Cyathostome-associated disease in the horse: Investigation and management of four cases

The Use of Innovative Diagnostics to Inform Sustainable Control of Equine Helminth Infections

Helminths are commonly found in grazing equids, with cyathostomin nematodes and the cestode Anoplocephala perfoliata being the most prevalent. Most horses harbour low burdens of these parasites and do not develop signs of infection; however, in a small number of animals, high burdens can accumulate and cause disease. Cyathostomins are associated with a syndrome known as larval cyathostominosis. This occurs when large numbers of larvae emerge from the large intestinal wall. This disease has a case fatality rate of up to 50%. A. perfoliata infection has been associated with various types of colic, with burdens of >20 worms associated with pathogenicity. Anthelmintic resistance is a serious problem in cyathostomins and is emerging in A. perfoliata. Control methods that reduce reliance on anthelmintics now need to be applied, especially as no new dewormer compounds are on the horizon. Sustainable control methods must employ diagnostics to identify horses that require treatment. Coprological tests (faecal egg counts, FECs) have been used for several decades to inform treatment decisions to reduce helminth egg shedding. These tests cannot be used to assess host burdens as FECs do not correlate with cyathostomin or A. perfoliata burdens. In the last decade, new tests have become available that measure parasite-specific antibodies, the levels of which have been shown to correlate with parasite burden. These tests measure antigen-specific IgG(T) and are available in serum (cyathostomin, A. perfoliata) or saliva (A. perfoliata) formats. Tests for other helminths have been developed as research tools and need to be translated to support equine clinicians in practice. A key element of sustainable control strategies is that diagnostics must be used in combination with management approaches to reduce environmental transmission of helminths; this will help limit the proportion of horses harbouring parasite burdens that need to be targeted by treatment. This manuscript provides a review of the development, performance and general utility of various diagnostic methods for informing equine helminth management decisions.

Outbreak of acute larval cyathostominosis - A "perfect storm" of inflammation and dysbiosis

Background

Cyathostomins are prevalent and pathogenic intestinal helminths of horses, causing acute and chronic disease, including acute larval cyathostominosis, which has a mortality rate of 50%. Factors determining individual susceptibility to acute larval cyathostominosis are unknown. Investigation of these factors could lead to novel treatment and prevention strategies.

Objectives

To investigate clinicopathological and faecal microbiota changes associated with disease in individual horses in an acute larval cyathostominosis outbreak.

Study design

Case series.

Methods

The study population was a herd of 23 mixed breed horses in Ireland. The outbreak occurred in November 2018. Fourteen horses were clinically affected. Clinical status was monitored and recorded. Blood and faecal sampling allowed clinicopathological, faecal 16s rRNA gene sequencing and faecal egg count analyses.

Results

Two horses were euthanised, whilst 12 recovered. Common clinical signs included loose faecal consistency, weight loss and pyrexia. Consistent clinicopathological findings were borderline anaemia, leucocytosis, thrombocytosis, hyperfibrinogenaemia, hyperglobulinaemia and a reverse A: G ratio. Decreased alpha‐diversity of the faecal microbiota and greater relative abundance of the genus Streptococcus, class Bacilli, order Lactobacillales and family Streptococcaceae, and family Prevotelleceae was found in clinically affected horses compared to their clinically normal cohorts. An increase in obligate fibrolytic bacteria was seen in the clinically normal group compared to the clinical group. Histopathological findings of the colon and caecum revealed a severe necrotising typhlocolitis associated with cyathostomin larvae and bacterial overgrowth in the mucosa of the large intestine.

Main limitations

The study population in this outbreak is small. There are several confounding factors limiting this to a descriptive case series. Faecal microbiota has been shown to reflect the large intestinal microbiota but do not represent changes directly.

Conclusions

These findings suggest that acute larval cyathostominosis is associated with dysbiosis of the gut microbiota as well as the inflammatory stimulus of numerous emerging larvae leading to structural and functional pathology of the large intestine.

Gastrointestinal and pulmonary parasites of working horses from Colombia

Working equids are used in different countries for numerous purposes and regularly are crucial for multiple communities' income and profit. Historically, in Bogotá D.C. they were used as animal-powered vehicles but in 2013 it was initiated a substitution and adoption program as a result of society pressure and lawful processes. Infectious diseases and mainly parasitic diseases, poses a threat in the health and productivity of these animals. Our aim was to identify, by coproparasitological methods, the gastrointestinal and pulmonary parasite species infecting working horses submitted to the mentioned substitution and adoption program. Between May and December 2013 and February and July 2014, fresh faecal samples were obtained from 1004 and 648 horses from Bogotá D.C. and other Colombian municipalities, respectively. They were processed by flotation and Baermann tests in order to visualize faecal parasitic forms (eggs, oocysts, and larvae). Prevalences were calculated for each gastrointestinal parasite at origin and one year after, at destination, and analysed by age group and coinfection. At origin (Bogotá D.C.), prevalence for at least one parasite species was 87.5% and one year later, at destination (other municipalities), was 89.5%. The most prevalent species were strongyles (86.4-89.4%) followed by Parascaris spp. (0.7-6.2%), cestodes (Anoplocephalidae) (3.7-4.9%) and Oxyuris sp. (2.8-4.3%). Other species detected were Eimeria sp., Strongyloides sp. and Dictyocaulus sp. Coinfection by two or more species ranged between 14.4 and 38.3% being strongyle, Parascaris spp., Oxyuris sp. and cestodes (Anoplocephalidae) the most common species involved. Some parasitic infections commonly associated with younger animals (e.g. Parascaris spp.) were detected in all age groups. Flow patterns of parasites linking Bogotá D.C. and other municipalities are presented. Finally, these results support the widespread distribution of most of species and the plausible health and welfare impact of this infections in working equids submitted to particular epidemiological risks.

Removal of adult cyathostomins alters faecal microbiota and promotes an inflammatory phenotype in horses

The interactions between parasitic helminths and gut microbiota are considered to be an important, although as yet incompletely understood, factor in the regulation of immunity, inflammation and a range of diseases. Infection with intestinal helminths is ubiquitous in grazing horses, with cyathostomins (about 50 species of which are recorded) predominating. Consequences of infection include both chronic effects, and an acute inflammatory syndrome, acute larval cyathostominosis, which sometimes follows removal of adult helminths by administration of anthelmintic drugs. The presence of cyathostomins as a resident helminth population of the equine gut (the "helminthome") provides an opportunity to investigate the effect helminth infection, and its perturbation, has on both the immune system and bacterial microbiome of the gut, as well as to determine the specific mechanisms of pathophysiology involved in equine acute larval cyathostominosis. We studied changes in the faecal microbiota of two groups of horses following treatment with anthelmintics (fenbendazole or moxidectin). We found decreases in both alpha diversity and beta diversity of the faecal microbiota at Day 7 post-treatment, which were reversed by Day 14. These changes were accompanied by increases in inflammatory biomarkers. The general pattern of faecal microbiota detected was similar to that seen in the relatively few equine gut microbiome studies reported to date. We conclude that interplay between resident cyathostomin populations and the bacterial microbiota of the equine large intestine is important in maintaining homeostasis and that disturbance of this ecology can lead to gut dysbiosis and play a role in the aetiology of inflammatory conditions in the horse, including acute larval cyathostominosis.

Small strongyle infection: consequences of larvicidal treatment of horses with fenbendazole and moxidectin

The study was undertaken to evaluate adverse effects of larvicidal treatment in horses naturally infected with cyathostomins. Out of 24 ponies kept on pasture, four animals were housed in September and anthelmintically cured to serve as worm-free controls (group C-0). The others were housed in December. Eight animals each were treated 8 weeks later with 5 x 7.5mg/kg fenbendazole (FBZ) or 1 x 0.4 mg/kg moxidectin (MOX). Four animals remained untreated (group C-i). Two, 4, 6 and 14 days after the end of treatment two animals of each of the treated groups were necropsied together with group C-0 and C-i animals. Infected animals before treatment showed weight loss, eosinophilia, increased plasma protein and globulin contents. Treatment was followed by weight gain and temporal plasma protein and globulin increase. Proportions of CD4+ and CD8+ T lymphocytes in the peripheral blood did not differ between the groups before treatment but dropped significantly temporally after FBZ treatment. Group C-0 was worm-free at necropsy. Group C-i animals contained variable numbers of luminal and tissue cyathostomins. Histological sections showed larval stages in the lamina propria und submucosa surrounded by macrophages. Either treatment was effective against luminal parasites and reduced the number of larvae in the bowel wall beginning 4-6 days after FBZ and 6-14 days after MOX treatment. Histologically, as a first reaction after FBZ application T lymphocytes accumulated around morphologically intact L4 in the submucosa. Subsequently T lymphocytes associated with eosinophils infiltrated the submucosa. Parasites became enclosed by granulomas with eosinophils adhering to and invading the larvae which started to disintegrate on day 4. Later on, particularly on day 14 inflammation extended into the mucosa and was frequently associated with ulcerations. Third stage larvae in general and L4 in the lamina propria, however, seemed not to be affected until day 14 and even then, parasites did usually not generate extensive inflammation. After MOX treatment severe morphologically detectable alterations of tissue larvae could not be observed earlier than day 14. Different from FBZ treatment, larvae disintegrated and were obviously resorbed without causing severe inflammation in the gut wall. In conclusion treatment with either drug was efficacious against tissue larvae of cyathostomins but there may be different clinical consequences: in contrast to MOX effects, killing of larvae due to FBZ was associated with severe tissue damage, which clinically may correspond to reactions caused by synchronous mass emergence of fourth stage larvae, i.e., may mimic larval cyathostominosis.

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.

Characterisation of IgG(T) serum antibody responses to two larval antigen complexes in horses naturally- or experimentally-infected with cyathostomins

Cyathostomins are the most common parasitic nematodes of horses. Larval stages, which inhabit the intestinal wall, are particularly pathogenic and can cause severe colitis and colic. Despite their clinical importance, diagnostic techniques for the prepatent stages do not exist. A method that could estimate mucosal infection intensity would have a major impact on the control and diagnosis of cyathostominosis. Here, serum IgG(T) responses to two larval antigen complexes of 25 and 20 kDa were quantified in horses with experimental infections, natural infections and in horses that presented with clinical larval cyathostominosis. In experimentally-infected animals, anti-25 kDa complex IgG(T) levels correlated positively with field exposure and with early third stage larval (r(s)=0.74, P=0.015) and total mucosal parasite (r(s)=0.78, P=0.010) burdens. In naturally exposed horses whose parasite burdens were quantified upon post-mortem examination, antigen-specific IgG(T) responses were significantly higher in infected than in uninfected horses (P=0.0001 and 0.002, for anti-25 and anti-20 kDa responses, respectively). In these animals, anti-25 kDa IgG(T) levels correlated positively with mucosal and lumenal burdens (P<0.05). IgG(T) responses to the 20 kDa antigen complex correlated positively with lumenal burdens (P=0.0043). In cases of larval cyathostominosis, antigen-specific IgG(T) levels were significantly higher than in uninfected ponies (P=0.002 and 0.0035, for anti-25 and anti-20 kDa responses, respectively). These results provide evidence that these two complexes contain antigens with potential as markers for prepatent cyathostomin infection.

Purification and analyses of the specificity of two putative diagnostic antigens for larval cyathostomin infection in horses

Cyathostomins are important equine gastrointestinal parasites. Mass emergence of mucosal stage larvae causes a potentially fatal colitis. Mucosal stages are undetectable non-invasively. An assay that would estimate mucosal larval stage infection would greatly assist in treatment, control and prognosis. Previously, we identified two putative diagnostic antigens (20 and 25 kDa) in somatic larval preparations. Here, we describe their purification and antigen-specific IgG(T) responses to them. Western blots confirmed the purity of the antigens and showed that epitopes in the 20 kDa complex were specific to larval cyathostomins. No cross-reactive antigens appeared to be present in Parascaris equorum or Strongyloides westeri species. Low levels of cross-reactivity were observed in Strongylus edentatus and Strongylus vulgaris species. Use of purified antigens greatly reduced background binding in equine sera. These results indicate that both antigen complexes may be of use in a diagnostic assay.

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.

A preliminary analysis of proteolytic activity of excretory-secretory products from Cyathostominea

The excretory-secretory product (ESP) derived from Cyathostominea in vitro was assessed, in terms of subunit composition, and proteolytic activity using as substrates azocasein and two synthetic fluorogenic peptides. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) resolved 13 subunits, and the presence of the protein cysteine proteinase activator dithiothreitol (DTT) revealed 21 subunits. DTT also enhanced azocaseinolysis, and hydrolysis of carbobenzoxy-phenylalanyl-arginine-7-amido-4-methylcoumarin (Z-Phe-Arg-NHMec) and carbobenzoxy-arginyl-arginine-7-amido-4-methylcoumarin (Z-Arg-Arg-NHMec). At the optimum pH of 5.5, hydrolysis of Z-Phe-Arg-NHMec was three-fold greater than that of Z-Arg-Arg-NHMec suggesting that the proteolytic specificities of the ESP are more like those of papain or cathepsin L, rather than cathepsin B. In SDS-PAGE gelatin gels, DTT was a requirement for proteolysis by the ESP. Optimum resolution was at pH=5.5, resolving six bands ranging from 114-20kDa. Cysteine proteinase inhibitors abolished all gelatinolytic activity at the pH values tested. Such data indicate the presence of cysteine-class proteinases in the ESP of Cyathostominea.

Caecocaecal and caecocolic intussusceptions associated with larval cyathostomosis in four young horses

This report describes the clinical and pathological features of 4 horses affected by caecocaecal or caecocolic intussusceptions associated with larval cyathostomiasis. In each case, there was clinical and/or pathological evidence of concurrent larval cyathostomosis. They all demonstrated variable signs including diarrhoea, pyrexia, weight loss and subcutaneous oedema and cyathostome larvae were identified in the faeces of 3 of the horses. These cases highlight the need to consider caecal intussusceptions as a possible complication in horses affected by larval cyathostomosis, especially where signs of abdominal pain are severe or persistent.

Treatment and control of gastrointestinal parasites

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

Pathogenicity of cyathostome infection

Cyathostomes are now the principle parasitic pathogen of the horse: a remarkable transformation during the last 25 years from virtual obscurity to focus of attention in equine parasitology. This rise to prominence coincides with the marked decrease in prevalence of large strongyle infections as a result of widespread use of modern anthelmintic compounds. On the basis that strongyle-associated diseases continue to commonly occur in the absence of these large strongyle species, clinical attention has turned to the pathogenicity of cyathostomes. Although many horses harbour burdens of tens of thousands of cyathostomes without developing detectable illness, these parasites can result in an inflammatory enteropathy affecting the caecum and colon. Although the principle clinical effect of cyathostomosis is weight loss, affected individuals may exhibit other signs including diarrhoea and/or subcutaneous oedema and/or pyrexia. Clinical cyathostomosis occurs more commonly in young horses in late winter/early spring but there is lifelong susceptibility to cyathostomes and they can cause clinical disease in any age of horse during any season. Animals with cyathostomosis often develop hypoalbuminaemia and/or neutrophilia but there are no clinicopathological features specific for the disease. Experimental infections with cyathostomes have resulted in both clinical and pathological features similar to those of naturally-occuring cyathostomosis cases. From the experimental infection studies, it is evident that cyathostomes are pathogenic at times of both penetration into and emergence from the large intestinal mucosa. An unusual feature of cyathostome biology is the propensity for arrested larval development within the large intestinal mucosa for more than 2 years. From limited studies it appears that this arrested larval development is favoured by: feedback from luminal to mucosal worms; larger size of challenge dose of larvae and trickle (versus single bolus) infection. During arrested larval development cyathostomes have minimal susceptibility to all anthelmintic compounds, thus, limiting the effectiveness of therapeutic and/or control strategies. Although, the relative importance of individual cyathostomes is not known, the development of species-specific DNA methods for identification of cyathostomes provides a means by which the pathogenicity of different species might be established.