'Tis the season. Botfly season. The flies themselves don't harm horses, but their larvae are common parasites of the equine digestive tract.
Ever see this on your horse's legs? (They may appear on other parts of the horse's body as well.) Marye brought these bot eggs with her when she arrived. They were immediately removed.
Last fall, I visited a herd of horses whose legs were literally covered in solid masses of bot eggs. Viewing the photo of "Bot Larvae" at the link listed below will illustrate why that was a disturbing sight. Aside from the potential masses of larvae in the stomachs, it must be tortuous to have that many larvae burrowing into the tissues of the gums and tongue.
Ivermectin will control bot larvae, but wouldn't preventing infestation be easier than managing the subsequent effects?
For more on bots, visit http://www.extension.org/pages/Stomach_bots_in_horses