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 are thought to show age-related immune resistance.

History

Recent history of being wormed with a product that causes sudden ascarid death.

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(37):

• Organophosphates
• Ivermectin
• Pyrantel pamoate (Strongid)
• Piperazine

Clinical Signs

• Acute onset mild to severe colic after administration of anthelmintic (onset varies with degree of obstruction (37))
• Signs compatible with small intestinal obtruction
  • Nasogastric reflux (occasionally dead parasites are seen in reflux)
• The animal may be in poor body condition

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%(37).

Prevention

• Regular worming programme that prevents build-up of large ascarid burdens in the foal:
  • Macrocyclic lactones (Ivermectin in Equimax or Eqvalan) 0.2mg/kg
  • Tetrahydropyrimidines (Pyrantel pamoate in Strongid) 6.6mg/kg
  • Benzimidazoles (Fenbendazole in Panacur) 10mg/kg
• For foals that have not been on a controlled programme or if a large burden is suspected:
  • Fenbendazole at 5mg/kg (lower dose with only 50% efficacy against Parasacaris)
  • Followed one week later by a full dose of Fenbendazole at 10mg/kg
• Pick up faeces