This mare at Assateague Island National Seashore died after eating a large amount of dog food/NPS

A 7-year-old mare that died at Assateague Island National Seashore had eaten "large quantities" of dog food that caused a blockage, and eventual rupture, of her intestine, according to seashore staff.

The mare, which had gotten to the dog food on July 17, was found dead three days later in a campground on the seashore that straddles the Virginia-Maryland coastline.

"Why would dog food be deadly for an Assateague horse?," a post on the seashore's Facebook page asked. "Most dog food contains corn, soybean, and animal products with a very high carbohydrate, protein and fat content. This recipe is too rich, and in large amounts is deadly for an Assateague wild horse whose natural forage consists of low nutrient, high fiber saltmarsh and beach grasses.

"Sadly, this incident demonstrates that 'a fed horse is a dead horse," the post continued. "While the dog food may not have been given directly to the horse, the dog food was not properly stored away from the horses and other wildlife. All food, including your pet’s food, must be properly stored. This tragic incident could have been prevented by simply storing pet food in a vehicle."

Seashore staff listed a few simple rules that visitors can follow to protect wildlife:

• Horses can open snap-on lids and latches. Coolers and containers “stored” under picnic tables are not secure from horses and wildlife. Secure all coolers with a nylon strap to prevent wildlife from opening.
• Secure all tote or beach bags with a zippered closure. Horses can easily access open totes and bags.
• Store all unattended food in your vehicle.
• Store all pet food in your vehicle. Do not leave your pet’s food and water bowls unattended. Horses, like your pets, are opportunists and will take advantage of a free meal. 
• Keep food stored if horses are in your immediate vicinity. Wait until they have moved on before beginning your meal. 
• Dispose of your trash immediately in dumpsters. The smell from food wrappers will attract horses and other wildlife, and if ingested could cause death.