Hence, two laboratory requirements defined viruses: passage through a porcelain filter and the need for living cells on which to grow.
Figure 4 Martinus Beijerinck in his laboratory, May 1921. Beijerinck, like Ivanowski, demonstrated that tobacco mosaic disease could be transmitted by sap which had passed through bacteriological filters. He also demonstrated the need for living cells to replicate the disease-causing factor, which he called contagium vivum fluidum. (Photo source: Wikimedia Commons, in the public domain.)
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For the first demonstration of a filterable agent in mammals, again the drive was commercial. Foot-and-mouth disease impaired cattle breeding and reduced milk production, bringing severe economic hardship to Prussian agriculture (35). Friedrich Loeffler, a collaborator of Koch and the discoverer of the diphtheria bacterium and toxin, was appointed in 1897 by the Prussian Ministry of Cultural Affairs to study foot-and-mouth disease. While readily transmissible experimentally, the agent was not visible microscopically. In collaboration with Paul Frosch, Loeffler demonstrated that foot-and-mouth disease could be transmitted to cows by bacterium-free lymph which had been passed through a Berkefeld filter (23). Experiments designed to detect the presence of a toxin resulted in retention of disease-producing activity, even at extremely high dilutions. This observation raised the possibility that the effect was produced by a germ which could multiply. The hope was that a vaccine could be developed. Unfortunately, as discussed by H.-P. Schmiedebach, a vaccine with persistent immunity-inducing capacity did not result, and foot-and-mouth disease remains a scourge to this day (35).
The work of Loeffler and Frosch stimulated work on yellow fever, a terrifying human viral disease which had been the subject of intense debate in the 19th century between the contagionists and the anticontagionists (1). Yellow fever was at once a political and commercial battle as well as a scientific matter. The politically liberal anticontagionists were against quarantines and the bureaucratic apparatus that supported quarantines. The French Academy of Medicine weighed in on the issue in 1828 against yellow fever quarantines. Thus, the matter of disease transmission had political overtones as well as commercial consequences. Walter Reed, as director of the U.S. Army Yellow Fever Commission, would definitively demonstrate the mode of yellow fever transmission. William Welch, the dean of American pathology, pointed out to Reed the possible relevance of the work of Loeffler and Frosch on foot-and-mouth disease to the etiology of yellow fever (30).
Walter Reed and the Yellow Fever Commission
By 1900, several pieces of the puzzle were in place that would help to explain the spread of yellow fever. Of principal interest was the mode of transmission; the precedent had been set in 1897 with malaria when mosquitoes were identified as vectors. Walter Reed acknowledged “. . . the splendid work of Ross, Bignami, and others with regard to the propagation of malarial fever. . .” (30). Reed also acknowledged the work of J. C. Nott in 1848 in suggesting “. . . that the spread of yellow fever could not be assumed by the assumption of a diffusible miasm in the atmosphere but required the presence of an intermediate host. . . .” The specific mosquito, then called Stegomyia fasciata, later called Culex fasciatus and finally Aedes aegypti (19), had been identified by Carlos Finlay in 1886. However, Finlay had failed to convince his colleagues that this mosquito was responsible for disease spread. One of the principal reasons for that failure was ignorance of an extrinsic incubation period, a time during which the virus matures in the mosquito.
Reed recognized the careful work of Henry Rose Carter (8) (Fig. 5) in two small towns in Mississippi in 1898, “‘demonstrating the interval between the infecting and secondary cases of yellow fever.’” Reed was gracious in declaring, “To Dr. Carlos J. Finlay, of Havana, must be given, however, full credit for the theory of the propagation of yellow fever by means of the mosquito” (30).
Figure 5 Henry Rose Carter, 1909. As a member of the Marine Hospital Service, he was able to deduce a delay between primary and secondary cases of yellow fever. This extrinsic incubation period implied the need for another, nonhuman, host, later shown to be the mosquito. He was assigned to the Panama Canal Zone in 1904 to work on yellow fever. (Courtesy of Historical Collections & Services, Claude Moore Health Sciences Library, University of Virginia.)
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Sill missing was a crucial piece of the puzzle: isolation of an agreed-upon etiological agent of yellow fever. Although this was a time of exciting discoveries in medical bacteriology, credit in disproving putative bacterial causes must go to George Miller Sternberg, a pioneer American bacteriologist. An author of early American textbooks of bacteriology in the 1890s, he spent most of his career in the U.S. Army and was largely self-taught in bacteriology. In 1890, he published Report on the Etiology and Prevention of Yellow Fever, in which he thoroughly disposed of the several candidate bacteria as the cause of yellow fever (36). In 1897, Sternberg appointed Reed and James Carroll to investigate yet another candidate, Bacillus icteroides (Sanarelli) along with his own candidate, Bacillus X. Reed and Carroll, who were joined by Aristedes Agramonte in Cuba in 1898, demonstrated that Bacillus icteroides “bore no relation to the disease” (7).
In 1900, by-then Surgeon General Sternberg appointed Reed, Carroll, Agramonte, and Jesse W. Lazear to a board of army medical officers to investigate yellow fever in Cuba (7). The board first met 25 June 1900 (2). Astonishingly, within 4 months the board was able to report at the Annual Meeting of the American Public Health Association in October 1900 that Culex fasciatus served as the intermediate host for yellow fever (29). In clearing the field of bacterial contenders and in appointing the U.S. Army Yellow Fever Commission in 1900, Sternberg can be credited as the catalyst of these findings on the transmission and etiology of yellow fever (Fig. 6).
Figure 6 George Miller Sternberg. Known as America’s first bacteriologist, he produced the first textbook of bacteriology in the United States. He was Surgeon General of the Army from 1893 to 1902, during which time he appointed the Yellow Fever Commission. (Courtesy of the Historical Collections & Services, Claude Moore Health Services Library, University of Virginia.)
