Equine Cyathostomins Research Articles

Evaluation of plant commercial feed additives for equine cyathostomin control

The increasing emergence of anthelmintic-resistant parasitic isolates prompts us to reassess the management of intestinal strongylosis in horses. Additionally, societal demand is shifting toward reducing the use of chemical treatments, aligning with environmentally-friendly practices and the exploration of natural alternatives. In this context, we provide an initial view of the antiparasitic activity and the effect on immune circulating blood cells of three commercialized plant-based feed additives in ponies. Three treatments, based either on mugwort (Artemisia vulgaris), echinacea (Echinacea purpurea) or curcumin (Curcuma longa) were administrated to 18 (six per treatment) Welsh female ponies naturally infected with cyathostomins to mimic their practical use in farming conditions. Another group of six untreated ponies was used as a control. Fecal egg count (FEC), the larval development percentage and the number of red blood cells, lymphocytes, monocytes, neutrophils, eosinophils and basophils were measured the first and the last day of each treatment, and compared with those characterizing the control group. None of the three treatments showed a significant effect on the studied parameters. Moreover, the efficacy of treatments, measured from the FEC reduction compared to the control group, was weak (≤ 38.6 %). Therefore, these results do not support the practical use of these additives in equine farming, even if the determination of Cohen's d values associated with the three treatments revealed some incidences on FEC and blood immune cell counts, as well as on larval development for mugwort.

Preliminary screening of plant essential oils from an oceanic climate zone (NW Spain) for the control of equine cyathostomins

Cyathostomins (Strongylida: Cyathostominae) are gastrointestinal nematodes (GIN) that affect equines in many parts of the world. Due to anthelmintic resistance (AR), it is necessary to search for control alternatives. The objective of the study was to carry out a screening of 26 essential oils (EOs) from plants to determine in vitro their action on cyathostomins. Essential oils were obtained by hydro-distillation from leaves, flowers, fruits, and seeds of the selected species and used against the eggs of cyathostomins by means of the tests of egg hatching and larval motility inhibition. For each EO, different concentrations were prepared (250, 125, 62.5, 31.3, 15.6, 7.8, 3.9, 2.0, 1.0, 0.5 and 0.24 mg/mL). Oxfendazole at 2.5% mixed with closantel 5% was used as positive control, and PBS served as negative control. Cyathostomin eggs were obtained from feces of infected horses and infective larvae were recovered after coproculture. The in vitro test was carried out 36 h after the challenge with the EOs. The highest production of EOs was obtained from leaves of bay, eucalyptus, mallow, and basil, as well as from seeds of black pepper, coriander, cinnamon, anise, and peel of orange and cloves, cumin, and Lawson’s cypress. The oils extracted from laurel leaves, ground cinnamon, anise, cumin, and coriander seeds had a very high effect on cyathostomins eggs up to concentrations lower than 3.9 mg/mL. Therefore, in addition to the uses against other pathogens such as fungi and bacteria, some EOs might attain great importance as an alternative control strategy in the control of nematode eggs.

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%.

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.

Faecal egg counts and nemabiome metabarcoding highlight the genomic complexity of equine cyathostomin communities and provide insight into their dynamics in a Scottish native pony herd

Understanding the composition of gastrointestinal nematode communities may help to mitigate or exploit parasite adaptations within their host. We have used nemabiome deep amplicon sequencing of internal transcribed spacer-2 (ITS-2) ribosomal DNA to describe the temporal and host species composition of gastrointestinal nematode communities following sampling of six Scottish ponies across 57 months. In the absence of parasite control, each horse showed seasonal trends of increases and decreases in faecal egg counts, consistent with the epidemiology of equine strongylid parasites, however, the composition of parasites within individuals changed over time. Sixteen presumptive strongylid species were identified in each of the horses, 13 of which were distributed in a complex clade together with small numbers of amplicon sequences which could not be classified beyond the Cyathostominae subfamily level. Egg shedding of seven trichostrongylid species, which had previously been identified in co-grazed Soay sheep, was identified during the early spring. Faecal egg counts and the percentage of amplicon sequences assigned to each gastrointestinal nematode species were combined to describe their relative abundance across both host and time. Significant differences in species diversity between horses and between months were observed, being greatest from March to May and least from October to December. The magnitude of the individual horse effect varied between months and, conversely, the magnitude of the seasonal effect varied between individual horses. The most abundant gastrointestinal nematode in each of the horses was Cylicostephanus longibursatus (46.6% overall), while the abundance of the other strongylid species varied between horses and relative to each other. Patent C. longibursatus infections over the winter months might represent a genetic adaptation towards longer adult worm survival, or a lower rate of developmental arrest in the autumn. This study provides insight into highly complex phylogenetic relationships between closely related cyathostomin species; and describes the dynamics of egg shedding and pasture contamination of co-infecting equine gastrointestinal nematode communities. The results could be applied to determine how climatic and management factors affect the equilibrium between hosts and their parasites, and to inform the development of sustainable gastrointestinal nematode control strategies for different host species.

Equine strongylids: Ivermectin efficacy and fecal egg shedding patterns.

Equine cyathostomins are ubiquitous in grazing horses around the world and a main target in parasite control programs. Anthelmintic resistance has been reported with increasing frequency in these parasites over the past decades, and recent findings of fulminant resistance to the macrocyclic lactone class have raised severe concerns. This study aimed to evaluate ivermectin efficacy in cohorts of yearlings and mares present on four different farms in Central Kentucky. Strongylid egg counts were determined with an automated egg counting system, and the percent fecal egg count reduction (FECR) was calculated using a hierarchical Bayesian analysis. Novel principles were used for classification of groups to either no signs of anthelmintic resistance, evidence of resistance, or inconclusive. Furthermore, an epidemiological analysis was conducted evaluating the possible influence of pregnancy status, time of foaling, and year of arrival on mare strongylid egg shedding levels. A total of 102 yearlings and 247 mares were enrolled in the study. Evidence of ivermectin resistance was documented in one group of yearlings with a mean FECR of 91.2% and a 95% credible interval of 84.0-95.8. The results from one mare group and one additional yearling group were deemed inconclusive, whereas the remaining five groups displayed no evidence of ivermectin resistance. Strongylid shedding in the mares was not positively associated with any of the evaluated factors. This study is the first to demonstrate ivermectin resistance in US bred horses, and the findings emphasize the need for routine monitoring of anthelmintic efficacy on horse farms.

A Survey of Control Strategies for Equine Small Strongyles in Lithuania.

SummaryAnthelmintic resistance (AR) in equine cyathostomins is being reported all over the world. In Lithuania, however, the last study on this subject was published more than fifteen years ago, thus little is known about the current situation. The aim of this study was to determine the factors that may associated with the development of AR on equine studs in Lithuania. A questionnaire containing seven open-ended and nine closed multiple-choice questions about worm control strategies, use of anthelmintic substances and stable management practices was posted to 71 randomly selected horse establishments in Lithuania. Replies were obtained from a total of 59 stables, representing 83 % of officially established stud farms in Lithuania.The results showed that more than 80 % of these establishments performed pasture management practices such as excrement removal from stables and pasture, 56 % mowed their pasture, 31 % practised mixed or rotational grazing with other species, and 97 % of the horses were routinely dewormed. Macrocyclic lactones (ML) (58 %, n=33) were the most commonly used drugs, followed by benzimidazoles (BZ) (24 %, n=14) and tetrahydropyrimidines (THP) (19 %, n=10). The majority of farms (60 %) treated horses four times per year and 68 % estimated the weight of the horses by eye before treatment. About 36 % of respondents had heard of faecal egg counts (FEC), but only 17 % used the test and as few as 9 % had tested their herds for AR with faecal egg count reduction tests (FECRT).The results demonstrate that there is scope for improving routines for worm control in many horse establishments in Lithuania. In order to increase knowledge and reduce the risk of the spread of AR, diagnostic methods should be adopted in a collaboration between stud farms and veterinary practitioners.

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.

Climate change is likely to increase the development rate of anthelmintic resistance in equine cyathostomins in New Zealand

Climate change is likely to influence livestock production by increasing the prevalence of diseases, including parasites. The traditional practice of controlling nematodes in livestock by the application of anthelmintics is, however, increasingly compromised by the development of resistance to these drugs in parasite populations. This study used a previously developed simulation model of the entire equine cyathostomin lifecycle to investigate the effect a changing climate would have on the development of anthelmintic resistance. Climate data from six General Circulation Models based on four different Representative Concentration Pathways was available for three New Zealand locations. These projections were used to estimate the time resistance will take to develop in the middle (2040–49) and by the end (2090–99) of the century in relation to current (2006–15) conditions under two treatment scenarios of either two or six yearly whole-herd anthelmintic treatments. To facilitate comparison, a scenario without any treatments was included as a baseline. In addition, the size of the infective and parasitic stage nematode population during the third simulation year were estimated. The development of resistance varied between locations, time periods and anthelmintic treatment strategies. In general, the simulations indicated a more rapid development of resistance under future climates coinciding with an increase in the numbers of infective larvae on pasture and encysted parasitic stages. This was especially obvious when climate changes resulted in a longer period suitable for development of free-living parasite stages. A longer period suitable for larval development resulted in an increase in the average size of the parasite population with a larger contribution from eggs passed by resistant worms surviving the anthelmintic treatments. It is projected that climate change will decrease the ability to control livestock parasites by means of anthelmintic treatments and non-drug related strategies will become increasingly important for sustainable parasite control.

Comparative H-gal-GP and H11 specific antibody binding in equine cyathostomins

The biological-based vaccine (Barbervax®) generates effective antibodies against the biologically essential H-gal-GP and H11 protein complex of the ruminant parasite Haemonchus contortus to target and kill the parasites after taking a blood meal. A comparative analysis of several parasite genera was performed to determine if a similar protein complex or one that is recognized by H-gal-GP and H11 specific antibodies was present. If so, it suggests the vaccine could be effective for other nematode parasites. Ancylostoma caninum, H. contortus, equine cyathostomins, bovine Bunostomum phlebotomum, Dracunculus lutrae, Parascaris sp., Ixodes scapularis, Amblyomma americanum, Dirofilaria immitis and Brugia malayi were evaluated for specific antibody binding using hyperimmunized antibodies against H-gal-GP and H11 native proteins. Of the parasites evaluated, specific and reproducible staining was observed in H. contortus and adult and encysted cyathostomins only. To further evaluate the similar reactivities between cyathostomins and H. contortus, cross-reactivity of equine serum with antibodies to cyathostomins on a H. contortus adult histology cross-section was observed using immunofluorescence. These findings pave the way for future studies on the safety and efficacy of H-gal-GP and H11 protein complex as a potential control for cyathostomins.

Cytokine and goblet cell gene expression in equine cyathostomin infection and larvicidal anthelmintic therapy.

The role of the immune response to cyathostomin infections in horses remains unknown. Intestinal goblet cell hyperplasia has previously been noted as a component in cyathostomin infection; however, the function is unclear. The goal of this study was to evaluate the local and systemic gene expression to cyathostomin infections following larvicidal treatment and explore their relation to goblet cells. Thirty-six ponies with naturally acquired cyathostomin infections were randomly allocated into three groups: fenbendazole-treated (10mg/kg PO 5days), moxidectin-treated (0.4mg/kg PO once) and untreated control. Whole blood from all horses was collected weekly, and tissue samples from the large intestine collected during necropsy at 2 and 5weeks post-treatment (WPT). Gene expression of interleukin (IL)-4, IL-5, IL-6, IL-10, IL-13, IL-17A, IL-22, IFN-γ, resistin-like molecule beta (RELM-β), Mucin 2 (MUC2) and tumour necrosis factor (TNF)-α was measured using qRT-PCR. There were statistically significant linear correlations between luminal worm burdens and MUC2 (r=-.2358) and RELM-β (r=-.2261). This suggests an active role of immune system post-treatment in parasite expulsion, specifically in goblet cells, and that the organs respond differently to treatment and the larvae themselves. This may have implications in the disease process and treatment.

Multiple resistance in equine cyathostomins: a case study from military establishments in Rio Grande do Sul, Brazil

Semi-intensive equine breeding system favors gastrointestinal nematode infections. The treatment of these infections is based on the use of anthelmintics. However, the inappropriate use of these drugs has led to parasitic resistance to the available active principles. The objective of this study was to evaluate the efficacy of the main classes of antiparasitic (ATP) used in control in adult and young animals, including: benzimidazoles (fenbendazole), pyrimidines (pyrantel pamoate), macrocyclic lactones (ivermectin and moxidectin), as well as the combination of active ingredients (ivermectin + pyrantel pamoate). The study was carried out in two military establishments, located in Rio Grande do Sul (RS), from January to December, 2018. The intervals between the treatments of the animals were performed from 30 to 90 days. Coproparasitological evaluations were determined by the egg count reduction in the faeces. Cyatostomine larvae were identified in pre and post-treatment cultures. The results demonstrated the multiple parasitic resistance of cyathostomins to fenbendazole, moxidectin in young animals, and to fenbendazole, pyrantel pamoate in adult animals. Thus, it is necessary to define or diagnose parasitic resistance to assist in the creation of prophylactic parasitic control, using suppressive treatment with ATP associated with integrated alternatives. The progress of parasitic resistance can be slowed.

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.

The importance of anthelmintic efficacy monitoring: results of an outreach effort.

Anthelmintic resistance in equine cyathostomin parasites is widespread. A surveillance-based parasite control program using fecal egg counts (FECs) and fecal egg count reduction tests (FECRTs) to decrease anthelmintic use and monitor treatment efficacy is recommended. The purpose of this study was to examine shifts in equine parasite control program management practices via a short course presented by the Penn State Extension, and to highlight how data collected from these programs is useful for monitoring anthelmintic efficacy on a large scale. Horse owners were enrolled after participating in a short course and filled out questionnaire surveys about their parasite management programs pre and post study, horse information, and farm information. FECs were performed at three time points, and horses above a 300 strongyle eggs per gram cut-off were treated with pyrantel pamoate, fenbendazole, or ivermectin. Two weeks post-treatment, FECRTs were performed to determine treatment efficacy, which included 29 farms with 513 individual treatments. Prior to the study, only 30.6% of farms used FECs, but after the study, 97.3% of farms said they would use FECs in the future. Horses were given an average of 4.1 anthelmintic treatments per year before the study, and post study 89.2% of farms were able to reduce the number of anthelmintic treatments used. Fenbendazole was effective on zero farms, pyrantel pamoate on 7.4% of farms, and ivermectin on 92.9% of farms. This outreach project helped generate information about anthelmintic efficacy levels, causing a shift in practices on participating farms, and collected useful anthelmintic resistance data.

Climate factors driving equine cyathostomin species distribution in the United States

Climate factors driving equine cyathostomin species distribution in the United States

The effect of climate, season, and treatment intensity on anthelmintic resistance in cyathostomins: A modelling exercise

Anthelmintic resistance is widespread in equine cyathostomin populations across the world, and with no new anthelmintic drug classes in the pharmaceutical pipeline, the equine industry is forced to abandon traditional parasite control regimens. Current recommendations aim at reducing treatment intensity and identifying high strongylid egg shedders in a targeted treatment approach. But, virtually nothing is known about the effectiveness of these recommendations, nor their applicability to different climatic regions, making it challenging to tailor sustainable recommendations for equine parasite control. This study made use of a computer model of the entire cyathostomin life-cycle to evaluate the influence of climate and seasonality on the development of anthelmintic resistance in cyathostomin parasites. Furthermore, the study evaluated the impact of recommended programs involving selective anthelmintic therapy on delaying anthelmintic resistance development. All simulations evaluated the use of a single anthelmintic (i.e., ivermectin) over the course of 40 model years. The study made use of weather station data representing four different climatic zones: a cold humid continental climate, a temperate oceanic climate, a cold semi-arid climate, and a humid subtropical climate. Initially, the impact of time of the year was evaluated when a single anthelmintic treatment was administered once a year in any of the twelve months. The next simulations evaluated the impact of treatment intensities varying between 2 and 6 treatments per year. And finally, we evaluated treatment schedules consisting of a combination of strategic treatments administered to all horses and additional treatments administered to horses exceeding a predetermined fecal egg count threshold. Month of treatment had a large effect on resistance development in colder climates, but little or no impact in subtropical and tropical climates. Resistance development was affected by treatment intensity, but was also strongly affected by climate. Selective therapy delayed resistance development in all modelled scenarios, but, again, this effect was climate dependent with the largest delays observed in the colder climates. This study is the first to demonstrate the value of cyathostomin parasite refugia in managing anthelmintic resistance, and also that climate and seasonality are important. This modelling exercise has allowed an illustration of concepts believed to play important roles in anthelmintic resistance in equine cyathostomins, but has also identified knowledge gaps and new questions to address in future studies.

A model for the dynamics of the parasitic stages of equine cyathostomins

A model was developed to reproduce the dynamics of the parasitic stages of equine cyathostomins. Based on a detailed review of published literature, a deterministic simulation model was constructed using the escalator boxcar-train approach, which allows for fully-overlapping cohorts of worms and approximately normally distributed variations in age/size classes. Key biological features include a declining establishment of ingested infective stage larvae as horses age. Development rates are constant for all the parasitic stages except the encysted early third stage larvae, for which development rates are variable to reflect the sometimes extended arrestment of this stage. For these, development is slowed in the presence of adult worms in the intestinal lumen, and when ingestion of infective larvae on herbage is high or extended. In the absence of anthelmintic treatments, the life span of adult worms is approximately 12 months, and the presence of an established adult worm burden largely blocks the transition of luminal fourth stage larvae to the adult stage, resulting in mortality of the larvae. This inhibition is removed by effective anthelmintic treatment allowing the rapid replacement of adult worms from the pool of mucosal stages.Within the model, the rate and seasonality at which infective stage larvae are ingested strongly influences the dynamics of the pre-adult stages. While the adult worm burden remains relatively stable within a year, due to the negative feedback they have on developing stages, the numbers and proportions of larval stages relative to the total worm burden increase with the numbers of infective larvae ingested. Further, within the model, the seasonal rise and fall of encysted stages is largely driven by the seasonal pattern of infective larvae on pasture. Because of this, the model reproduces the contrasting seasonal patterns of mucosal larvae, typical of temperate and tropical environments, using only the appropriate seasonality of larvae on pasture. Thus, the model reproduces output typical of different climatic regions and suggests that observed patterns of arrested development may simply reflect the numbers and seasonality of free-living stages on pasture as determined by different management practices and weather patterns.

Anthelmintic resistance and novel control options in equine gastrointestinal nematodes.

Control of equine nematodes has relied on benzimidazoles (BZs), tetrahydropyrimidines and macrocyclic lactones. The intensive use of anthelmintics has led to the development of anthelmintic resistance (AR) in equine cyathostomins and Parascaris equorum. Field studies indicate that BZ and pyrantel resistance is widespread in cyathostomins and there are also increasing reports of resistance to macrocyclic lactones in cyathostomins and P. equorum. The unavailability of reliable laboratory-based techniques for detecting resistance further augments the problem of nematode control in horses. The only reliable test used in horses is the fecal egg count reduction test; therefore, more focus should be given to develop and validate improved methodologies for diagnosing AR at an early stage, as well as determining the mechanisms involved in resistance development. Therefore, equine industry and researchers should devise and implement new strategies for equine worm control, such as the use of bioactive pastures or novel feed additives, and control should increasingly incorporate alternative and evidence-based parasite control strategies to limit the development of AR. This review describes the history and prevalence of AR in equine nematodes, along with recent advances in developing resistance diagnostic tests and worm control strategies in horses, as well as giving some perspective on recent research into novel control strategies.

First report of anthelmintic resistance of equine cyathostomins in Cuba.

Anthelmintic resistance in equine cyathostomins has been described worldwide, with resistance to the benzimidazole class being particularly widespread. The status of anthelmintic efficacy in Cuba has been virtually unknown due to the lack of equine labelled products. One recent report documented suboptimal efficacy levels of extra-label albendazole products against cyathostomins, but it remains unknown to which extent benzimidazole resistance exists in the population. The aim of the present study was to evaluate the anthelmintic efficacy of two benzimidazole products labelled for equines, fenbendazole and oxibendazole. A fecal egg count reduction test (FECRT) was carried out on 132 horses aged 4 months to 18 years in 14 herds, belonging to six provinces. Ten herds exhibited signs of resistance to at least one of the benzimidazoles (mean FECRT<90%). Overall, oxibendazole exhibited higher efficacy than fenbendazole (p = 0.0062), and higher efficacy levels were found in horses never dewormed before compared to those treated within 3-12 months prior to the study (p = 0.0015). Pre-treatment larval cultures revealed the presence of large strongyles and cyathostomin larvae in all herds, while only cyathostomin larvae were detected post treatment. The present work is the first report of anthelmintic resistance in equine cyathostomins in Cuba, and suggests pre-selection for resistant strains by extra-label use of albendazole on the studied farms.

Combination deworming for the control of double-resistant cyathostomin parasites – short and long term consequences

Equine cyathostomin are pervasive gastrointestinal parasites with wide-spread resistance to the benzimidazole and tetrahydropyrimidine drug classes worldwide. Combination deworming has been proposed as a more sustainable parasite control strategy. Simulation studies have found combination deworming to be effective in controlling drug resistant ovine trichostrongylid parasites. One equine study demonstrated an additive effect of a combination of oxibendazole and pyrantel pamoate against cyathostomins. However, this is the only equine study evaluating combination therapy, and the effects of repeated combination treatments administered over time remain unknown. The purpose of this study was to observe the efficacy of repeated oxibendazole/pyrantel pamoate combination therapy administered over one year against a cyathostomin population with resistance to benzimidazole and pyrantel products. Fecal egg counts were determined for the entire herd (N = 21) at the day of anthelmintic treatment and at two-week intervals for eight weeks post treatment. Starting efficacies of oxibendazole (OBZ, 10 mg/kg) and pyrantel pamoate (PYR, 6.6 mg base/kg) were 66.7% and 63.3%, respectively. Hereafter, the herd was treated four times with an oxibendazole/pyrantel pamoate combination, eight weeks apart, followed by repeating the single active treatments before concluding the study. While the first combination treatment exhibited an additive effect of the two active ingredients, this efficacy was not sustained over the course of the study. Mean fecal egg count reduction (FECR) was significantly greater for the first combination treatment (76.6%) than the second (42.6%, p = 0.0454), third (41.6%, p = 0.0318), and fourth (40.7%, p = 0.0372) combination treatments. The final single active mean FECRs were 42.3% for oxibendazole, and 42.7% for pyrantel pamoate. These efficacies were not significantly different from the initial single active efficacies (OBZ, p = 0.4421; PYR, p = 0.8361). These results suggest that combination therapy against double resistant equine cyathostomin populations is not sustainable, when using actives with markedly decreased starting efficacies.