Metro Animal Control is investigating the sudden deaths of 10 horses on a property west of Casper in April, its manager said Thursday.

Metro learned of the deaths on April 11 from the veterinarian of the horses' owner, Rick Sulzen said.

Six horses were already dead by that date and the other four, which were lethargic and unable to stand, died within the next 24 hours, he said.

The causes and manners of their deaths are unknown, Sulzen said. "We didn't see anything out of the ordinary."

The horses appeared well cared for, investigators did not find any apparent environmental problems, and there was no evidence of foul play, he said.

The horses drank from Casper Creek, which has selenium in the water, Sulzen said. But other animals drink from the stream and don't have problems, he said.

The owner kept the horses in an area near 7 Mile Road and U.S. Highway 20-26.

The owner, Sulzen said, did not want to talk to the press about the deaths.

Metro officials sent specimens of the horses to the state veterinary laboratory in Laramie, and to a lab at Colorado State University in Fort Collins, Colo., he said.

Metro is still waiting for the results of the necropsies from the labs, Sulzen said.

State Veterinarian Dr. Walter Cook learned of the deaths by e-mails sent from local veterinarians.

Although the tests on the specimens are not finished, a tentative cause of the deaths may have been botulism, Cook said.

Botulism is a form of food poisoning caused by the bacterium botulinum, which secrets a virulent nerve toxin called botulin.

It is not infectious, Cook said.

Spoiled food often is the culprit for animals and humans, he said.

Botulism grows in an anaerobic -- oxygen-free -- environment such as canned goods consumed by humans, Cook said.

With livestock, botulism can occur when a dead animal is trapped in a hay bale and the bacteria grow because of the lack of oxygen in the compressed feed, he said.

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