To Catch a Virus. John Booss
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yellow fever. It was concluded that “The mosquito acts as the intermediate host for the parasite of yellow fever, and it is highly probable that the disease is only propagated through the bite of this insect” (italics in the original) (29).
There followed the construction of two small buildings in an open field to compare the transmission of yellow fever by fomites with transmission by the bites of infected mosquitoes or inoculation of infected blood. The “Infected Mosquito Building” was well ventilated and divided into two compartments by a screen. The “Infected Clothing and Bedding Building” was purposely not well ventilated so as to retain any noxious effects of bed clothing, pajamas, and other items from previously infected cases. After some early discouraging results, John R. Kissinger, a soldier who Reed praised for having volunteered “solely in the interest of humanity and the cause of science” and who would accept no payment, came down with experimental yellow fever from the bites of infected mosquitoes (30). In these experiments, six of seven “non-immunes” bitten by infected mosquitoes in the Infected Mosquito Building became ill with yellow fever (32). None of the seven subjects in the Infected Clothing and Bedding Building exposed to fomites from cases of yellow fever became ill, nor did subjects become ill who had remained behind the screen, not bitten by mosquitoes.
The clarity of the design of comparison groups and the results were decisive: 85.71% infected by mosquitoes versus 0% by fomites. In the definitive publication in JAMA, “The Etiology of Yellow Fever: an Additional Note,” Reed, Carroll, and Agramonte ended with several major conclusions. In addition to confirming that “C. fasciatus serves as the intermediate host,” they determined that 12 days or more was required after contamination for the mosquito to transmit the infection. Thus, they determined experimentally what Carter had observed epidemiologically. They found that yellow fever could be transmitted by blood subcutaneously inoculated when taken from a patient on the first 2 days of the illness. They concluded that yellow fever resulting from a mosquito bite “confers immunity” against attempted reinfection with infected blood (32).
In memory of Lazear, the experimental station established by Reed, where the crucial studies were conducted demonstrating the transmission of yellow fever by mosquitoes and not by fomites, was christened Camp Lazear. Ironically, although Carroll recovered from acute yellow fever infection, he tragically died 7 years later of myocarditis attributed to that attack of yellow fever.
An important piece of the puzzle still remained to fall in place. Walter Reed and his colleagues’ final conclusion of their JAMA report was that “. . . the specific cause of this disease remains to be discovered” (32). Having turned away from that goal in their transmission studies, Carroll returned to the project. Initially confronted with local objections to further experimentation, Carroll resumed his studies in September 1901 in Cuba on the nature of the infecting agent (7). In the crucial experiment, six individuals were exposed to the bites of infected mosquitoes (33). Four did not develop yellow fever, but two did. Blood was taken from patients I and II for further transmission study, but due to an accident to the vacuum pump, the blood from patient I could not be used. The blood from patient II was divided into three aliquots of partially defibrinated and diluted serum. The first aliquot, a positive control, was left untreated and successfully transmitted yellow fever to patient III. The second aliquot was heated to 55°C for 10 minutes and failed to transmit disease to patients IV, V, and VI. Based on previous work on heat stability with toxins, Reed and Carroll argued against a toxin. The third aliquot was “slowly filtered through a new Berkefeld laboratory-filter” and the filtrate was inoculated into patients VII, VIII, and IX. Patients VII and VIII developed “unmistakable” attacks of yellow fever; patient IX remained well. The scientific data were presented at the annual meeting of the Society of American Bacteriologists, 31 December 1901 and 1 January 1902. Thus, clinical virology can be said to have started in the first years of the 20th century.
Presciently, they noted that the most effective means of controlling the spread of yellow fever was through destruction of mosquito breeding areas and prevention of mosquitoes biting the sick (31). This strategy was employed with extraordinary success by William Crawford Gorgas of the U.S. Army, also present in Havana at that time. A brilliant story unto himself, Gorgas cleared Havana of yellow fever. An interesting connection can be noted here between Gorgas and J. C. Nott, mentioned above, who had suggested that yellow fever transmission required an intermediate host. Nott, coincidentally, was the doctor who delivered the infant Gorgas, whose success in the story of yellow fever eradication was based on Nott’s theory. A few years later, Gorgas also cleared the Canal Zone of yellow fever and malaria, allowing the successful construction of the Panama Canal (17, 18, 25). Thus, the yellow fever-mosquito story was intimately wound into America’s expanding international role (26).
It was clear from the classic studies of the Yellow Fever Commission that transmission experiments had to be performed in human subjects. However, that presented significant ethical issues, not only for potentially lethal viral infections such as yellow fever but also for permanently disabling anterior poliomyelitis.
Although Carroll reported in 1904 that others had also shown that the agent of yellow fever was filterable (7), attempts to identify a bacterial cause continued (2). It was not until a successful experimental animal host, the rhesus monkey, was demonstrated in 1928 (37) and then the successful use of intracerebral inoculation of white mice (38, 39) that large-scale studies of the yellow fever virus could be undertaken and the bacterial candidates dismissed.
References
1 Ackerknecht, E. H. 1948. Anticontagionism between 1821 and 1867. Bull. Hist. Med. 22:562–593.
2 Agramonte, A. 1928. A review of research in yellow fever. Ann. Intern. Med. 2:138–154.
3 Beijerinck, M. W. 1942. Concerning a contagium vivum fluidum as cause of the spot disease of tobacco leaves, 1898, with 2 plates, p. 33–52. In J. Johnson (ed. and translator), Phytopathological Classics, No. 7. American Phytopathological Society, St. Paul, MN.
4 Bradbury, S. 1967. The Evolution of the Microscope. Pergamon, Oxford, United Kingdom.
5 Brock, T. D. 1999. Robert Koch: a Life in Medicine and Bacteriology. American Society for Microbiology, Washington, DC.
6 Carey, M. 1959. Yellow fever, p. 114–118.