The disease is no longer a threat, but the smallpox virus remains a potential weapon

microbe of the month: smallpox virus Why do stocks of this killer survive? By Bernard Dixon

Doctors working for the Pentagon are still administering smallpox vaccine to military personnel - long after routine vaccination ended throughout the world. It is well over a decade since the World Health Organisation completed a triumphant campaign against this disfiguring and lethal disease so that smallpox was obliterated from the face of the Earth.

Nearly two decades have now elapsed since the last natural case (in Somalia in October 1977) and the last smallpox death (following the accidental release of virus at the University of Birmingham in August 1978). Why, then, is the Pentagon continuing to use smallpox vaccine?

The short answer is that the US Army needs to replenish its stock of vaccine immune globulin (VIG). This is an antibody that appears in the bloodstream after smallpox vaccination - a procedure in which the vaccine (vaccinia virus) triggers off the formation of antibodies that protect the recipient against the closely related smallpox virus.

VIG can be used to treat individuals who develop complications such as brain damage after vaccination. These side-effects are rare, but can prove fatal. Although the Pentagon terminated the routine vaccination of recruits in 1990, certain active-duty personnel are still immunised under circumstances involving hostile governments or terrorist groups. So volunteers are being paid to accept vaccination, and the precious VIG is being extracted from their blood.

The rather longer explanation is that the story of smallpox is by no means as complete as one might assume. Anxieties persist. The US government at least believes that some long-concealed stock(s) of smallpox virus might exist in unknown hands somewhere around the world. Human immunity having dwindled away over the past two decades since regular immunisation came to an end, this would be a devastating weapon. Hence the Pentagon's concern in maintaining the essential ingredients of an immunisation programme.

The only known stocks of smallpox virus since the early Eighties have been in the Centers for Disease Control and Prevention in Atlanta, Georgia, and the Research Institute for Viral Preparations in Moscow, Russia. Although the WHO decreed that both would be destroyed in December 1993, action was delayedbecause experts could not agree about taking such an irrevocable step. The WHO's latest guesstimate is that the deadly material will be destroyed in May next year. That would be exactly 200 years since the Gloucestershire physician Edward Jenner launched the era of vaccination by successfully immunising a young boy, James Phipps, against the disease.

The final extermination of smallpox virus can now be contemplated because scientists in the Russian and the US laboratories have determined the entire coded sequence of nearly 200,000 DNA sub-units comprising its genetic material. This removes a key argument deployed by those experts who had been insisting that we should retain the two stocks of virus because they might be required as a benchmark for comparison if smallpox were ever to return.

The likelihood of this happening seems extremely remote, not least because there is no reservoir of the virus, now eliminated from the human population, in any other animal. The historian Peter Razzell has argued that the virus could exist in desiccated burial vaults, based on cases in the past when the infection may have been acquired in locations of this sort. However, most experts discount this possibility.

But the sequencing of the smallpox virus DNA does pose a dilemma of a different kind. Writing in Perspectives in Biology and Medicine, Carol Shepherd McClain of the University of California, San Francisco, points out that the sequencers are due to publish their data shortly. Should they do so, true to the open spirit of science? Or should they desist, in the wider public interest? The DNA of smallpox virus is not itself infectious. However, it seems highly likely that knowledge of the DNA sequence could be used in future to engineer genetically a hybrid with at least some of the horrendous potential of natural smallpox virus.

So we have assuredly not seen the end of the challenges posed by a microbe which, over the centuries, has brought misery and death to millions of people. Although the disease has probably disappeared for ever as a natural phenomenon, the virus is still posing some daunting questions - questions arising directly out of the power and success of modern medical science.

After smallpox, as McClain observes, there will be other foci of concern. In recent times, the WHO has been showing great optimism regarding three other communicable diseases that could realistically be rendered extinct in the near future. These are poliomyelitis, measles and dracunculiasis (guinea-worm disease). The cattle infection rinderpest is another plausible candidate. Once these diseases have been eliminated, should their agents, duly sequenced, also be committed to the flames? Or should they be preserved, in microbial equivalents of the Chamber of Horrors, in recognition of their former odious impact on human history?

Finally, a question for those who believe that, given the opportunity, we ought to exterminate disease-causing microbes as far as possible and without a moment's hesitation. How can this attitude be reconciled with our modern consciousness that all forms of life are unique components of the planet's genetic diversity, that they have potential value which cannot always be seen in advance of its realisation, and that they thus justify preservation irrespective of what we feel about their most obvious attributes?