At least one major study suggests that dogs can be infected with the deadly virus without having symptoms. But whether or how likely they are to spread it to people is less clear.
Lab experiments on other animals suggest their urine, saliva or stool might contain the virus. That means that in theory, people might catch it through an infected dog licking or biting them, or from grooming.
"Clearly we want to look at all possibilities. We have not identified this as a means of transmission," said Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention.
The nursing assistant and her husband have been in isolation since she tested positive for Ebola earlier this week. She was part of team at a Madrid hospital that cared for a missionary priest who died of Ebola.
The Madrid regional government got a court order to euthanize their dog, saying "available scientific information" can't rule out it could spread the virus.
The dog's owners don't want it killed. Carlos Rodriguez, a Spanish veterinarian and host of a talk show about animals, said the husband messaged him from the hospital, trying to grant him temporary custody of the mixed-breed dog.
But now that there is a court order, "I can't stop this happening," Rodriguez said. The husband "asked me, crying, to at least make sure the animal does not suffer."
The Spanish animal rights group Animal Equality complained that authorities wanted to "sacrifice the animal without even diagnosing it or considering the possibility of placing it in quarantine."
It's not clear how effective quarantine would be, since infected dogs don't show symptoms and it's not known how long the virus can last in them, or how long tests would have to be done to check for it.
Dr. Peter Cowen, a veterinarian at North Carolina State University who has advised global health experts on animal infection disease risks, says killing the dog is "clearly an overreaction."
"I think it's very unfortunate they are thinking of euthanizing that dog. They should really study it instead," he said. "Ebola has never been documented to be spread by a dog," and that's clearly not a major route of spread in the outbreak in Africa, he said.
Ebola's source in nature hasn't been pinpointed. The leading suspect is a certain type of fruit bat, but the World Health Organization lists chimpanzees, gorillas, monkeys, forest antelope and porcupines as possibly playing a role in spread of the disease. Even pig farms may amplify infection because of bats on farms.
The possibility of spread by dogs at least in Africa was raised by a 2005 report. Researchers tested dogs during the 2001-2002 Ebola outbreak in Gabon after seeing some of them eating infected dead animals. Of the 337 dogs from various towns and villages, 9 per cent to 25 per cent showed antibodies to Ebola, a sign they were infected or exposed to the virus.
"I think it's possible" that dogs might spread Ebola, but it's not likely in the U.S. or other places where dogs aren't near corpses or eating infected animals, said Sharon Curtis Granskog, a spokeswoman for the American Veterinary Medical Association.
In Dallas, health officials are monitoring 48 people who may have had contact with Ebola patient Thomas Duncan, but "we are not monitoring any animals at this time," said Dr. David Lakey, commissioner of the Texas Department of State Health Services.
Associated Press Madrid Bureau Chief Alan Clendenning contributed to this report.
Dog study: http://wwwnc.cdc.gov/eid/article/11/3/pdfs/04-0981.pdf
Marilynn Marchione can be followed at http://twitter.com/MMarchioneAP