Pro-vivisection organisations often cite the conquest of polio as a triumph of animal experiments. In fact, emphasis on animal research rather than human studies delayed a proper understanding of the disease for over 25 years.(1)
In 1908 Landsteiner and Popper announced they had discovered the polio virus: tissue from an infected patient who had died produced spinal cord disease when injected into two monkeys. The animals died with one developing paralysis in both legs. Negative results had been obtained with rabbits, guinea pigs and mice and indeed the researchers had been "fortunate" in choosing Old World monkeys such as the rhesus who are highly susceptible to the disease. New World monkeys are relatively resistant.
Although obviously important to discover the virus, the means by which it was achieved had a devastating impact on polio research. Believing they now had an exact replica of the human infection, scientists focussed their main attention on the artificially induced disease in monkeys. Based on these experiments it was generally believed that poliovirus enters the body through the nose and that it only attacks the central nervous system (CNS) producing spinal cord damage.(1,2)
Yet by 1907 careful epidemiological analysis of actual human cases had shown that poliomyelitis was not entirely or even chiefly a disease of the CNS. The studies, based on over 1000 Swedish cases, were carried out by Dr. Ivar Wickman, who also correctly concluded that the gastrointestinal tract was the probable route of infection.(1) By 1912 other clinical studies also established the intestinal tract as the means of infection.
Tragically, animal experiments so dominated research that prior to 1937 most scientists rejected the notion that polio is an intestinal disease. As Dr Paul explains in A History of Poliomyelitis, "...with the discovery of the virus and the rush of enthusiasm for experimental work, the mainstream had soon been diverted away from Wickman's correct concepts of the human disease gained from clinical epidemiological work carried out so painstakingly in the field."
Whether the virus entered the body by the mouth or nose was of great practical importance, for it determined strategies for preventing the spread of disease. By 1937 researchers had produced a nasal spray that prevented infection in monkeys. It was widely promoted for human use but inevitably failed.(1) The only result was to abolish the children's sense of smell, in some cases permanently.(2)
Support for the nasal route of infection gradually waned after further clinical observations while animal researchers were no doubt reassured with the finding that chimpanzees, unlike the rhesus monkeys used earlier, could be infected via the intestinal tract. It was only when scientists understood that poliovirus enters the mouth and first resides in the intestines that it was possible to develop an orally administered vaccine, and this formed the basis of Sabin's approach.
For years, monkeys were also used for diagnostic purposes, to test for the presence of virus. Tissue samples from patients (or other monkeys) were innoculated into the animals who were then assessed for damage to the spinal cord. The procedure was laborious, time consuming and expensive but during the early years of the 20th century, scientists had only learnt to grow viruses in living animals. However, in 1949 Enders, Weller and Robbins showed that polio virus could be grown in human tissue culture. Most significantly, the virus produced a specific change in the infected cells which could be recognised under the microscope. It was therefore easy to detect the presence of polio virus in tissue samples. Had such a quick and simple alternative been introduced at an earlier stage, progress would surely have been more rapid. Indeed, by suggesting that virus would only grow in the CNS, the misleading monkey model of polio delayed the development of tissue culture techniques which were ultimately critical to the discovery of a vaccine.(1,3)
1) J.R.Paul, A History of Poliomyelitis (Yale University Press,1971).
2) H.F.Dowling, Fighting lnfection (Harvard University Press, 1977).
3) A Critical Look at Animal Research (Medical Research Modemization Committee, New York, 1990).
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