In December of 2019, the Department of Fish and Wildlife confirmed that six fawns on the Vineyard, all male deer that had been born the previous spring, were found paralyzed in a broad region between Chilmark and West Tisbury.

Three months later, after significant testing, there is still no definitive cause, although state biologists have been able to narrow their investigation to a specific type of chemical present in medicated livestock and poultry supplements.

The chemical has many different trade names, but the most common form is called Monensin, said David Stainbrook, a Department of Fish and Wildlife (MassWildlife) deer biologist who is responsible for assessing the local and statewide health of the deer herd.

“If deer are exposed to a high level of these compounds, it can cause behavior like what we are seeing,” he said, speaking to the Gazette by phone. “We are not certain . . . [but] it is reasonable to think that these could be related.”

The mysterious paralysis was the same for each animal. The fawns lay on the ground unable to move their legs, but able keep their heads in the air. State officials sent the fawns to a veterinary diagnostic laboratory in New Hampshire. The paralyzed fawns were either dead on arrival or euthanized.

Testing officially ruled out chronic wasting disease, eastern equine encephalitis, epizootic hemorrhagic disease, leptospirosis, listiriosis, rabies and tick paralysis.

Biologists have also ruled out certain toxins, such as glycophosphates , selenium, holly and inkberry plants.

Monensin, which is used to control parasites (coccidian) and improve feed efficiency in cows and chickens, became a likely possibility as it is known to harm horses, causing generalized weakness in the limbs and the loss of control over body movements, making the animal unable to stand, Mr. Stainbrook said, adding that the chemical is likely to have the same effect on deer.

The chemical is present in a commonly used livestock feed called Rumensin, Mr. Stainbrook said. Other feed brands that use the chemical include Avatec and Purina. The substance usually comes in pellet or powder form to be mixed into traditional feed, which he clarified does not often contain the chemical.

“The additive comes in a number of different chemical forms and it’s not easy to test for,” Mr. Stainbrook said. “If we could get some information from the public to help explain it, we may be able to confirm or rule it out.”

Mr. Stainbrook said his department is asking members of the public to come forward with any information on the accidental or intentional use of these substances to feed deer.

Citing a poor year for acorns, a staple of the deer diet, Mr. Stainbrook said it was possible that people may have been putting livestock or poultry feed out to supplement the deer. He also said it was possible hunters may have been using the feed to bait deer, which is illegal under state law.

“It was such a poor acorn year,” he said. “It’s reasonable to think people may have thought the deer needed some help. And, for the first time, went and bought something to feed them.”

Mr. Stainbrook said it was also possible the deer could have been looking for other food supplies due to the lack of acorns and stumbled upon a backyard chicken coop or an open cattle pasture.

“It is not illegal to feed deer, the only illegal part about it would be to purposely feed deer during hunting season to take the deer,” he said. “As an agency, we are not in support of people feeding wildlife, for any reason.”

Mr. Stainbrook recommended not eating deer meat from animals that exhibited signs of paralysis.

Mr. Stainbrook said that after spending the last three months ruling out a diverse spread of diseases and substances from consideration, this is the only lead his department is still investigating. He added that his department is working with local police and grain stores to isolate the incidents and ensure that it does not happen again.