Prevalence of benzimidazole-resistant small strongyles in horses in a southeastern Pennsylvania practice

A survey was conducted to determine the prevalence of benzimidazole (BZ)-resistant small strongyles in horses in a southeastern Pennsylvania practice. Resistant parasites were found in 291 of 342 horses surveyed. Anthelmintic practices and pasture management factors in use for 3 to 6 years did not correlate with the presence of resistant small strongyles. Benzimidazole-resistant small strongyles were recovered in horses that had been treated alternately with BZ and non-BZ products and in horses receiving BZ products as infrequently as twice a year. However, inasmuch as the horses may have been infected with resistant small strongyles before the various anthelmintic schedules were implemented, it was not possible to attribute BZ-resistance to any particular pattern of drug use. Fifty-one (14.9%) horses had BZ-susceptible small strongyles: these horses were on poor overall parasite control programs and had received BZ products no more than once a year. Benzimidazole-piperazine and non-BZ drugs were effective in herds infected with BZ-resistant small strongyles.     

Effectiveness of oxibendazole against benzimidazole-resistant strongyles in horses

Twenty-eight horses with a residual burden of strongyle eggs in the faces after treatment with mebendazole (MBZ) paste were treated with a suspension of either MBZ or oxibendazole (OBZ). Fecal samples were collected before and 14 days after these treatments. The number of strongyle eggs/g (epg) of feces for each horse was estimated using the Cornell-McMaster dilution and the Cornell-Wisconsin double centrifugation procedures. The epg for each horse was transformed using log (× + 1) and in an analysis of variance of the reduction in egg count for each horse on the logarithmic scale, there was a highly significant difference between the treatments. The mean epg was increased in the MBZ-treated horses and reduced in the OBZ-treated horses, but the reduction was only by 82% with an upper confidence limit of 89%. Subsequently, the horses were retreated with MBZ and OBZ suspensions without significant reduction in the mean epg for OBZ-treated horses.     

A field evaluation of oxibendazole in horses infected with benzimidazole resistant small strongyles

Horses with a history of frequent benzimidazole (BZD) treatment were used in a trial to assess the effectiveness of oxibendazole (OBZ). Initial investigations established that these animals were infected with small strongyles resistant to thiabendazole (TBZ), mebendazole (MBZ), cambendazole (CBZ), oxfendazole (OXF) and fenbendazole (FBZ).The herd was subsequently divided into four groups and received TBZ, CBZ, OBZ, or no treatment. OBZ effectly reduced fecal egg counts. Similar reductions were not observed with TBZ or CBZ.     

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%).     

Critical tests in equids with fenbendazole alone or combined with piperazine: particular reference to activity on benzimidazole-resistant small strongyles

Seven critical tests in equids were conducted with single doses of fenbendazole (5 mg kg-1) alone (Panacur--American Hoechst, Somerville, NJ); (2 tests with paste and 1 with suspension formulation) or in combination with piperazine (American Hoechst); (40 mg base kg-1); (4 tests with paste formulation). The main purpose of the tests was evaluation of activity against benzimidazole-resistant small strongyles (Cyathostomum catinatum, Cyathostomum coronatum, Cylicocyclus nassatus, Cylicostephanus goldi, and Cylicostephanus longibursatus). Natural infections of 2 populations of benzimidazole-resistant small strongyles were evaluated; 1 was population B in 2 horses and the other was population S in 5 ponies. Removal of the 5 species of population B was 49-91% in the animal treated with fenbendazole paste alone and 100% (4 of these species present) in the animal treated with the combination. For population S, 2 of the 5 resistant species were present in small numbers in 1 animal treated with fenbendazole paste alone and all were removed; the 1 animal receiving fenbendazole suspension alone had removals of 0-70% for the 5 benzimidazole-resistant species. Also for population S, the 5 resistant species were present in 2 animals treated with the paste combination and removal was 98-100% and of 4 of the 5 resistant species in 1 animal, removal was 76-99%. Removal of large strongyles (Strongylus vulgaris and Strongylus edentatus) was 92-100% for fenbendazole paste alone or in combination with piperazine in the 5 infected animals. For Oxyuris equi, present in 1 animal treated with the combination, there was 91% removal of immature and 100% removal of mature specimens. There probHably was no activity by fenbendazole alone or the combination against bots, tapeworms, and parenteral stages of S. vulgaris and S. edentatus. The combination may have had some activity against immature Habronema spp. and mature abronema muscae.     

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.     

Resistance to benzimidazole anthelmintics in small strongyles (Cyathostominae) of horses in Denmark.

This study was undertaken to establish whether anthelmintic resistance was present in nematode parasites of horses in Denmark. Sixteen horse farms were selected for faecal egg count reduction (FECR) tests to measure the efficacy of the anthelmintic used. Resistance to benzimidazole anthelmintics was found on 13 of the 16 farms, with FECR values ranging from 80.0% to -101.3%. On the remaining 3 farms FECR was 100.0%, 99.3% and 97.2%. Results of a questionnaire study on anthelmintic usage, parasite control measures and management practices showed that horses in this study were treated on average 7.1 times/year. Horse owners changed between preparations of drugs but almost only within the same class of anthelmintics. Nine owners gave an anthelmintic treatment to purchased horses before they were introduced on the farm. On 14 farms, the same paddock was grazed every year and the average stocking rate was estimated to be 2.4 horses/ha. Strategies to avoid development of anthelmintic resistance are discussed and recommendations of parasite control on horse farms are presented.     

Continuance of studies on Population S benzimidazole-resistant small strongyles in a Shetland pony herd in Kentucky: effect of pyrantel pamoate (1992-1999)

Research on benzimidazole-resistant Population S small strongyles began in a Shetland pony herd in 1974 at the University of Kentucky and has continued for over 25 years. The present update, for the period 1992-1999, evaluated activity of pyrantel pamoate (PRT) in field tests in the pony herd. Additional critical tests with PRT and oxibendazole (OBZ) were done in foals born in the herd. Activity of PRT was initially excellent in field tests, based on epg/lpg count data, but declined rapidly during the second full year of pyrantel treatments. Critical test data for small strongyles indicated efficacies of PRT were about 60% at the beginning of the present observations and this intermediate level of removal continued throughout the seven-year period except for 1994 (75%). Unusual was the finding that field test epg/lpg data on small strongyles indicated much lower activity of PRT than found in worm count data in critical tests. The previously reported ineffective activity of OBZ on this population of small strongyles continued. Data are presented on prevalence and drug activity on several species of internal parasites besides small strongyles.     

Comparison of in vitro methods and faecal egg count reduction test for the detection of benzimidazole resistance in small strongyles of horses

The objective of the study was to compare the in vitro egg hatch test (EHT), larval development test (LDT) and in vivo faecal egg count reduction test (FECR test) for the detection of benzimidazole resistance in equine strongyles. The presence of resistant or susceptible strongyle populations was determined in 25 stud farms using the in vivo FECR test and in vitro EHT. On the basis of the FECR values, resistance to fenbendazole was detected on 15 of the 25 farms (60%). The ED₅₀ value (anthelmintic concentration producing 50% inhibition of hatching) for suspected resistant populations varied from 0.110 to 0.222 g/ml thiabendazole (TBZ). Final LD₅₀ values (anthelmintic concentration inhibiting development of 50% of eggs into L₃ infective larvae) above 0.029 g/ml TBZ in the in vitro larval development test on samples from 11 stud farms revealed the presence of populations of small strongyles suspected of being benzimidazole-resistant.     

Helminths in horses: use of selective treatment for the control of strongyles

The current level of anthelmintic resistance in the horse-breeding industry is extremely high and therefore more emphasis is being placed on studies that focus on the judicious use of anthelmintic products. The aims of the study were to: 1) establish if there is variation in the egg excretion pattern of strongyles between the different age classes of Thoroughbred horses in the Western Cape Province (WCP), 2) test if a selective treatment approach successfully reduces the number of anthelmintic treatments and maintains acceptably low helminth burdens in adult Thoroughbred horses, and 3) evaluate the efficacy of subsampling large horse herds for faecal egg counts (FECs) to monitor the strongyle burden. In 2001 the FECs of 4 adult mare, 5 yearling and 3 weanling herds from 8 different farms were compared in the WCP Within the mare herds there were generally fewer egg-excreting individuals with lower mean FECs compared with the younger age classes. Individual faecal samples were collected every 3-4 weeks from 52 adult Thoroughbred mares from 1 farm in the WCP during a 12-month period (2002/2003). Animals with strongyle FECs > or =100 eggs per gram (epg ) were treated with an ivermectin-praziquantel combination drug (Equimax oral paste, Virbac). The mean monthly strongyle FEC for the entire group was <300 epg throughout the study and the number of treatments was reduced by 50 %. Resampling methods showed that an asymptote to mean FEC was reached at 55 animals for each of the pooled weanling, yearling and mare egg counts. Resampling within 4 different mare herds recorded asymptotes of between 24 and 28 animals. Subsampling entire herds for FECs therefore provided an effective approach to treatment management. This study demonstrates that selective treatment is both a practical and an effective approach to the management of anthelmintic resistance.     

In vitro field screening for anthelmintic resistance in strongyles of sheep and horses

Simplified in vitro field methods are described for the detection and assay of benzimidazole-resistance in sheep trichostrongylids and horse strongyles. Worm eggs are recovered from fresh faeces, within 1 hour of collection, by flotation in sugar solution. The separated eggs are then incubated for 20–24 hours at 27–30° C in solutions of thiabendazole in distilled water ranging from 0.1 to 1.1 p.p.m. for sheep trichostrongylids and from 0.05 to 0.5 p.p.m. for horse strongyles. Under controlled conditions, eggs from thiabendazole-susceptible individuals of both sheep and horse nematodes rarely hatch at thiabendazole concentrations of 0.1 p.p.m. Eggs from resistant individuals will hatch at 0.1 p.p.m. and above. Semi-quantitative estimates of the level of resistance can be determined by measuring the % egg-hatch at varying concentrations of thiabendazole. A field method for selecting test animals with low egg-counts, and an in vitro method for the culture of eggs or first-stage larvae to third stage for identification are described.