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Disease - shaping history in a mobile world

By Peter Curson - posted Thursday, 29 September 2005


Infectious disease has for at least a thousand years, and probably longer, played a major role in shaping the course of human history, and there is every indication that despite the medical and public health advances of the last 50 years, it will continue to do so.

Have we reached a critical phase in human development, and will we all be engulfed in what has been called the approaching microbial storm? Much of this storm is driven by emerging and re-emerging zoonotic or animal diseases. The link between zoonoses and human health has been brought to the forefront in recent years with HIV, mad cow disease, SARS, Ebola, and now bird flu, all of which originated from long established natural reservoirs among wild animals.

Looking back on the past 40 years, probably 75 per cent of all newly emerged infections have been zoonotic in origin and probably at least 60 per cent of all the infectious diseases that affect human populations owe their origin to animal diseases. Currently bird flu reveals the very thin line that divides animal and human health.

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There would seem little doubt that we have all become much more vulnerable to cross species infection. That this has come about by virtue of a human encounter with an animal disease agent that already has the ability to infect humans, or because the zoonotic micro-organism has reassorted or mutated and travelled to human populations via an intermediary species, such as domesticated poultry or pigs is the question.

SARS provides an excellent example. There is good evidence that the virus may have normally circulated among wild animals in China, before spreading to other animals and then to humans. SARS also had something else going for it, that currently bird flu doesn’t, and this was the ability to be transmitted from one human to another. If bird flu makes this leap, or indeed somehow combines with the human flu virus, then we are all in for an interesting time, and the perfect microbial storm may indeed be upon us.

Given all this, it is surprising that there is no single agency in Australia responsible for the surveillance of diseases that affect humans, domestic animals and wildlife both here and abroad.

The transmission of zoonotic infections to human populations depends largely on factors such as: whether or not humans or domesticated animals intrude on natural disease reservoirs; whether wild and domesticated animals are free to intermingle, as is often the case in Asia with farmed poultry and pigs; whether humans disturb or in some way modify natural disease reservoirs; whether the particular zoonotic disease agent can easily jump to infect other species; and whether or not it has a mode of transmission that places humans at risk.

Bird flu is an interesting case in point. Influenza viruses have probably been circulating among aquatic wild birds in parts of China for perhaps thousands of years. The virus over this period established an ideal relationship with its host, primarily as a low level benign infection, rarely threatening its host’s life. Such birds also transported the virus over thousands of kilometres in the course of their normal seasonal migrations, excreting the virus in their faeces while remaining perfectly healthy. Occasionally, the virus would penetrate domesticated chickens but usually only resulting in a mild illness causing ruffled feathers and reduced egg production.

Recently, however, it appears the virus has mutated into a much more virulent form that could kill chickens within 48 hours, and perhaps more critically, the virus has shown considerable versatility in its ability to jump species and has been found in a wide variety of different animals.

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The epidemic of bird flu which is currently sweeping through Asia and further afield, is the largest and most devastating the world has seen, and threatens to become a global health crisis of huge proportions. Hundreds of millions of chickens have been killed in order to contain it, and so far more than 120 people have contracted the virus, with more than 60 deaths.

While bird flu has turned out be much more infective than previously thought, there is still no evidence of it spreading from person to person, and all the human cases seem to have come from people who were in close contact with infected birds. In Australia as elsewhere, there is concern that if the virus comes into contact with the human flu virus and mutates into a new and virulent human flu virus, then the world might be looking at a cataclysmic health event.

Why this has not yet happened remains something of a mystery, given that the virus must have had ample opportunity to combine with human flu. Many think that it is only a matter of time before a new and lethal flu is unleashed on the world and if this happens then Australia may be looking at between 25 and 30 per cent of its population having flu, and possibly as many as 20,000 deaths. A frightening thought. How well we are prepared for such an eventuality, is another story.

The real reasons for the emergence of infections like bird flu, lie not in the seemingly random nature of the biophysical environment, as they do with our behaviour, our collective life styles, our farming methods and our general attitude to the environment and animals. Crowded living conditions, with people, domestic animals, birds and wild life all sharing living space, has undoubtedly encouraged viral intermixing and the emergence of “new” viral infections. The overuse of antibiotics, particularly in animal populations has further contributed to the growing problem of antibiotic resistance and the emergence of new disease strains. The tremendous growth and expansion of worldwide trade, human movement and travel has added another dimension, allowing greater opportunities for disease agents to adapt, change and spread to new hosts and environments.

Bird flu, as SARS did before it, underscores just how fragile our relationship is with our biophysical environment, how we tend to underestimate the importance of zoonotic diseases for our own health, how we continue to modify and disturb natural reservoirs of disease, and how, in a closely interconnected and intensely mobile world, the next infectious disease is only an aircraft ride away.

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About the Author

Peter Curson is Emeritus Professor of Population and Health in the Faculty of Medicine and Health Sciences at Macquarie University.

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