The vets agree with you: horses cannot throw up. That's what they are taught in veterinary school; that's what the books say. Horses have a band of muscle around the esophagus as it enters the stomach. This band operates in horses much as in humans: as a one-way valve. Food freely passes down the esophagus into the stomach as the valve relaxes but the valve squeezes down the opening and cuts off the passage for food going back up.
Horses, however, differ from us because their valve really works. Humans can vomit. Horses almost physically can't because of the power of the cut-off valve muscle. Also, the esophagus meets the stomach at an angle which enhances the cut-off function when the horse's stomach is bloated with food or gas. Then the stomach wall pushes against the valve, closing the esophagus even more completely from the stomach. Normally, the mechanics are such that the horse's stomach ruptures before the valve yields.
If material does pass from stomach out the esophagus, the horse is dead or nearly so. That's why horses can't vomit. But, sometimes they do. Rarely, to be sure.
Brent Kelley, veterinarian for a valuable mare during a difficult delivery, tells of one incident. After the mare delivered her foal, she laid there as if dead — not even responding to her baby's nickers. Worse: she threw up and stopped moving. Brent thought she was gone.
"But then the old girl rolled up on her sternum and called to her foal," says Kelley.
The mare lived another six years, had four more foals, and died well into her 20s.
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