The first report of macrocyclic lactone resistant cyathostomins in the UK

In recent years, resistance to the benzimidazole (BZ) and tetrahydropyrimidine (PYR) anthelmintics in global cyathostomin populations, has led to reliance on the macrocyclic lactone drugs (ML-of which ivermectin and moxidectin are licensed in horses) to control these parasites. Recently, the first confirmed case of resistance to both ivermectin (IVM) and moxidectin (MOX) was reported in the USA in yearlings imported from Ireland. This suggests that ML resistance in cyathostomins has emerged, and raises the possibility that regular movement of horses may result in rapid spread of ML resistant cyathostomins. Resistance may go undetected due to a lack of surveillance for ML efficacy. Here, we report anthelmintic efficacies in cyathostomins infecting UK Thoroughbreds on four studs. Faecal egg count reduction tests (FECRT) were performed to define resistance (resistance = FECR <95% lower credible interval (LCI) < 90%). Stud A yearlings had FECRs of 36.4-78.6% (CI:15.7-86.3) after three IVM treatments, 72.6% (CI: 50.8-85.2) after MOX, and 80.8% (CI: 61.9-90.0) after PYR. Mares on stud A had a FECR of 97.8% (CI: 93.3-99.9) and 98% (95.1-99.4) after IVM and MOX treatment, respectively. Resistance to MLs was not found in yearlings or mares on studs B, C or D with FECR after MOX OR IVM treatment ranging from 99.8 to 99.9% (95.4-100); although yearlings on studs B, C and D all had an egg reappearance period (ERP) of six weeks for MOX and stud C had a four-week ERP for IVM. This study describes the first confirmed case of resistance to both licensed ML drugs on a UK Thoroughbred stud and highlights the urgent need for a) increased awareness of the threat of ML resistant parasites infecting horses, and b) extensive surveillance of ML efficacy against cyathostomin populations in the UK, to gauge the extent of the problem.

Fasciola hepatica in UK horses

Background

Fasciola hepatica (liver fluke) affects grazing animals including horses but the extent to which it affects UK horses is unknown.

Objectives

To define how liver fluke affects the UK horse population.

Study design

Descriptive, cross‐sectional, observational study.

Methods

An F. hepatica excretory‐secretory antibody detection ELISA with a diagnostic sensitivity of 71% and specificity of 97% was validated and used to analyse serum samples. An abattoir study was performed to determine prevalence. A case‐control study of 269 horses compared fluke exposure between horses with liver disease and controls. Data on clinical signs and blood test results were collected for sero‐positive horses. Genotyping of adult fluke was used to produce a multilocus genotype for each parasite.

Results

Four (2.2%) of 183 horses registered in the UK, sampled in the abattoir, had adult flukes in the liver, and the sero‐prevalence of F. hepatica was estimated as 8.7%. In the case‐control study, horses showing signs consistent with liver disease had significantly higher odds of testing positive for F. hepatica on ELISA than control horses. In 23 sero‐positive horses, a range of non‐specific clinical signs and blood test abnormalities was reported, with a third of the horses showing no signs. Genotypic analysis of liver flukes from horses provided evidence that these came from the same population as flukes from sheep and cattle.

Main limitations

Bias could have arisen in the prevalence and case‐control studies due to convenience sampling methods, in particular the geographic origin of the horses. Only a small number of horses tested positive so the data on clinical signs are limited.

Conclusions

Exposure to liver fluke occurs frequently in horses and may be an under‐recognised cause of liver disease. Flukes isolated from horses are from the same population as those found in ruminants. When designing and implementing parasite control plans, fluke should be considered, and horses should be tested if appropriate.

Fasciola and fasciolosis in ruminants in Europe: Identifying research needs

Fasciola hepatica is a trematode parasite with a global distribution, which is responsible for considerable disease and production losses in a range of food producing species. It is also identified by WHO as a re-emerging neglected tropical disease associated with endemic and epidemic outbreaks of disease in human populations. In Europe, F. hepatica is mostly associated with disease in sheep, cattle and goats. This study reviews the most recent advances in our understanding of the transmission, diagnosis, epidemiology and the economic impact of fasciolosis. We also focus on the impact of the spread of resistance to anthelmintics used to control F. hepatica and consider how vaccines might be developed and applied in the context of the immune-modulation driven by the parasite. Several major research gaps are identified which, when addressed, will contribute to providing focussed and where possible, bespoke, advice for farmers on how to integrate stock management and diagnosis with vaccination and/or targeted treatment to more effectively control the parasite in the face of increasing the prevalence of infection and spread of anthelmintic resistance that are likely to be exacerbated by climate change.

The relationships between faecal egg counts and gut microbial composition in UK Thoroughbreds infected by cyathostomins

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We profiled the faecal microbial communities of horses with cyathostomin infections, pre- and post-anthelmintic treatment.

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Methanomicrobia and Dehalobacterium were expanded in the microbiota of horses with low cyathostomin faecal egg counts.

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A reduction in TM7 and an expansion in Adlercreutzia followed anthelmintic treatment in horses with high faecal egg counts.

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Novel intervention strategies against cyathostomins based on the manipulation of the gut flora may be developed.

A growing body of evidence, particularly in humans and rodents, supports the existence of a complex network of interactions occurring between gastrointestinal (GI) helminth parasites and the gut commensal bacteria, with substantial effects on both host immunity and metabolic potential. However, little is known of the fundamental biology of such interactions in other animal species; nonetheless, given the considerable economic losses associated with GI parasites, particularly in livestock and equines, as well as the global threat of emerging anthelmintic resistance, further explorations of the complexities of host-helminth-microbiota interactions in these species are needed. This study characterises the composition of the equine gut commensal flora associated with the presence, in faecal samples, of low (Clow) and high (Chigh) numbers of eggs of an important group of GI parasites (i.e. the cyathostomins), prior to and following anthelmintic treatment. High-throughput sequencing of bacterial 16S rRNA amplicons and associated bioinformatics and statistical analyses of sequence data revealed strong clustering according to faecal egg counts (P = 0.003). A trend towards increased populations of Methanomicrobia (class) and Dehalobacterium (genus) was observed in Clow in comparison with Chigh. Anthelmintic treatment in Chigh was associated with a significant reduction of the bacterial Phylum TM7 14 days post-ivermectin administration, as well as a transient expansion of Adlercreutzia spp. at 2 days post-treatment. This study provides a first known insight into the discovery of the intimate mechanisms governing host-parasite-microbiota interactions in equines, and sets a basis for the development of novel, biology-based intervention strategies against equine GI helminths based on the manipulation of the commensal gut flora.

P-glycoproteins play a role in ivermectin resistance in cyathostomins

Anthelmintic resistance is a global problem that threatens sustainable control of the equine gastrointestinal cyathostomins (Phylum Nematoda; Superfamily Strongyloidea). Of the three novel anthelmintic classes that have reached the veterinary market in the last decade, none are currently licenced in horses, hence current control regimens focus on prolonging the useful lifespan of licenced anthelmintics. This approach would be facilitated by knowledge of the resistance mechanisms to the most widely used anthelmintics, the macrocyclic lactones (ML). There are no data regarding resistance mechanisms to MLs in cyathostomins, although in other parasitic nematodes, the ABC transporters, P-glycoproteins (P-gps), have been implicated in playing an important role. Here, we tested the hypothesis that P-gps are, at least in part, responsible for reduced sensitivity to the ML ivermectin (IVM) in cyathostomins; first, by measuring transcript levels of pgp-9 in IVM resistant versus IVM sensitive third stage larvae (L3) pre-and post-IVM exposure in vitro. We then tested the effect of a range of P-gp inhibitors on the effect of IVM against the same populations of L3 using the in vitro larval development test (LDT) and larval migration inhibition test (LMIT). We demonstrated that, not only was pgp-9 transcription significantly increased in IVM resistant compared to IVM sensitive L3 after anthelmintic exposure (p < 0.001), but inhibition of P-gp activity significantly increased sensitivity of the larvae to IVM in vitro, an effect only observed in the IVM resistant larvae in the LMIT. These data strongly implicate a role for P-gps in IVM resistance in cyathostomins. Importantly, this raises the possibility that P-gp inhibitor-IVM combination treatments might be used in vivo to increase the effectiveness of IVM against cyathostomins in Equidae.

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Pgp-9 transcript levels were higher in ivermectin resistant versus susceptible cyathostomin populations.

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P-gp inhibitors increased ivermectin effect against cyathostomins in vitro.

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P-gp activity may play a role in ivermectin resistance in cyathostomins.

A questionnaire study on parasite control practices on UK breeding Thoroughbred studs

REASONS FOR PERFORMING STUDY

Improved education of veterinarians and equine owners/managers is essential in implementing parasite control strategies that are less reliant on chemicals.

METHODS

This questionnaire study, conducted on 61 UK Thoroughbred (TB) establishments during 2009 and 2010, was designed to obtain an understanding of current helminth control practices on studs. To our knowledge, this is the first occasion that statements obtained from TB studs via questionnaire have been supported by statistical analysis.

RESULTS

Despite many respondents indicating high levels of concern regarding anthelmintic resistance, 56% of these establishments that received visiting equines co-grazed these animals with permanent stock and <74% administered anthelmintics prior to integration. In the 12 months preceding the study, most respondents administered frequent macrocyclic lactone (ML) treatments, with none appearing to leave any animals in groups untreated at each administration. Indiscriminate whole group treatments with MLs and movement of animals to 'clean grazing' post treatment (reported by >25% of respondents), indicates that many stud owners/managers are not aware of the strong risk factors for the development of anthelmintic resistance. Few studs had conducted faecal egg count (FEC) analysis in the past and only 22% indicated that they considered this form of analysis beneficial in determining anthelmintic choice.

CONCLUSIONS AND POTENTIAL RELEVANCE

The challenge now is to convince stud owners/managers to deviate from their current practices to control strategies that are more likely to preserve anthelmintic efficacy. Veterinarians need to get more involved in implementing these control strategies, with better emphasis placed on the role of diagnostic tests in facilitating targeted treatments and in investigating anthelmintic sensitivity in the associated nematode populations.

Evolution of acaricide resistance: phenotypic and genotypic changes in field populations of Rhipicephalus (Boophilus) microplus in response to pyrethroid selection pressure

There have been few, if any, studies of arthropod field populations quantifying the kinetics of evolution of phenotypic and genotypic resistance to chemicals in response to the presence or absence of selection pressure. A prospective intervention study was undertaken over 2 years in Mexico to measure changes in resistance phenotype and genotype in the presence or absence of pyrethroid selection pressure on field populations of Rhipicephalus (Boophilus) microplus ticks on 11 farms with varying degrees of pyrethroid resistance. The resistance phenotype was evaluated by bioassay in a larval packet test expressed as the resistance factor (RF) derived from probit analysis of dose mortality regressions, and resistance genotype by an allele-specific PCR (AS-PCR) to determine the frequency of a sodium channel mutation (F1550I) associated with pyrethroid resistance. To validate the AS-PCR, a Pyrosequencing™ method was developed to detect the F1550I mutation. There was good concordance with the genotypes identified by both Pyrosequencing™ and AS-PCR (Kappa: 0.85). On five farms cypermethrin (CY) was exclusively used at intervals and on six farms amitraz was used. On two of the five CY-treated farms, the experiment was prematurely terminated due to unacceptably high levels of tick resistance. For all five farms, after 8-24 months of continued selection pressure with CY, the RF had increased 2-125-fold. The frequency of the resistance allele increased on all five farms from a starting range of 5-46% to a range of 66-95% after 8-24 months. On six farms treated with amitraz neither the RF nor the frequency of the resistance allele changed. A clear correlation between the phenotype and genotype was found in three of four treated farms confirming that the F1550I mutation is a major cause of synthetic pyrethroid resistance in Mexico. These results show that the pyrethroid resistance trait is stable (> 2 years) and that resistance is acquired much faster than it is lost. Hence, alternation of pyrethroid acaricide with other chemicals is likely to lead to the stepwise acquisition of synthetic pyrethroid resistance but not additional prolongation of its efficacious lifespan.

PCR-based differentiation of Fasciola species (Trematoda: Fasciolidae), using primers based on RAPD-derived sequences

The zoonotic liver flukes Fasciola hepatica and F. gigantica co-exist in parts of Africa and Asia. The two species have similar life-cycles but different transmission characteristics. Although the identification of adult Fasciola to species level is traditionally based on differences in size and shape, recent studies have demonstrated this method to be unreliable. Species of Fasciola can be distinguished by staining and comparing the morpho-anatomy of the gut and ovaries or by iso-enzyme analysis but such approaches are time-consuming and require specialist skills. Two primer sets, based on RAPD-derived sequences from English F. hepatica and Ghanaian F. gigantica, can now be used, in two separate PCR, to distinguish F. hepatica from F. gigantica. When the PCR were used to investigate 10 flukes (five from the U.K. and five from Peru) morpho-anatomically identified as F. hepatica and 10 (five from Ghana and five from Sudan) morpho-anatomically identified as F. gigantica, all 20 flukes were correctly identified to species level. The PCR were validated using 175 flukes collected, over a 12-year period, from different countries and both cattle and sheep.

Molecular diagnosis and equine parasitology

The future implementation of improved and sustainable control strategies for the major equine parasites will be dependent on a greater insight into their basic biology, pathogenicity and epidemiology together with an enhanced ability for accurate diagnosis. This paper will provide a review of the current molecular methods under development for the detection of equine parasites and their application to current scientific questions. In particular, the strongyles are recognised as important pathogens of horses and recent advances made in the study of this parasitic group at the single species level will be addressed. The ribosomal (r)DNA region of the parasite genome has been employed to distinguish between closely related species. Molecular probes designed to this target region were used in combination with PCR technology to allow the identification of individual species within mixed infections. They have been applied to all parasite stages to look at the role of individual species in natural infection, disease and drug resistance. Similar techniques have been developed to detect other equine parasites and these will also be discussed. Further opportunities for employing existing techniques and the need for new diagnostic tools will be highlighted.

Isolation and characterisation of a beta tubulin isotype 2 gene from two species of cyathostomin

This study describes the isolation and characterisation of beta tubulin isotype 2 cDNA sequences from two common species of cyathostomin, Cylicocyclus nassatus and Cyathostomum catinatum. The full-length cDNAs for these species were 1709 and 1753 bp in length, respectively, including 1350 bp of sequence inferred to encode 450 amino acids of peptide sequence. They had greatest identity with previously characterised isotype 2 sequences from Trichostrongylus colubriformis, Cooperia oncophora and Haemonchus contortus (96% for C. nassatus and 95% for C. catinatum), and grouped together with these sequences in phylogenetic analysis. Both cyathostomin beta tubulin isotype 2 sequences contained the isotype-specific carboxyl terminal region described previously in other nematode species. Alignment with beta tubulin isotype 1 proteins from other trichostrongyloids, indicated 95 and 94% identity for the isotype 2 sequences of C. nassatus and C. catinatum, respectively. This comparison revealed 14 isotype-specific amino acid substitutions. Also, 2605 bp of beta tubulin isotype 2 genomic DNA sequence were isolated from C. nassatus. Comparison with the previously published isotype 1 gene of C. nassatus indicated differences in genomic organisation between the two isotypes. Reverse transcriptase (RT)-polymerase chain reaction analysis revealed constitutive temporal expression of beta tubulin isotype 1, whilst isotype 2 appeared to be developmentally expressed, with transcripts detected only in RNA derived from adult parasites.

Cytokine responses to Cyathostominae larvae in the equine large intestinal wall

To investigate cytokine responses in cyathostomin infection, we quantified mucosal interleukin-4 (IL-4), interleukin-10 (IL-10), tumour necrosis factor (TNF)-alpha and interferon (IFN)-gamma by reverse transcriptase-competitive polymerase chain reaction. The analysis was performed on large intestinal wall samples obtained from six anatomical sites spanning the caecum and colon of 17 naturally exposed horses. The numbers of developing larvae (DL) and early third stage larvae (EL3) were ascertained using transmural illumination and pepsin digestion techniques, respectively. Levels of each cytokine transcript were correlated with local intestinal wall burdens of Cyathostominae larvae. IL-4 and IL-10 levels showed significant correlations with EL3 and DL burdens at several sites. No significant correlations were observed with IFNgamma. A pro-inflammatory response, typified by detection of TNFalpha transcript, was observed at a few sites in some horses with inflammatory enteropathy associated with emerging or emerged larvae. However, this cytokine was measured at an insufficient number of sites to enable statistical analysis. Levels of IL-4, IL-10 and IFNgamma transcript were compared between two groups: one group consisting of horses with low to high mucosal burdens (Group A) and the other, of horses with negative/negligible mucosal burdens (Group B). Significant differences in IL-4 (P<0.001) and IL-10 (P<0.001) transcript levels were observed between the groups, with higher levels observed in Group A. No significant differences in IFNgamma were observed. Taken together, these results indicate that Th2 responses predominate in mucosal Cyathostominae infection prior to larval reactivation.

Identification of strongyle eggs from anthelmintic-treated horses using a PCR-ELISA based on intergenic DNA sequences

The efficacy of five daily fenbendazole (FBZ) treatments was tested against benzimidazole-resistant cyathostomins in naturally infected horses (n=13). Horses were treated with pyrantel embonate (PYR) to remove adult strongyles followed, 7 days later, by a 5-day course of FBZ. The PYR treatment produced an average faecal egg count reduction of 98%. All samples were negative by faecal egg count 7 days after the start of the FBZ treatment. Positive egg counts were observed from 28 days after the start of FBZ treatment and all horses displayed positive faecal egg counts by 77 days after treatment. Strongyle eggs were harvested from the faeces of the horses prior to treatment and then weekly from 42 to 70 days post-treatment. DNA was obtained from eggs in groups of ten. A PCR-ELISA, based on species-specific differences in intergenic DNA sequences, was used to identify the presence of six cyathostomin species. In pre-treatment samples, Cyathostomum catinatum was detected in nine out of the 13 horses and Cylicostephanus longibursatus, Cylicostephanus goldi and Cylicocyclus nassatus, were found in samples from eight animals. Cylicocyclus ashworthi and Cylicocyclus insigne were not detected pre-treatment. After anthelmintic treatment, C. catinatum and C. longibursatus were most frequently detected, followed by C. nassatus, C. goldi and C. ashworthi. C. insigne was detected at only one time point in a sample from a single horse.

Molecular systematics and diagnosis

This collection of articles provides an account of six presentations delivered at the 19th International Conference of the World Association for the Advancement of Veterinary Parasitology(WAAVP) (held in New Orleans, Louisiana, USA, from 10 to 14 August 2003) in a symposium session on Molecular Systematics and Diagnosis, organised and chaired by R.B. Gasser and D.S. Zarlenga. The focus was on recent advances in molecular tools for specific and genotypic identification,diagnosis, systematics and population genetics, with special emphasis on investigations of parasitic nematodes and protists.

Seasonal development of Cyathostominae larvae on pasture in a northern temperate region of the United Kingdom

A study following the development of Cyathostominae from egg to the infective larval third stage was conducted from April to December 2001 in west central Scotland. Duplicate samples (1 kg) of naturally infected faeces were placed on a 78 cm2 plot each week on a cyathostomin-free pasture. Subsamples of the grass surrounding the faecal plot were collected weekly on four occasions and the number of larvae obtained determined. Few larvae were recovered in the first week of development of individual plots, followed by a rise in the numbers of larvae in second, third and fourth weeks of development of each sample. The climatic conditions were seen to have an effect on the rate of development. Specifically, from multilevel, multivariable linear regression models it was evident that the factors associated with numbers of infective larvae recovered from pasture were the time since the faeces samples were laid down, the average temperature and rainfall during the previous week, as well as the interaction between temperature and rainfall. Conversely, from the model, the number of larvae recovered from pasture was associated with neither the number of eggs within the faeces samples placed on the plots nor with the viability of these eggs.

A PCR–ELISA for the identification of cyathostomin fourth-stage larvae from clinical cases of larval cyathostominosis

We report the use of six oligoprobes designed from intergenic spacer region sequences to identify fourth-stage larvae (L4) of the tribe Cyathostominae. Oligoprobes were designed for identification of the following species: Cylicocyclus ashworthi, Cylicocyclus nassatus, Cylicocyclus insigne, Cyathostomum catinatum, Cylicostephanus goldi, and Cylicostephanus longibursatus. A seventh probe was designed as a positive control to identify all these members of the Cyathostominae. The intergenic spacer region was amplified by PCR using conserved primers. Initially, three oligoprobes were used in Southern blot analysis. To facilitate high-throughput identification, these and a further four oligoprobes were developed for use in a PCR-ELISA. All probes were validated for their ability to detect cyathostomin PCR products in the PCR-ELISA, using DNA from morphologically identified adult parasites. Initially, 712 L4 were isolated from the diarrhoeic faeces from horses (n=17) with clinical larval cyathostominosis. PCR products from 522 of these L4 were subjected to analysis, with 413 L4 being identified as one of the aforementioned species. With reference to individual species analysis, 28.5% of the 522 L4 were identified as C. longibursatus, 25.7% as C. nassatus, 15.9% as C. ashworthi, 7.3% as C. goldi and 1.7% as C. catinatum. No L4 were identified as being C. insigne species. When L4 within faeces from individual horses were compared, no sample was found to comprise parasites of one species. The least number of species identified in a single sample was two. This study suggests that clinical larval cyathostominosis is predominantly caused by mixed-species infections.

Evaluation of the specificity of five oligoprobes for identification of cyathostomin species from horses

Here, we report evaluation of five oligoprobes designed from intergenic spacer (IGS) region sequences for identification of cyathostomin species. Oligoprobes were designed for identification of Cylicocyclus ashworthi, Cylicocyclus nassatus, Cylicostephanus longibursatus, Cylicostephanus goldi and a fifth probe designed to identify all members of this tribe. PCR amplification of IGS DNA from 16 cyathostomin species allowed sequence comparison and identification of four putative species-specific probes. Southern blotting of amplified products from 16 species showed that all probes were species-specific. The fifth probe recognised all 16 cyathostomin species but did not bind to members of the genus Strongylus. Furthermore, these probes were used to identify individual infective L3, eggs and L4 indicating that they will be invaluable to furthering the study of the epidemiology and pathogenesis of these important equine nematodes.

Expression of a human homeobox-containing gene is regulated by 1,25(OH)2D3 in bone cells

The murine msh-like homeobox-containing gene Msx-2 (Hox-8) has a multiphasic pattern of embryonic expression which includes the developing skull bones and teeth. In order to determine whether this gene might be expressed in adult mineralizing tissues, an adult human bone cell (osteoblastic) cDNA library was screened with a murine Msx-2 probe. One of the positive clones obtained was sequenced in full and shown to be highly homologous to the murine Msx-2 gene. Expression of this gene, MSX2, was studied in human bone-derived cells, where a low level of expression was detected which was greatly induced by stimulation with 1,25(OH)2D3. Little or no increase in expression was detected with retinoic acid or TPA stimulation. In cultures of skin fibroblasts no stimulation of expression with 1,25(OH)2D3 was detected. MSX2 is the first homeobox-containing gene to be shown to be regulated by 1,25(OH)2D3 and to be expressed in adult human bone cells. These results suggest a possible role for MSX2 in bone cell differentiation and/or calcium homeostasis.