Equis ISSN 2398-2977

Diarrhea: parasitic

Contributor(s): Vetstream Ltd, Jamie Prutton


  • Diarrhea due to endoparasitic infections may be common in certain equine populations. In 27% of cases of chronic diarrhea Cyathostomiasis (small strongyle infestation) was diagnosed as the cause in one study.
  • Regular anthelmintic treatment schedules have led to an ever-increasing anthelmintic resistance, therefore targeted deworming approaches should be utilized.
  • Cause: potential pathogens include the large and small Strongyles, Strongyloides westeriParascaris equorum, Cryptosporidium spp and Anoplocephala spp, although most might lead to colic with cyathostomins being the most likely to lead to diarrhea.
  • Signs: colic, diarrhea, weight loss, ill thrift, and even dehydration, hypoproteinemia and edema.
  • Diagnosis: history, clinical signs and further tests such as fecal examination and blood tests.
  • Treatment: larvicidal endoparasiticides. It is important to consider the emerging resistance to anthelmintic and therefore careful consideration should be undertaken when choosing treatment. 
  • Prognosis: variable - severe cases associated with mass emergence of encysted cyathostomins can be fatal.
Print off the Owner factsheets All about worms and Diarrhoea to give to your clients.



Predisposing factors


  • When considering cyathostomes and Cryptosporidium horses gain a resistance as they become older and therefore disease is more common in younger animals. For Cryptosporidium it is <6 months and for cyathostomes somewhere in the region of <10 years.
  • Parascaris Parascaris equorum and Strongyloides westeri Strongyloides westeri infection tend to be a problem in foals under 6 months.
  • Tapeworms and strongyles tend to be a problem in young horses, typically between 1 and 3 years old but can affect horses of any age.
  • Due to the high rate of anthelmintic use, large strongyle infections are rarely seen.


  • High stocking density with poor parasite control will increase the risk of infection. Therefore, techniques should be implemented to reduce parasitic burden on land. 


  • The exact pathophysiology behind diarrhea induced by most parasites is unknown other than for encysted cyathostomes.
  • During the larval penetration of the mucosa the ex-sheathed L3 will cause direct damage to the intestinal mucosa leading to a reduction in nutritional metabolism as well as mucosal damage and inflammation. This stage is relatively mild but can induce weight loss, lack of weight gain or mild diarrhea.
  • Mass emergence of the encysted L3 normally occurs during late winter/early spring in Northern temperate zones.
  • The encysted form emerges as L4 and during this stage there will be severe mucosal inflammation, hemorrhage, ulceration and necrosis. Due to the villous destruction there will be a reduced reabsorption of fluids.
  • A secretory type diarrhea also occurs with loss of protein and electrolytes leading to many of the clinical signs seen.
  • Chronic cases can have mucosal thickening with fibrosis.


  • Often insidious lack of weight gain but can be acute onset during the late winter or spring or following use of anthelmintics.


  • Seen when horses are at grass with poor parasitic egg control.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Matthews J B (2014) Anthelmintic resistance in equine nematodes. Int J Parasitol Drugs Drug Resist (3), 310-315 PubMed.
  • Stratford C H, Lester H E, Pickles K J et al (2014) An investigation of anthelmintic efficacy against strongyles on equine yards in Scotland. Equine Vet J 46 (1), 17-24 PubMed.
  • Corning S (2009) Equine cyathostomins: a review of biology, clinical significance and therapy. Parasit Vectors (2), S1 PubMed.
  • Hillyer M (2004) A practical approach to diarrhoea in the adult horse. In Pract 26 (1), 2-11 VetMedResource.
  • Love S, Mair T S & Hillyer M H (1992) Chronic diarrhoea in adult horses - a review of 51 referred cases. Vet Rec 130 (11), 217-219 PubMed.
  • Mair T S, de Weserlaken L V, Cripps P J & Love S (1990) Diarrhoea in adult horses - a survey of clinical cases and an assessment of some prognostic indices. Vet Rec 126 (19), 479-481 PubMed.
  • Vercruysse Jet al (1998) Persistence of the efficacy of a moxidectin gel on the establishment of cyathostominae in horses. Veterinary Record. 143307-309 PubMed.

Other sources of information

  • Reed S, Bayly W & Sellon D (2010) Equine Internal Medicine.
  • Rose R J & Hodgson D R (1993) Manual of Equine Practice. Saunders. ISBN 0 7216 3739 6.
  • Urquhart G M, Armour J, Duncan J L et al (1988) Veterinary Parasitology. Longmann Scientific and Technical. ISBN 0 5824 0906 3.