Demonstrators with pets hold signs in support of Excalibur, the dog of the Spanish nurse who contracted Ebola, outside her apartment. Excalibur has since been euthanized, but the American patient's dog will live. (REUTERS/Sergio Perez)

The first Ebola infections to take place in Spain and the United States had one unfortunate detail in common — both of the young nurses who contracted the illness were dog owners, and their pets were exposed to the virus.

But while the Spanish patient's dog was put down in the interest of human safety, American Nina Pham's Cavalier King Charles Spaniel will await its owner's recovery in the special care of the SPCA. The CDC is working on official guidelines for Ebola cases involving pets.

Unfortunately, we don't know enough about dog-to-human Ebola transmission to know whether Spanish or U.S. officials made the right call. Here's what we do know:

Ebola most certainly can be transmitted to humans by way of other mammals.

Excalibur, the dog belonging to Spain's first Ebola patient, was taken by Madrid authorities Wednesday and reportedly put down. Animal rights activists rallied outside the home of Maria Teresa Romero Ramos as the dog was being driven away. (PACMA - Partido Animalista)

That's how we first got the virus. Ebola is a zoonotic disease, one that jumped into a human host from an animal one. The practice of hunting and eating bushmeat is especially risky.

At least one major study suggests that dogs can get Ebola.

Of over 300 dogs tested after a 2001 outbreak in Gabon, 9 to 25 percent showed antibodies to the virus (and only 2 percent of dogs in the control group, who lived in France, showed the same). That means they were likely infected or at least exposed.

But it's important to remember that we have no straightforward diagnostic test to show that a dog is actively contagious, so it's not as simple as testing them and sending them back home.

We don't know how easy it is for dogs to get Ebola from humans.

In the study mentioned above, scientists had observed many dogs eating from the corpses of Ebola victims left in the street. That's some pretty direct contact with the virus, so we can't say for sure that petting and cuddling would transmit, too. (But we also don't know that it can't.)

The dogs in that study didn't show symptoms.

So no, there's no point in freaking out (any more than you normally would) if your dog feels a little toastier than usual. The only reason to worry that a dog has Ebola is if its owner has been diagnosed, and they've spent time together since the infection.

We're not sure that dogs can transmit Ebola to humans.

In other animals studied in lab settings, the virus was spread by bodily fluids — saliva, urine and feces — that dog owners come into contact with all the time. But David Moore, an infectious disease researcher at the London School of Hygiene and Tropical Medicine, told Time that scientists have no evidence that dogs secrete viral cells in the same fluids. "There is absolutely no evidence to support a role for dogs in transmission," he said.

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You can keep exposed dogs in isolation — but when do you let them out?

We know so little about dogs and Ebola that we've got no idea how long to quarantine a suspected carrier of the disease. There just hasn't been enough research done.

In an interview with NPR, University of Pittsburgh infectious disease specialist Amesh Adalja pointed out the tragic futility of keeping exposed dogs alive. None of our current research — on incubation periods and transmission risks — applies to canines.

"I don't know what life that dog could have," Adalja told NPR (speaking hypothetically, after the death of the Spanish dog and before Pham's diagnosis). "Perhaps as an experimental animal to understand what the disease does. I do hope we learn from this experience irrespective of the outcome about what Ebola does in dogs."

No one is saying that keeping Pham's dog alive comes risk-free.

The risks posed by an Ebola-exposed dog remain totally unclear — and that's why Spanish officials decided to put their patient's pup down.

In the United States, we're taking the opposite gamble. "The dog's very important to the patient and we want it to be safe," Mayor Mike Rawlings told USA Today.

No details have been released as to specific plans for the dog, but representatives from the team tasked with decontaminating Pham's apartment said they'd help the SPCA and Dallas animal control retrieve the pet properly.

Related: CDC chief: U.S. must ‘rethink’ Ebola protocols