All birds and mammals can be infected with a form of influenza virus, of which there are three types (A, B and C). Humans can be infected by forms of all three, but most flu varieties in animals and humans that cause serious health concerns are Influenza Type A. Viruses can mutate rapidly, and because hosts' immune systems do not initially protect against new mutations, new strains can subsequently cause widespread infection. Often new strains result from the spread of an existing flu virus from one species to another, which provides the virus with the necessary tools to transmit between members of a different species to it's usual host.
The latest flu strain to hit the headlines (H1N1) – commonly known as "Swine Flu" is a strain of Influenza Type A. While the normal version of "Swine Flu" causes outbreaks of influenza with low mortality rates in pigs, the strain which is currently causing human deaths is not the same virus. The new strain combines genes from human, pig, and bird flu and is similar to the strain that caused "Spanish Flu", responsible for a human pandemic in 1918. "Swine Flu" is an entirely different virus to the "Bird Flu" which was widely talked about last year, and among the most important differences is that "Bird Flu" infected humans who had direct contact with infected birds, where as "Swine Flu" is being transmitted from human to human.
Flu in Horses
Influenza is widespread in horses and is believed to have a nearly 100% infection rate in unvaccinated populations. Flu in horses is primarily caused by the H7N7 and H3N8 strains. In 2007, an outbreak caused the Sydney Races in Australia to be suspended.
Flu in Cats
An avian strain (H5N1) of Influenza Type A, which was given the popular name "Bird Flu", had until recently posed the greatest risk for a new pandemic pandemic since it first killed humans in Asia in the 1990s, but it did not mutate into a form that spreads easily among people. H5N1 is unusual in being deadly to many species, including domestic cats which were never previously susceptible to any influenza virus. Aside from when infected with H5N1, the term "Feline Flu" does not actually refer to infection by influencing, but instead generally refers to the symptoms of an upper respiratory tract infection. Because cats have little exposure to influenza viruses, any case of flu which was able to transmit between humans or dogs and cats would probably lead to a widespread infection, since cats have no natural immunity to any influenza virus.
Flu in Dogs
Type A Influenza viruses also include equine influenza (H3N8), which in 2004 was discovered to be responsible for Canine Influenza. Because of the lack of previous exposure to this virus, dogs have no natural immunity to this virus.
Flu in Pigs
Although this new influence is being called "Swine Flu," it is being spread from person to person, not from pigs to people. None of the infected humans in North America have had contact with pigs, and no pigs in North America have been found to be infected with this strain. Pet pigs are there before not known to be able to contract the strain of "Swine Flu" which is being talked about in the news, however they are able to contract normal "Swine Flu", which does not normally have any more serious consequences than seasonal flu does for humans.
In general, influenza viruses are not transmitted from one species to another. For example, dogs and cat do not develop flu after exposure to owners with a seasonal flu virus. While it is theoretically possible for a new influenza strain to be visible between humans and other domestic animals, it is likely that such a strain would be transmissible between only humans and one other animal. Because the "Swine Flu" virus contains genetic elements of human, pig and avian flu viruses, it would seem very unquestionably that this strain would have the ability to infect hosts which are not humans, pigs or birds. And, according to the American Veterinary Medicine Association (AVMA), "there is no evidence that pets are susceptible to this new strain of influenza; it appears to be transmitted solely from person to person".