Topic:Anthelmintic Resistance
Anthelmintic resistance in horses refers to the reduced effectiveness of deworming agents, known as anthelmintics, in controlling parasitic worm populations. This phenomenon arises when parasites develop the ability to survive treatments that previously were effective at standard doses. Anthelmintic resistance is a growing concern in equine management, as it can lead to increased parasite burdens and associated health issues in horses. Common parasites affected by resistance include small strongyles (cyathostomins) and ascarids (Parascaris spp.). This page compiles peer-reviewed research studies and scholarly articles that explore the mechanisms, prevalence, and management strategies related to anthelmintic resistance in equine populations.
Cylicocyclus species predominate during shortened egg reappearance period in horses after treatment with ivermectin and moxidectin. The normal time interval between treatment of horses with a macrocyclic lactone (ML) and reappearance of strongyle eggs in the feces, or 'Egg Reappearance Period (ERP)', is at its shortest 8 weeks for ivermectin (IVM) and 12 weeks for moxidectin (MOX). Nowadays, it is not uncommon to find shorter ERPs, potentially indicating the beginning of the development of drug resistance. Whether all cyathostomin species contribute equally to a shortened ERP is not known. In the present study a Reverse Line Blot (RLB) on individual infective larvae was used to compare species composition before and after ...
Strongyle infections and parasitic control strategies in German horses – a risk assessment. As a consequence of the increasing levels of anthelmintic resistance in cyathostomes, new strategies for equine parasite control are being implemented. To assess the potential risks of these, the occurrence of strongyles was evaluated in a group of 1887 horses. The distribution of fecal egg counts (FECs), the frequency of anthelmintic drug use, and the deworming intervals were also analyzed. Between June 2012 and May 2013, 1887 fecal samples from either selectively or strategically dewormed horses were collected at 195 horse farms all over Germany and analyzed quantitatively with a modified Mc...
Anthelmintic resistance in equine nematodes. Anthelmintics have been applied indiscriminately to control horse nematodes for over 40 years. Three broad-spectrum anthelmintic classes are currently registered for nematode control in horses: benzimidazoles (fenbendazole, oxibendazole), tetrahydropyrimidines (pyrantel) and macrocyclic lactones (ivermectin, moxidectin). Generally, control strategies have focused on nematode egg suppression regimens that involve the frequent application of anthelmintics to all horses at intervals based on strongyle egg reappearance periods after treatment. The widespread use of such programmes has substantial...
Parascaris univalens–a victim of large-scale misidentification? The equine ascarid parasite Parascaris equorum is well known as a ubiquitous parasite infecting foals. A sibling species, Parascaris univalens, was first described over 130 years ago, but very little attention has been given to its existence and possible implications for anthelmintic resistance, clinical disease, or host age spectrum. P. univalens only possesses one germ line chromosome pair as opposed to two for P. equorum, but the two species are otherwise considered morphologically identical. For the present study, live worms obtained from the University of Kentucky parasitology horse herd ...
The efficacy of ivermectin, pyrantel and fenbendazole against Parascaris equorum infection in foals on farms in Australia. This study was performed to estimate the prevalence of patent Parascaris equorum infections and determine the efficacy of ivermectin, pyrantel and fenbendazole against P. equorum infection in foals on farms in southern Australia. Foals aged >3 months on five farms in the south-western slopes region of New South Wales were used. Faeces were collected from each foal and foals with a P. equorum faecal egg count (FEC) of >100 eggs per gram (EPG) were used to measure anthelmintic efficacy using the FEC reduction (FECR) test, after random allocation to a control group or an ivermectin, pyrante...
Deep amplicon sequencing of preselected isolates of Parascaris equorum in β-tubulin codons associated with benzimidazole resistance in other nematodes. The development of anthelmintic resistance (AR) to macrocyclic lactones in the equine roundworm Parascaris equorum has resulted in benzimidazoles now being the most widely used substance to control Parascaris infections. However, over-reliance on one drug class is a risk factor for the development of AR. Consequently, benzimidazole resistance is widespread in several veterinary parasites, where it is associated with single nucleotide polymorphisms (SNPs) in drug targets encoded by the β-tubulin genes. The importance of these SNPs varies between different parasitic nematodes, but it has been h...
Anthelmintic resistance of intestinal nematodes to ivermectin and pyrantel in Estonian horses. There is evidence of resistance in horses to anthelmintic treatment using ivermectin and pyrantel. However, little information is available about the parasites, treatment practices or anthelmintic resistance in the horse population in Estonia. In the present study, we examined 41 trotting and riding horses aged < 3 years from four stables in Estonia. Faecal samples were collected, and horses were selected for treatment if the nematode egg count per gram faeces exceeded 200. Horses (n= 32) that shed strongyle-type eggs were treated with pyrantel, whereas Parascaris equorum-positive animals...
Anthelmintic efficacy on UK Thoroughbred stud farms. Anthelmintic drugs have been applied indiscriminately to control horse nematodes for over 40 years. We undertook a comprehensive study to investigate efficacy of the four available broad-spectrum anthelmintic drugs on 16 Thoroughbred stud farms using the faecal egg count reduction test. Efficacy against strongyles was determined by calculating the percentage of reduction in faecal egg count between the group mean at Day 0 and Days 14-17 post-treatment and the 95% lower confidence intervals estimated by non-parametric bootstrapping. Individual strongyle faecal egg count reduction tests (n=429) ...
Selective therapy in equine parasite control–application and limitations. Since the 1960s equine parasite control has relied heavily on frequent anthelmintic treatments often applied with frequent intervals year-round. However, increasing levels of anthelmintic resistance in cyathostomins and Parascaris equorum are now forcing the equine industry to change to a more surveillance-based treatment approach to facilitate a reduction in treatment intensity. The principle of selective therapy has been implemented with success in small ruminant parasite control, and has also found use in horse populations. Typically, egg counts are performed from all individuals in the pop...
[Helminth control in the adult horse: the need for a re-orientation]. The epidemiological situation of strongyle infections in adult horses in Switzerland is characterized by a strong dominance of small strongyles (Cyathostominae) and an overall low level of egg shedding in the faeces. The prevailing attitude towards anthelmintic therapy considers neither husbandry conditions nor pasture hygiene measures. Instead, calendar-based routine medication, comprising usually 3 to 4 annual treatments, is the typical strategy. Such an approach, however, often results in an excessive administration of anthelmintics. With respect to the continuous spread of drug resistant c...
The effectiveness of faecal removal methods of pasture management to control the cyathostomin burden of donkeys. The level of anthelmintic resistance within some cyathostomin parasite populations has increased to the level where sole reliance on anthelmintic-based control protocols is not possible. Management-based nematode control methods, including removal of faeces from pasture, are widely recommended for use in association with a reduction in anthelmintic use to reduce selection pressure for drug resistance; however, very little work has been performed to quantitatively assess the effectiveness of such methods. Methods: We analysed data obtained from 345 donkeys at The Donkey Sanctuary (Devon, UK), m...
Anthelmintic resistance in important parasites of horses: does it really matter? Parascaris equorum and cyathostomins are currently considered the most important parasites of horses and have traditionally been controlled with anthelmintics belonging to three drug classes: benzimidazoles, the tetrahydropyrimidine pyrantel, and macrocyclic lactones. Unfortunately, resistance to benzimidazoles, and to a lesser extent pyrantel, is widespread in cyathostomins around the world. Furthermore, resistance to macrocyclic lactones appears to be in the early stages of development in cyathostomins in multiple locations. In contrast, P. equorum populations have remained susceptible to th...
Oxyuris equi: lack of efficacy in treatment with macrocyclic lactones. Whilst anthelminthic resistance of small strongyles is well documented, anthelmintic failures against infections with Oxyuris equi have scarcely been published so far. We describe two cases of equine oxyurosis and the anthelminthic failure of macrocyclic lactones (moxidectin, ivermectin) resulting in persistent O. equi infections with continuous egg shedding. The horses were kept in two different herds in the federal state of Hessia, Germany. Herd A kept two geldings: an 8-year-old Welsh-Cob-Mix and a 7-year-old Haflinger. Herd B was composed of four animals: 2 Connemara-mares, 31 and 19 years...
The management of anthelmintic resistance in grazing ruminants in Australasia–strategies and experiences. In many countries the presence of anthelmintic resistance in nematodes of small ruminants, and in some cases also in those infecting cattle and horses, has become the status quo rather than the exception. It is clear that consideration of anthelmintic resistance, and its management, should be an integral component of anthelmintic use regardless of country or host species. Many years of research into understanding the development and management of anthelmintic resistance in nematodes of small ruminants has resulted in an array of strategies for minimising selection for resistance and for dealin...
Anthelmintic resistance in equine parasites–current evidence and knowledge gaps. Anthelmintic resistance is becoming increasingly prevalent among equine nematode parasites. The first reports documenting resistance were published in the 1960s, just a short time after introduction of the first modern anthelmintics phenothiazine and thiabendazole. Several factors are known to influence development of resistance, but evidence specific to equine parasites is limited. Most current knowledge and applications have been extrapolated from research with trichostrongylid parasites of sheep. The number of cyathostomin species co-infecting horses adds to the complexity of investigating ...
Faecal worm egg count analysis for targeting anthelmintic treatment in horses: points to consider. Equine gastrointestinal nematodes are ubiquitous; in horses that graze contaminated pasture and that are not treated appropriately, large numbers of worms can accumulate, which can lead to serious clinical disease. Nematode control has traditionally followed interval treatment regimens, which involve regular anthelmintic administration to all horses based on the strongyle egg reappearance periods of each drug, usually defined around the time of licensing. Interval treatment programmes have resulted in substantial reductions in large strongyle disease, but have made major contributions to the d...
Equine parasite control under prescription-only conditions in Denmark–awareness, knowledge, perception, and strategies applied. Due to widespread development of anthelmintic resistance in equine parasites, recommendations for their control are currently undergoing marked changes with a shift of emphasis toward more coprological surveillance and reduced treatment intensity. Denmark was the first nation to introduce prescription-only restrictions of anthelmintic drugs in 1999, but other European countries have implemented similar legislations over recent years. A questionnaire survey was performed in 2008 among Danish horse owners to provide a current status of practices and perceptions with relation to parasite control....
Sub-optimal efficacy of ivermectin against Parascaris equorum in foals on three Thoroughbred stud farms in the Manawatu region of New Zealand. To ascertain whether resistance was present in Parascaris equorum to the macrocyclic lactone anthelmintic ivermectin, using faecal egg count reduction tests. Methods: Thirty-nine foals aged between 11 and 28 weeks on three Thoroughbred stud farms (Farms A; n=20, B; n=5 and C; n=14) were treated with ivermectin (Day 0) and faecal egg counts (FEC) were monitored before and for 21 (Farms A and B) or 14 (Farm C) days after treatment. On Farms A and B, the foals were treated with a macrocyclic lactone/benzimidazole/praziquantel combination on Day 21 and FEC assessed on Day 35. The three farms were ...
Transcriptome analysis of a parasitic clade V nematode: comparative analysis of potential molecular anthelmintic targets in Cylicostephanus goldi. Clade V nematodes comprise several parasitic species that include the cyathostomins, primary helminth pathogens of horses. Next generation transcriptome datasets are available for eight parasitic clade V nematodes, although no equine parasites are included in this group. Here, we report next generation transcriptome sequencing analysis for the common cyathostomin species, Cylicostephanus goldi. A cDNA library was generated from RNA extracted from 17 C. goldi male and female adult parasites. Following sequencing using a 454 GS FLX pyrosequencer, a total of 475,215 sequencing reads were generate...
A questionnaire study of equine gastrointestinal parasite control in Scotland. Anthelmintic resistance in equine gastrointestinal nematodes is a threat to equine health and welfare. Detailed knowledge of anthelmintic use and parasite control methods is a prerequisite to identification of potential risk factors for resistance. Objective: To identify parasite control practices employed by equine owners in Scotland and investigate management factors associated with anthelmintic resistance. Methods: Questionnaire study of equine parasite control in Scotland. Methods: Questionnaires were available electronically, distributed at a conference and mailed to clients. Key areas ex...
A xenodiagnostic method using Musca domestica for the diagnosis of gastric habronemosis and examining the anthelmintic efficacy of moxidectin. Equine habronematidosis has a global distribution and is caused by three spirurid species, Habronema muscae, Habronema microstoma and Draschia megastoma. A case of cutaneous habronematidosis in a stallion in a stable in Dubai, UAE gave occasion to investigate the parasite situation on the farm. Patent H. muscae infections were diagnosed in 18 out of 49 horses in a stable in Dubai, UAE with a xenodiagnostic test using houseflies as indicator host. All horses in the stable were treated with a single dose of moxidectin administered orally as 2% gel in a dosage of 0.4 mg/kg body weight and the eff...
Anthelmintic efficacy against cyathostomins in horses in Southern England. Cyathostomins are considered to be the most important group of helminths to affect equids due to their high prevalence, potential pathogenicity and ability to develop anthelmintic resistance. Their control relies almost exclusively on frequent anthelmintic use. Currently, fenbendazole (FBZ), pyrantel embonate (PYR), ivermectin (IVM) and moxidectin (MOX) are licensed for use in horses in the UK. With no new anthelmintics likely to be licensed in the near future, it is essential that investigations into the efficacy of current anthelmintics in different locations are performed to help inform con...
Association between large strongyle genera in larval cultures–using rare-event poisson regression. Decades of intensive anthelmintic treatment has caused equine large strongyles to become quite rare, while the cyathostomins have developed resistance to several drug classes. The larval culture has been associated with low to moderate negative predictive values for detecting Strongylus vulgaris infection. It is unknown whether detection of other large strongyle species can be statistically associated with presence of S. vulgaris. This remains a statistical challenge because of the rare occurrence of large strongyle species. This study used a modified Poisson regression to analyse a dataset fo...
An investigation of anthelmintic efficacy against strongyles on equine yards in Scotland. Cyathostomins comprise 50 helminth species, considered the most problematic equine endoparasites. Three classes of anthelmintic are currently licensed for their control, namely the benzimidazoles (fenbendazole), tetrahydropyrimidines (pyrantel) and macrocyclic lactones (ivermectin and moxidectin). Anthelmintic resistance in cyathostomins is common. With no new classes expected in the near future, it is essential to determine the efficacy of the available anthelmintics to inform future control programmes. Objective: To determine the efficacy of all 3 anthelmintic classes against strongyles in e...
Hierarchical model for evaluating pyrantel efficacy against strongyle parasites in horses. Anthelmintic resistance is an increasing challenge for the control of equine parasites. The fecal egg count reduction test (FECRT) is the practical gold standard method for evaluating reduction in anthelmintic efficacy, but the interpretation is complicated due to high levels of variability. A hierarchical statistical model was described for analysis of FECRT data from multiple farms to evaluate the role of biological factors in determining the strongyle efficacy of pyrantel pamoate in a study performed in Denmark. The model was then used to describe two notions of farm efficacy, namely condit...
Genetic variants and increased expression of Parascaris equorum P-glycoprotein-11 in populations with decreased ivermectin susceptibility. Macrocyclic lactones (MLs) represent the major drug class for control of parasitic infections in humans and animals. However, recently reports of treatment failures became more frequent. In addition to human and ruminant parasitic nematodes this also is the case for the horse-nematode Parascaris equorum. Nevertheless, to date the molecular basis of ML resistance is still not understood. Unspecific resistance mechanisms involving transporters such as P-glycoproteins (Pgps) are expected to contribute to ML resistance in nematodes. Here, complete sequences of two P. equorum Pgps were cloned and i...