HERTFORD, N.C. (WAVY) — For three weeks, horses at stables in Hertford, North Carolina, have been getting sick.

“It’s like a roller coaster ride, you think you’re over the threshold and then another one goes down and another one goes down,” Carla Bundy, owner of New Hope Stables, told NewsNation affiliate WAVY-TV.

The stables had a total of 17 horses, of which 15 became sick with abdominal issues and four died. The sick horses were hooked up to IVs and get their temperatures taken daily. The horses have been tested for infectious diseases both common and uncommon to the area.

“After all that came back negative, they started pulling feed, water samples. Ok maybe it was accidentally something in the feed? No one trying to harm them deliberately, but all of that stuff is coming back negative,” said volunteer Nancy Harrell.

Now they worry the horses could have been intentionally poisoned. They say the horses haven’t been left alone since, and the stables have increased security measures.

“We are taking steps to put in security cameras, put in security lights,” said Harrell.

The liver and heart from one of the horses that died was sent to a Pennsylvania lab for further testing. Just one IV bag is $150, and one horse needed more than 20 bags. Emergency vet calls are $500. They say blood tests to look for a disease are $1,000 a horse.

The Oaks Veterinary Clinic set up a GoFundMe page to help the owners with the vet bills. The staff at the stables say they’re grateful for the veterinarians, who have been working around the clock to care for the horses.

“We’ve been completely and totally blown away by the support,” said Harrell.