IT MIGHT BE HARD to find a nail in a haystack, but sometimes horses do just that — with unhealthy results.

Consider the case of the horse in Indiana that apparently swallowed a shoeing nail with its feed. The nail sat in the animal’s intestinal tract for a long time, perhaps years, without puncturing  any organs. The horse’s system gradually built layer upon layer of a hard shell on the nail, and eventually the nail was encased in what amounted to a rock.

“It’s called an enterolith,” says Nickie Baird, a veterinarian at Purdue University in West Lafeyette, Ind. He notes that enteroliths can grow around any foreign object (shoeing nails are an uncommon source) or even without one.

A Recurring Problem

“Eventually in such cases, the horses show signs of colic,” Baird says. “While these horses might require surgery the first time they colic, more often they experience recurring colic episodes.

“The enterolith acts like a ball valve, obstructing the large intestine, wedging in a narrow point in the digestive tract. The flow of feedstuffs is blocked by the obstruction. Many horses respond to treatment with pain medication and mineral oil to loosen up the feces, and everything seems fine.

“The enterolith may fall back down into the dorsal colon and open up the pathway, and the horses will go on about their business,” Baird notes. “But a week or a month or so later, they may colic again. It’s not unusual to see a horse do that two…