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Smallpox extinction – a note of caution

This is a classic article from New Scientist's archive, republished as part of our 50th anniversary celebrations

Smallpox is now confined as an active infectious disease to a few villages in Ethiopia, and the World Health Organisation is expected shortly to announce its total elimination. Much of the confidence that the disease can be rendered extinct rests on the absence of an animal "reservoir" in which the virus is carried forward as a pool for potential human infection in the future.

In this situation, the survival of the virus outside of its normal host is a matter of some practical interest. Dr A. C. Mitra and his colleagues published recently an analysis of the survival of the virus in different areas of the body (Bulletin of the World Health Organisation, vol 51, p 106). Virus in the urine, and throat and conjunctival secretions of smallpox patients diminished rapidly with time, whereas in scabs (dried-pustules) "The quantum of virus was found to have been unaffected by the passage of time"

The authors concluded that "variola virus is fairly stable when in the dried state". Indeed, the virus can be preserved for many years in the laboratory in a dried-frozen state. The Department of medical Microbiology at Liverpool University for example, holds smallpox virus which has been preserved for about 20 years. The survival time outside of the laboratory is unknown. However, work on the history of smallpox inoculation suggests that the virus can survive for some years even under natural conditions.

Smallpox virus has been used at least since the 17th century as a source of "vaccine", which was inoculated from the arm of a smallpox patient to that of the person being "variolated". My own researches suggest that much of what has been thought to be vaccination using cowpox virus has indeed been a form of attenuated variolation with smallpox virus itself (Medical History, vol 10, p 216). This practice was found in nearly every country of the world.

According to some historical accounts, inoculators were able to preserve smallpox virus for several years, before using it to good effect (producing mild cases of smallpox). For example, the Reverend Dishington described in 1792 how an amateur inoculator by the name of John Williamson preserved the virus: "He Keeps it a long time before he puts it to use - sometimes seven or eight years; and, in order to lessen its virulence, he first dries it in peat smoke, and then puts it underground, covered with camphor."

Similarly, J. Z. Howell described how inoculators in India frequently used virus "four or five years old", again finding that its virulence was weakened with the passage of time.

There is also evidence that smallpox can survive accidentally for long periods of time under natural conditions. Dr A. W. Downie has described how some years ago a demolition worker taking down what five years previously had been a smallpox isolation hospital in Aberdeen, caught smallpox at a time when there was no other smallpox in the area. And at the beginning of the 19th century, one of the English medical journals published the following macabre account of a smallpox epidemic originally described in Walpole's British Traveller (1784 edition, p 365):

"The following very singular occurrence happened in the year 1759 at Chelwood a village near Pensford (Somerset); the sexton of the place opened up the grave in which a man, who died of smallpox, had been interred near 30 years before. The coffin was of oak, and so firm, that it might have been taken out whole; but the man forced his spade through the lid, when there issued a most nauseous stench. The person who was to be buried being of eminence, most of the inhabitants of the village attended the funeral: in a few days afterwards, 14 persons were seized with the smallpox in one day; and in three days later, all but two in the whole village, who had not had it, were seized in like manner. It is remarkable, the disease was of so favourable a nature, that no more than two persons died of it." (Medical and Physical Journal, No. 199)

Given the practical relevance of knowledge of the survival of smallpox virus, historical evidence of this kind may be worth investigating in a more systematic and thorough fashion. Moreover, is the need for further knowledge of the survival time of smallpox virus under various conditions, the best argument against destroying the world's remaining stocks of laboratory smallpox virus?

First published in New Scientist on 1 July 1976

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