They are beetles that live near alfalfa field in certain areas. They die during cutting and are left in the hay which when consumed by horses they release the toxin Cantharidin. Even just a few beetles can kill a horse.
"Cantharidin is a terpenoid compound that is a strong vesicant that is extremely irritating to skin and mucous membranes and is rapidly absorbed and eliminated in the urine. Severe damage to the oral and gastrointestinal mucous membranes, urinary tract, and other organs occurs and in severely poisoned horses shock and death can occur within a few hours. The severity of poisoning is dose-related and less-severely-affected animals can be treated successfully. There is some uncertainty whether residual effects occur in horses that survive poisoning; however, given the fact that the toxin has effects on many organ systems surviving horses should be monitored closely."
In the case of my yearling we scoped her stomach (she lived at the vet's place thankfully) and her entire G.I tract was blistered from the inside out. There were 3 other horses in the barn at the same time, older horses that most have gotten the parts of the bale that were less/not infested. Its more common in certain areas than others, Texas is known for it, which was where my mare was at, but we had specially ordered hay shipped in from non-prone states but turned out the hay dealer didnt get his full shipment and replaced our order with locally grown hay without saying anything
