Symptoms of equine parasite infection vary a lot. Mild parasitism leads to failure to thrive, weight loss, reduced food conversion efficiency (competition for nutrients), impaired growth and predisposition to other infectious diseases. On the other hand, a higher level of parasitism may cause colic, diarrhea, emaciation or unexpected sudden death. Certainly, many horses that appear to be healthy could be harboring a substantial parasite burden and represent an important source for pasture contamination by parasite eggs.
For more information, please read the article Equine Gastrointestinal Parasites.