Microbe of the Month: Endangered, but not quite extinct: The virus that causes polio could go the same way as smallpox and be history by 2000, writes Bernard Dixon
Monday 11 October 1993
At present, only one disease-causing microbe has been rendered extinct in nature - the smallpox virus, whose eradication the WHO announced in 1987 following a triumphant worldwide vaccination programme. The only surviving samples of the virus are held in secure laboratories in Atlanta and Moscow. Following discussions at the International Congress of Virology in Glasgow in August, the decision whether or not to destroy those stocks will be taken at the World Health Assembly in Geneva next May.
There are two principal grounds for believing that polio will follow smallpox into the textbooks of medical history. First, immunisation in both cases confers solid, long-lasting immunity. Even if the virulent virus is introduced into a vaccinated community, therefore, it cannot cause disease and has negligible chances of spreading or surviving. Second, poliovirus, like smallpox virus, does not infect or persist in any other living creature (as does the malarial parasite in mosquitoes).
Where immunisation has been widely deployed, it has already vanquished polio. In 1955, there were more than 76,000 cases of the disease in the United States, Canada, Australia, New Zealand, the Soviet Union and 23 European countries. By 1967 that figure had fallen to 1,013, and today polio is very rare in industrialised regions. These advances have come from two vaccines. The first, consisting of 'killed' poliovirus and given by injection, was developed by Jonas Salk and introduced in the late Fifties.
The second, a live but weakened virus taken by mouth, was devised by Albert Sabin and came into use in the early Sixties.
Today, each year, more than 80 per cent of the world's infants are immunised against polio. The problem for the WHO is covering the remaining 20 per cent, many of whom live in parts of the world that are
not easy to reach by mobile immunisation teams. Yet this was precisely the challenge that was faced and overcome in the crusade against smallpox.
But no battle is won until the enemy has been defeated. That is why public- health specialists have been warning that as long as even the smallest pockets of unvaccinated, or inadequately vaccinated individuals, exist, even the richest countries face the risk of sudden outbreaks. Dr David Carrington and colleagues at the Ruchill Hospital in Glasgow recently reported the results of a survey in which they showed that only a third of people tested in their city had adequate levels of antibodies against one of the three types of poliovirus. To ensure full protection, it is essential that everyone has received the full sequence of three doses of vaccine.
There is another problem, illustrated by the outbreak of poliomyelitis in the past year in the Netherlands, a country with one of the most efficient health services in Europe. As occurred in 1978-79, the epidemic arose in Protestant communities whose allegiance to the Dutch Reform Church leads them to refuse immunisation on religious grounds. Although the outbreak has been contained by offering immediate vaccination to school pupils and other people likely to have been in contact with members of the affected group, at least 68 people have had the disease, some suffering paralysis. The 1978-79 incident eventually affected 110 people, 80 of whom were paralysed. In neither case has the original source of the virus been pinpointed, although presumably it was imported from a part of the world where the disease is still endemic.
Ironically, religious leaders are one group the WHO has been urging to become involved in its global eradication efforts. The campaign has brought together disparate forces, from the cricketer Imran Khan, who has promoted immunisation in Pakistan, to Rotary International, which is raising funds towards the elimination of polio by 2005. Religious leaders would be a welcome addition to these forces.
A little subversion might help, too. There is a good chance that if the Netherlands had chosen the Sabin vaccine for its routine immunisation programme, at least some of the children now paralysed by polio would have escaped. This is because the living Sabin virus, unlike the killed Salk virus, is shed in the faeces of vaccinees. It is thus passed to other children, who become immunised without either their or their parents' knowledge.
Which prompts the question: should vaccine manufacturers, using the new techniques of genetic manipulation, now begin to design vaccines against other infections with just this aim in mind - the incidental vaccination of other children? Would that be common sense or an infringement of religious freedom?
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