Dogs are the latest to join the list, with reports out of China suggesting two dogs have tested positive for the same virus which has infected millions of people around the world.
The virus has so far shown an ability to infect pigs, birds, cats, dogs and ferrets.
The Xinhua news agency reports thyat China's Ministry of Agriculture has called for intensified investigation of A/H1N1 flu in animals after two samples from sick dogs tested positive for the virus.
The positive tests were returned from a total of 52 samples taken from sick canines. The veterinary clinic of the College of Veterinary Medicine at the China Agricultural University said the virus was a 99 per cent match for the strain currently circulating in humans.
Influenza is a relatively recent phenomenon in dogs. It was unknown until 1999, when an equine flu strain made the jump to racing greyhounds in the United States.
It then managed to jump from dog to dog, and is now known as equine influenza A H3N8 virus.
Meanwhile, in Colorado, two cats from different households have tested positive for H1N1.
Colorado State University's Veterinary Diagnostic Laboratory confirmed the cases in testing. The cats were diagnosed with lower respiratory tract infections. Both cats remain ill and have been unwell for some weeks, with one illness beginning on October 2 and the other beginning in early November.
"We expect that both cats will recover, but these cases serve as a reminder to pet owners to seek veterinary attention as soon as possible if their pet seems ill," said Kristy Pabilonia, a veterinarian and expert on H1N1 testing in animals at the university.
"This flu has also been identified in ferrets, exotic cats and birds in the United States, so any companion animal that appears to have the flu should get immediate attention.
"We are very interested in understanding more about H1N1 in cats and other pets and providing tests to veterinarians who suspect H1N1 in companion animals."
Swabs from each cat's mouth and nose were tested by university staff and additional blood serum confirmation testing from Iowa State University confirmed that they have the pandemic H1N1 strain.
At least one cat and one ferret in other US states are known to have died from the flu strain.
Scientists cannot rule out other species being affected. Horses are susceptible to some strains of influenza, but no H1N1 infections have been reported.
Dick Hesse, a flu expert and director of virology in Kansas State University's Veterinary Diagnostic Laboratory, said the companion animals infected to date appear to have become ill after being in close contact with their owners.
He said there was no evidence of pets passing H1N1 to people at this time.
Gary Anderson, director of the Kansas university's Veterinary Diagnostic Laboratory, suggests pet owners see their veterinarian about keeping their pets healthy over the flu season.
"Although rare, transmission of influenza viruses from people to dogs and cats can occur," said Ken Harkin, associate professor of small animal internal medicine.
"This is true with both H1N1 as well as H3N2, but the magnitude of the H1N1 pandemic dramatically increases the pet population exposure to influenza virus.
"We don't think the pet owner should be overly concerned. However, if pet owners are infected with H1N1 they should notify their veterinarian immediately if the pet becomes ill."
Flu-affected pet owners should take the same precautions against infecting their animals as they would in preventing healthy members of their household becoming ill.