`Beaten' viruses set to fight back

Steve Connor reports from the American Association for the Advancement of Science's meeting in Atlanta, Georgia

Mass vaccination, which has all but eliminated medical scourges of the past, has created a climate in which the same infectious diseases could re-emerge with a vengeance, Professor Roy Anderson of Oxford University told the American Association for the Advancement of Science.

Professor Anderson, a world authority on the spread of disease, warned that the suc- cess of modern vaccines in controlling infections such as measles, whooping cough and rubella had produced immense evolutionary pressure on the viruses to mutate into vaccine-resistant forms.

"There is a very urgent need at the moment to put in place an international surveillance system in the developed world, where these vaccination programmes have worked very successfully, to keep track on the genetic evolution of this set of viruses because we are exerting with mass vaccination the most enormous selective pressure on the genetic rates of change."

Professor Anderson said that present-day vaccines were up to 95 per cent effective and so exerted a strong pressure on new forms of old viruses to come about. "The greater the selective pressure you apply, the greater the probability that a resistant form will emerge. One needs caution and to take viruses from an outbreak and continually check that they are not genetically changing."

Vaccination had contributed to the tremendous complacency over infectious disease, he said. "Although the developed world perceive these to be problems of the past, infectious diseases have remained a leading cause of human mortality throughout the world right from past history through to today. The deaths dwarf all those due to human conflict."

Modern genetics had demonstrated that infectious diseases were not stable entities, Professor Anderson said. "They are genetically very diverse beasts and we've been lucky with our vaccines in the past because all our successful ones have been against infectious agents that have been relatively genetically stable: measles, mumps, rubella and polio."

The emergence of antibiotic-resistant bacteria that were once thought to have been largely conquered was evidence of strong evolutionary forces in infectious disease, Robert Tauxe, of the US Centers for Disease Control in Atlanta, said.

"Once a pathogen becomes resistant, it has a consistent advantage wherever antibiotics are used. To humans and to public health, this is the growing threat, to the pathogen it is an evolutionary success story."

Dr Tauxe said the most virulent and resistant forms of the bacterium shigella, which causes dysentery, had appeared throughout Africa and was killing many thousands of people. "The most recent epidemic of dysentery among the Rwandan refugees at Goma is the most visible episode of this ongoing epidemic...the disease can only be treated with the most expensive oral antibiotics."

John La Montagne, head of the infectious disease division of the Centres for Disease Control, said new infectious diseases were certain to emerge, adding that HIV and Aids had shattered the illusion that infectious diseases were no longer problems.

"Among the newest microbes is a new cholera bacteria strain, 0139, that emerged in South-east Asia in 1992 and affected more than 500,000 people...Compared to people, viruses and bacteria have very little genetic material and one mutation can change a microbe's ability to infect, spread or cause disease. Also microbes can transform themselves rapidly because they exist in large numbers and quickly reproduce."

Leading article, page 18