by Perry McCarney

Created on: February 10, 2009   Last Updated: August 14, 2010

Animals can develop disease conditions due to a viral infection of the respiratory system that are very similar to those suffered by a human with the flu. But it is highly unlikely they will be directly attributable to any influenza being experienced by a person the animal has been in contact with.

The various respiratory disease conditions that we give the general name of "flu" are caused by members of the Orthomyxoviridae family of viruses. These are given names starting with the primary host species of animal they infect, followed by "influenza virus". When there are multiple virus types they are also given a letter. When outbreaks are being discussed they may be identified by a sub-type. For example, the outbreak of canine influenza that occurred in greyhounds at a racetrack in Florida in January, 2004 was caused by a strain of canine influenza A virus, sub-type H3N8. There can be multiple strains of each sub-type and quite a number of sub-types. These viruses, despite being highly mutagenic (easily change their genetic code), tend to be very species specific, which is why their names start with the species affected. The viruses and their multitude of strains usually only infect and impact the species of animal they are attuned to.

The only domestic animals that may be susceptible to human influenza viruses are pigs. This is because both pigs and humans are omnivorous animals with very similar digestive or gastro-intestinal tracts. The same diet is extremely suitable for both of us, whether we wish to admit that or not, because we both have similar dietary requirements and the same vulnerabilities to poisons and toxins. This is why research into xenotransplantation, using animal organs to surgically replace human organs, primarily revolves around experimentation involving pigs.

Even so, swine influenza viruses demonstrate little ability to infect humans; a boy in Canada was found to have become infected with a strain of swine influenza A H3N2 in 2007, but such cases are rare. There are apparent links between canine (dog) influenza and equine (horse) influenza, with a potential transference in both directions, that are currently being studied in veterinary circles, but these are yet to be confirmed.

Avian influenza or bird flu has come to the world's attention since 1997 because strains of a sub-type of avian influenza A virus called H5N1resulted not only in significant deaths in domestic poultry species in Hong Kong at that time, but infection and eight deaths in human poultry workers there as well. The effects of this initial infection were minimized because the entire domestic poultry population of Hong Kong, some 1.3 million birds, were killed and buried.

Further outbreaks of similar H5N1 strains have occurred since 1997 in several parts of the world, causing deaths in wild and domestic bird populations as well as more human deaths. These strains are highly unusual, not only can they infect people as well as birds, they have been found infecting canine and feline species too. Stray dogs in southeast Asia and domestic cats feed the carcasses of infected domestic fowl have become ill. Two tigers (Panthera tigris) and two leopards (Panthera pardus) at a zoo in Suphanburi, Thailand, died after being fed raw chicken carcasses. Tigers at Thailand's largest tiger zoo became infected in 2004, with five dying within three days. Unexpectedly, and of worrying concern, direct tiger to tiger infection occurred and by the time the outbreak was dealt with, 147 tigers had died or been euthanazed to halt the spread to healthy animals.

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