interior page header


ascarids parascaris equorumLarge Roundworms (ascarids)Photo kindly provided by University of Liverpool Diagnosteq Lab

Ascarids, (also known as roundworms) are parasites which are mainly a problem to foals. However, small numbers can also be carried by adults.

The adult worms are very large and can be up to 40cm in length. Once the larvae of this parasite are swallowed, they pass through the gut wall, via the liver to the lungs. Heavy infestations can cause coughing as the larvae travel through the lungs.

Adult worms can cause intestinal impaction. They pose a considerable threat to young horses and their developing immune systems, and they have to potential to kill by triggering colic. Even if a young horse escapes colic, a heavy ascarid burdened young horse will appear depressed and its normal growth will be affected.

The most common ascarid to infect horses is Parascaris equorum.The eggs laid by the large females pass out into pasture protected by a tough shell which equips them well for survival.  They can withstand drying conditions and even freezing; waiting for that moment when a passing horse ingests them with grass.

Symptoms of Ascarid Infection In Foals

It goes without saying that the migratory habits of ascarids cause damage. However, the impact of ascarids in the horse will depend upon the degree of infection and the whether the larvae are in their migratory phase.

  • When migrating, a horse may well show signs of respiratory problems. It may have a nasal discharge or a cough and possibly a fever. Antibiotics will make no difference as they are not effective against ascarids.
  • A heavy burden in the gut will likely show in the classic signs of a poor coat, a loss of weight, a pot-bellied appearance and sluggishness.
  • Ascarids present one other notable danger. Their size is such that collectively they have the ability to block the intestinal tract of a young horse and trigger a potentially fatal bout of colic.

Ironically, this colic can be triggered by a dose of drench. The worms are killed, fall away from the intestinal lining and cause a blockage. This is the primary reason that you not only need to drench young horses, but do so regularly to ensure there is no potentially fatal build-up of mature ascarids in the gut.

Symptoms of Ascarid Infection In Adult Horses

Adult horses are symptomless but can carry them and provide a reservoir for infestation to foals.

Which Actives Treat Large Roundworms (Ascarids)?

Ivermectin Moxidectin Ivermectin/Praziquantel Moxidectin/ Praziquantel
Pyrantel Praziquantel Fenbendazole Mebendazole


Bimeda Equine is proud to promote best practice targeted worming regimes, to help protect both today’s horses and future generations.

To find out more about best targeted worming regimes, visit our Best Practice Advice section.

To view and download our materials, such as our Horse Owner’s Guide To Worming, visit out Library Section.

Contact Us - UK

Bimeda UK
Unit 2, Bryn Cefni Industrial Park,
Llangefni, Anglesey, Wales LL77 7XA
Phone: +44 (0) 1248 725400