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 Field Survey on Anthelmintic Resistance in Equine Small Strongyles in Norway

A field survey at 17 stables involving 221 horses was performed to evaluate the presence of anthelmintic resistance in the equine small strongyles (cyathostomes). The horses were allocated into treatment groups, and resistance to fenbendazole (FBZ), pyrantel pamoate (PYR) and ivermectin (IVM) was tested by the faecal egg count reduction test (FECR-test). Faecal samples were collected at the time of treatment, 14 days post treatment and 90 days post treatment.Resistance to FBZ, which was defined as a faecal egg count reduction <95%, was found in 14 out of 17 stables. In 2 of the 14 stables the egg reductions were close to the limit of 95%, 91 and 93%, respectively. In 1 stable the egg reductions indicated resistance to PYR as well as detection of resistance to FBZ, 94% reduction for PYR and 85% for FBZ. No signs of resistance were detected to IVM.The investigation was performed in late autumn and winter, and due to the climatic conditions and cleaning procedures in the stables no reinfection took place during this period. The faecal egg count reduction from treatment till day 90 post treatment was used as an expression of the effect of PYR and IVM on the early stage (hypobiotic), late third stage and fourth stage larvae in the gut wall. This was justified because there was no reinfection and because the 14 day post treatment egg counts were zero or close to zero for the PYR and IVM treatment groups. The effects of PYR and IVM on the larval stages were compared and no statistically significant differences were found.     

Cases of reduced cyathostomin egg-reappearance period and failure of Parascaris equorum egg count reduction following ivermectin treatment as well as survey on pyrantel efficacy on German horse farms

In 2003 and 2004, on a total of 63 different German horse farms, a survey using the faecal egg count reduction (FECR) test was performed to investigate the efficacy of ivermectin (IVM, Ivomec) and pyrantel (PYR, Banminth) treatment against gastro-intestinal nematodes in a total of 767 horses. IVM treatment resulted in 100% reduction of the cyathostomin egg production 14 and 21 days post-treatment (d.p.t.) on 37 farms. On the remaining five farms, the mean faecal egg count reduction ranged between 97.7 and 99.9%. The mean cyathostomin FECR following PYR treatment ranged between 92.2 and 100% on the 25 farms tested. Therefore, based on the 90% FECR threshold suggested for detection of anthelmintic resistance in horses, neither IVM nor PYR anthelmintic resistance was detected. However, if the thresholds recommended for the detection of resistance in small ruminants were applied, on one and four farms signs of reduced IVM and PYR efficacy, respectively, were observed. In 2005, to further investigate these findings, the cyathostomin egg-reappearance period (ERP) following IVM treatment was examined on six selected farms, two of which were found to show less than 99.8% FECR in the previous survey. On these two latter farms, the ERP was less than 5 weeks, while on the other four it was at least 8 weeks. Earlier investigations described IVM cyathostomin ERP of at least 9 weeks. The efficacy of IVM to reduce Parascaris equorum egg excretion was also studied. On one farm in 2 consecutive years, IVM treatment did not lead to a significant reduction in P. equorum faecal egg counts in one and five young horses, respectively.     

A combination treatment strategy using pyrantel pamoate and oxibendazole demonstrates additive effects for controlling equine cyathostomins

Anthelmintics of the avermectin/milbemycin class continue to demonstrate the highest efficacy against cyathostomin nematodes; however, over‐reliance on these drugs is increasing the likelihood that recent emerging resistance to these drugs will worsen. Use of benzimidazole and pyrantel compounds given concurrently may provide sufficiently high efficacy to serve as a viable treatment alternative to the avermectin/milbemycin drugs, thus reducing the selective pressures for resistance development. The study was conducted to determine if oxibendazole (OBZ) and pyrantel pamoate (PYR) would have greater efficacy when used in combination vs. what either drug could achieve individually, and to determine if the combination protocol would consistently achieve faecal egg count reduction (FECR) rates >90%, the general threshold for acceptable efficacy. Horses of various ages and breeds on 11 horse farms were assigned randomly to treatment with OBZ (n = 34), PYR (n = 35), or a combination of both (n = 61). A faecal egg count was performed for each horse prior to treatment and 9–14 days post treatment. Mean FECR percentages were calculated for each treatment group on individual farms, and for each treatment group across all farms. Combination treatment achieved >90% mean FECR on all 11 farms, and >95% on 9 of the 11 farms. Overall arithmetic mean FECR rates were 90.03%, 81.10% and 96.35% for horses treated respectively with OBZ, PYR, and the 2 drugs in combination. Combination treatment with OBZ and PYR demonstrated an additive effect whereby horses given both anthelmintics concurrently had mean FECR percentages that were significantly greater than in horses given either drug alone. Combination treatment with OBZ and PYR can be an efficacious and viable treatment choice for controlling cyathostomins, thereby reducing the over‐reliance on avermectin/milbemycin drugs.     

Evaluation of a larval development assay (DrenchRite) for the detection of anthelmintic resistance in cyathostomin nematodes of horses

A larval development assay (LDA, DrenchRite) was evaluated to determine the effectiveness of this method in detecting anthelmintic resistance in cyathostomin nematodes of horses. A total of 15 horse farms from Georgia and South Carolina (USA) and Population S ponies from the University of Kentucky (USA) were included in this study. Nematode eggs were extracted from pooled fecal samples and placed into the wells of a DrenchRite plate for testing against thiabendazole (TBZ), levamisole (LEV) and 2 ivermectin (IVM) analogs (IVM-1, IVM-2). After a 7-day incubation larvae in each well were counted and data were analyzed by logistic regression. Resistance status of each farm for different drugs was determined in a separate study using a fecal egg count reduction test. LDA were performed on the 15 farms once, however, the Population S cyathostomins were assayed on 3 separate occasions to estimate the consistency of results between assays. Mean TBZ LC50 for oxibendazole resistant, suspected resistant and sensitive farms were 0.2015, 0.1625, and 0.1355 microM, respectively. For LEV, mean LC50 for PYR resistant, suspected resistant and sensitive farms were 1.590, 1.8018 and 1.4219 microM, respectively. All 15 farms had worms susceptible to IVM; mean LC50 for IVM-1 and for IVM-2 were 7.5727 and 87.9718 nM, respectively. A linear mixed model was fitted to the data to determine the relationship between LC50 and LC95 and resistance status for each farm. No meaningful relations were found. Consistency of assays varied between drugs, being best for TBZ and worst for LEV and IVM-1. All farms in this study had benzimidazole-resistant nematodes; therefore usefulness of DrenchRite for discriminating susceptibility versus resistance to this drug class could not be accurately assessed. Moreover, since all farms tested were sensitive to IVM and resistance to this drug class has not yet been reported in cyathostomins, it is not possible to assess accurately the usefulness of DrenchRite LDA for detecting IVM resistance at this time. Assay results for LEV suggest that LEV in a LDA does not yield data that is useful in estimating PYR efficacy in vivo. Based on results for PYR/LEV, the current high prevalence of benzimidazole resistance, no known cases of IVM resistance, and the sometimes extreme variation in results seen in many of the assays, DrenchRite LDA cannot be considered a useful tool for the diagnosis of resistance in cyathostomins of horses at present.     

Endoparasite control management on horse farms – lessons from worm prevalence and questionnaire data

Reasons for performing study: Increasing prevalence of anthelmintic resistance in equine nematodes calls for a reexamination of current parasite control programmes to identify factors influencing control efficacy and development of resistance. Objectives: To investigate if associations occur between prevalence of parasitic nematodes and management practices. Methods: German horse farms (n = 76) were investigated in 2003 and 2004. Information on farm and pasture management with respect to endoparasite control measures obtained using a questionnaire survey. Faecal examinations were performed in parallel. Results: Horses (n = 2000) were examined by faecal nematode egg counts, grouped into foals, yearlings and mature individuals for statistical analyses. Farms were categorised into 3 types, riding, stud farms and small holdings. Count regression models were used to analyse strongyle faecal egg count data. Following dichotomisation of faecal egg count (FEC) data, prevalence of strongyle and Parascaris equorum infections were assessed by logistic regression models as a function from various management factors. Yearlings on stud farms showed a 2‐fold higher risk of being positive for strongyle FEC, higher (i.e. ≥3 per year) anthelmintic drug treatment frequencies were associated with reduced strongyle infection rates only in mature individuals but not in foals or yearlings, foals on farms fertilising pastures with horse manure had a significantly higher risk of being P. equorum FEC positive and yearlings on stud farms were more often showing incomplete FECR following anthelmintic treatment compared to yearlings on other farm types. The mean yearly treatment frequencies per age group were: foals 4.52, yearlings 3.26 and mature horses 2.72 times, respectively. Conclusion and potential relevance: To delay the development of anthelmintic, resistance management should include additional nonchemotherapeutic parasite control strategies, FEC‐monitoring, controlled quarantine treatment of new arrivals and control of efficacy by the faecal egg count reduction test on a regular basis.     

Efficacy of Ivermectin Pour-on Against Nematodes Infecting Foals on Pasture: Coprological and Biochemical Analysis

The efficacy of topical ivermectin (IVM) on foals naturally infected by parasitic nematodes was evaluated. Two dosages of IVM were applied pour-on (F-Nor0.5; 0.5 mg/kg body weight [BW] and F-Nor1; 1 mg/kg BW) and results compared with the oral administration (F-Eq0.2; 0.2 mg/kg BW of IVM). The efficacy was measured by estimating the reduction in the fecal egg counts (fecal egg count reduction) and in the numbers of horses shedding parasite eggs (positive horse reduction). Several biochemical and enzymatic serum parameters were measured in the groups F-Eq0.2 and F-Nor1. Before the deworming of the horses, eggs of Parascaris equorum, Cyathostomum, Gyalocephalus spp, and Oxyuris equi were identified. In all the treated groups, the excretion of ascarid eggs ended 4 days after the treatment. The orally administered IVM suppressed the egg output of strongyles and pinworms 4 days after the treatment, whereas for the F-Nor1 group this occurred 8 days after the treatment. Eggs of strongyles were detected in the F-Nor0.5 group throughout the study. The levels of blood urea nitrogen, creatinine, total proteins, albumin, globulins, and lactate dehydrogenase (LDH) reduced significantly after the administration of IVM, but values not within the normal range were only achieved for LDH. A significant positive correlation between the fecal egg output of cyathostomins and the LDH was investigated. Clinically, no adverse reactions in the horses receiving the topical IVM were observed. It was concluded that the pour-on administration of 1 mg/kg BW IVM provides similar results to the oral administration, and offers a very useful tool to control infestation by the intestinal nematodes affecting wild grazing horses.     

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.     

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.     

A field evaluation of anthelmintics for control of cyathostomes of horses in Brazil

The anthelmintic efficacy of ivermectin (IVM), piperazine citrate (PPZ) and fenbendazole (FBZ) alone, and combinations of FBZ with varying dosages of PPZ, was evaluated for the control of strongyles of Mangalarga equids. In all cases of positive strongly egg counts per gram of faeces (EPG) before and after treatment, pure populations of cyathostome larvae with eight gut cells were consistently observed in faecal cultures. The faecal egg count reductions (FECR) were evaluated between Day 7 and Day 56 post-treatment. Significant differences (P less than or equal to 0.05) were found between Day 7 and Day 49 when comparing the mean EPG values of IVM with those of the other anthelmintics. PPZ alone and the combination of FBZ (7.5 mg kg-1) and PPZ (55 mg base kg-1) did not show significant differences during the trial; both groups exhibiting an FECR of greater than 90% between Day 7 and Day 30. Horses treated with the combinations of FBZ (7.5 mg kg-1) and PPZ at either 40 or 25 mg base kg-1, and with FBZ alone at either 7.5 or 15 mg kg-1, showed an FECR of less than 90% 2 weeks after treatment. The emergence of a degree of resistance of strongyles to FBZ and PPZ in a segment of the cyathostomes in this population is suggested.     

Prevalence of strongyles and efficacy of fenbendazole and ivermectin in working horses in El Sauce, Nicaragua

Horses, mules and donkeys are indispensable farming and working animals in many developing countries, and their health status is important to the farmers. Strongyle parasites are ubiquitous in grazing horses world-wide and are known to constitute a threat to equine health. This study determined the prevalence of strongyle infection, the efficacy of ivermectin and fenbendazole treatment, and strongyle re-infection rates of working horses during the dry months in Nicaragua. One hundred and five horses used by farmers for transport of people and goods were randomly allocated into three treatment groups, i.e., the IVM group treated with ivermectin, the FBZ group treated with fenbendazole and the control group treated with placebo. Determined by pre-treatment faecal egg counts (FECs), horses showed a high prevalence (94%) of strongyle parasites with high intensities of infection (mean FEC of 1117 eggs per gram (EPG) with an SD of 860 EPG, n=102). Body condition scores of all horses ranged from 1.5 to 3.5 with a mean of 2.4 (scales 1-5). Fourteen days after treatment faecal egg count reductions (FECRs) were 100% and 94% in the IVM and the FBZ groups, respectively. The egg reappearance period (ERP) defined as the time until the mean FEC reached 20% of the pre-treatment level, was estimated as 42 days for the FBZ group and 60 days for the IVM group. Individual faecal cultures were set up and the larval differentiation revealed a 36% prevalence of Strongylus vulgaris before treatment (n=45). In the FBZ group, 25% of the horses were S. vulgaris-positive 70 days post treatment compared to 11% in the IVM group. Our results indicate that strongyle infection intensities in Nicaragua are high and that S. vulgaris is endemic in the area. Furthermore, efficacies and ERPs of IVM and FBZ were within the expected range with no signs of anthelmintic resistance.     

Characterisation of two triple resistant field isolates of Teladorsagia from Scottish lowland sheep farms

The anthelmintic resistance status of two field isolates derived from farms (farm A and B) located near Edinburgh were examined using both controlled efficacy tests (CET) and faecal egg count reduction tests (FECRT). Efficacies against fenbendazole (FBZ), levamisole (LEV) and ivermectin (IVM) and, for one isolate, against combinations of these anthelmintics and moxidectin were determined in na?ve lambs, artificially infected with the isolates and treated with the compounds at the manufacturers recommended dose rates. (FBZ, 5mg/kg bodyweight (BW); LEV, 7.5mg/kg BW; IVM, 0.2mg/kg BW; Moxidectin (MOX) 0.2mg/kg BW). In both field isolates, the predominant species found pre-treatment and the only species found post-treatment was Teladorsagia circumcincta. Resistance to FBZ, LEV and IVM was confirmed in CET and FECRT on farm A and to the latter two compounds on farm B, which had a history of benzimidazole resistance and where TBZ resistance was also demonstrated using an egg hatch assay (EHA). For the farm A isolate CET efficacies against FBZ; IVM; LEV; FBZ + IVM; FBZ + LEV; FBZ, LEV + IVM and MOX were 59, 60, 88, 94,93, 92 and 98%, respectively. The CET efficacies for the farm B isolate were 51% and 72% for LEV and IVM, respectively.     

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.     

Comparative plasma and milk dispositions,faecal excretion and efficacy of per os ivermectin and pour‐on eprinomectin in horses

The horse milk gains increasing interest as a food product for sensitive consumers, such as children with food allergies or elderly people. We investigated the plasma and milk disposition, faecal excretion and efficacy of per os ivermectin (IVM) and pour‐on eprinomectin (EPM) in horses. Ten mares were divided into two groups. The equine paste formulation of IVM and bovine pour‐on formulation of EPM were administered orally and topically at dosage of 0.2 and 0.5 mg/kg bodyweight. Blood, milk and faecal samples were analysed using high‐performance liquid chromatography. The plasma concentration and persistence of IVM were significantly greater and longer compared with those of EPM. Surprisingly, EPM displayed a much higher disposition rate into milk (AUC_milk/plasma: 0.48) than IVM (AUC_milk/plasma: 0.19). IVM exhibited significantly higher faecal excretion (AUC_faeces: 7148.54 ng·d/g) but shorter faecal persistence (MRT_faeces: 1.17 days) compared with EPM (AUC_faeces: 42.43 ng·d/g and MRT_faeces: 3.29 days). Faecal strongyle egg counts (EPG) were performed before and at weekly intervals after treatment. IVM reduced the EPG by 96–100% for up to 8 weeks, whereas the reduction in the EPM group varied from 78 to 99%. In conclusion, due to the relatively low excretion in milk, EPM and IVM may be used safely in lactating mares if their milk is used for human consumption. Nevertheless, much lower plasma and faecal availabilities of EPM could result in subtherapeutic concentrations, which may increase the risk of drug resistance in nematodes after pour‐on EPM administration compared with per os IVM.     

Further characterisation of a triple resistant field isolate of Teladorsagia from a Scottish lowland sheep farm

Anthelmintic efficacies against juvenile developing populations of Teladorsagia species that were known to be resistant to anthelmintics from all three broad spectrum families were examined using a controlled efficacy test. Fenbendazole (FBZ), levamisole (LEV), ivermectin (IVM), combinations of these anthelmintics and moxidectin (MOX) were assessed in parasite na?ve lambs artificially infected with 8,000 third stage larvae (Tci5) and treated orally 8-day post-infection with the compounds at the manufacturers recommended dose rates, FBZ, 5 mg/kg body weight (BW); LEV, 7.5 mg/kg BW; IVM, 0.2 mg/kg BW; MOX (0.2 mg/kg BW). The lambs were slaughtered 14-day post-treatment. The arithmetic mean worm burden reductions resulting from oral treatments with FBZ; IVM; LEV; FBZ+IVM; FBZ+LEV; FBZ, LEV+IVM or MOX were 36%, 82%, 38%, 86%, 60%, 88% and 97%, respectively. The results illustrate that combination treatments showed improved efficacies against the juvenile population compared to individually administered treatments but that these improvements were not wholly effective. Moxidectin was the only treatment that was over 95% effective, though caution should be noted when advising the use of MOX prophylactically since 3% of the infection still survived this treatment. Treatments directed at juvenile stages of Tci5 were less effective, with the exception of IVM, compared to a similar trial using Tci5 where the same treatments were directed against a predominantly adult population. No interaction was detected comparing the timings of treatments and its effectiveness with the exception of IVM (two-way ANOVA, p < 0.05). These findings suggest that, on the whole, the selection processes for anthelmintic resistance (AR) may occur at an early stage of development within the parasites, having severe implications for the early detection of AR.     

Assessing resistance against macrocyclic lactones in gastro-intestinal nematodes in cattle using the faecal egg count reduction test and the controlled efficacy test

The main objective of the present study was to evaluate the accuracy of the faecal egg count reduction test (FECRT) to assess the resistance status of ivermectin (IVM)-resistant isolates of the cattle nematodes Ostertagia ostertagi and Cooperia oncophora, using the controlled efficacy test (worm counts) as a reference. The second objective was to investigate whether both IVM-resistant isolates showed side-resistance against moxidectin (MOX) under controlled conditions. Thirty male Holstein calves were experimentally infected with 25,000 L3 of an IVM-resistant O. ostertagi isolate and 25,000 L3 of an IVM-resistant C. oncophora isolate. Twenty-eight days later the calves were randomly divided into 2 treatment groups and 1 untreated control group. Animals in groups 1 and 2 received MOX (Cydectin(?) 1%, Pfizer) and IVM (Ivomec(?) 1%, Merial) respectively, by subcutaneous injection at a dose rate of 0.2mg/kg bodyweight. Faecal samples were collected 7 and 14days after treatment and animals were necropsied 14/15days post-treatment. Both the FECRT and the controlled efficacy test demonstrated that the O. ostertagi and C. oncophora isolates were resistant against IVM, with efficacies below 90%. The IVM-resistant O. ostertagia isolate was still susceptible to MOX treatment, as shown by over 99% reduction in egg counts and worm burden. The FECRT suggested borderline resistance against MOX in the IVM-resistant C. oncophora isolate, with egg count reductions between 97% (95% CI: 76; 100) at day 7 and 86% (95% CI: 49; 96) at day 14. However, the controlled efficacy test clearly showed MOX-resistance, with a decrease of only 31% (95% CI: -12; 57) in C. oncophora worm numbers. After MOX treatment, a significantly lower number of eggs per female C. oncophora worms was counted compared to the control group (43% reduction). Due to this reduced fecundity, the FECRT may fail to detect MOX-resistance.     

Evaluation of tests for anthelmintic resistance in cyathostomes

Resistance, especially to the anthelmintic benzimidazoles (BZ), has been reported in horse cyathostomes world-wide. Diagnosis of resistance has traditionally been made by faecal egg count reduction (FECR) trials, however, this technique has limitations. Some of the shortcomings may be resolved by refining the test or by using an in vitro test. FECR tests and the larval development assay (LDA) were performed on adult horses held on 15 different horse properties across a wide geographical area of NSW, Australia. FECR were measured before and 10-14 after days treatment with oxibendazole (OBZ), morantel (MOR) or ivermectin (IVM) at recommended dose rates. Eight properties were rejected following low pre-treatment egg counts, leaving seven in the study. On these, the majority of larvae recovered from faecal cultures were cyathostomes. Using a definition of resistance as a FECR of <90%, resistance to OBZ was present on six properties and to MOR on two properties. Resistance to IVM was not detected. An alternative method of calculating FECR based on individual horse egg counts pre- and post-treatment was developed and results from the same properties compared with the results of the LDA. For example, for the BZ, correlation coefficients of values of lethal concentration to kill 50% of population (LC50) on LDA and FECR percentages were -0.536 before and -0.704 after OBZ treatment. We conclude that the LDA has the potential to be a single visit test for detection of anthelmintic resistance in horse cyathostomes but requires further investigation and standardisation.     

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).     

Impact on beef cattle productivity of infection with anthelmintic-resistant nematodes

ABSTRACT

Aims: The main goal of the current study was to evaluate, on a commercial beef cattle farm, the impact of infection with gastrointestinal nematodes resistant to both ivermectin (IVM) and moxidectin (MXD) on the productivity of calves.

Methods: Male Aberdeen Angus calves, aged 9–11 months, with faecal nematode egg counts (FEC) ≥200 epg and body weight ≥190?kg, were allocated to two herds. Herd A (n?=?90) grazed a maize-winter forage crop rotation and Herd B (n?=?90) grazed a 2-year-old Agropyrum pasture. On Day 0 in each herd, calves were randomly allocated into five groups (n?=?18), which were treated with 0.2?mg/kg IVM; 0.2?mg/kg MXD; 3.75?mg/kg ricobendazole (RBZ), both IVM and RBZ, or remained untreated. Faecal samples collected on Days ?1 and 19 were used to determine the percentage reduction in FEC, and genera of the nematodes were determined by the identification of the third-stage larvae recovered from faecal cultures. Total weight gain was determined from body weights recorded on Days ?1 and 91.

Results: Overall mean reduction in FEC was 42% for IVM, 67% for MXD, 97% for RBZ and 99% for IVM?+?RBZ. The reduction in FEC for Cooperia spp. was ≤78% for IVM and MXD, and for Haemonchus spp. was 0 and 36% for IVM and MXD, respectively, confirming the presence of parasites resistant to both anthelmintics. Only IVM?+?RBZ treatment resulted in 100% efficacy against Haemonchus spp. The overall estimated mean total weight gain for calves treated with IVM was 15.7 (95% CI?=?11.9–19.7) kg and for calves treated with IVM?+?RBZ was 28.8 (95% CI?=?25–32.5) kg (p?<?0.001). Mean total weight gain for calves treated with MXD was 23.5 (95% CI?=?19.7–27.2) kg.

Conclusions and clinical relevance: In calves naturally infected with resistant nematodes, under the production system assessed here, weight gains were lower in calves treated with anthelmintics that were moderately or highly ineffective compared to those treated with highly effective anthelmintics. These results demonstrate to farmers and veterinarians the importance of a sustainable and effective nematode control under field conditions.     

Anthelmintic resistance in cattle nematodes on 13 south-west Victorian properties

Objective To report the level of anthelmintic resistance on 13 commercial cattle properties in south-west Victoria, Australia. Procedure Between 2006 and 2009 worm egg count reduction tests were conducted on calves on the 13 properties. Samples were collected 10–14 days post anthelmintic treatment and worm egg counts and larval differentiation tests were conducted. Resistance was defined if there was less than 95% reduction (lower confidence limit <90%) in the faecal worm egg count for the particular genus. Results The percentage of properties with anthelmintic resistance in at least one species was 54% for benzimidazole (BZ), 100% for levamisole (LEV) and for ivermectin (IVM) it was 100% for the half-dose (0.1 mg/kg) and 62% for the full dose (0.2 mg/kg). A substantial frequency of resistance was detected in Ostertagia ostertagi to BZ (5/11), LEV (3/3) and IVM (5/11), in Trichostrongylus spp. to BZ (4/7) and in Cooperia spp. to IVM (6/11). No resistance to LEV was detected in Trichostrongylus or Cooperia spp. Suspected IVM-resistant Trichostrongylus spp. and BZ-resistant Cooperia spp. were only detected on one property each. Conclusion This is the first Australian report of macrocyclic lactone-resistant O. ostertagi in the refereed literature. The frequency of resistance in O. ostertagi to BZ, LEV and IVM and in Trichostrongylus spp. to BZ in the present study appears higher than levels detected in the 2004–05 New Zealand survey, whereas the resistance frequency in Cooperia spp. to IVM and BZ was less.