Back in the 1990s, the development of antibody testing for tapeworm infestations allowed researchers to screen large numbers of horses easily and correlate exposure to tapeworms with clinical effects. Prior to this time, tapeworms were largely considered to be a minor parasite and not cause much harm.
However, this research quickly found a strong association between tapeworm infestations and the risk of impaction of the ileum or spasmodic colic (aka ”gas colic”).
Tapeworms are known to preferentially attach to the intestinal lining of the ileum, which is the last portion of the small intestine, the ileocecal valve between the ileum and the cecum, and in the cecum.
In one study, the risk of spasmodic colic was determined to be eight times higher for horses with heavy tapeworm infections. That same study estimated that 80% of ileal impactions are related to tapes.
While the connection was confirmed in several studies, the mechanism by which tapeworms cause colic remained unclear. An Italian study published in May 2010 has shed some light. Investigators found that the severity and depth of damage to the intestinal wall was directly related to the number of tapeworms present.
They also found that the muscular portion of the intestinal wall is thickened in horses with tape damage and found evidence of direct damage to the nerve supply. This would explain changes in motility leading to impaction or spasm.
Bottom Line. Most surveys have determined that approximately 50% of horses will be harboring tapeworms at any given time. The risk is higher for horses maintained on pasture. Grazing season is the peak time of year for picking up tapeworms, but we don’t know how long they remain in the horse.
Tapeworms are difficult to detect on fecal examination. The antibody tests are more sensitive, but they don’t tell you if the horse currently has them or had them in the past.
Routine deworming seems to be the best approach, at least once a year for stabled horses. Horses on pasture should be dewormed for tapes at the beginning and end of the grazing season, rationale being to remove any before contaminating the pasture, and later in case the horse has picked up more.