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  • Parascaris equorum is the most common species of ascarid in the UK to infect horses

  • Ascarids, also known as roundworms, are nematodes

  • They mainly infect youngstock (up to 18 months old), older animals generally gain their own immunity to this parasite

  • Resistance has been seen to ivermectin and moxidectin


Parasite lifecycle


The eggs are excreted in the dung; the larvae develop into L2 stage within the egg and remain on the pasture. The horse ingests the eggs while grazing, the larvae hatch out and migrate to the small intestine. Within 48 hours they migrate to the liver and moult to L3 stage. From here the larvae migrate to the lungs, they are then coughed up and swallowed returning back to the small intestine approximately 2 weeks after being ingested. At this point they complete their L4 and L5 moults to become adults and start reproducing. Eggs are excreted in the dung and the cycle starts again.























The eggs have a sticky outer layer enabling them to stick firmly to blades of grass and be easily ingested by horses. Eggs can also stick to horse’s legs and the mare’s udders, a particular problem for nuzzling foals. Ascarid eggs are surrounded by a thick outer shell which enables them to survive outside the horse. They can withstand dry and freezing conditions to survive in the pasture for years and remain infective



What do they look like?


Long white roundworms 15-40cm in length


















What damage can they cause?


  • The main problem is impaction or even blockage of the small intestine due to the size of the parasites

  • They can cause impaction colic and intestinal rupture in severe cases


Clinical Signs


Although adult horses can ingest eggs when grazing, infection is only clinically significant in young horses, up to 18 months old. Infection can be characterised by the following symptoms:


  • Poor growth

  • Pot-bellied appearance

  • Diarrhoea

  • Fever

  • Coughing

  • Nasal discharge

  • Colic

  • Death


In severe cases acute colic due to build up of the worms in the intestine, may result in the horse needing surgery.


Diagnosis and Detection


  • Ascarids can be detected through faecal egg counts

  • Foal and youngstock need to be monitored the closest as this age group are the most vulnerable

  • Faecal egg counts only indicate the presence of mature, egg laying adults and will not show infection with other larval stages.






















  • ALWAYS follow the advice of a vet or a qualified advisor when worming against ascarids. Sometimes after worming a build of dead worms can cause intestinal impaction and blockages which in some cases can only be treated with surgery.

  • Ivermectin, moxidectin and benzimidazole can be used as they remove both the migrating larvae and the adult worms

  • Foals should be treated from 4 months of age


Preventative Measures


  • Routine faecal egg counts should be performed to allow the earliest detection of ascarid infections

  • As faecal egg counts do not detect all larval stages youngstock should be wormed regularly from 4 weeks of age to prevent ascarids from developing to egg laying age

  • Frequent poo picking to remove eggs from the pasture

  • Compost heaps may generate enough heat to kill off the eggs

  • Feeding young horses off the ground using feed buckets, hay nets etc. to reduce exposure


Current Issues in 2015


  • Resistance to ivermectin and moxidectin has been seen




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