Clinical trial of moxidectin oral gel in horses

Shortened strongylid egg reappearance periods in horses following macrocyclic lactone administration - The impact on parasite dynamics

Over the past three decades, equine strongylid egg reappearance periods (ERPs) have shortened substantially for macrocyclic lactone anthelmintics. The ERPs of ivermectin and moxidectin were originally reported in the 8-10 and 12-16 week ranges, respectively, but several recent studies have found them to be around 4-5 weeks for both actives. This loss of several weeks of suppressed strongylid egg output could have substantial implications for parasite control. This study made use of a computer simulation model to evaluate the impact of shortened ERPs on the anthelmintic performance of ivermectin and moxidectin against equine cyathostomins. The original ERPs were set to 7.1 and 15.4 weeks for ivermectin and moxidectin, respectively, while the reduced ERP was set to 4.6 weeks for both actives. Simulations were set to compare model outputs between original and reduced ERP scenarios and results expressed as percent increase in strongylid egg output, infective third stage larvae on herbage (L3h), and encysted early third stage larvae (EL3). For each drug, simulations were evaluated for two different treatment scenarios (2 and 4 treatments annually), two different age groups (yearlings and adults), and for four different climates (cold humid continental, temperate oceanic, humid subtropical, and hot/cold semi-arid). Across all simulations, there was a substantial increase of the three evaluated parameters. With the ivermectin simulations, all three parameters increased in the 100-300% range across climates, age groups and treatment intensities. The moxidectin simulations displayed a wider range of results with parameters increasing from a few hundred to several thousand percent. The increases were most pronounced for L3h in the two cooler climates, reaching as high as 6727%. Overall, the loss of anthelmintic performance was at a magnitude of 10 times larger for moxidectin compared to ivermectin. This performance loss was climate dependent, and was also affected by treatment intensity, but not by horse age. This is the first study to evaluate consequences of shortened ERPs in horses and demonstrated a substantial loss in anthelmintic performance resulting from this development. The results illustrate that anthelmintic efficacy is more than the percent reduction of fecal egg counts at 14 days post treatment, and that substantial anthelmintic performance can be lost despite FECRTs remaining at 100%.

Egg reappearance periods of anthelmintics against equine cyathostomins: The state of play revisited

Cyathostomins are the most common and highly prevalent parasites of horses worldwide. Historically, the control of cyathostomins has mainly relied on the routine use of anthelmintic products. Increasing reports on anthelmintic resistance (AR) in cyathostomins are concerning. A potential method proposed for detecting emerging AR in cyathostomins has been estimating the egg reappearance period (ERP). This paper reviews the data available for the ERP of cyathostomins against the three major classes of anthelmintics, macrocyclic lactones, tetrahydropyrimidines, and benzimidazoles. Published peer-reviewed original research articles were obtained from three databases (PubMed, CAB Direct and Web of Science) and were evaluated for their inclusion in a systematic review. Subsets of articles were then subjected to a review of ERP data. A total of 54 (of 134) studies published between 1972 and 2022 met the criteria for inclusion in the systematic review. Until the beginning of 2022, there was no agreed definition of the ERP; eight definitions of ERP were identified in the literature, complicating the comparison between studies. Additionally, potential risk factors for the shortening of the ERP, including previous anthelmintic use and climate, were frequently not described. Reports of shortened ERP for moxidectin and ivermectin are frequent: 20 studies that used comparable ERP definitions reported shortened moxidectin and ivermectin ERPs of 35 and 28 days, respectively. It is unclear whether the ERPs of these anthelmintics reduced to such levels are due to the development of AR or some biological factors related to horses, cyathostomin species, and/or the environment. The ERPs for other anthelmintics, such as fenbendazole and pyrantel, were frequently not reported due to established resistance against these drugs. Future research in horses is required to understand the mechanism(s) behind the shortening of ERP for cyathostomins. Based on this systematic review, we propose recommendations for future ERP studies.

Shortened egg reappearance periods of equine cyathostomins following ivermectin or moxidectin treatment: morphological and molecular investigation of efficacy and species composition

Macrocyclic lactones have been the most widely used drugs for equine parasite control during the past four decades. Unlike ivermectin, moxidectin exhibits efficacy against encysted cyathostomin larvae, and is reported to have persistent efficacy with substantially longer egg reappearance periods. However, shortened egg reappearance periods have been reported recently for both macrocyclic lactones, and these findings have raised several questions: (i) are egg reappearance period patterns different after ivermectin or moxidectin treatment? (ii) Are shortened egg reappearance periods associated with certain cyathostomin species or stages? (iii) How does moxidectin's larvicidal efficacy affect egg reappearance period? To address these questions, 36 horses at pasture, aged 2-5 years old, were randomly allocated to three treatment groups: 1, moxidectin; 2, ivermectin; and 3, untreated control. Strongylid fecal egg counts were measured on a weekly basis, and the egg reappearance period was 5 weeks for both compounds. Strongylid worm counts were determined for all horses: 18 were necropsied at 2 weeks post-treatment (PT), and the remaining 18 at 5 weeks PT. Worms were identified to species morphologically and by internal transcribed spacer-2 (ITS-2) rDNA metabarcoding. Moxidectin and ivermectin were 99.9% and 99.7% efficacious against adults at 2 weeks post treatment, whereas the respective efficacies against luminal L4s were 84.3% and 69.7%. At 5 weeks PT, adulticidal efficacy was 88.3% and 57.6% for moxidectin and ivermectin, respectively, while the efficacy against luminal L4s was 0% for both drugs. Moxidectin reduced early L3 counts by 18.1% and 8.0% at 2 or 5 weeks, while the efficacies against late L3s and mucosal L4s were 60.4% and 21.2% at the same intervals, respectively. The luminal L4s surviving ivermectin treatment were predominantly Cylicocyclus (Cyc.) insigne. The ITS-2 rDNA metabarcoding was in good agreement with morphologic species estimates but suggested differential activity between moxidectin and ivermectin for several species, most notably Cyc. insigne and Cylicocyclus nassatus. This study was a comprehensive investigation of current macrocyclic lactone efficacy patterns and provided important insight into potential mechanisms behind shortened egg reappearance periods.

Anthelmintic resistance in equine nematodes: Current status and emerging trends

Anthelmintic resistance is reported in equine nematodes with increasing frequency in recent years, and no new anthelmintic classes have been introduced during the past 40 years. This manuscript reviews published literature describing anthelmintic resistance in cyathostomins, Parascaris spp., and Oxyuris equi with special emphasis on larvicidal efficacy against encysted cyathostomin larvae and strongylid egg reappearance periods (ERP). Resistance to benzimidazoles and pyrimidines is highly prevalent in cyathostomin populations around the world, and macrocyclic lactone resistance has been documented in cyathostomins in recent years as well. Two recent studies have documented resistance to the larvicidal regimen of fenbendazole, whereas the larvicidal efficacy of moxidectin is variable, but with no evidence of a reduction from historic levels. In the 1990s, ERP estimates were 8-10 and 12-16 weeks for ivermectin and moxidectin, respectively, while several studies published after year 2000 found ERPs to be 5 weeks for both compounds. This is a clear change in anthelmintic performance, but it remains unclear if this is due to development of anthelmintic resistance or selection for other biological traits leading to a quicker resumption of strongylid egg shedding following anthelmintic treatment. Macrocyclic lactone resistance is common in Parascaris spp. around the world, but recent reports suggests that resistance to the two other classes should be monitored as well. Finally, O. equi has been reported resistant to ivermectin and moxidectin in countries representing four continents. In conclusion, multi-drug resistance is becoming the norm in managed cyathostomin populations around the world, and a similar pattern may be emerging in Parascaris spp. More work is required to understand the mechanisms behind the shortened ERPs, and researchers and veterinarians around the world are encouraged to routinely monitor anthelmintic efficacy against equine nematodes.

The Use of Molecular Profiling to Track Equine Reinfection Rates of Cyathostomin Species Following Anthelmintic Administration

Simple Summary

Cyathostomins (small strongyles) are a multispecies group of intestinal parasites in horses and the main target of deworming efforts by horse owners. It is not known whether species of cyathostomins have individual responses to dewormers. The objective of this study was to identify differences between cyathostomin species in reemergence rates following commercial dewormer treatment. This study used gene sequencing to profile the presence/absence of cyathostomin species in fecal samples at 2-week intervals following deworming to determine how quickly each species reinfected horses. Moxidectin was found to be the most effective at slowing the overall reemergence of these parasites, followed by Ivermectin, then Pyrantel. Seven species were resistant to all three deworming products. This study demonstrates that dewormer sensitivity differs between cyathostomin species, which could lead to more targeted control measures.

Abstract

Cyathostomins are a multispecies parasite ubiquitous in Equids. Cyathostomins have developed resistance to all but one class of anthelmintics, but species-level sensitivity to anthelmintics has not been shown. This study measured reinfection rates of cyathostomin species following the administration of three commercial dewormers. Nine treated horses were compared with 90 untreated controls during June-September 2017–2019. Ivermectin (IVM) (n = 6), Moxidectin (MOX) (n = 8) or Pyrantel (PYR) (n = 8) were orally administered. Fecal samples were collected every 14 d for 98 d. Fecal egg count reductions (FECR) were calculated using a modified McMaster technique. Nineteen cyathostomin species were identified by 5.8S-ITS-2 profiling using amplicon sequencing. Data were analyzed in QIIME1 and R statistical software using presence/absence methods. MOX had the lowest numbers of species present over the time course, followed by PYR then IVM (7.14, 10.17, 11.09, respectively); however, FECR was fastest for PYR. The presence of seven species: Coronocyclus labiatus, Cyathostomum catinatum, Cyathostomum tetracanthum, Cylicocylus elongatus, Cylicodontophorus bicoronatus, Cylicostephanus minutus, and Cylicostephanus goldi were unaffected by treatment (p > 0.05) points to species-specific differences in dewormer sensitivity and environmental persistence. Identifying resistance patterns at the species level will enable mechanistic understandings of cyathostomin anthelmintic resistance and targeted approaches to control them.

Importation of macrocyclic lactone resistant cyathostomins on a US thoroughbred farm

Anthelmintic resistance in equine cyathostomins is both widespread and highly prevalent in the benzimidazole and tetrahydropyrimidine classes; however, reports of resistance to macrocyclic lactone (ML) drugs are sparse and sporadic. This study reports a case of clear ML resistance in a group of Thoroughbred yearlings imported from Ireland to the US in 2019. Fecal egg count reduction (FECR) following ivermectin administered in February 2020 demonstrated 100% reduction in the US bred yearlings, but 93.5%, 70.5%, and 74.5% reduction in three groups of the imported yearlings. The two former groups were then retreated with ivermectin, yielding FECRs of 33.8% and 23.5%, respectively. Horses from these two groups were then assigned randomly to two possible treatments; moxidectin or a triple combination of moxidectin, oxibendazole, and pyrantel pamoate. The groups treated with moxidectin had FECRs of 90.2%, 57.3%, and 50.0%, while the triple combination had a 100% FECR in all treated groups. Subsequently, the efficacy of ivermectin was reassessed in June 2020 yielding FECRs of 99.8%, 87.7%, and 62.0% in the three imported groups. The FECRs of the US bred yearlings all remained in the 99–100% range. This is the first study to clearly demonstrate ML resistance in cyathostomins and to confirm the suspicion through reassessment. These data demonstrate that ML-resistant cyathostomins were imported from Ireland and serve to illustrate that the global movement of horses has the potential to quickly spread ML-resistant parasite isolates around the world. The equine industry is strongly encouraged to routinely monitor anthelmintic efficacy, so occurrence of ML resistant cyathostomins can be detected and appropriate interventions implemented as early as possible.

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Reduced macrocyclic lactone efficacy in yearlings imported from Ireland.

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Ivermectin efficacy was retested twice and was reduced on all occasions.

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100% strongyle fecal egg count reduction in US bred yearlings on all occasions.

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A combination of moxidectin, oxibendazole and pyrantel pamoate was 100% effective.

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Global movement of horses likely to spread macrocyclic lactone resistance quickly.

Dealing with double trouble: Combination deworming against double-drug resistant cyathostomins

An alternative control regimen for drug-resistant parasites is combination deworming, where two drugs with different modes of action are administered simultaneously to target the same parasite. Few studies have investigated this in equine cyathostomins. We previously reported that an oxibendazole (OBZ) and pyrantel pamoate (PYR) combination was not sustainable against a cyathostomin population with high levels of OBZ and PYR resistance. This study consisted of a field study and two computer simulations to evaluate the efficacy of a moxidectin-oxibendazole (MOX-OBZ) combination against the same cyathostomin population. In the field study, anthelmintic treatments occurred when ten horses exceeded 100 eggs per gram. Fecal egg counts and efficacy evaluations were performed every two weeks. The two simulations utilized weather data as well as equine and parasite population parameters from the field study. The first simulation repeated the treatment schedule used in the field study over a 40 year period. The second evaluated efficacies of combination treatments using selective therapy over 40 years. In the field study, efficacies of MOX and both combination treatments were 100%. The egg reappearance period for MOX was 16 weeks, and the two combination treatments were 12 and 18 weeks. The first (46.7%) and last (40.1%) OBZ efficacies were not significantly different from each other. In the simulation study, the combination treatment delayed MOX resistance development compared to when MOX was used as a single active. This occurred despite the low efficacy of OBZ. The second set of simulations identified combination treatments used with selective therapy to be the most effective at delaying MOX resistance. Overall, this study supports the use of combination treatment against drug-resistant cyathostomins, when one of the actives exhibits high efficacy, and demonstrates benefits of this approach despite substantially lowered efficacy of the other active ingredient.

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Oxibendazole-moxidectin combination treatments were 100% effective.

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Oxibendazole efficacies (<50%) did not differ pre and post combination treatment.

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The model observed oxibendazole-moxidectincombinationto delaymoxidectin resistance.

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Combination use in selective therapy delayed resistance most effectively.

Anthelmintic drugs used in equine species

Internal parasites of horses comprise an intractable problem conferring disease, production and performance losses. Parasitism can rarely be controlled in grazing horses by management alone and anthelmintic drugs have formed the basis of therapy and prophylaxis for the last sixty years. The pharmacology of the anthelmintic drugs available dictate their spectrum of activity and degree of efficacy, their optimal routes of administration and characteristics which prevent some routes of administration, their safety tolerance and potential toxicities and as a consequence of their persistence in the body at effective concentrations their use in epidemiological control programmes. Their use has also resulted in the selection of parasites with genetically controlled characteristics which reduce their susceptibility to treatment, characteristics which are often common to whole chemical classes of anthelmintics. Pharmacological properties also confer compatibility in terms of safety and persistence with other anthelmintic drugs and thus the potential of combinations to treat parasites from different phylogenetic groups such as nematodes, cestodes and trematodes and also the potential by agency of their different molecular mechanisms of action to delay the selection of resistant genes. The major groups of anthelmintics now available, the benzimidazoles (BZD), macrocyclic lactones (MLs) and tetrahydropyrimidines are all highly effective against their targeted parasites (primarily nematodes for BZD's and ML's and cestodes for tetrahydropyrimidines) easily administered orally to horses and are well tolerated with wide margins of safety. Nevertheless, some parasitic stages are inherently less susceptible such as hypobiotic stages of the small strongyles (cyathostomins) and for some such as the adult stages of cyathostomins resistance has developed. Furthermore, for some less common parasites such as the liver fluke unlicensed drugs such as the salicylanilide, closantel have been used. A deep understanding of the pharmacology of anthelmintic drugs is essential to their optimal use in equine species.

Anthelmintic therapy of equine cyathostomin nematodes - larvicidal efficacy, egg reappearance period, and drug resistance

Cyathostomins are ubiquitous in grazing horses across the world, and anthelmintic resistance has been reported with increasing levels over past decades. The aims of the present study were (i) to investigate the efficacy against encysted larval stages of moxidectin (0.4 mg/kg) and fenbendazole (10 mg/kg daily for five consecutive days) and compare these regimens at 2 and 5 weeks post-treatment, (ii) to investigate individual cyathostomin species associated with shortened egg reappearance periods, and (iii) to document species exhibiting decreased susceptibility to the evaluated compounds. Thirty-six ponies were allocated to treatment groups with half euthanatized 2 weeks post-treatment, and the remainder necropsied after 5 weeks. Luminal and mucosal worm counts were conducted and strongyle egg counts were determined at weekly intervals. At 2 weeks, mean reductions of early L3s were 50.4% and 73.8% for fenbendazole and moxidectin, respectively. At 5 weeks, the respective efficacies were 51.3% and 71.8%. Two week efficacies against late L3s and L4s (LL3s/L4s) were 70.8% and 74.6% for fenbendazole and moxidectin, respectively, whereas very low numbers were found in all three groups at 5 weeks. None of the mucosal counts were significantly different between treatment groups. Fenbendazole and moxidectin reduced luminal worm counts by 93.2% and 98.3% at 2 weeks following administration, with moxidectin group adult counts being significantly lower than the other two groups (P < 0.0001). Both treatment groups had increased counts 3 weeks later (P = 0.0415). A moxidectin ERP of 4 weeks was associated with surviving luminal L4s, and adult species contributing to this were Cyathostomum catinatum, Cylicostephanus longibursatus, Cylicocyclus ashworthi and Cylicocyclus nassatus. This study documented (i) larvicidal efficacy of fenbendazole much lower than historical standards, (ii) survival of luminal immatures (L4) following moxidectin administration, and (iii) new information about cyathostomin species associated with these phenomena.

Strongyle egg reappearance period after moxidectin treatment and its relationship with management factors in UK equine populations

Parasitic nematodes, particularly cyathostomins, are ubiquitous in grazing horses world-wide. Considerable burdens of cyathostomin larvae can encyst in the large intestinal wall. The most recommended treatment against these pathogenic stages is moxidectin. Information is required on how effective moxidectin is against cyathostomin populations in different regions. The objectives here were to determine the efficacy of moxidectin treatment and estimate the strongyle egg reappearance period (ERP) after treatment in several equine populations, to confirm the type of strongyle nematodes present and to identify other (i.e. management) factors associated with shortened ERP. Eight yards were recruited and moxidectin in combination with praziquantel administered to all horses (n=261). Faecal egg count (FEC) analysis was performed at weeks 0, 2, 6, 10 and 12 after treatment to determine efficacy and ERP. The ERP was estimated using two previously published methods. Morphological identification of cultured third stage larvae from the sample population was compared to a Strongylus vulgaris-specific end-point PCR to examine the presence of S. vulgaris in samples before and after treatment. Strongyle egg shedding patterns were also compared to worm management practices at each site. At 2 weeks post-treatment, moxidectin was highly effective (faecal egg count reduction range, 99.9-100%). The strongyle ERP ranged from 6 weeks to >12 weeks depending on the calculation method applied. Only cyathostomin larvae were detected by morphological identification. The results from the coprocultures and PCR showed that S. vulgaris was absent before and after treatment. Analysis revealed that regular faecal removal from pasture was associated with lower average FEC and lower prevalence of egg shedding.

Species composition of larvae cultured after anthelmintic treatment indicates reduced moxidectin susceptibility of immature Cylicocyclus species in horses

For the control of cyathostomins in horses, the macrocyclic lactones (MLs), moxidectin (MOX) and ivermectin (IVM) are the most commonly used anthelmintics. However, reduced activity, observed as shortening of the egg reappearance period (ERP) has been described. Shortening of the ERP may be caused by a decreased susceptibility of immature worms for MLs. Alternatively, immature worms may develop faster into egg producing adults as a result of repeated ML treatments. The species composition of the larval cultures obtained shortly after ML and pyrantel (PYR) treatment can confirm the hypothesis of decreased ML susceptibility, as this is often class-specific, whereas faster development would also occur after treatment with anthelmintics with a different mode of action. From 3 farms with a known history of shortened ERP, 8 horses per farm were selected and divided into 2 groups. The MOX-PYR-MOX group was treated twice with MOX (day 0 and 126) and once with PYR (day 84) and the IVM-PYR-IVM group was treated twice with IVM (day 0 and 98) and once with PYR (day 56). Cultured infective larvae (L3s) were counted and differentiated with the reverse line blot on pooled samples. Per cyathostomin species, the number of larvae per gram was calculated. The efficacy of all ML treatments was 100% and a shortened ERP was found on all 3 farms. The species composition of the larval cultures after ML treatment did not differ significantly from that after PYR treatment in the IVM-PYR-IVM group, but it did differ in the MOX-PYR-MOX group. The larval cultures obtained after MOX treatment consisted mostly of Cylicocyclus nassatus, while after PYR treatment Cylicostephanus longibursatus was the most abundant species. In the cultures from 42days after MOX treatment 6 cyathostomin species from 3 genera were found on the farm with the lowest activity (farm 1), while on the farm with the highest activity (farm 3) only 3 species from one genus were found in the same number of examined L3s. The high numbers of L3s of Cylicocyclus species 42days after MOX treatment and the low numbers 42days after PYR treatment can be explained by reduced susceptibility of the immature worms to MOX, but not by a faster development. In conclusion, shortening of the ERP following MOX treatment is most likely a process in which an increasing number of immature worms from an increasing number of species is becoming less susceptible to the active compound.

Development of the larval migration inhibition test for comparative analysis of ivermectin sensitivity in cyathostomin populations

Cyathostomins are the most prevalent parasitic pathogens of equids worldwide. These nematodes have been controlled using broad-spectrum anthelmintics; however, cyathostomin resistance to each anthelmintic class has been reported and populations insensitive to more than one class are relatively commonplace. The faecal egg count reduction test (FECRT) is considered the most suitable method for screening anthelmintic sensitivity in horses, but is subject to variation and is relatively time-consuming to perform. Here, we describe a larval migration inhibition test (LMIT) to assess ivermectin (IVM) sensitivity in cyathostomin populations. This test measures the paralysing effect of IVM on the ability of third stage larvae (L3) to migrate through a pore mesh. When L3 from a single faecal sample were examined on multiple occasions, variation in migration was observed: this was associated with the length of time that the L3 had been stored before testing but the association was not significant. Half maximal effective concentration (EC50) values were then obtained for cyathostomin L3 from six populations of horses or donkeys that showed varying sensitivity to IVM in previous FECRTs. Larvae from populations indicated as IVM resistant by FECRT displayed significantly higher EC50 values in the LMIT than L3 from populations classified as IVM sensitive or L3 from populations that had not been previously exposed to IVM or had limited prior exposure. The analysis also showed that EC50 values obtained using L3 from animals in which IVM faecal egg count reduction (FECR) levels had been recorded as <95% were significantly higher than EC50 values obtained using L3 from animals for which FECR was measured as >95%. For one of the populations, time that had elapsed since IVM administration had an effect on the EC50 value obtained, with a longer time since treatment associated with lower EC50 values. These results indicate that the LMIT has value in discriminating IVM sensitivity amongst cyathostomin populations, but several factors were identified that need to be taken into account when executing the test and interpreting the derived data.

Comparison of a single dose of moxidectin and a five-day course of fenbendazole to reduce and suppress cyathostomin fecal egg counts in a herd of embryo transfer-recipient mares

OBJECTIVE

To compare larvicidal regimens of fenbendazole and moxidectin for reduction and suppression of cyathostomin fecal egg counts (FEC) in a transient herd of embryo transfer-recipient mares.

DESIGN

Randomized, complete block, clinical trial.

ANIMALS

120 mares from 21 states, residing on 1 farm.

PROCEDURES

An initial fecal sample was collected from each mare; mares with an FEC ≥ 200 eggs/g were assigned to treatment groups. Eighty-two horses received fenbendazole (10.0 mg/kg [4.5 mg/lb], PO, q 24 h for 5 days) or moxidectin (0.4 mg/kg [0.18 mg/lb], PO, once); FEC data were analyzed 14, 45, and 90 days after treatment.

RESULTS

Mean FEC reduction was 99.9% for moxidectin-treated mares and 41.9% for fenbendazole-treated mares 14 days after treatment. By 45 days, mean FEC of fenbendazole-treated mares exceeded pretreatment counts; however, FECs of moxidectin-treated mares remained suppressed below pretreatment values for the duration of the 90-day study. Fecal egg counts were significantly different between groups at 14, 45, and 90 days after treatment.

CONCLUSIONS AND CLINICAL RELEVANCE

Failure of the 5-day regimen of fenbendazole to adequately reduce or suppress FEC suggested inadequate adulticidal and larvicidal effects. In contrast, a single dose of moxidectin effectively reduced and suppressed FEC for an extended period. Given the diverse geographic origins of study mares, these results are likely representative of cyathostomin-infected mares in much of the United States, confirming previous findings indicating that fenbendazole resistance in cyathostomins is widespread and that moxidectin remains an effective treatment for control of these important parasites.

Characterisation of the inflammatory cytokine response to anthelmintic treatment in ponies

REASONS FOR PERFORMING STUDY

Anthelmintic treatments have been associated with local inflammatory reactions. Since each class of anthelmintic has unique mechanisms of action affecting different subpopulations of parasites, we hypothesised that they will also induce characteristic proinflammatory responses.

OBJECTIVES

To determine the effect of anthelmintic class on the proinflammatory response post treatment.

STUDY DESIGN

Ponies naturally infected with cyathostomins and other parasites after pasture grazing were left untreated or treated with representatives of 3 different classes of anthelmintics: fenbendazole (benzimidazole); pyrantel tartrate (pyrimidine); and moxidectin (macrocyclic lactone). All were monitored for the expression of proinflammatory genes in the peripheral blood using real-time PCR.

METHODS

The ponies were divided into 4 treatment groups: Group 1 (n = 4) were untreated controls; Group 2 (n = 5) received 5 daily doses of fenbendazole (10 mg/kg bwt); Group 3 (n = 4) received daily treatment of pyrantel tartrate 2× (2.65 mg/kg bwt); and Group 4 (n = 5) received a single dose of moxidectin (400 μg/kg bwt). Blood samples were collected daily for 2 weeks to determine the effect of deworming on proinflammatory gene expression. Faecal egg counts were used to evaluate the efficacy of each drug.

RESULTS

While treatment with the benzimidazole significantly reduced egg counts up to 14 days post treatment, it also stimulated proinflammatory gene expression. Treatment with pyrantel salt also reduced faecal egg counts with less of a proinflammatory response. Treatment with the macrocyclic lactone was the most successful in reducing faecal egg counts and produced no signs of increased proinflammatory cytokine expression.

CONCLUSIONS

This study revealed pronounced differences in the cytokine responses to anthelmintic treatment. This inflammatory reaction may play a role in the development of parasitic disease post anthelmintic treatment.

Shortened strongyle-type egg reappearance periods in naturally infected horses treated with moxidectin and failure of a larvicidal dose of fenbendazole to reduce fecal egg counts

Deworming horses with anthelmintics that have activity against encysted small strongyle larvae (L(3) and L(4)) is a common practice in parasite control programs. The two drugs currently available for this use are moxidectin (MOX) administered in a single dose of 0.4 mg/kg and fenbendazole (FBZ) given at the larvicidal dose (10mg/kg for 5 days). Here, we report the efficacy of MOX and the larvicidal dose of FBZ for reducing counts of strongyle-type eggs per gram of feces in naturally infected horses. Fecal egg counts (FECs) of 15 yearlings were observed following deworming. On day 0, 6 of the 15 yearlings were administered a larvicidal dose of FBZ; 14 days later, all 15 yearlings received MOX at a single dose of 0.4 mg/kg. Feces were collected on day 0 for pre-treatment egg counts. Feces were collected at weekly intervals thereafter during FEC observation periods. FECs of FBZ-treated horses were compared at day 0 and 14 days post-treatment. The difference in means pre- and post-treatment with FBZ was not statistically significant (p=0.65). On days 0 and 42 of the MOX treatment observation period the mean FEC of the yearlings that had not received the FBZ treatment did not differ significantly from that of the FBZ-treated yearlings. MOX was effective in reducing fecal egg counts to 0 EPG for 21 days. At day 35 all but 2 of the yearlings had some eggs present (range=4-361 EPG) and at day 42 all but 1 yearling had eggs present (range=3-432 EPG). At day 42 the group mean FEC reduction had fallen from 100% to 67%. Results of this study do not support the use of the larvicidal dose of FBZ for small strongyle control. Larger field studies will be needed to investigate whether egg reappearance periods are shortening for MOX-treated horses.

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 multicenter evaluation of the effectiveness of Quest Gel (2% moxidectin) against parasites infecting equids

Controlled trials with a common protocol were conducted in Idaho, Illinois and Tennessee to evaluate anthelmintic effectiveness of Quest Gel (QG; 2% moxidectin) against lumenal parasites in horses. Candidate horses were required to have naturally acquired nematode infections, as confirmed by presence of strongylid eggs in feces. At each site, 24 equids were blocked on the basis of pretreatment strongyle fecal egg counts (EPG) and randomly assigned to treatments within blocks. Within each block of two animals, one received QG on Day 0 at a dosage of 0.4 mg moxidectin/kg b.w. and one was an untreated control. Body weights measured the day before treatment served as the basis for calculating treatment doses. Horses assigned to treatment with QG received the prescribed dose administered orally with the commercially packaged Sure Dial syringe. Horses were necropsied 12-14 days after treatment, and lumenal parasites and digesta were harvested separately from each of five organs, including the stomach, small intestine, cecum, ventral colon and dorsal colon. Parasites from stomachs and small intestines were identified to genus, species and stage. Micro- (i.e., < 1.5 cm) and macroparasites (i.e., > 1.5 cm) in aliquots from the cecum, ventral colon and dorsal colon were examined in aliquots of approximately 200 parasites until at least 600 parasites had been identified to genus, species and stage or until all parasites in the 5% aliquot were examined, whichever occurred first. Data were combined across sites and analyzed by mixed model analysis of variance to assess the fixed effect of treatment and random effects of site and block within site. Because QG does not contain a cestocide, efficacy of QG against tapeworms was not significant (P > 0.05). Based on geometric means, however, efficacy of QG was greater than 90% (P < 0.05) against 38 species and developmental stages of cyathostomes, strongyles, bots, larval pinworms and ascarids encountered in at least 6 of 36 control horses in the combined data set. None of the horses treated with moxidectin exhibited evidence of adverse effects. Study results demonstrate QG, administered to horses with naturally acquired endoparasite infections at a dosage of 0.4 mg moxidectin/kg b.w., was highly effective against a broad range of equine parasitic infections.

Advances in developing molecular-diagnostic tools for strongyloid nematodes of equids: fundamental and applied implications

Infections of equids with parasitic nematodes of the order Strongylida (subfamilies Strongylinae and Cyathostominae) are of major veterinary importance. In last decades, the widespread use of drugs against these parasites has led to problems of resistance within the Cyathostominae, and to an increase in their prevalence and intensity of infection. Novel control strategies, based on improved knowledge of parasite biology and epidemiology, have thus become important. However, there are substantial limitations in the understanding of fundamental biological and systematic aspects of these parasites, which have been due largely to limitations in their specific identification and diagnosis using traditional, morphological approaches. Recently, there has been progress in the development of DNA-based approaches for the specific identification of strongyloids of equids for systematic studies and disease diagnosis. The present article briefly reviews information on the classification, biology, pathogenesis, epidemiology of equine strongyloids and the diagnosis of infections, highlights knowledge gaps in these areas, describes recent advances in the use of molecular techniques for the genetic characterisation, specific identification and differentiation of strongyloids of equids as a basis for fundamental investigations of the systematics, population biology and ecology.

Plasma profiles of ivermectin in horses following oral or intramuscular administration

A study was undertaken in order to evaluate and compare ivermectin's (IVM) plasma disposition kinetic parameters after oral or intramuscular (IM) administration in horses. Ten clinically healthy adult horses, weighing 380-496 kg body weight (BW), were allocated to two experimental groups of five horses. Group I, was treated with an oral paste formulation of IVM at the manufacturer's recommended dose of 0.2 mg/kg BW. Group II, was treated IM with an injectable 1% formulation of IVM at a dose of 0.2 mg/kg BW. Blood samples were collected by jugular puncture at different times between 0.5 h and 75 days post-treatment. After plasma extraction and derivatization, samples were analysed by high-performance liquid chromatography with fluorescence detection. A computerized kinetic analysis was performed, and data were compared using the Wilcoxon signed rank test. The parent molecule was detected in plasma between 30 min and either 20 (oral) or 40 (IM) days post-treatment. Significant differences were found for the time corresponding to peak plasma concentrations (tmax) and for absorption half-life. Peak plasma concentrations (Cmax) of 51.3 +/- 16.1 ng/ml (mean +/- SD) were obtained after oral administration and of 31.4 +/- 6.0 ng/ml for the IM route. The values for area under concentration-time curve were 137.1 +/- 35.9 ng day/ml for the group treated orally, and 303.2 +/- 4.3 ng day/ml for the IM treated group. The mean plasma residence times were 4.2 +/- 0.4 and 8.9 +/- 0.7 days for oral and IM-treated groups, respectively. The results of this study show that the route of administration considerably affects the disposition of IVM. A significant difference in bioavailabilty and half-life of elimination of IVM was observed after IM administration compared with oral administration. A close relationship between pharmacokinetic profiles and the clinical efficacy of IVM was established.

Impact of management interventions on helminth levels, and body and blood measurements in working donkeys in South Africa

The aim of the study was to determine the effect of alternative management interventions on levels of nematodes and the condition of working donkeys in South Africa. Twenty-four adult donkeys (Equus asinus) within an area of 200km radius were randomly allocated to eight paddocks. Two replicates each of three management interventions together with a control group were tested in a 16-month study. The interventions included monthly removal of feces from paddocks where the donkeys grazed, a pre-winter moxidectin treatment, and a combination of a pre-winter moxidectin treatment and monthly fecal removal. The influence of the different interventions on the nematode fecal egg counts, animal live weights, body condition scores and general blood chemistry were compared. In addition, herbage samples were collected from the pastures in each paddock to determine the number of third-stage larvae (L(3)) per kg dry matter. At the end of the study worm recoveries and counts were performed on eight of the animals following euthanasia. The cyathostomes represented the largest portion of the helminth species composition in both the fecal egg counts and larval cultures. Monthly fecal removal alone did not significantly reduce the L(3) on pasture and consideration of more frequent removal is discussed. Pre-winter moxidectin treatment resulted in a 100% reduction in fecal egg counts, an average egg reappearance period of 42-55 days, a reduced average egg count for up to 8 months, and reduced total helminth burdens in all the treated donkeys. It also resulted in improved live weights, hemoglobin concentration, packed cell volumes and to some extent body condition score of the donkeys.

Cyathostome fecal egg count trends in horses treated with moxidectin, ivermectin or fenbendazole

Commercial preparations of fenbendazole (Safe-Guard, Intervet), ivermectin (Eqvalan, Merial) or moxidectin (Quest, Fort Dodge) were administered once to horses scheduled for routine parasiticide treatment. In total, 93 horses from six cooperating farms were used in the study. Computer generated, random allocation of horses to treatment group was conducted at each farm. Fecal egg counts were determined for all horses on trial days 0, 56, 84 and 112, with corresponding calendar dates that were unique to each farm. Only strongyle egg counts from animals which were positive at day 0 were used for analysis of variance and comparisons. Counts for the three treatment groups were similar at day 0, moxidectin<ivermectin<fenbendazole for days 56 and 84, and moxidectin<ivermectin=fenbendazole on day 112 (P<0.05). Reductions of geometric mean egg counts from day 0 levels were 99.1, 97.6 and 94.9% for moxidectin, 16.4, -27.0 and -32.0% for fenbendazole and 85.9, 24.2 and -8.1% for ivermectin on trial days 56, 84 and 112, respectively. Adverse reactions to treatment were not observed for any of the parasiticides.

Comparative efficacy, persistent effect, and treatment intervals of anthelmintic pastes in naturally infected horses

Eighty horses were involved in a comparative, controlled, and randomised field study conducted in Australia and Brazil. This study was undertaken to address the duration of efficacy (by faecal egg count reduction) of four anthelmintic pastes and to measure the time required between treatments on horses naturally infected by gastrointestinal nematodes. The treatment interval was based on the egg reappearance period (ERP), defined as "the period after treatment when horses have reached a positive egg count equal or superior to 200 eggs per gram (epg) of faeces". Horses were ranked according to pre-treatment faecal egg counts and randomly allocated on Day 0 to one of the four treatment groups (n=16). Group A received a combination of ivermectin at 200 microg/kg and praziquantel at 1.5mg/kg, Group B received an ivermectin paste at 200 microg/kg, Group C received a reference product containing ivermectin at 200 microg/kg, Group D received a moxidectin paste at 400 microg/kg, and Group E received a placebo. Horses were individually faecal sampled at weekly interval from Days 0 to 70 after treatment and coprocultures were made on pooled samples at the pre-treatment time on D-7 in Brazil and D-6 in Australia. The nematode population was mainly composed of small strongyles (Cyathostominae, Gyalocephalus spp., Triodontophorus spp.). All products were efficient (>90% efficacy) until Day 42 with no statistical difference between groups. From Day 49 onwards, Group C reached the threshold, while Group B exceeded this threshold on Day 56. Groups A and D remained below 200 epg for the entire study period (70 days). The interval between two anthelmintic treatments can vary according to the threshold. The ERP was defined as the period after treatment while the output of eggs is negligible or considered as acceptable. The mean number of days calculated to recurrence of 200 epg and more was, respectively, 60 days for product A, 56 days for products B and C, and 64 days for product D. If treatments are combined with other methods of limiting exposure to infective larvae on pasture, the number of treatments required will be reduced even further.

Efficacy of moxidectin 2 per cent gel against naturally acquired strongyle infections in horses, with particular reference to larval cyathostomes

The efficacy of moxidectin 2 per cent equine gel against naturally acquired strongyle infections was assessed in 18 ponies which had grazed on contaminated pasture before being housed for eight weeks. Twenty-four hours before the treatment, two randomly selected ponies were euthanased and their worm burdens were determined. Eight of the remaining 16 ponies were treated with moxidectin 2 per cent gel while the other eight were given a placebo gel. Eight weeks later the 16 animals were necropsied and their worm burdens established. A 100 per cent efficacy was recorded against adult and lumenal L4 cyathostomes and adult Strongylus and Triodontophorus species. Digest recoveries of larval cyathostomes indicated a 90.8 per cent (P<0.002) reduction in early L3 and a 99.9 per cent (P<0.001) reduction in developing stages. There was a reduction in faecal egg output of between 96 and 100 per cent in the treated animals compared with the controls.

Treatment and control of gastrointestinal parasites

Routine anthelmintic treatments are one of the most important components of an equine wellness program used by horse owners and veterinarians today. Thirteen different compounds are available in the United States in the treatment of gastrointestinal parasites, most of which are available over the counter. As a result, there is a decreased reliance on the veterinarian to perform routine tube dewormings. Therefore, the future of the veterinarian's role in the management of gastrointestinal parasites is likely to be in the consultation and design of parasite control programs. With this in mind, this article covers all of the equine anthelmintics and their clinical applications.

Efficacy of a 2 per cent moxidectin gel against gastrointestinal parasites of ponies

The efficacy of moxidectin was evaluated in young ponies naturally infected with gastrointestinal parasites. Eight animals were treated orally with moxidectin at 0.4 mg/kg bodyweight and eight received only the vehicle. They were all necropsied two weeks later. Faecal samples were examined daily for egg counts and larval cultures. Parasites were recovered from total faecal samples collected daily and from the gastrointestinal tracts at necropsy. Moxidectin reduced the strongyle egg counts by > 99 per cent from three days after treatment but some individuals remained positive for 10 days. The drug had little or no ovicidal activity. As evaluated in the critical-controlled test, moxidectin was 99 to 100 per cent effective (P < 0.05) against luminal stages of parascaris equorum, Strongylus vulgaris, Triodontophorus species, Craterostomum acuticaudatum, 19 cyathostome species and Oxyuris equi. Adult S edentatus were also completely removed. Its efficacy against third stage larvae of Gasterophilus intestinalis was 95 per cent (P < 0.05). Luminal nematode stages were removed within a few days, and bots continued to be eliminated for at least two weeks after treatment. No activity was observed against Anoplocephala perforliata. As evaluated in the controlled test, moxidectin was 100 per cent effective against Habronema muscae (P < 0.05) and had a 76 per cent but not significant efficacy against encysted small strongyle larvae.