Ascarid Impaction - Horse

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Also known as: Colic in foals — Impaction — Simple Obstruction — Small Intestinal Simple Obstruction — Small Intestinal Obstruction — Small Intestinal Impaction


Foals younger than 6 months in poor condition, on a poor worming schedule and with a heavy parasite burden.[1] Adult horses demonstrate age-related immunity which starts to develop from around 6 months[2] and is substantial by about 15 months depending on worm exposure.


Recent history (within the previous 24 hours) of being wormed with a product effective against ascarids.


Products that cause sudden death[1] or paralysis[3] of ascarids, particularly Parascaris equorum within the small intestine. Adult ascarids are large and thus readily cause small intestinal impaction.[1] It has been suggested that disruption of the surface of the ascarid releases antigenic fluids that inhibit intestinal muscular activity, thereby increasing the likelihood of intestinal obstruction.[3] Anthelmintics that are suspected to induce episodes of ascarid impaction include[4]:

  • Organophosphates
  • Ivermectin
  • Pyrantel pamoate
  • Piperazine

Clinical Signs

  • Acute onset mild to severe colic after administration of anthelmintic (onset varies with degree of obstruction[4])
  • Signs compatible with colic due to small intestinal obstruction[1]
  • The foal may be in poor body condition with a poor coat, pot-bellied appearance and stunted growth.[3]


  • History
  • Clinical signs referable to small intestinal obstruction
  • Abdominal imaging (radiography and ultrasonography) may demonstrate multiple loops of distended small intestine, but imaging is not required if the clinical presentation indicates immediate surgery.[1]


  • Medical treatment: immediate treatment of hypovolaemic shock resulting from sequestration of fluid in small intestine
  • Surgical treatment: enterotomy made over the intraluminal impaction and removal of ascarids


Fair in cases that are rapidly treated but poor in foals with evidence of hypovolaemia and septic shock. Surgical cases also carry a poorer prognosis than cases which are resolved medically.[3] In a recent study, long term survival of 25 affected horses was 33%.[4]: Failure to relieve the impaction may lead to intestinal rupture.[3]



Ensure a regular worming programme that prevents a build-up of large ascarid burdens in the foal:

  • Macrocyclic lactones such as ivermectin. NB: resistance may be widespread.[5]
    • Equimax Oral Paste for Horses: (also contains praziquantel) administered PO. Treatment is recommended at least twice a year. Not for foals under 2 weeks of age. Withdrawal period 35 days.
    • Eqvalan Paste for Horses: Foals should be treated initially at 6 to 8 weeks of age, and routine treatment repeated as appropriate. Meat withdrawal 21 days.
  • Tetrahydropyrimidines such as pyrantel embonate.
    • Strongid®-P Paste: For foals over 4 weeks of age up to 8 months dose every 4 weeks. For horses over 8 months of age dose every 6 to 8 weeks, but when at grass dose every 4 to 6 weeks. Always dose 3 to 4 days before turning out after in-wintering.
  • Benzimidazoles such fenbendazole
    • Panacur Equine Oral Paste: It is recommended that horses and ponies are routinely wormed with a single dose of this product every 6 to 8 weeks. It is recommended that horses are administered the 5-day Panacur treatment in the Autumn (ideally late October/November) and again in the Spring (ideally in February) to cover for encysted small redworms. All new horses whose worming history is unknown should also be administered the 5-day Panacur treatment. Pregnant mares and young foals may also be safely treated with Panacur at the recommended dosage levels. Not to be used in horses intended for human consumption. For foals that have not been on a controlled programme or if a large burden is suspected it has been suggested that fenbendazole be given at a lower dose (only 50% efficacy against Parasacaris), followed one week later by a full dose. This may avoid ascarid impaction.


  • Clean pasture management should be encouraged including the regular removal of faeces.
  • Stables that have been occupied by foals or weanlings should be thoroughly cleaned.
  • Mares should be bathed and their udders washed before arrival at foaling premises to remove any sticky ascarid eggs on the coat.

Avoiding Anthelmintic Resistance

Care should be taken to avoid the following practices because they increase the risk of development of resistance and could ultimately result in ineffective therapy:

  • Too frequent and repeated use of anthelmintics from the same class, over an extended period of time.
  • Underdosing, which may be due to underestimation of bodyweight, misadministration of the product, or lack of calibration of the dosing device (if any).

Suspected clinical cases of resistance to anthelmintics should be further investigated using appropriate tests (e.g. Faecal Egg Count Reduction Test). Where the results of the test(s) strongly suggest resistance to a particular anthelmintic, anthelmintics belonging to another pharmacological class and having a different mode of action should be used.

  • Regular faecal worm egg counts (FECs) should be conducted to monitor the effects of the anthelmintic schedule and the presence of resistance
  • The worming programme for adult horses should reflect the circumstances and efficiency of clean pasture management. A strategic or targeted strategic dosing plan is recommended.

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Ascarid Impaction in Horses publications


NOAH Compendium of Data Sheets for Animal Medicines. At

  1. 1.0 1.1 1.2 1.3 1.4 1.5 Blikslager, A.T (2010) Obstructive Disorders of the Gastrointestinal Tract in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) Equine Internal Medicine (Third Edition), Chapter 15, Saunders.
  2. Clayton, H.M (1986) Ascarids. Recent Advances. Vet Clin North Am Equine Pract, 2(2):313-328.
  3. 3.0 3.1 3.2 3.3 3.4 Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition), Merial.
  4. 4.0 4.1 4.2 Cribb, N.C, Cote, N.M, Boure, L.P, Peregrine, A.S (2006) Acute small intestinal obstruction associated with Parascaris equorum infection in young horses: 25 cases (9185-2004). N Z Vet J, 54:338-343. In: Blikslager, A.T (2010) Obstructive Disorders of the Gastrointestinal Tract in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) Equine Internal Medicine (Third Edition), Chapter 15, Saunders.
  5. Lind, E.O, Cristensson, D (2009) Anthelmintic efficacy on Parascaris equorum in foals on Swedish studs. Acta Vet Scand, 51:45.

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