METHUEN — In what is being called an extremely rare occurrence, a dog adopted from the MSPCA's Animal Care and Adoption Center at Nevins Farm in Methuen had undetected rabies and bit its new owners in Haverhill.

The dog, a 16-month-old golden retriever-wheaten terrier cross named Chewy, was at the Methuen center April 7 through 15, but was actually on the adoption floor for only one or two days, according to Mike Keiley, manager of the center.

The MSPCA and the Massachusetts Department of Public Health last night announced that the dog received a failed rabies vaccination while under the care of its previous owners.

In a public health alert, the agencies said anyone who "may have been bitten, scratched or had contact with saliva" from the dog while it was at the adoption center should immediately contact their doctor, local board of health or the state Department of Public Health at 617-983-6800.

For most of the time in Methuen, the dog was in the center's "holding room" where he was being evaluated, Keiley said.

Keiley said that because of the popularity of the breeds, Chewy was quickly adopted.

He said it is also possible that people may have come in contact with the dog while it was being walked outdoors by center volunteers. "It's possible someone could have petted him," he said.

Keiley said it's possible dozens of people had contact with Chewy and that the center last night was contacting its staff and volunteers to tell them about the situation. Persons who did not have contact with the dog are not at risk.

"I've been here 14 years and never heard of this," he said.

Following Chewy's adoption, he was returned to the Methuen center on Tuesday after biting his new owner.

He was euthanized because of his "overt aggressive behavior" and sent to the Department of Public Health for rabies testing. Results of the tests were received yesterday afternoon, the agencies said in a statement.

The MSPCA staff in Methuen had verified Chewy's veterinary records. They confirmed with his former veterinarian that Chewy was current on his rabies vaccines, having received two of them in less than a year.

Chewy had received an initial rabies vaccination May 5, 2007, which lasts one year. He received a booster March 1, 2008, which lasts three years.

"In very rare instances the rabies vaccine can fail and it has been concluded that Chewy's booster vaccine did indeed fail to protect him against infection," the statement said.

"Two prior shots; one would have thought it would be enough protection," Keiley said.

Brian Adams, a spokesman for the MSPCA in Boston, said it is unlikely that the failed vaccination was the result of a bad batch. "We do not want people to feel that the vaccinations their animals received are in question," he said.

Rabies is spread from the saliva of an infected animal through a bite, scratch or mucous membrane exposure. Treatment for exposed persons should begin as soon as possible and includes both rabies immune globulin and five doses of rabies vaccine.

Complete information about rabies and rabies treatment is available from the state Department of Public Health at 617-983-6800 or online at www.mass.gov/dph/cdc/epii/rabies/rabies.htm.