomly allocated to one of two equal-sized groups and treated with either half of the recommended dose rate of ivermectin (half of 0.2 mg/kg), or with the full recommended dose rate of oxfendazole (4.5 mg/kg). The ivermectin treatment only was used on a further six properties. Faecal egg counts, accompanied by pooled larval cultures, were conducted on all samples at the time of treatment and 7-10 days later. RESULTS: Resistance, as indicated by a <95% faecal egg count reduction (FECR) in both instances, was found to oxfendazole on 13/30 (43%) farms and to a half dose of ivermectin on 12/36 (33%) properties. For oxfendazole, such resistance was found to involve all six nematode genera whereas for ivermectin it was almost entirely restricted to Ostertagia and Cooperia infections. CONCLUSIONS: These results indicate that emerging ML resistance may be more common on sheep farms in New Zealand than is generally realised. They also suggest that the half-dose ivermectin faecal egg count reduction test (FECRT) may offer some very practical benefits for parasite control by providing early warning of developing resistance to ML drenches and by signalling the possible imminent failure of these at their therapeutic dose rates. The sensitivity and reliability of this procedure may be further enhanced by the inclusion of larval cultures.     

Assessing resistance of ivermectin and moxidectin against nematodes in cattle naturally infected using three different methodologies

The objective of this study was to evaluate the accuracy of the faecal egg count reduction test (FECRT) and the faecal egg count efficacy test (FECET) to assess the resistance status of ivermectin (630 μg/kg) and moxidectin (200 μg/kg), using the controlled efficacy test as a reference, and whether the results of the EPG are equivalent to the efficacy results from the parasitological necropsies. Two experiments were conducted. The results demonstrate that it was not possible to demonstrate that the EPG values were equivalent with the ivermectin and moxidectin efficacy obtained by parasitological necropsies, mainly if the phenomenon of parasites resistance is not advanced in a determined field population. Maybe the FECET technique would be possibly better than the FECRT. The high anthelmintic efficacy of 200 μg/kg moxidectin, in naturally infected cattle, against field population of nematodes that are resistant to 630 μg/kg ivermectin, was observed in this study.     

A Field Study on the Effect of Some Anthelmintics on Cyathostomins of Horses in Sweden

The objective of the study was to investigate different aspects on the efficacy of three anthelmintics on cyathostomin nematodes of Swedish horses. A faecal egg count reduction (FECR) test was performed on 26 farms. Horses were treated orally with recommended doses of ivermectin, pyrantel pamoate or fenbendazole. Faecal samples were collected on the day of deworming and 7, 14 and 21 days later. No resistance was shown against ivermectin; the FECR was constantly >99%. The effect of pyrantel was assessed as equivocal in 6 farms 14 days after treatment; the mean FECR was 99%. As many as 72% of the fenbendazole-treated groups met the criteria for resistance; the mean FECR was 86%, ranging from 56% to 100%. A re-investigation of two farms where pyrantel resistance had been suspected clearly revealed unsatisfactory efficacy of pyrantel on one of these farms; the FECR varied from 72% to 89%. Twenty-six of the horses previously dosed with pyrantel or fenbendazole, and which still excreted ≥150 eggs per gram of faeces 14 days after treatment, were dewormed with ivermectin and fenbendazole or pyrantel in order to eliminate the remaining cyathostomins. A total of 13 cyathostomin species were identified from horses that initially received fenbendazole and seven species were identified from pyrantel-treated individuals. The egg reappearance period (ERP) following treatment with ivermectin and pyrantel was investigated on two farms. The shortest ERP after ivermectin treatment was 8 weeks and after pyrantel was 5 weeks. We conclude that no substantial reversion to benzimidazole susceptibility had taken place, although these drugs have scarcely been used (<5%) in horses for the last 10 years. Pyrantel-resistant populations of cyathostomins are present on Swedish horse farms, but the overall efficacy of pyrantel is still acceptable.     

Assessment of serum protein electrophoresis for monitoring therapy of naturally acquired equine cyathostomin infections

Serum protein electrophoresis (SPE) has previously been suggested as a means of assessing cyathostomin burdens in horses, although SPE used for that purpose is supported by little evidence. This clinical research report describes a study that objectively evaluated the use of SPE on a population of 38 horses following the administration of different anthelmintics. The population was subdivided into three groups, Groups F, M and P: 7.5 mg/kg bwt fenbendazole was administered to Group F on day -12; on day 0 0.4 mg/kg bwt moxidectin was administered to Group M and 19 mg/kg bwt pyrantel was administered to Group P. Faecal worm egg counts were obtained on days -14, 0 and 10. Groups M and P acted as controls for the Group F faecal egg count reduction test (FECRT) in which a high level of benzimidazole resistance was demonstrated. Group F was then used as a control group for the FECRT for both Groups P and M. A high anthelmintic efficacy of moxidectin and pyrantel was detected. SPE was performed on venous blood collected on days 0, 10, 30, 56 and 80. As the cyathostomins infecting the horses had been shown to be highly resistant to fenbendazole, Group F then served as a control group for comparison of any changes in protein fractions. Serum proteins did not vary significantly between groups on any of the sampling dates. No significant changes in serum proteins were observed in any group and no patterns were apparent on qualitative assessment of SPE profiles. SPE was therefore concluded to be an insensitive tool for the monitoring of cyathostomin treatment in horses in a clinical environment.     

Anthelmintic resistance in cyathostomins of brood horses in Ukraine and influence of anthelmintic treatments on strongylid community structure

In 2004-2006, 322 brood horses from 11 horse farms were examined using the faecal egg count reduction test (FECRT) to determine the presence and distribution of anthelmintic resistance in strongylids in Ukraine. The anthelmintic drugs "Albendazole-7.5" (7.5mg of albenazole, Ukraine) at a dose of 5mg per kg body weight and "Univerm" (0.2% aversectin C, Russia) at a dose of 0.5mg per kg body weight were used. Seventy-one horses from six farms were examined in vivo to investigate the influence of anthelmintic treatment on the gastrointestinal strongylid community structure. Horses were treated with anthelmintics; faecal sampling (200 g in each sample) for strongylid expulsion was performed 24, 36, 48 and 60 h after treatment; and all strongylids expelled (25,292 specimens) were collected and identified. Fourteen horses from the Dubrovsky horse farm were also examined to determine the benzimidazole-resistant cyathostomin species; 5208 specimens of benzimidazole-resistant cyathostomins were collected and identified. According to the FECRT data, benzimidazole resistance in strongylids was observed only at the Dubrovsky horse farm (FECRT=68.7%). No resistance to macrocyclic lactones in strongylids or in Parascaris equorum was observed. Twenty-nine strongylid species were found in horses from six horse farms. The number of species per horse ranged from 4-9 (5.8+/-1.5) to 10-20 (14.4+/-2.9) and depended on horse anthelmintic treatment strategies. From 4 to 13 strongylid species predominated (prevalence>66.7%) in the strongylid community. Eleven cyathostomin species (Cylicocyclus nassatus, C. ashworthi, C. leptostomum, Cyathostomum catinatum, C. pateratum, Cylicostephanus calicatus, C. longibursatus, C. goldi, C. minutus, Coronocyclus coronatus and C. labiatus) were found to be resistant to benzimidazoles at the Dubrovsky horse farm. Ten of these were the dominant species in the strongylid community; only C. labiatus was a rare species (prevalence 29.4%). Species richness and species diversity were significantly higher in horses from farms without treatment or with occasional treatments than from farms with regular treatments. The shape of the prevalence frequency distribution of strongylid species from farms with regular treatments was bimodal ("core" and "satellite" mode). This distribution was multimodal at farms without treatment or with occasional anthelmintic treatments. The results of the current study indicated the possibility of the further spread of anthelmintic resistance on horse farms in Ukraine and the necessity of monitoring the development of resistance in horse parasitic nematodes.     

A survey of the prevalence of emerging macrocyclic lactone resistance and of benzimidazole resistance in sheep nematodes in the lower North Island of New Zealand

AIM: To investigate the occurrence of emerging macrocyclic lactone (ML) resistance and of resistance to benzimidazole anthelmintics on a number of sheep farms in the North Island of New Zealand.

METHODS: On commercial sheep farms (n=30) in the Taihape district in the North Island of New Zealand, 30 animals were randomly allocated to one of two equal-sized groups and treated with either half of the recommended dose rate of ivermectin (half of 0.2 mg/kg), or with the full recommended dose rate of oxfendazole (4.5 mg/kg). The ivermectin treatment only was used on a further six properties. Faecal egg counts, accompanied by pooled larval cultures, were conducted on all samples at the time of treatment and 7–10 days later.

RESULTS: Resistance, as indicated by a <95% faecal egg count reduction (FECR) in both instances, was found to oxfendazole on 13/30 (43%) farms and to a half dose of ivermectin on 12/36 (33%) properties. For oxfendazole, such resistance was found to involve all six nematode genera whereas for ivermectin it was almost entirely restricted to Ostertagia and Cooperia infections.

CONCLUSIONS: These results indicate that emerging ML resistance may be more common on sheep farms in New Zealand than is generally realised. They also suggest that the half-dose ivermectin faecal egg count reduction test (FECRT) may offer some very practical benefits for parasite control by providing early warning of developing resistance to ML drenches and by signalling the possible imminent failure of these at their therapeutic dose rates. The sensitivity and reliability of this procedure may be further enhanced by the inclusion of larval cultures.     

A small scale survey of ivermectin resistance in sheep nematodes using the faecal egg count reduction test on samples collected from Scottish sheep

Thirty-eight sheep flocks, predominantly from the south/central Scotland, were examined using a faecal egg count reduction test (FECRT) for the presence of ivermectin (IVM) resistant nematodes. Efficacies of less than 95%, 14-17 days post-treatment, were identified in 6 of 17 naturally grazing flocks where pre-treatment faecal egg counts were in excess of 150 eggs per gram. Efficacies on these IVM resistant farms ranged from 66 to 92%. One other suspected cases of IVM resistance was also identified in returned material. The larvae detected in post-treatment coprocultures from resistant flocks were from the genera Teladorsagia (4 from 6) and Trichostrongylus (2 from 6).     

Prevalence of anthelmintic resistance on 62 beef cattle farms in the North Island of New Zealand

AIM: To establish the prevalence of anthelmintic resistance in parasitic nematodes on a random sample of beef cattle herds in the North Island of New Zealand. METHODS: A cross-sectional prevalence study was conducted using a standardised faecal nematode egg count (FEC) reduction (FECR) test (FECRT) for ivermectin, levamisole and albendazole on 60 calves on each of 62 farms in the North Island chosen at random from farms that conformed with the selection criteria. Resistance to an anthelmintic was inferred when there was <95% reduction in FEC 7-10 days after treatment. Larval cultures were performed for all control groups and for treated groups for which resistance was evident. RESULTS: Of the farms that completed the FECRT, 4/61 (7%) showed > or =95% reduction in FEC for all anthelmintics tested. Resistance to ivermectin was evident on 56/61 (92%) farms, to albendazole on 47/62 (76%) farms, and to both ivermectin and albendazole on 45/61 (74%) farms. Resistance to levamisole was evident on only 4/62 (6%) farms. The parasites most prevalent in resistant populations cultured were Cooperia spp. On 45/61 (74%) farms where Cooperia spp were present in sufficient numbers, resistance to both ivermectin and albendazole was evident. No cases of levamisole-resistant Cooperia spp were detected. Resistance of Ostertagia spp to ivermectin was evident on 4/45 (9%) farms, to albendazole on 15/46 (35%) farms, and to levamisole on 4/46 (9%) farms. CONCLUSION: Anthelmintic resistance in parasitic nematodes of cattle is common in the North Island of New Zealand. Beef farmers need to be aware of the risks posed by anthelmintic resistance, and routine FECR testing is recommended to ensure optimal productivity and to guide decision-making when purchasing anthelmintics to be used on-farm.