In 1884, “. . . Pasteur did concede that he and his collaborators had still not managed to isolate and cultivate a rabies microbe in artificial media” (21). While rabies virus was incapable of growth on artificial media, one of the criteria that defined a viral agent, rabies was transmissible to susceptible animal hosts. The stage was set for the demonstration that rabies was a filterable, “ultravisible” agent. That task would prove to be a technical challenge. However, in 1903, Paul Remlinger, the Director of the Imperial Institute of Bacteriology at Constantinople, reported “Le passage du virus rabique à travers les filtres” (51). His success depended on the techniques of filtration and inoculation and on the number of experimental animals tested. As summarized by a contemporary, John McFadyean, a veterinarian, the most porous of the Berkefeld filters would allow passage of the virus, whereas more finely porous filters would hold the agent back. The filtrates were deposited under the dura, and several rabbits were inoculated with each filtrate (41). The success in filtration was confirmed by others (41). Remlinger was gracious, attributing to Pasteur the idea of rabies as ultramicroscopic (51). It was a fitting tribute to Pasteur.
Coincidentally, in 1903, A. Negri described inclusions in the neurons of the CNS of rabid animals and humans (44). While the inclusions were misinterpreted as parasites, they became in the following years “an extremely important method of diagnosis” (28). Thus, 1903 was a seminal year in the study of rabies. Using the animal model developed by Galtier and refined by Pasteur and his colleagues, Remlinger demonstrated the filterable, that is, viral, nature of the agent of rabies. Negri developed a histopathological hallmark for rabies which, when positive, was a “rapid viral diagnostic technique.”
Polio: Monkeys
Thought to exist for centuries as a sporadic disease, polio was a rare crippler of infants and children. John Paul, a student of polio, used a figure of an ancient Egyptian stele showing a young priest with a withered leg as evidence of such early observations in his 1971 volume, A History of Poliomyelitis (47). As Paul described, “Under such endemic conditions paralytic poliomyelitis could have remained under cover for centuries in populations in which infant mortality was high.” Yet the individual experience of the paralytic form of the disease, from sporadic to epidemic times, has been consistent. Walter Scott was born in 1771 in an area characterized as “. . . a cramped dimly lit alleyway with poor sanitation and little fresh air” (15a). He described the onset of polio at 18 months: “One night, I have often been told, I showed great reluctance to be caught and put to bed, and after being chased about the room, was apprehended and consigned to my dormitory with some difficulty. It was the last time I was to show such personal agility. In the morning I was discovered to be affected with the fever which often accompanies the cutting of large teeth. It held me three days. On the fourth, when they went to bathe me as usual, they discovered that I had lost the power of my right leg” (47). The contagiousness of polio was not recognized in the centuries when polio was understood to be a sporadic disease, and Scott’s fever was attributed to the cutting of teeth.
The evolution of polio from an endemic, sporadic disease to an epidemic pattern occurred toward the end of the 19th century. In the United States, for example, Charles Solomon Caverly of the Vermont State Board of Health reported in November 1894 that “Early in the summer just passed, physicians in certain parts of Rutland County, Vermont, noticed that an acute nervous system disease, which was almost invariably attended with some paralysis, was epidemic” (7). Caverly investigated this “‘new disease” that was affecting the children and initially found 123 cases. He reported that “The territory covered is mainly the narrowest part of the Otter Creek Valley in Rutland County, bounded on the east by the Green Mountain range and on the west by the Taconic range. . . .” Of 110 cases in which paralysis occurred, 50 persons recovered fully and 10 died, “leaving fifty who are apparently permanently disabled.” It is of note that a microbe was suspected: “. . . the microbic or infectious nature of the disease is generally believed in by us. . . .” However, Caverly reported no evidence of contagiousness, since it “affected almost invariably but a single member of a household.” He published a fuller report of his investigations in 1896 (8). It is remarkable that in his first report just months after the epidemic, Caverly noted the apparent absence of communicability. Presciently, however, Caverly suspected the microbial nature of the disease, which was to be shown by Karl Landsteiner and Erwin Popper 14 years later (38). Thus, with reports by others, the stage was set for proving the contagious and filterable virus nature of this affliction.
Communicability was demonstrated by Ivar Wickman 11 years later in Sweden, showing the role of nonparalytic cases in sustaining the epidemic (67). John Paul emphasized Wickman’s contribution: “Considering the importance of the contributions of Ivar Wickman, I do not believe that his work is fully appreciated today” (47). Later, Wickman failed to be appointed the Chair of Pediatrics at Stockholm, a position formerly held by his mentor in polio studies, Karl Oscar Medin. Perhaps in despair, though the reason was never ascertained, Wickman took his own life at the age of 42. John Paul emphasizes Wickman’s contributions as identifying the contagious nature of the disease, describing the importance of nonparalytic forms, estimating the incubation period as 3 to 4 days, and classifying different forms of the disease (67). Polio transmission to monkeys was soon to be shown by others. While crucial in demonstrating the viral nature of the disease, studies with monkeys may have deflected attention from key pathogenic aspects of the human disease.
The first decade of the 20th century proved to be decisive in the demonstration of polio as caused by a contagious, filterable virus. Unfortunately, those proofs were dependent on the emergence of epidemics of polio in Europe and North America. Wickman’s extensive studies of the epidemic in Sweden in 1905 were published as journal reports and as a monograph which was translated into English in 1913 (67). While the infectious nature of the disease had been suspected, no bacterial cause had been successfully demonstrated. Small laboratory animals also had not yielded an agent. However, in a meeting in December 1908 in Vienna, Austria, Karl Landsteiner, acknowledging the assistance of collaborator Erwin Popper, reported transmission of polio to monkeys, demonstrating for the first time a virus as the etiological agent of polio (Fig. 4). Two monkeys inoculated by the intraperitoneal route with spinal cord from a fatal human case demonstrated the symptoms and microscopic lesions of polio (38). A fuller presentation of Landsteiner and Popper’s findings was published the following year (33). Clinical findings of paralysis were observed in one of the monkeys, and both showed the characteristic histopathological findings of polio in the spinal cord exactly like those observed in human cases. In contrast, rabbits, guinea pigs, and mice, also inoculated by the intraperitoneal route, were negative. Thus, the importance of the monkey for isolation of poliovirus was established.