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Origins of the viruses

June 30 2003

Viral diseases have always leapt from animals to humans and it seems the next pandemic is just around the corner. Ruth Pollard tracks the rise of interspecies illness.

THE common cold is thought to have jumped from horses to humans, leprosy from water buffalo, measles from the virus that causes distemper in dogs. The bubonic plague of the 1600s originated in rats.

Disease experts say that beyond death and taxes, the only other certainty is that viruses will continue to leap from animals to humans, accelerated by the trappings of modern life - tourism, trade, war, climate change, population movements, crop production and intensive livestock farming.

Air travel allows an infective agent to move from one side of the globe to the other in about 24 hours. The urban sprawl pushes humans in on animals' territory, while climate change is driving mosquitoes - the most efficient disease carriers of all - into new areas, bringing new infections with them.

"That scenario is probably occurring in major and minor ways all around the world," says a microbiologist and infectious diseases specialist, Peter Collignon, director of microbiology at the Canberra Hospital. "It means we need to be really circumspect with what we do to the environment, what we do with medical procedures that involve animal tissue. People underestimate viruses and microbes - they are everywhere, and they will always be everywhere."

What separates the weak from the strong in the virus world is those which can not only jump from animal to human, but those which can be spread from person to person. The most recent examples - SARS, the severe acute respiratory syndrome, and HIV, the human immunodeficiency virus - were the strong ones.

"All the available evidence suggests that HIV came from monkeys, which carried SIV, a similar virus," says Collignon.

And, unlike the case with influenza, which is highly infectious but obvious, people with HIV can remain relatively well for some time, so the virus is transmitted silently. In the 22 years since its discovery, HIV has killed 28 million and infected 42 million.

Dominic Dwyer, a medical virologist at Westmead Hospital, says 75 per cent of emerging viral diseases in humans in the past decade have been passed from animals, known as zoonosis. "I see no reason why this won't continue," says Dwyer, who worked on Australia's response to SARS.

He says SARS jumped from wild or farmed animals and spread quickly in an unprepared human population. "It was exacerbated by modern things, such as rapid travel and hospital environments, where amplification of the infection could occur."

SARS is thought to have originated in the civet cat - a delicacy in southern China - and spread to humans as they farmed, slaughtered or cooked them. "It is possible those animals were the origins of SARS, but it is also possible those animals were infected just like humans were," Dwyer says.

Yet, amid outbreaks of exotic diseases such as the Australian bat lyssa virus, the Hendra virus and SARS, it is the inevitable threat of a new flu epidemic that most worries Australian scientists. "We are terribly concerned about new pandemics of influenza. These arise out of birds and animals and get into humans. If there is human-to-human spread then you get the pandemic," Dwyer says.

The emergence of the Hong Kong bird, or avian, flu in 1997 sent a collective shiver down the spines of infectious disease experts around the world. Was this the next big one?

"Fortunately, that did not appear to be incredibly infectious," says Dwyer of the flu that infected 17 people and resulted in the slaughter of more than a million chickens, ducks and other edible fowl.

Influenza is always in the community, emerging as an epidemic a few times each hundred years. The disease appeared in pandemic form three times last century - in 1918, 1957 and 1968. The 1918 virus killed an estimated 50 million people. Another one is virtually inevitable - if not this year, then soon after.

Dwyer says the SARS outbreak, which infected about 8500 people and killed 809, showed the importance of international co-operation and openness at every level in the control of outbreaks. In just 100 days, an epidemic was spotted, the virus was identified and the management of individual patients was established. For many infectious diseases specialists, SARS was both a wake-up call and a chance to better prepare for the next pandemic of influenza, which is expected to come out of Asia.

NSW Health's director of communicable diseases, Jeremy McAnulty, agrees influenza is the one to watch.

In preparation, NSW Health is planning an exercise later this year to assess how the system would cope.

"Every few decades a new strain of flu emerges from an animal, then adapts to infect people," McAnulty says. "Luckily, it is often very hard for viruses to jump the species barrier - they are just not genetically built that way, as we saw in the Hong Kong flu virus."

McAnulty points to the little-known Menangle virus, which affected a piggery in south-western Sydney in 1997, as another that originated in wildlife (bats), moved to livestock and humans, then died out quickly.

The adult pigs suffered no ill health, but the females miscarried their litters, and the foetuses were deformed.

Two humans who worked very closely to the pigs were quite sick with a flu-like illness. They recovered completely but tests showed they were antibody-positive to Menangle virus.

McAnulty says: "You would have to guess that these strains have been around for decades and decades [and now] we are able to investigate and track them as technology ... improves."

Australia is in a good position to guard against the introduction of some exotic diseases, he says.

The appearance of the monkeypox virus earlier this month in the United States Midwest would have been prevented by our strict importation laws, he says. The US outbreak appears to have started when a Gambian rat was brought into the country, then housed with prairie dogs in a pet shop. The rat transmitted the disease to the dogs, which were then sold in 15 states. There are now at least 12 confirmed human cases of monkeypox, a close relative of the deadly smallpox virus.

Bryan Eaton is a senior virologist at the Australian Animal Health Laboratory, which is part of CSIRO and a key player in the identification of the Hendra virus. A distant relative of the family of viruses that incudes measles, mumps, distemper in dogs and other respiratory illnesses, Hendra was transmitted via flying foxes (also known as fruit bats), to horses, then to humans in the mid-1990s. But unlike SARS, humans proved to be a "dead end host" to Hendra, says Eaton.

Killing two Australians, including the racehorse trainer Vic Rail, and 13 horses in Brisbane, Hendra lived deep in the lungs of infected horses.

"There is no evidence at all that Hendra can be spread by one horse breathing on the other," Eaton says.

Instead, it is probably spread to humans by the simple act of hand-feeding - you get the virus on your hand. If you have cuts on your hand, it goes into your bloodstream.

But because it is not easily transmissible, Hendra doesn't travel very far.

The opposite was true of the Nipah virus - a close relative of the Hendra virus - discovered in 1999 in Malaysia. Both viruses are transmitted by flying foxes to other livestock species - with Nipah it was bat, to pig, to human.

But in this case, the virus caused an upper respiratory tract infection which meant it spread very quickly between pigs, and then to humans, Eaton says.

Causing encephalitis, Nipah killed 105 people who had been in contact with the infected pigs. More than a million pigs were slaughtered.

As with many viruses, removing the cause of the virus cross-over was easy. Pig farmers had planted mango trees on their farms. Bats fed on the mangoes, chewing and then spitting out the pulp. The pigs ate the discarded pulp and the virus crossed species. The solution, says Eaton, was to remove the mango trees and "break the chain".

There have been seven new virus movements in a decade from wildlife to livestock to humans, he says.

In 1996, the rabies-like Australian bat lyssa virus infected and killed two women in Queensland. Eaton calls this a "chaotic, random event". You cannot control the event, but you can prepare for the inevitable. "We can be prepared by enhancing our diagnostic capacities [to be] faster, more specific, more sensitive and more discriminatory, and better surveillance capabilities."

Eaton says Australia has taken a step in that direction, with the recent establishment of the Australian Biosecurity Co-operative Research Centre in Emerging Infectious Diseases.

It was thousands of years ago, when human settlements became denser, dirtier and entailed close contact with herded animals, that many diseases emerged, says Professor Tony McMichael, director of the National Centre for Epidemiology and Population Health. "As the scale of social-demographic change and human impact on the ecosphere escalates, so the probabilities of infectious diseases, new and resurgent, increase," he warns.




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