Equis ISSN 2398-2977

Cyathostomins: small strongyles

Contributor(s): Maggie Fisher, Sheelagh Lloyd, Craig Reinemeyer




  • Phylum: Nematoda.
  • Superfamily: Strongyloidea.
  • Subfamily: Cyathostominea.
  • Genus:Cyathostomum, Cylicostephanus, Gyalocephalus, Cylicocyclus, Cylicodontophorus, Posteriostomum although there are other genera and over 50 different species.

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Clinical Effects



  • Equidae throughout the world.
  • Donkeys grazing with horses may have heavier burdens.
  • Prevalence and importance of cyathostomins has increased probably due to:
    • Decreased prevalence of the largeStrongylusspp   Strongylus spp  with use of macrocyclic lactone anthelmintics.
    • Development of widespread benzimidazole resistance among the cyathostomins.


  • Adult worms in cecum/colon horse.
  • Eggs passed in feces.
  • L1 hatch from egg.
  • L2/L3 develop in feces.
  • L3 migrate onto pasture.
  • L3 ingested.
  • Some L3 undergo hypobiosis.
  • L3/L4 in mucosa (lamina propria and submucosa) of large intestine (particularly cecum and ventral colon).


  • Direct via pasture contamination with L3   Cyathostomes: epidemiology - graph  .
  • In northern temperate climates, horses ingest overwintered L3. These develop to egg laying adults in the autumn, but most of these eggs are produced too late to develop and many die overwinter. Eggs are able to develop in areas with milder winters, ie sometimes in southern England.
  • Eggs produced following the activation of hypobiotic larvae in spring are pivotal to the life cycle. These eggs begin to develop and L3 are produced on pasture beginning in about July (a few in May and June), through August. These L3 when ingested might:
    EitherDevelop directly to adults.
    OrUndergo hypobiosis.
    OrRemain on pasture to overwinter. These larvae therefore begin the cycle in the following year.

Pathological effects

  • Some horses appear to develop protection.
  • Other older horses can still carry a sizeable cyathostomin population but clinical signs are less common although older horses may be unthrifty if carrying an extremely heavy burden of adults and occasionally show cyathostominosis Type II.
  • Intestinal immune reaction with infiltration of mucosa with lymphocytes, eosinophils and mast cells.
  • Type I cyathostominosis (late autumn/early winter northern temperate climates):
    • L3 are invading the intestinal mucosa even as far as the muscularis mucosa and developing or undergoing hypobiosis. They induce inflammation, goblet cell hyperplasia.
    • Accumulating adults grazing on the mucosa induce some erosion and some inflammation.
    • Very large number may produce diarrhea (intermittent)   Diarrhea: parasitic  , poor condition, sometimes edema and occasionally colic   Abdomen: pain - adult  and even deaths.
    • Migration of small numbers ofS. vulgarislarvae in the arteries may contribute.
  • Type II cyathostominosis (late winter/early spring, northern temperate climates), but can occur through winter and at other times of year:
    • The larvae when they emerge are as much as 1 cm long   Cyathostomes: infection - mucosa  . In heavy infections, the mature larvae occupy a lot of mucosa   Cyathostomiasis 01: Type II    Cyathostomes: heavy infection - mucosa    Intestine: ulceration - punctate - pathology  . Mass emergence of larvae almost simultaneously will destroy the overlying intestinal wall. In some horses there is a marked inflammatory response, predominantly eosinophils and mast cells, and edema.
    • The signs are very variable.
    • The diarrhea can be acute, watery and may contain blood   Diarrhea: parasitic  , but diarrhea may be absent.
    • The course may be longer with recurrent diarrhea   Diarrhea: parasitic  , anorexia, weight loss and edema of the limbs   Cyathostomiasis 02: Type II  , possibly colic.
    • Neutrophilia, hyperbetaglobulinemia and hypoalbuminemia. 
    • Prognosis can be poor - about 50% case fatality rate.
    • L3/L4 frequently found in the feces.

Other Host Effects

  • Light or moderate infections are tolerated well but reduce conditon.
  • Recent deworming to remove adult worms occasionally can precipitate disease.
  • It has been suggested that removal of adult worms may precipitate development of hypobiotic L3. Alternately, killing parasites (adults or mucosal L3/L4) could provoke severe inflammation precipitating clinical signs.


Control via animal


  • Type I:
  • Type II:

    Recent preliminary information has suggested that, at least in light to moderately infected horses fenbendazole causes disintegration of submucosal larvae with marked T cell and eosinophilic inflammation producing eosinophilic masses and neutrophils resulting in abscessation moxidectin produced resorption of larvae with slight nuetrophil inflammation).

    Note: Moxidectin has a relatively low therapeutic index. As it is absorbed into fat and released, there is potential for higher blood levels and there are reports of toxicity in debilitated horses such as those with larval cyathostominosis and thin horses.

  • Immunosuppression.
  • Corticosteroids: for anti-inflammatory and immunosuppressive effects. Dexamethasone    Dexamethasone   5 ug/kg IM then prednisolone    Prednisolone   1 mg/kg SID.
  • Fluid therapy    Fluid therapy: overview  .
  • Treat all grazing cohorts of horses as they are likely to be heavily infected.

Control via environment

  • Pick up feces - this will also prevent roughs and increase grazing area   Parasite control programs  .
  • Do not overgraze as this makes horses graze in the roughs containing the highest numbers of L3.
  • Pasture rotation with sheep or cattle.
  • Dose and move in early July has been shown to have effect experimentally (consider anthelmintic resistance).
  • Dose and quarantine new animals (despite deworming they may still carry hypobiotic L3 for some time).
  • Do not allow other horses' feces on your field and vice versa (pick up the feces).

Other countermeasures

Pick up feces

  • Mechanical pasture sweeping or vacuuming or manual removal.
  • This measure carried out from daily to twice weekly or even weekly in cooler climates is highly effective in reducing pasture contamination with L3.
  • It also increases grazing areas reducing the development of roughs   Pasture: lawn and roughs  . It is here that the horses defecate and prefer not to graze resulting in rank grass.
  • This is the best control measure as no anthelmintics are needed and so prevents the development of resistance.

Worm only horses with worms

  • Carry out fecal egg counts, at initially 2 week-intervals and later at >1-3 month-intervals (for </=200 epg) and at egg re-appearance period (for susceptibles), beginning in early spring (or cool months in hotter climates or wet months in tropics).
  • This identifies "susceptible" horses for deworming as they develop high counts and "resistant" horses that have egg counts </=200 epg (or </=100 epg for youngstock) that do not require treatment.
  • This means that while deworming the treated horses may have selected for resistant worms, worms in the untreated horses have not been selected and help to dilute out the resistant worms.

Strategic anthelmintic treatment

  • Beginning in Jan/Feb (northern temperate climates) or as above in other climates, deworm at the interval recommended by the manufacturer through until August/September.
  • This prevents egg production at the time (spring/early summer) most pivotal in the epidemiology for development of L3.
  • The manufacturer's time interval reflects the "egg reappearance period" or this can be determined for individual horses by regular fecal egg counts. Some horses have partial resistance and a longer egg re-appearance period.

Deworming at the egg reappearance period is the most likely to select for resistance among worms that have the genetic mutation for resistance.

  • Benzimidazoles and morantel/pyrantel   Pyrantel  : 4-5 weeks (kill only adults and immature adults).
  • Ivermectin   Ivermectin  : 6-8 weeks - kills immature adults and 34-42% effective against larvae mainly late L4.
  • Moxidectin   Moxidectin  : 12-13 weeks - kills immature adults and can be >90-95% effective against larvae mainly EL3, L3 and L4).
  • 5-day fenbendazole   Fenbendazole  : 10-12 weeks - kills immature adults and can be up to 90-95% effective against EL3, L3 and L4.
  • Continuous treatment- low dose pyrantel tartrate in feed during period of exposure (USA).
  • Likely to promote resistance to pyrantel, may prevent development of immunity.

Resistance to benzimidazoles and probenzimidazoles is widespread. Resistance to morantel/pyrantel and 5-day fenbendazole is appearing   Anthelmintics: efficacy - graph  .

Use slow annual rotation of classes of anthelmintics

  • Class 1: benzimidazoles and probenzimidazoles   Therapeutics: parasiticides  .
  • Class 2: morantel and pyrantel   Pyrantel  , and in some countries levamisole   Levamisole  is available.
  • Class 3: ivermectin   Ivermectin  and moxidectin   Moxidectin  .
  • Preferably control bots and tapeworms only in the years that the appropriate drug is used and control bots by non-chemotherapeutic measures. Blood test ELISA to identifyAnoplocephalainfected horses for treatment as required leaving others untreated.

Anthelmintic treatment

  • Where horses are known to have been grazing contaminated pastures, treatment for larvae (moxidectin   Moxidectin  or 5-day fenbendazole   Fenbendazole  ) should be instituted.



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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Geurden T et al (2013) Determination of anthelmintic efficacy against equine cyathostomins and Parascaris equorum in France. Equine Vet Educ 25 (6), 304-307 VetMedResource.
  • Matthews J (2010) Clinical forum: Drug resistance in cyathostomins. UK Vet 15 (3), 9-17 VetMedResource
  • Lloyd S (2009) Effects of previous control programmes on the proportion of horses shedding small numbers of strongyle-type eggs. Vet Rec 164 (4), 108-111 PubMed.
  • Matthews J B (2008) An update on cyathostomins: Anthelmintic resistance and worm control. Equine Vet Educ 20 (10), 552-560 VetMedResource.
  • Molento M B, Antunes J, Bentes R N & Coles G C (2008) Anthelmintic resistant nematodes in Brazilian horses. Vet Rec 162 (12), 384-385 PubMed.
  • Steinbach T, Bauer C, Sasse E et al (2006) Small strongyle infection: consequences of larvicidal treatment of horses with fenbendazole and moxidectin. Vet Parasitol 139, 115-131 PubMed.
  • Ramsey Y H, Christley R M, Matthews J B et al (2004) Seasonal development of Cyathostominae larvae on pasture in the northern temperate reqion of the United Kingdom. Vet Parasitol 119, 307-318 PubMed.
  • Love S (2003) Treatment and prevention of intestinal parasite-associated disease. VetClin North Am Equine Pract 19, 791-806 PubMed.
  • Lloyd S et al (2000) Parasite control methods used by horse owners - factors predisposing to the development of anthelmintic resistance in nematodes. Vet Rec 146, 487-492 PubMed.
  • Vercruysse J, Eysker M, Demeulenaere D, Smets K & Dorny P (1998) Persistence of the efficacy of a moxidectin gel on the establishment of cyathostominae in horses. Vet Rec 143, 307-309 PubMed.
  • Paul J W (1998) Equine larval cyathostomiasis. Comp Cont Educ 20, 509-514.
  • Love S & McKeand J B (1997) Cyathostomiasis. Equine Vet Educ 9, 253-256.
  • Murphy D, Keane M P, Chandler K J & Goulding R (1997) Cyathostome associated disease in the horse - investigations and management of four cases. Equine Vet Educ 9, 247-252 WileyOnline.
  • Reid S W, Mair T S, Hillyer M H & Love S (1995) Epidemiological risk factors associated with a diagnosis of clinical cyathostomiasis in the horse. Equine Vet J 27 (2), 127-130 PubMed.
  • Lichtenfels J R (1975) Helminths of domestic equids. Illustrated keys to genera and species with emphasis on North American forms. Proc Helminthol Soc Washington 42, 165-170 (The standard key used by most investigators studying strongyles of horses) VetMedResource.

Other sources of information

  • Little D & Gardner S Y (2003) Current Therapy in Equine Medicine 5. Ed: Robinson N E. W B Saunders. pp 161-164. ISBN: 072169540X.
  • Bowman D D (1999) Georgi's Parasitology for Veterinarians. W B Saunders Co, Philadelphia (Contians a useful set of photographs for identifications of equine strongyles).