Are small strongyles (Cyathostominae) involved in horse colic occurrence?

Strongyle infections have been traditionally regarded as a possible cause of colic in horses. Aim of the study was the comparison of parasitological status between subjects with or without colic syndrome, with particular attention to small strongyle infections. Coprological analyses were performed on 86 horses: 43 with colic and 43 controls. Strongyle eggs were found in 34/86 horses (prevalence 39.5%), the mean number of strongyles eggs per gram of faeces (EPG)² was 145.34 (standard deviation 398.28). All those 34 positive animals had small strongyles infections. Negative binomial multiple regression highlighted no influence of horse sex on strongyle EPG, while there was a negative relationship between age and EPG (p <0.05); the same analysis revealed a significant difference of EPG (p <0.05) between control horses (mean EPG=178.1; standard deviation: 411.4) and horses with surgical colic (mean EPG=68.6 standard deviation: 259.8) when controlling for S. vulgaris presence including it in the model. On the contrary, the intensity of infection in horses with non-surgical colic (mean EPG=154.5; standard deviation: 480.4) did not significantly differed from controls. Similar results were obtained having estimated cyathostomine EPGs as dependent variable. Multinomial logistic regression confirmed the negative relationship between cyathostonine presence and surgical colic occurrence. It is possible that (1) the presence of adult luminal parasites, would have a protective effect against the pathogenic action exerted by the development and emergence of small strongyles larvae from intestinal mucosa; (2) the management practices able to reduce the risk of colics are the same that cause higher exposure to strongyle infective larvae.

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.