Small Redworm

Posted by Wormers-direct on 7th Jan 2020

Small Redworm


SMALL REDWORM (Cyathostomes)

Small redworm (small strongles/cyathostomes) are the most common parasite to affect your horse. They are up to 2.5cm long, thin and can range in colour from white to red. Small redworm larvae are picked up by horses whilst grazing. Historically, the key period of risk was spring and summer. However, recent climatic changes have led to hot, dry summers and warm, wet autumns and winters. Due to these changes, spring and autumn have now become the periods of greatest risk. Furthermore, recent studies that have involved monitoring pasture infectivity have shown high levels of infective larvae present on turnout paddocks, even in January after snow. This means that “winter worming” is now just as important as worming during the summer grazing season. Small redworm larvae can develop into adults within a period of five weeks but in autumn however, their development is prolonged and they remain in cysts as tiny inhibited larvae (early L3 larvae) or as larger late 3rd and 4th stage larvae within the gut wall. These are broadly termed “encysted larvae” but in the late winter or spring, an unknown trigger causes these larvae to resume their development. The symptoms of simultaneous emergence of large numbers of 4th-stage larvae are colic, weight loss, diarrhoea and devastating damage to the large intestine, sometimes resulting in death. Even before they emerge, large numbers of encysted larvae can cause fatal disease. The encysted larvae stages can account for over 90% of the total small redworm burden in a horse, so controlling these larval stages is an important part of any equine worm control programme. Horses particularly at risk are those that have grazed on heavily stocked and contaminated pasture, have not been regularly wormed, or have mixed with other horses that have not been treated regularly. In addition, horses under the age of 6 are at an increased risk.

Horses that are subject to effective worming procedures and graze in areas where good pasture management is practised throughout the year will pass fewer small redworm eggs in their droppings, this means the levels of worm burden will be lower and therefore the risk of encysted larvae will be reduced. However, these worms have a life cycle of between 6 weeks and up to 2 years so caution must be exercised even when pastures have been rested.

It is possible for horses with an early EL3 infestation to appear healthy and well. Furthermore, if a Faecal Egg Count (FEC) test is carried out on an EL3-infected horse, the small redworm burden may not appear to be high because it is mainly present in the dormant EL3 form.

Only 2 anthelmintics have claims of efficacy against encysted small redworms and they are:

Fenbendazole (5 day course)

Moxidectin (single dose)


Diarrhoea, rapid and severe weight loss, colic and can even KILL your horses. As cyathostomes attach themselves to the gut and are absorbed into it, they reduce the efficacy of the gut wall to absorb essential nutrients which can lead to general ill thrift in the horse and an incapability to utilise properly a balanced diet.