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This is a clinical term encompassing these superfamilies of bursate nematodes:





Their eggs are very similar and all contribute to parasitic gastroenteritis.

Strongyles (Red worms)

The strongyles that occur in the horse can be divided on the basis of size into two groups

  • Large strongyles
    • Strongylus species (3 species; used to be widespread prior to the introduction of worm control programmes; now uncommon)
    • Triodontophorus species (common)
  • Small strongyles
    • Also known as Cyathostomins (preferred term), cyathostomes, trichonemes or small redworms
    • Cyathostomins (widespread, including 4 genera and over 40 species of worms)

General epidemiology of large and small strongyles

Strongylosis occurs in

  • Young horses
  • Adult animals (especially if overcrowding, poor hygiene)
  • Animals on permanent pasture

Sources of infection

  • Overwintered L3 on pasture
  • Many adult horses pass significant numbers of strongyle eggs throughout their lives
  • "Spring rise" in faecal egg output occurs in both breeding and non-breeding horses

Pattern of infection on pasture

  • Pattern of L3 on pasture is similar to gastrointestinal worms in cattle
  • Main difference is that the mare makes a major contribution to pasture contamination (c.f. cow)

Hypobiosis of cyathostomin larvae

  • Occurs throughout the year, but particularly in late summer/autumn
  • EL3 may remain arrested for years
  • Resumption of normal development can occur
    • seasonally in late winter/early spring
    • following removal of adult worm population via anthelmintic treatment

Larval cyathostominosis

  • Sudden onset diarrhoea and/or weight-loss
  • Diagnosis difficult, prognosis guarded
  • Generally in late winter/spring
  • Usually <5 years old
  • Sporadic, but increasing in incidence
  • Hyperglobulinaemia, especially IgG(T)
  • Hypoalbuminaemia
  • Leukocytosis
  • Sometimes peripheral oedema
  • Faecal egg-count low (disease caused by emerging larvae)
  • Larvae may be found in faeces or on faecal glove


Resumed development of massive numbers of larvae → subsequent emergence of bright red L4 → massive eosinophilic infiltration of mucosa → catarrhal and haemorrhagic colitis

Control of cyathostomin infections in horses


  • Only 3 chemical groups currently available
    • Avermectin/milbemycins
    • Benzimidazoles
    • Pyrantel
  • Resistance is an emerging problem (especially to benzimidazoles)

Target life-cycle stages

  • These are not all equally susceptible to each anthelmintic
  • Pyrantel is affective against
    • Adult worms in the lumen
  • Ivermectin or a one off administration of Fenbendazole is affective against
    • Adult worms and L4 in the lumen
  • Moxidectin or a 5 day course of Fenbendazole is affective against
    • Adult worms and L4 in the lumen
    • Developing and hypobiotic L3 in the mucosa

Egg reappearance period

  • This is the time from treatment until eggs reappear in the faeces. It is determined by
    • degree of activity against mucosal larval stages
    • persistency of anthelmintic treatment

Prevention of pasture contamination

  • The objective is to create safe grazing by preventing depostion of strongyle eggs onto pasture
  • Treat all grazing horses at intervals determined by
    • Egg reappearance time of chosen anthelmintic
    • Risk level
  • Treat all new arrivals and stable for 48-72 hours so that eggs are not passed onto pasture
  • Adopt strategy that will minimise risk of resistance developing (you may need to include tapeworm and stomach bots in your scheme)
  • No new eggs passed → no new L3 developing, however it is important to use epidemiological knowledge to predict how long existing L3 will survive as the pasture will not be safe for use before then
  • Remove faeces from paddocks at least weekly:
    • This markedly reduces dependence on anthelmintics
    • Increases available grazing
    • But is labour intensive and less effective in rainy weather
  • Examine faecal samples twice yearly to monitor effectiveness of your chosen strategy

Pasture management

  • Reserve clean grazing for nursing mares and foals
  • Rest pastures used the previous year until overwintered L3 have gone
  • Mixed or alternate grazing with cattle or sheep
    • These are refractory to most horse worms, except T.axei

Chemoprophylaxis of larval cyathostominosis

  • Needed if a horse is known to have grazed heavily contaminated pasture and may therefore be harbouring massive numbers of hypobiotic larvae
  • Fenbendazole treatment given daily for 5 consecutive days in autumn or winter will reduce the risk of clinical disease developing.


  • horses
  • it is common to find fibrous tags incidentally on the liver surface and adjacent diaphragm
  • these are remnants of fibrous repair following the egress of the parasites from the liver

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