Ascarid Impaction

Also known as:

Colic in foals Impaction Simple Obstruction Small Intestinal Simple Obstruction Small Intestinal Obstruction Small Intestinal Impaction

Signalment

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

History

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

Aetiology

Products that cause sudden death or paralysis of ascarids, particularly Parascaris equorum within the small intestine. Adult ascarids are large and thus readily cause small intestinal impaction. 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. Anthelmintics that are suspected to induce episodes of ascarid impaction include:

• Organophosphates
• Ivermectin
• Pyrantel pamoate
• Piperazine

Clinical Signs

• Acute onset mild to severe colic after administration of anthelmintic (onset varies with degree of obstruction)
• Signs compatible with small intestinal obtruction
  • Nasogastric reflux (occasionally dead parasites are seen in reflux)
• The foal may be in poor body condition with a poor coat, pot-bellied appearance and stunted growth.

Diagnosis

• 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.

Treatment

• 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

Prognosis

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. In a recent study, long term survival of 25 affected horses was 33%.: Failure to relieve the impaction may lead to intestinal rupture.

Prevention

• Regular worming programme until at least 15 months of age that prevents a build-up of large ascarid burdens in the foal:
  • Macrocyclic lactones (Ivermectin in Equimax® or Eqvalan® paste) 0.2mg/kg at 60 day intervals (NB: NOT the drug of choice because resistance is widespread)
  • Tetrahydropyrimidines (Pyrantel embonate in Strongid-P® paste) 19mg/kg PO: for foals over 4 weeks of age up to eight months - dose every four weeks. For horses over eight months of age routinely dose every 6 to 8 weeks, but during the summer and

autumn when at grass dose every 4 to 6 weeks. Always dose three to four days before turning out after in-wintering.

• • Benzimidazoles (Fenbendazole in Panacur®) 10mg/kg PO at 56 day intervals
• For foals that have not been on a controlled programme or if a large burden is suspected:
  • Fenbendazole at 5mg/kg PO (lower dose with only 50% efficacy against Parasacaris)
  • Followed one week later by a full dose of Fenbendazole at 10mg/kg PO
• Regular faecal worm egg counts should be conducted to monitor the effects of the anthelmintic schedule and the presence of resistance
• Clean pasture management including the regular removal of faeces
• The worming programme for adult horses should reflect the circumstances and efficiency of clean pasture management. A strategic or targeted startegic dosing plan is recommended.
• Thorough cleaning of stables that have been occupied by foals or weanlings
• Bathe mare and clean udder before arrival at foaling premises to remove any ascarid eggs on the coat

References

Pasquini, C, Pasquini, S, Woods, P (2005) Guide to Equine Clinics Volume 1: Equine Medicine (Third edition), SUDZ Publishing