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  • Cyathostomins is the collective name given to over 50 species of small strongyle

  • The most prevalent and pathogenic group of parasites in horses today

  • These parasites have an encysted stage in their lifecycle when larvae can remain dormant in the gut wall for up to 2 years

  • Encysted burdens cannot be detected by faecal egg counts and mass emergence of encysted larvae causes disease

  • This group of parasites has documented resistance to a variety of chemicals




  • Non migratory lifecycle

  • Contains stages of encysted development within the gut wall


The horse ingests 3rd stage larvae (L3) from the pasture. These migrate to the large intestine where the larvae encyst in the gut wall. At certain times of year and under certain conditions they can become inhibited and lie dormant in the gut wall in this state. Inhibited larvae can take up to 2 years to reactivate and continue their lifecycle from the point of encysting. Under normal conditions the encysted L3 develop into L4 before re-emerging in the large intestine. Here they moult to L5 adults and the females start laying eggs, the eggs are excreted in the dung. The eggs hatch to L1 larvae in the dung and moult to L2 before moulting to L3 before the cycle starts again.























What do they look like?


  • Small red worms between 5 and 12 millimetres in length

  • Feed on blood so often dark red but can also be white























What damage can they cause?


  • Most dangerous if present in high numbers and emerge from encysted nodule in the gut wall en masse, this condition is called larval cyathosominosis and causes extensive damage to the lining of the large intestine

  • High encysted burdens can impair nutrient uptake


Clinical Signs


  • Loss of condition

  • Increased or decreased appetite

  • Diarrhoea

  • Dramatic weight loss

  • Colic

  • Low levels of protein in the blood leading to oedema (fluid swelling) under the abdomen


Diagnosis and Detection


  • Cyathostomin infection by adult worms can be detected and monitored with faecal egg counts.

  • Encysted stages cannot be monitored with faecal egg counts because they are immature larvae not egg laying adults.

  • A common problem is encysted emergence. Your horse could have a history of very low or clear egg count results and then for no apparent reason the results can go up. If there has been no change in the horse’s management or grazing conditions this could be indicative of an emergence of inhibited larvae that have continued their lifecycle to become egg laying adults. Over time the pattern of faecal egg counts results will help to indicate if emergence has occurred or not.


















If a Cyathostomin burden is detected by a faecal egg count the horse will be treated according to the number of eggs seen.

  • A LOW reading of between 0-200 eggs per gram (epg) does not require treatment. The horse’s natural immune system should be able to cope with the infection, however, it is recommended to continue carrying out faecal egg counts regularly.

  • A MEDIUM (200-500epg) or HIGH (500+ epg) reading indicates that the horse has a parasite burden that the horse cannot deal with alone and needs treating to get this under control.


Adult worms are easy to kill using a range of wormers, however, encysted infections are not as easily cleared. The only chemicals effective against encysted infections are moxidectin (Equest/Equest Pramox) and fenbendazole (Panacur Equine Guard).


Preventative Measures


  • Routine faecal egg counts to detect and monitor infection

  • Reducing risk in the grazing environment through good pasture management e.g.poo picking and resting land


Current issues in 2015


  • The most prevalent and pathogenic group of parasites in horses today

  • Typically over 95% of a horse’s parasite burden will be Cyathostomins

  • This group of parasites has documented resistance to the benzimidazoles group of anthelmintics worldwide

  • Some resistance to pyrantel and ivermectin has also been documented

  • Reduced efficacy has been seen with moxidectin treatments

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