A sobering study from the University of Guelph, Ontario Veterinary College, reports on the short- and long-term outcome of 25 cases of Ascarid (roundworm) small-intestinal impactions in foals. Sixteen had simple impactions with the worms causing bowel obstruction, but nine had other associated problems such as intussception (a telescoping of one segment of bowel inside another), or volvulus/twist. All except one were less than 12 months old and all required surgery. Short term survival living long enough to be discharged was 64% for the entire group, close to 80% in the uncomplicated obstructions. However, long-term survival, for greater than a year, was only 27%.
This study drives home the point in our page 3 deworming article that young horses are a high-risk group for parasites. The study found that 72% of these horses had been dewormed within 24 hours of the colic/obstruction. The authors wondered if roundworm resistance to ivermectin, which has been reported in Canada, may have been part of the reason for the large worm burdens. However, the close timing between deworming and obstruction may well also mean that the drugs were effective and the problem arose because a large number of parasites were killed and swept down the intestine at one time.
This means that it is important to start deworming at an early age and set up a frequent enough schedule to prevent large burdens from occurring in the first place. Consult with your veterinarian about a program suitable for your situation, and check fecals periodically to make sure it is working. By the time you see the outward signs of a parasite problem (see article), it may be too late.
Ascarids should be considered a health threat to all young horses. The eggs can survive on soil under very adverse conditions, for periods as long as 10 years. The larvae remain protected within the egg until it is swallowed.