Can my cat, dog or horse catch swine flu?

May 4, 2009

Influenza is, first and foremost, a disease of birds. The world's birds provide a massive reservoir for the virus, and the heart of that reservoir is in Asia, centred on China.

Neil Clarkson looks at the prospects faced by your pets and other livestock as the so-called "swine flu" spreads around the globe.

Influenza is on the march and the media is all over the story like a rash. Truth is, influenza is never far from the news.

Health authorities around the globe are watching the progress of H5N1 bird flu like a hawk. Closer to home, Australians and New Zealanders know plenty about equine influenza, which dealt a nasty economic blow to the Aussie equine industry in 2007.

Now "swine flu" is big news as the threat of a worldwide outbreak grows by the day.

While people will understandably be concerned about their own prospects of an unhealthy encounter with the flu virus, they will equally be concerned for their pet cat, dog or horse - indeed, any livestock under their care.

Should there be concern? Why, for example, is the World Organisation for Animal Health currently awaiting test results to determine if the new virus is capable of infecting horses and other livestock?

Influenza affects a number of species, but before we deal with them individually, let's take a quick overview.

Influenza is, first and foremost, a disease of birds. The world's birds provide a massive reservoir for the virus, and the heart of that reservoir is in Asia, centred on China.

There are many viruses that are specific to species, but influenza virus has shown an occasional and worrying trait: an ability to jump the species barrier into mammals.

There are three types of influenza virus - A, B and C. People can can be infected by all three, but the most troublesome is type A.

Current thinking suggests that all influenza A strains that affect mammals originated from the wild aquatic bird population.

It's all in the genes

It took a perfect storm of circumstances for the latest influenza outbreak to occur, said a Texas AgriLife Research scientist.

There's nothing new about H1N1 influenza, said Dr. Jason Osterstock, an AgriLife Research infectious disease epidemiologist. In fact, the latest human flu vaccine and most all flu vaccines for humans and swine include a strain of H1N1, which is considered the most common of the human and swine influenzas.

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New strains jumping the species barrier worry disease experts, as humans are likely to have limited or no resistance to these new infections.

Humans have been badly affected by the flu virus. Ten to 20 human influenza pandemics are believed to have swept the globe in the last 250 years, the worst being the so-called Spanish flu of 1918 to 1919 that caused more than 20 million deaths and affected some 200 million people.

Once a flu virus manages the jump to a new species, its long-term survival largely depends on its ability to infect more of the same species. If successful in this regard, it can develop to the point where it becomes specific to that species.

Hence, the world is home to a raft of different flu strains which are species-specific. Some, such as the highly dangerous H5N1 bird-flu strain can infect people, but it has thus far shown very limited ability to jump from person to person. Such spread has, in the words of the US Centers for Disease Control, been "limited, inefficient and unsustained".

Experts are worried that a person suffering human influenza could become infected by bird flu, raising the risk that DNA can be shared, resulting in a new strain capable of transmission from person to person.

While scientists have been watching the comings and goings of H5N1 bird flu - with countries at times culling millions of birds to minimise the threat of cross-species transmission - the so-called swine flu, which is a type A influenza, has emerged.

While the world should be worried by the threat of any major flu outbreak, there is no evidence to suggest we are on the verge of anything like the major pandemics of the past. For a start, this is not a totally new virus. It is a tricked-up version of the H1N1 virus. Yes, a similar virus caused the 1918 pandemic. It was also responsible for another outbreak in 1977, meaning there is likely to be some immunity in the human population.

Indeed, one reason the Mexican cases seem to hit younger people harder is that the immune systems of older people may well have tangled with the virus in the past.

Finally, the flu spreading around the world is clearly milder than the strain that has killed in Mexico, most likely because travellers that have carried it around the globe did not have close contact with the sickest Mexicans. They were rubbing shoulders with sick people well enough to get on with their lives in public.

So why are scientists testing horses and other livestock to determine if they are susceptible to the virus?

The issue comes down to whether other species are capable of harbouring the virus, perhaps with limited symptoms, or no symptoms at all, and then potentially passing it on to susceptible species - in this case, humans.

Research published only this month pointed to the possible role of pigs in helping the transmission of the Spanish flu of 1918.

The study showed the virus was infecting pigs at the same time, causing a mild respiratory infection, and may ultimately have played a role in the spread among people.

So how do other species stack up?


New Zealand, Australia and Iceland are the only remaining nations free of equine influenza. The 2007 outbreak in Australia showed just how explosively the disease can spread in a population never before exposed to it.

Left unchecked, past experience shows the infection rate among a naive population would be 60-90%, with a potential fatality rate of 1-20%, with foals and weaker animals most susceptible. Flu in horses causes similar symptoms to those experienced in humans.

Among equines, two major sub-types of influenza A virus are known to circulate. The first (H7N7) was isolated in Czechoslovakia in 1956, the second (H3N8) in Miami in 1963. The H7N7 sub-type is believed to be extinct or present only at very low levels.

The H3N8 sub-type is almost certainly a mutation of a bird flu virus, and changes in the virus have seen the development of two lineages - American and European. The H3N8 sub-type was behind a severe 1989 outbreak in China, which affected 80% of horses exposed to it and killed 20%. It went on to cause outbreaks in Trinidad (1979), Argentina (1985), South Africa (1986), and Jamaica (1989). H3N8 was behind the recent Australian outbreak.

So what are your chances of catching flu from flu-affected horse with a runny nose and a hacking cough? The chance is considered extremely low because horse flu strains have proved themselves to be very species-specific. That said, around 1999, an equine strain made the jump into dogs - the first time that horse flu was shown to have jumped the species barrier.

The flu story in horses does not end there, however. Dalva Mancini and a team of researchers in Brazil have looked for evidence pointing to the transmission of influenza between horses and people. Their work revealed high levels of antibodies in horses to two human influenza A strains - H1N1 and H3N2.

The researchers had taken fluids and mouth and nasal swabs from 46 horses at studs and racing stables in Sao Paulo and Rio de Janeiro. About 78% of the horses carried antibodies to two strains of equine flu. Remarkably, they tested positive in even higher numbers to human influenza A virus - 80.43% for H1N1 and 93.47% for H3N2. Similar numbers had antibodies to type B influenza.

The results meant that the immune systems of the horses recognised and generated antibodies against the human flu viruses, but did not become sick. And it's worth noting that more than 80% of the horses in the study had generated just such a response against an H1N1 strain.

"A high incidence of human influenza virus infection was observed among the horses evaluated," the researchers said. "These results demonstrated that horses are reservoirs of the circulating influenza viruses of both the equine-specific and the non-specific strains."

The researchers said: "It may be concluded ... the high incidence of influenza viruses among animals and humans surely represents a problem to both public health and animal protection all over the world."

"The occurrence of interspecies transmission is of concern since this allows the maintenance of the influenza virus in different reservoirs."

The evidence, they said, led to the conclusion that interspecies transmission probably occurs between humans and horses.

Horses, however, can probably rest easy in their stalls over "swine flu". Scientists have said that the new flu circulating in humans contains genes from flu viruses found in pigs, birds, and people. With no equine flu input in its makeup, the chances of it infecting horses would be extremely low.


Birds, as discuseed before, are the main reservoir for influenza and some of the strains have shown the worrying ability to jump the species barrier. Asia is considered ground zero for bird flu, although outbreaks occur in various places around the globe, tied in largely to species' migratory habits. H5N1 is of particular concern, and it has killed several hundred people during the last 10 years. To date, those affected tend to live or work closely with birds and the disease has so far failed to generate the ability to easily transmit from human to human.


Influenza was unknown in dogs until 1999, when an equine flu strain made the jump to canines. It then managed to jump from dog to dog, and is now known as equine influenza A H3N8 virus. It was not until 2004 that the canine influenza strain was discovered as the cause of a disease outbreak among racing greyhounds in the US. Testing of older samples revealed it was behind the outbreak of a respiratory disease in greyhounds dating back to 1999. The new flu's derivation from pig, bird and human flu-virus DNA would make the risk to dogs very low.


Cats can catch the H5N1 strain of Type A influenza - this is the same dangerous strain which has infected and killed people. Like dogs, until recently cats were not thought to be susceptible to any flu viruses at all. This is bird flu - pure and simple - and cats have caught it by eating the remains of infected dead birds. Tigers and domesticated cats are known to have been laid low by the infection. However, as in people, the virus may be able to infect cats, but it has not yet managed to jump from cat to cat. If anyone talks to you about "feline flu", they're not actually talking about a real influenza. Feline flu is the name for a general upper respiratory infection. The new H1N1 flu strain would have to be remarkably resourceful to make any cat sick.


Pigs are getting a pretty bad rap over the new influenza, not helped by the common use of the term "swine flu". Health authorities are now trying to call it H1N1 influenza to dispel the notion that porkers are to blame. Scientists have yet to find evidence that pigs can even be infected by the new virus, but they're looking. We know only too well that it spreads from person to person, but pigs are thus far in the clear. Given that new flu carries genes from human, pig, and bird flu, there is a greater risk for pigs than the likes of cats and dogs and horses. Pigs can still get good, old fashioned dose of "swine flu", which will lay a pig low for a while, but they'll bounce back to snort another day.

So, it seems, household pets are most likely safe.

The fact the new virus combines flu DNA from pigs, bird and humans brings into sharp focus the observations of influenza researcher Dr Laurent Kaiser.

Kaiser has explored the question of influenza reservoirs and discussed the issues at an international symposium on influenza and other respiratory diseases in Memphis, Tennessee.

The main reservoir, he observed, was the wild aquatic bird population, which can transmit influenza viruses directly to species such as chickens and pigs.

Wild aquatic birds can spread influenza to pigs, and the two species should never be kept together in close quarters, he explained.

Generally, he said, pigs acted as a mixing vessel between birds and humans, but avian influenza had shown an ability in recent years for direct transmission to humans, as evidenced by outbreaks in Asia.

"Transmission of influenza viruses from birds to mammals has probably occurred for centuries. However, increased opportunities for transmission, larger chicken and pig populations, and overall growth of human populations are associated with a higher risk of interspecies reassortment.

"This situation is a possible start for a new pandemic. The timing of the next influenza pandemic cannot be predicted," he said, "but we know that it will occur eventually."