The development of the virus concept as reflected in corpora of studies on individual pathogens. 2. The agent of fowl plague--a model virus.

FowL PLAGUE is a devastating disease of poultry which was first described in the form of outbreaks in Italy in the last two decades of the nineteenth century.1 Until then no clear aetiological distinction had been made between it and chicken cholera, and only after Perroncito2 in 1878 and Pasteur" in 1880 had turned their attention to chicken cholera did the difference in clinical manifestations come to be recognized. In 1901 there was another outbreak of fowl plague in Italy. It spread in the early months of the year from the area around Ferrara to Modena, and was subsequently brought from northern Italy over the Alps with the stock of an itinerant poultry merchant to cause an epizootic in Austria, where it travelled the length of the upper Inn valley.4 It also made an appearance at the Brunswick poultry show, where the authorities unfortunately panicked, closed the show, and insisted that all birds be returned immediately to their place of origin.5 The inevitable result was numerous outbreaks throughout the German states, as far apart as Oldenburg6 and Wiirttemberg.7 All of the outbreaks were duly recorded, and as there was at the time a great deal of interest in the recently established group of pathogens referred to variously as "invisible"8 or "filterable" viruses,9 pathologists working on the epizootic were quick to test the ability of the agent to pass through bacteria-proof filters. Three papers were published in rapid succession, reflecting accurately and chronologically the spread of the disease through Italy and into Austria. The respective authors were Centanni and Savonuzzi,10 Maggiora and Valenti,"' and Lode and Gruber.12 In all three studies was the agent shown to be filterable. The clinical picture and the obvious neurotropic character of the agent led some pathologists in the early years to compare fowl plague to rabies;13 initially the severity of the symptoms and the very high mortality gave no indication that the virus might be related to that of influenza in man, as was finally proved to be the case in 1955 when Schiifer"4 demonstrated the close relationship between the viruses of fowl plague and influenza A. It is interesting that more recently morphological similarities have been observed between rabies virus and members of the myxoviruses which include the influenza group.15