Just in case you weren't panicking enough over the swine flu, it turns out that the virus that causes it, H1N1, can make dogs, cats and ferrets sick, too.

H1N1 got its nickname, the swine flu, due to its origin in pigs, but it's really a genetic mashup of human, swine and bird viruses. Because it can cause serious illness and even death in susceptible humans, particularly infants and pregnant women, veterinarians greeted the recent news that it could affect family pets with alarm.

The first cases in pets were reported in ferrets, which are notoriously prone to influenza viruses. But when the American Veterinary Medical Association (AVMA) reported last month that a sick cat in Iowa had the disease, epidemiologists and virologists took notice. It was the first time a cat had become ill from an influenza virus.

Since then, two more sick cats, one of whom died, tested positive for H1N1, and Chinese officials announced Saturday that they had isolated the virus in two sick dogs.

Dr. Tony Johnson, a clinical assistant professor at the Purdue University School of Veterinary Medicine, said that it doesn't look like our pets are a risk to us.

In fact, we're a risk to them -- every cat that's been diagnosed with H1N1 lived with a human who had a respiratory illness shortly before the cat became sick.

"So far there is no evidence that this virus can be passed from cats to humans," Johnson said. "Sometimes a virus can make a host sick, but not reproduce and become infectious in that host."

That appears to be the case with the H1N1-positive dogs in China as well. The Chinese Ministry of Agriculture reported that the virus found in the dogs, whose breeds and ages were not given, was 99 percent identical to the human swine flu, suggesting that they contracted it from people.

"Only when the virus mutates within dogs will it be a new threat to humans," Feng Zijian of the Chinese Center for Disease Control and Prevention told China Daily.

Johnson acknowledged that feline to human transmission of the virus can't be ruled out, but he asked cat owners not to over-react. "The humans who gave the virus to their cats recovered, and so will most cats," he said. "Common sense and a cool head are better than flipping out and putting your cat out with the garbage."

There may be many more undiagnosed cases of H1N1 in pets, because it's rarely tested for and has symptoms similar to other canine and feline diseases, which are usually mild.

The AVMA describes those symptoms as "lethargy, loss of appetite, fever, runny nose and/or eyes, sneezing, coughing or changes in breathing (including difficulty breathing)." But as with humans, some dogs and cats with respiratory infections will go on to develop pneumonia, and some of them will die.

With doomsday scenarios about mutant pathogens making headlines around the globe, a certain amount of over-reaction isn't a surprise.

It doesn't help that one of the viruses that can be transmitted between humans and dogs and cats is a pretty terrifying one: rabies. But rabies in domesticated animals is very rare in the United States, and most pets are vaccinated against the disease.

A handful of other diseases and parasites can affect humans, dogs and cats, like fleas, intestinal worms and skin diseases like ringworm.

But until five years ago, no influenza virus had ever been known to cause illness in a dog or cat, which is somewhat surprising given that influenza viruses typically originate in farm animals and wildlife, and dogs and cats have always lived closely with humans and their livestock.

The H1N1 outbreak isn't the first time an influenza virus has adapted to be able to cause illness in a dog, however.

That occurred in 2004, when an equine influenza virus, H3N8, was identified as the cause of a deadly outbreak of respiratory disease in racing greyhounds in Florida.

Canine influenza

Woody, a flat-coated retriever, is a healthy adult dog today, but when he was just a puppy in Texas, he came down with what his breeder thought was "kennel cough," the dog version of a cold.

But what Woody had was something no one thought to look for, a brand new virus that posed a serious risk to very young, very old and immune-compromised dogs.

"He spent the next month at the vet school at Texas A&M, fighting for his life," said his owner, Gina Spadafori, who lives in Sacramento. "He survived, but he was weak and spent many weeks recuperating after that."

The bug that almost killed Woody was a viral changeling dubbed H3N8. Originally thought only to affect horses, it had adapted itself to be able to cause disease in another species, the dog.

Once researchers started looking for H3N8, they found dogs carrying the virus in 30 states (including California) and the District of Columbia, a wide distribution suggesting it had been spreading without detection for quite some time.

The virus, now dubbed "canine influenza virus" or CIV, probably went undiscovered for so long because its symptoms in most dogs mimic the usually-mild kennel cough, officially known as "canine respiratory disease complex."

"Canine influenza virus is generally a mild disease, with typical symptoms of cough, some lethargy, fever and perhaps nasal discharge," said Dr. Melissa Kennedy, a clinical virologist at the University of Tennessee Veterinary Teaching College and an infectious disease and immunology consultant for the Veterinary Information Network.

But for some dogs, like Woody, that risk is much greater. "As with the human influenza, there is a risk for secondary bacterial infections which can be serious," Kennedy said. "This risk is highest among puppies and elderly dogs, where immunity may not be as good as in healthy adult animals."

Researchers at Iowa State University estimate around 80 percent of infected dogs become ill, and even the 20 percent who don't can still infect other dogs. Around 10 percent of sick dogs develop pneumonia, and somewhere between 1 and 5 percent die.

Although a vaccine for CIV came on the market in May, it's far from perfect. It can protect the dog from getting sick, but sometimes it only makes symptoms less severe or shortens the period of time that the dog is sick, as well as the length of time he's infectious.

Unfortunately, it doesn't prevent infection with CIV, which means vaccinated dogs can carry the disease home to other dogs even if they're not showing any symptoms themselves.

Kennedy said she does not consider the new canine influenza vaccine a "core" vaccine that should be given to every dog, but rather a tool that might be helpful in shelters, kennels or other environments where dogs are housed in close quarters and high numbers.

"Dogs that board or frequently co-mingle with other dogs could also be at risk," she added. So owners of dogs who go to dog parks, training classes, doggie daycare centers, dog shows or other places where dogs gather should discuss the vaccine with their veterinarians.

Dog owners concerned about CIV or H1N1 should keep an eye out for their early symptoms, such as a cough or nasal discharge. A dog with a fever of more than 104 degrees, lethargy, a green nasal discharge or difficulty breathing is at high risk of developing dangerous complications and needs immediate veterinary attention.

The AVMA is actively tracking instances of H1N1 in animals and posting updates on its Web site. The organization also provides comprehensive information on canine influenza, as well as the new CIV vaccine.