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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 were dissected and identified using karyotyping techniques. With no exception, all specimens (n = 30) were identified to be P. univalens. Further, the karyotyping technique was adapted to ascarid eggs derived from fecal samples and carried out on samples collected from 25 Thoroughbred foals from three farms in Central Kentucky. P. equorum was not identified among these, whereas P. univalens was found in 17 samples, with the remaining being inconclusive. The mitochondrial genome was sequenced, assembled, and annotated from one male worm identified as P. univalens, and comparison with available sequence reads labeled as P. equorum revealed only 0.16 % nucleotide differences. However, it is unlikely that the sequences available in public databases have been unequivocally identified to species level by karyotyping. Taken together, these data suggest that P. univalens is likely the main species now observed in equines and that perhaps the designation Parascaris spp. should be used unless cytological characterization has confirmed the species.

Strongylus vulgaris is regarded as the most pathogenic helminth parasite infecting horses. Migrating larvae cause pronounced endarteritis and thrombosis in the cranial mesenteric artery and adjacent branches, and thromboembolism can lead to ischemia and infarction of large intestinal segments. A recently developed serum ELISA allows detection of S. vulgaris-specific antibodies during the six-month-long prepatent period. A population of horses has been maintained at the University of Kentucky without anthelmintic intervention since 1979, and S. vulgaris has been documented to be highly prevalent. In 2012, 12 foals were born in this population, and were studied during a 12-month period (March-March). Weekly serum samples were collected to monitor S. vulgaris specific antibodies with the ELISA. Nine colts underwent necropsy at different time points between 90 and 300 days of age. At necropsy, Strongylus spp. and Parascaris equorum were identified to species and stage and enumerated. Initial statistical findings indicate a significant interaction between foal age and ELISA results (p

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 population, and those exceeding a predetermined cutoff threshold are treated. Several studies document the applicability of this method in populations of adult horses, where the overall cyathostomin egg shedding can be controlled by only treating about half the horses. However, selective therapy has not been evaluated in foals and young horses, and it remains unknown whether the principle is adequate to also provide control over other important parasites such as tapeworms, ascarids, and large strongyles. One recent study associated selective therapy with increased occurrence of Strongylus vulgaris. Studies are needed to evaluate potential health risks associated with selective therapy, and to assess to which extent development of anthelmintic resistance can be delayed with this approach. The choice of strongyle egg count cutoff value for anthelmintic treatment is currently based more on tradition than science, and a recent publication illustrated that apparently healthy horses with egg counts below 100 eggs per gram (EPG) can harbor cyathostomin burdens in the range of 100,000 luminal worms. It remains unknown whether leaving such horses untreated constitutes a potential threat to equine health. The concept of selective therapy has merit for equine strongyle control, but several questions remain as it has not been fully scientifically evaluated. There is a great need for new and improved methods for diagnosis and surveillance to supplement or replace the fecal egg counts, and equine health parameters need to be included in studies evaluating any parasite control program.

Migrating Strongylus vulgaris and encysted cyathostomin larvae cause a localized inflammatory response in horses. It is unknown whether these larvae elicit a systemic acute phase response (APR), evidenced by changes in serum amyloid A (SAA), haptoglobin (Hp), iron (Fe), albumin, or albumin/globulin (A/G) ratio. In this study, 28 horses were randomly allocated to receive either pyrantel tartrate or a pelleted placebo formulation in their daily feed. Concurrent with treatment, all the horses were administered 5000 pyrantel-susceptible cyathostomin infective larvae once daily, 5 days a week, for 24 weeks. Beginning in the fifth week, the horses also received 25 S. vulgaris larvae once weekly for the remainder of the study. At regular biweekly intervals, fecal samples were collected for quantitative egg counts, and whole blood and serum samples were collected for measurement of packed cell volume, total protein, albumin, globulin, A/G ratio, SAA, Hp, and Fe. On days 161–164, all the horses were euthanatized and necropsied. Samples were collected for enumeration of total luminal worm burdens, encysted cyathostomin larval populations, and migrating S. vulgaris larvae. Concentrations of Hp, Fe, and A/G ratio were associated significantly with strongyle burdens. Only treated male horses had significant increases in serum albumin. Larval S. vulgaris did not associate with Fe, whereas Fe was associated negatively with both total cyathostomin burdens and encysted L4s. The A/G ratios differed significantly between the two treatment groups. Significant differences between groups and individual time points were also observed for Hp and Fe, whereas SAA concentrations remained low throughout the study. In general, this study illustrated that experimental inoculations with S. vulgaris and cyathostomins may be associated with changes in Hp, Fe, and serum proteins, but not with SAA. Overall, these changes suggest that mixed strongyle infections elicit a mild acute phase reaction.

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 drug resistance but, given their apparent limited biological diversity, viewing these in a unispecific context remains a pragmatic approach. Factors affecting resistance development in cyathostomins include parasite seasonality, life span and fecundity, host immunity, and the existence of encysted stages. Further, parasite refugia have been shown to play a vital role in resistance development in other parasites, and likely is also important in equine parasites. Specific genetic factors for drug resistance and possible modes of inheritance have been identified for trichostrongylid nematodes, but it is widely accepted that several more remain undiscovered. Current evidence with equine and ruminant parasites suggests that fitness is not significantly compromised in drug resistant strains. Attempts to develop in vitro and molecular assays for diagnosing anthelmintic resistance in equine nematodes have had only limited success, standardized guidelines are sorely needed for performing the fecal egg count reduction test in horse populations. Taken together, this review illustrates the complexity of understanding anthelmintic resistance in equine nematodes, and emphasizes the need for further research.

BACKGROUND: Strongyle parasites are ubiquitous in grazing horses. Strongylus vulgaris, the most pathogenic of the large strongyles, is known for its extensive migration in the mesenteric arterial system. The lifecycle of S. vulgaris is characterised by a long prepatent period where the migrating larvae are virtually undetectable as there currently is no test available for diagnosing prepatent S. vulgaris infection. Presence of S. vulgaris larvae in the arterial system causes endarteritis and thrombosis with a risk of non-strangulating intestinal infarctions. Emergence of anthelmintic resistance among cyathostomins has led to recommendations of reduced treatment intensity by targeting horses that exceed a predetermined strongyle faecal egg count threshold. One study suggests an apparent increase in prevalence of S. vulgaris on farms where reduced anthelmintic treatment intensity has been implemented. These issues highlight the need for an accurate and reliable assay for diagnosing prepatent S. vulgaris infection. METHODS: Immunoscreening of a larval S. vulgaris cDNA library using hyperimmune serum raised against S. vulgaris excretory/secretory antigens was performed to identify potential diagnostic antigens. Immunoreactive clones were sequenced, one potential antigen was characterised, expressed as a recombinant protein, initially evaluated by western blot (WB) analysis, the diagnostic potential of the IgG subclasses was evaluated by ELISA, and the diagnostic accuracy evaluated using serum from 102 horses with known S. vulgaris infection status. RESULTS: The clone expressing the potential antigen encoded a S. vulgaris SXP/RAL2 homologue. The recombinant protein, rSvSXP, was shown to be a potential diagnostic antigen by WB analysis, and a target of serum IgGa, IgG(T) and total IgG in naturally infected horses, with IgG(T) antibodies being the most reliable indicator of S. vulgaris infection in horses. Evaluation of diagnostic accuracy of the ELISA resulted in a sensitivity of 73.3%, a specificity of 81.0%, a diagnostic odds ratio of 11.69; a positive likelihood ratio (LR) of 3.85 and a negative LR was 0.33. The area under the ROC curve was 0.820. CONCLUSION: IgG(T) antibodies to recombinant SvSXP show potential for use as an antigen for prepatent diagnosis of migrating stages of S. vulgaris with moderate to good diagnostic accuracy. Strongyle parasites are ubiquitous in grazing horses. Strongylus vulgaris, the most pathogenic of the large strongyles, is known for its extensive migration in the mesenteric arterial system. The lifecycle of S. vulgaris is characterised by a long prepatent period where the migrating larvae are virtually undetectable as there currently is no test available for diagnosing prepatent S. vulgaris infection. Presence of S. vulgaris larvae in the arterial system causes endarteritis and thrombosis with a risk of non-strangulating intestinal infarctions. Emergence of anthelmintic resistance among cyathostomins has led to recommendations of reduced treatment intensity by targeting horses that exceed a predetermined strongyle faecal egg count threshold. One study suggests an apparent increase in prevalence of S. vulgaris on farms where reduced anthelmintic treatment intensity has been implemented. These issues highlight the need for an accurate and reliable assay for diagnosing prepatent S. vulgaris infection.

Parasites infecting horses are ubiquitous and clinically important across the world. The major parasitic threats to equine health are cyathostomins, Parascaris equorum, Anoplocephala perfoliata, and Strongylus vulgaris. Increasing levels of anthelmintic resistance reported world wide in equine parasites have led to recommendations of constructing sustainable parasite control programmes based on systematic surveillance of parasite levels. Regulations at the European Union level now make anthelmintics available on prescription-only basis and disallow prophylactic treatment. This emphasizes the needs for reliable and practical diagnostic tools for detection of major parasites infecting equines. The current, widely used coprological techniques are important and useful, but they do have considerable limitations as they are incapable of diagnosing the pathogenic migrating stages. Species-specific molecular assays have been developed for diagnosing patent infections with 21 cyathostomin species, A. perfoliata, and S. vulgaris, but none of these have found use in practice. An antibody-directed enzyme-linked immunosorbent assay (ELISA) has been developed, validated and made commercially available for diagnosing A. perfoliata infection, but interpretation is complicated by the fact that horses not harbouring tapeworms can maintain elevated antibody titres. Recent work with a coproantigen ELISA has shown promise for reliable detection of current A. perfoliata infection. Perhaps most remarkable is the fact that the pathogenic larval stages of cyathostomins and large strongyles cannot be detected by any of the available diagnostics. With the lengthy prepatency periods characterizing these parasites, there is a huge need for developing such assays. The recent identification of a possible diagnostic marker for encysted cyathostomins holds great promise, and could become very useful in clinical practice. Several attempts have been made to construct assays for diagnosing the highly pathogenic migrating larvae of S. vulgaris, but none of these have performed sufficiently to make a useful test. The present review illustrates that classical coprological techniques remain the cornerstone of equine parasitology diagnosis and surveillance, and will remain so in a foreseeable future. However, promising progress has been made for developing assays capable of diagnosing prepatent stages of strongyle infection, and there is reason to hope for validated and useful assays in the relative near future.

The large roundworm of horses, Parascaris equorum is considered ubiquitous in breeding operations, and is regarded as a most important helminth pathogen of foals. Over the past decade, this parasite has been reported increasingly resistant to anthelmintic drugs worldwide. This paper reports analysis of the population genetic structure of P. equorum . Adult parasites ( n = 194) collected from Sweden, Norway, Iceland, Germany, Brazil and the USA were investigated by amplified restriction fragment length polymorphism (AFLP) analysis. The genetic variation was low (Hj = 0.12–0.4), for the global population of worms. This was accompanied by a weak degree of population structure (Fst = 0.2), low gene flow (Nm = 1.0) and low mutation rate (4 Nμ = 0.07). Thus, the low genetic diversity is probably a result of a low mutation rate in DNA, although the gene flow (due to global movement of horses) is large enough to allow the spread of novel mutations. Surprisingly, isolates from Icelandic horses were not found to be different from other isolates, in spite of the fact that these have been isolated for thousands of years. The study indicates that the global P. equorum population is essentially homogenous, and continents do not appear to be strong barriers for the population structure of this species. Consequently, the potential spread of rare anthelmintic resistance genes may be rapid in a homogenous population.

Strongyle and trichostrongyle parasites are ubiquitous nematodes of grazing livestock. Several molecular diagnostic tests are based upon measuring and quantifying DNA obtained from parasite eggs. It is well known that such eggs undergo development during storage, but it remains unknown to which extent developmental stages can affect the variation of diagnostic test results. This study investigated the influence of developmental stages of strongyle eggs on the variation real-time polymerase chain reaction (PCR) results. Mixed species strongyle eggs were obtained from the faeces of a naturally infected horse. Eggs were isolated and placed in microtiter plates with demineralized water. A total of 25 wells containing 100 eggs each were set up and kept refrigerated for up to five days. Once daily, five wells were examined on an inverted microscope at 100× magnification, where the developmental stages of the eggs were noted, and then eggs harvested for DNA extraction. The protocol was repeated three times. Genomic DNA was extracted using a commercial kit previously validated for strongyle type eggs. PCR reactions were performed with a primer set specific for the ribosomal DNA region for all strongyle type parasites (NC1, NC2). SYBR Green Real-Time PCRs were performed in triplicates. Results revealed a statistically significant increase in PCR yield after three days, which was statistically associated with beginning embryonation of the eggs. In conclusion, storage time and developmental stage of strongyle eggs are significant sources of error in studies based on quantitative real-time PCR analysis. This study suggests that for refrigerated storage of more than three days, eggs should be inactivated and preserved for further analysis.

Nematodes belonging to the order Strongylida are ubiquitous in grazing horses, and the large strongyle Strongylus vulgaris is considered the most pathogenic. This parasite was originally described widely prevalent in equine establishments, but decades of frequent anthelmintic treatment appears to have reduced the prevalence dramatically. Increasing levels of anthelmintic resistance in cyathostomin parasites have led to implementation of selective therapy to reduce further development of resistance. It has been hypothesized that S. vulgaris could reoccur under these less intensive treatment circumstances. The aim with the present study was to evaluate the occurrence of S. vulgaris and the possible association with usage of selective therapy. A total of 42 horse farms in Denmark were evaluated for the presence of S. vulgaris using individual larval cultures. Farms were either using a selective therapy principle based on regular fecal egg counts from all horses, or they treated strategically without using fecal egg counts. A total of 662 horses were included in the study. Covariate information at the farm and horse level was collected using a questionnaire. The overall prevalence of S. vulgaris was 12.2% at the individual level and 64.3% at the farm level. Farms using selective therapy had horse and farm prevalences of 15.4% and 83.3%, respectively, while the corresponding results for farms not using selective therapy were 7.7% and 38.9%. These findings were found statistically significant at both the horse and the farm level. Stud farms using selective therapy were especially at risk, and occurrence of S. vulgaris was significantly associated with the most recent deworming occurring more than six months prior. The results suggest that a strict interpretation of the selective therapy regimen can be associated with an increased prevalence of S. vulgaris. This suggests that modifications of the parasite control programs could be considered on the studied farms, but it remains unknown to which extent this can be associated with increased health risks for infected horses.

The purpose of this study was to evaluate whether aged horses demonstrate statistically higher fecal egg counts (FECs) compared with middle-aged horses and to investigate systemic expression of proinflammatory cytokines in old and middle-aged horses treated with moxidectin compared with horses treated with pyrantel pamoate. Old horses (n = 21) and middle-aged horses (n = 19) were blocked by levels of inflammation and randomly allocated to one of the following treatment groups: group 1 (n = 8) old treated with moxidectin gel; group 2 (n = 9) old treated with pyrantel pamoate paste; group 3 (n = 4) old receiving no treatment; group 4 (n = 8) middle-aged treated with moxidectin gel; group 5 (n = 7) middle-aged treated with pyrantel pamoate paste, and group 6 (n = 4) middle-aged receiving no treatment. Fecal samples were collected on day 0 and again 14 days after treatment to determine FECs and presence of Strongylus vulgaris by polymerase chain reaction. Peripheral blood was also collected on day 0 and again at days 3, 5, and 14 after deworming for inflammatory cytokine analysis, along with routine hematological analyses. Results indicated that old horses have significantly higher FEC than middle-aged adults. Fecal egg counts declined significantly after anthelmintic treatment in both age groups of horses. Inflammatory markers exhibited differences between age groups and by anthelmintic treatment. In summary, this study provided evidence of different inflammatory and immunologic reactions to anthelmintic treatment in old horses. Moreover, the higher FECs found in the old horse group may have practical implications for parasite management routines on farms with representation of this age group.

Strongyle parasites are ubiquitous in grazing horses. Of these, the bloodworm Strongylus vulgaris is regarded as most pathogenic. Increasing levels of anthelmintic resistance in strongyle parasites has led to recommendations of decreased treatment intensities, and there is now a pronounced need for reliable tools for detection of parasite burdens in general and S. vulgaris in particular. The only method currently available for diagnosing S. vulgaris in practice is the larval culture, which is laborious and time-consuming, so veterinary practitioners most often pool samples from several horses together in one culture to save time. Recently, molecular tools have been developed to detect S. vulgaris in faecal samples. The aim of this study was to compare the performance of a conventional polymerase chain reaction (PCR) assay with the traditional larval culture and furthermore test the performance of pooled versus individual PCR for farm screening purposes. Faecal samples were obtained from 331 horses on 18 different farms. Farm size ranged from 6 to 56 horses, and horses aged between 2 months and 31 years. Larval cultures and PCR were performed individually on all horses. In addition, PCR was performed on 66 faecal pools consisting of 3–5 horses each. Species-specific PCR primers previously developed were used for the PCR. PCR and larval culture detected S. vulgaris in 12.1 and 4.5% of individual horses, respectively. On the farm level, eight farms tested positive with the larval culture, while 13 and 11 farms were positive with the individual and pooled PCRs, respectively. The individual PCR method was statistically superior to the larval culture, while no statistical difference could be detected between pooled and individual PCR for farm screening. In conclusion, pooled PCR appears to be a useful tool for farm screening for S. vulgaris.

Increasing levels of anthelmintic resistance in equine nematodes have led to recommendations of more sustainable anthelmintic treatment protocols with emphasis on parasite surveillance and diagnosis, rather than prophylactic calendar-based treatments. This requires knowledge of the diagnostic test performance of techniques for counts of eggs per gram of feces (EPG) as well as methods for culturing, counting and identifying third stage (L(3)) strongyle larvae per gram of feces (LPG). For horses, such information does not exist in the published literature. The aim of this study was to examine the relationship between worm count and fecal egg count (FEC) data for strongyle and Parascaris equorum infections as well as larval culture counts for diagnosing Strongylus spp. infections. Necropsy data from 693 horses used for critical or controlled tests, including information on total worm counts, fecal egg counts (FEC) and larval culture results collected at the University of Kentucky over a period of 50 years were analyzed. Sensitivity, specificity, positive and negative predictive values, and receiver operator characteristic (ROC) curves were generated for the larval cultures and ascarid egg counts. For the strongyle egg counts, potential FEC cutoff values for treatment were evaluated statistically by comparing the total strongyle worm counts below and above chosen cutoff values. All tests had high positive predictive values (>0.95), but moderate negative predictive values (

The Faecal Egg Count Reduction Test (FECRT) is the most widely used method of assessing the efficacy of anthelmintics, and is the only in vivo technique currently approved for use with horses. Equine Faecal Egg Count (FEC) data are frequently characterised by a low mean, high variability, small sample size and frequent zero count observations. Accurate analysis of the data therefore depends on the use of an appropriate statistical technique. Analyses of simulated FECRT data by methods based on calculation of the empirical mean and variance, non-parametric bootstrapping, and Markov chain Monte Carlo (MCMC) are compared. The MCMC method consistently outperformed the other methods, independently of the distribution from which the data were generated. Bootstrapping produced notional 95% confidence intervals containing the true parameter as little as 40% of the time with sample sizes of less than 50. Analysis of equine FECRT data yielded inconclusive results in 53 of 63 (84%) datasets, suggesting that the routine use of prior sample size calculations should be adopted to ensure sufficient data are collected. The authors conclude that computationally intensive parametric methods such as MCMC be used for analysis of FECRT data with sample sizes of less than 50, in order to avoid erroneous inference about the true efficacy of anthelmintics in the field.

For 9 consecutive months (March-November 2008), faecal samples were collected monthly from 129 horses residing within 40 km of Salzburg, Austria. Samples were analysed quantitatively using a modified McMaster egg counting technique. Whenever a faecal egg count (FEC) result exceeded 250 eggs per gram (EPG), the horse was treated with pyrantel, ivermectin or moxidectin. In 52 of 129 horses (40.3%), no strongyle eggs were ever detected over the course of 9 months. In 39 horses (30.2%), strongyle eggs were detected in at least 1 sample, but the egg count never exceeded 250 EPG. The remaining 38 (29.5%) horses were treated at least once in response to a FEC that exceeded 250 EPG. As a result of this selective anthelmintic scheme, the total number of anthelmintic treatments was reduced to 54% of the number of treatments administered to the same horses in the previous year. Both the maximum and mean FEC dropped significantly after initiation of the study. A statistically significant, negative correlation was demonstrated between the maximum and mean FEC of a horse and its age. Pasture hygiene appeared to reduce FECs, but the effect was not statistically significant. The magnitude of the initial FEC was significantly correlated with the maximum FECs in the subsequent 8 months (p

Fecal analyses are becoming increasingly important for equine establishments as a means of parasite surveillance and detection of anthelmintic resistance. Although several studies have evaluated various egg counting techniques, little is known about the quantitative effects of pre-analytic factors such as collection and storage of fecal samples. This study evaluated the effects of storage temperature, storage time and airtight versus open-air storage on fecal egg counts. The experimental protocols were replicated in two study locations: Copenhagen, Denmark and Athens, Georgia, USA. In both locations, the experiment was repeated three times, and five repeated egg counts were performed at each time point of analysis. In experiment A, feces were collected rectally and stored airtight at freezer (-10 to -18 degrees C), refrigerator (4 degrees C), room (18-24 degrees C), or incubator (37-38 degrees C) temperatures. Egg counts were performed after 0, 6, 12, 24, 48, and 120h of storage. In experiment B, feces were collected rectally and stored airtight or in the open air in the horse barn for up to 24h. Egg counts were performed after 0, 3, 6, 12, and 24h of storage. In experiment A at both locations, samples kept in the refrigerator showed no decline in egg counts, whereas storage in the freezer and incubator led to significantly declining egg numbers during the study. In contrast, storage at room temperature yielded marked differences between the two study locations: egg counts remained stable in the U.S. study, whereas the Danish study revealed a significant decline after 24h. In experiment B, the Danish study showed no differences between airtight and open-air storage and no changes over time, while the U.S. study found a significant decline for open-air storage after 12h. This difference was attributed to the different barn temperatures in the two studies. To our knowledge, this is the first study to evaluate the pre-analytic factors affecting egg counts in horses using an experimental protocol replicated in two contrasting geographic and climatic locations. Our results demonstrate that refrigeration is the best method for storage of fecal samples intended for egg count analysis, but that accurate results can be derived from fecal samples collected from the ground within 12h of passage.

Summary Reasons for performing study: Increased knowledge is needed to assist in the interpretation of presently available diagnostic techniques for infection by the tapeworm Anoplocephala perfoliata in horses. Hypothesis: The suggested cut-off level of an A. perfoliata specific ELISA may not adequately reflect the actual infection level. Hence, faecal egg counts may be a more useful diagnostic test for individual horses than previously reported. Methods: Eighty-four horses admitted for slaughter at a Danish abattoir were examined for the presence of A. perfoliata. The number of tapeworms, their stage of development and gross pathological mucosal lesions were recorded and compared with serum antibody responses and faecal egg counts. Faecal egg counts were determined in samples from A. perfoliata infected horses using a semi-quantitative centrifugation/flotation technique. Blood samples collected at slaughter were analysed by ELISA to determine serum antibody levels against A. perfoliata 12/13 kDa excretory/secretory antigens. Results: Macroscopically visible tapeworms were detected in 24 (29%) of the horses. The overall sensitivity of the faecal egg count was found to be 0.46; however, if the detection limit was increased to above 20 tapeworms, sensitivity increased to 0.89. There was a correlation of 0.71 between worm burden and egg count. The antibody levels correlated significantly with infection intensity despite a wide variation among horses with similar levels of infection. The optimal cut-off value was determined using receiver operating characteristic analysis. If cut-off was chosen at optical density (OD) = 0.7, sensitivity was 0.68 and specificity 0.71. Conclusions: Both diagnostic methods were capable of revealing potentially pathogenic infections, with the faecal egg count being more applicable on the individual horse level. Potential relevance: In the population of Danish horses investigated the serum ELISA test should be interpreted such that horses in need of anti-Anoplocephala treatment have an OD = 0.7 or above.

Development of resistance to anthelmintic drugs by horse strongyles constitutes a growing threat to equine health because it is unknown when new drug classes can be expected on the market. Consequently, parasite control strategies should attempt to maintain drug efficacy for as long as possible. The proportion of a parasite population that is not exposed to anthelmintic treatment is described as being “in refugia” and although many factors affect the rate at which resistance develops, levels of refugia are considered the most important as these parasites are not selected by treatment and so provide a pool of sensitive genes in the population. Accordingly, treatment should be avoided when pasture refugia are small because such treatments will place significant selection pressure for resistance on worm populations. Given this new paradigm for parasite control, it has become important to identify seasons and circumstances wherein refugia are diminished. Free-living stages of equine strongyles are highly dependent on climatic influences, and this review summarises studies of strongyle development and survival under laboratory and field conditions in Northern (cool) temperate, Southern (warm) temperate and subtropical/tropical climates. In Northern temperate climates, refugia are smallest during the winter. In contrast, refugia are lowest during the summer in warm temperate and subtropical/tropical climates. Although adverse seasonal changes clearly have significant effects on the ability of free living stages of strongyle nematode parasites to survive and develop, available data suggest that climatic influences cannot effectively “clean” pastures from one grazing season to the next.

STRONGYLID egg counts are gaining increasing importance for monitoring parasite infection and anthelmintic resistance in horses. The effects of storage on faecal egg count (FEC) measurements may therefore be important in obtaining accurate and comparable results, and the identification of procedures

Given the increasing levels of anthelmintic resistance in equine parasites, parasitologists now recommend traditional treatment approaches to be abandoned and replaced by more sustainable strategies. It is of crucial importance to facilitate veterinary involvement to ensure that treatment decisions are based on parasitic knowledge. Despite recommendations given for the past two decades, strategies based on the selective therapy principle have not yet been implemented on a larger scale in equine establishments. In contrast, treatment regimens appear to be derived from recommendations originally given in 1966. The province of Quebec in Canada, and an increasing number of European countries, have implemented prescription-only restrictions on anthelmintic drugs. Denmark introduced this legislation ten years ago, and some evidence has been generated describing potential consequences. It is without dispute that Danish veterinarians are now deeply involved with parasite management in equine establishments. However, little is known about the impact on levels of anthelmintic resistance and the risk of parasitic disease under these circumstances. In addition, the legislation makes huge demands on diagnosis and parasite surveillance. No data have been published evaluating fecal egg count techniques and larval culture methods as clinical diagnostic tools, and very little is known about potential correlations with actual worm burdens. This article provides a general review of anthelmintic strategies currently used in equine establishments and outlines the recommendations now given for parasite control. Preliminary experience with prescription-only restrictions in Denmark is presented and current research needs to further evaluate this approach are discussed.

Knowledge of horses that shed the same number of strongyle eggs over time can lead to the optimization of parasite control strategies. This study evaluated shedding of strongyle eggs in 424 horses on 10 farms when a selective anthelmintic treatment regime was used over a 3-year period. Faecal egg counts were performed twice yearly, and horses exceeding 200 eggs per gram (EPG) of faeces were treated. The results are presented as probabilities of the egg count outcome, when two previous egg counts are known. A horse with no strongyle eggs detected in the two previous faecal examinations had an 82% probability of a zero, and a 91% of being below 200 eggs per gram in the third examination. A horse with the two previous egg counts below 200 EPG had an 84% probability of being below 200 EPG the third time as well. When faecal egg counts exceeded 200 EPG on the previous two counts, the probability for a horse exceeding 200 EPG the third time was 59%. In conclusion, these data demonstrate consistent shedding from one grazing season to another in a majority of horses despite treatment of horses exceeding 200 EPG.