AUGUSTA – The seven horses found dead last month at a Searsport farm died from what now is being considered an accidental poisoning, the state veterinarian said Wednesday.

Tests on the dead horses’ manure sent to the University of Wisconsin validated the presence of monensin, an anti-bacterial additive in cattle feed that is deadly to horses.

State veterinarian Dr. Don Hoenig said the test was “extremely significant” and represented the first case of monensin poisoning he had seen in his 21 years in Maine.

“This is so unusual that it points out the need for some public education,” he said.

Another seven horses were removed live from Kathy Hecht’s home and taken to Goose River Farm, the home of Lynn Boynton of Swanville.

Boynton said Wednesday three of the seven have been put down, but the other four remain at her farm. Hecht could not be reached Wednesday.

No decision has been made on whether to return those horses to Hecht.

Hoenig said he did not know how the horses got the tainted feed at Hecht’s farm in Searsport.

Hoenig said testing of Hecht’s commercial horse feed revealed no contamination and no traces of monensin.

The investigation began in February at Hecht’s small house on Mount Ephraim Road.

At a recent meeting of the Animal Welfare Advisory Committee, Animal Welfare Division veterinarian Christine Fraser said a necropsy of one of the dead horses was conducted by a pathologist at the University of Maine in Orono and “no cause of death was found.”

Tissue, blood and manure samples were sent to a variety of university laboratories across the country after veterinarians said they believed the deaths were not from an infectious disease or mistreatment.

“Right from the start, the local veterinarian was not concerned that this was an animal cruelty issue,” Hoenig said. “Monensin fits the clinical pictures and points out this was not a cruelty case.”

Hecht reportedly brought 35 horses to Maine from North Dakota in 2004 and almost immediately caught the attention of animal welfare agents.

Fraser said she had been working with Hecht on the horses’ care and that Hecht had complied with every suggestion. “She was working with us and following through,” Fraser said.

Hoenig said there were some problems with Hecht’s horses. “They were not perfect, but they were not emaciated. They had food, water and shelter, and she was cooperative. This didn’t reach the threshold for seizure.”