According to "The Tapeworm Threat" (Marcia King, Horse Illustrated, May 2005), tapeworms were not considered a problem in horses until the 1990s, when a test for tapeworms became available. Horses that were considered "poor keepers" (chronically underweight) or had severe colic actually had tapeworms. The problem was more widespread than people realized and is considered a very real threat to horses living nearly anywhere in the world.
According to "Horse Owner's Veterinary Handbook" (Thomas Gore, DVM, et al; 2008), middle aged horses (between 3 and 15 years old) rarely get symptoms of tapeworm infestations. Horses with tapeworm problems are generally under 3 and over 15.
A horse in the mild stage of tapeworm infestation will often have diarrhea, some weight loss and mild colic after exercise.
A horse with tapeworms will eventually have severe colic. Colic symptoms include looking at or kicking the belly, sweating, stamping, trying to roll, swollen abdomen and constipation.
The only reliable way to test for tapeworms is through a blood test which notes a peculiar symptom. This detects an antigen in the horse's blood that is only present when tapeworms are in a horse.
Using a worming medication every six months can reduce a horse's chances of developing symptoms of tapeworms. Medications are generally a mixture of praziquantel and macrocylic lactone drugs such as ivermectin.