Many foals are born healthy and are growing well until about three months of age and then growth seems to slow, according to Dr. Kathleen Crandall, a nutritionist with Kentucky Equine Research. Certainly, heavy burdens of roundworms can cause weight loss and all foals should be dewormed every month for three months starting at two months of age to control roundworms. Many cases of roundworms occur because ivermectin is used as a dewormer and ivermectin is not very effective against roundworms, regardless of the label claim.
The other causes of weight loss include respiratory and digestive infections. Intestinal disease is usually caused by E. coli, Salmonella, Clostridium and Lawsonia; Lawsonia is really an interesting bacteria in that many of the horses do not have diarrhea but still lose weight. Many of them will have intermittent colic and may be lethargic as well as accumulating fluid in the lower legs due to intestinal protein loss. As far as respiratory causes of weight loss, Rhodococcus is a bacterium that causes pneumonia and abscesses to develop in the foal’s lungs. These foals lose weight and have a fever, loss of appetite, are lethargic. Dr. Crandall indicates that it is important to determine the body condition score of foals differently than adult horses. Foals generally add fat along the withers and behind the elbows and only then does fat accumulate along the back and ribcage. Because of this, foals that have their ribs showing may be normal weight while adult horses appearing in this manner are too thin.