There have been in Europe two general routes followed by pandemics, a Northern one through Russia and following the lines of travel into Germany and through the countries of Europe; and a Southern path coming from Asia, through Constantinople, and entering Europe from the South, particularly Italy. With the latter, after reaching Europe, the spread is northerly; with the former it is southerly, and usually Spain was the country last infected.
In the United States as well, pandemic influenza usually has spread from East to West, entering the country at or near New York or Boston, and spreading West and South. This was true in the autumn epidemic of 1918.
Spread in recurrences.—As a rule the manner of spread of a secondary epidemic following the primary pandemic wave is quite different. At a longer or shorter interval following the first spread the disease breaks out anew in one locality or another, sometimes simultaneously in widely separated districts. Sometimes we can distinguish a direction of spread in the relatively small community affected, it frequently being observed that the disease will start up in a large city which has experienced the illness during the first pandemic, and from there will spread to small nearby localities which may have remained free until that time. Again, any clearcut direction of spread may be entirely lacking. It is rare indeed that an epidemic following another by a short interval will follow a definite line over an entire country or continent. Such an example is, however, to be found in the epidemic of 1833, which traveled over Europe from Russia, spreading to the west and the south and following practically the identical path that it had taken in 1830. Even so it was not as widespread, for while the epidemic of 1830 had covered the entire earth, America appears to have escaped the second epidemic.
These disseminated and independent outbreaks are believed to arise from endemic foci in which the virus has been deposited during the progress of its first spread and in which the germ has survived until it has acquired once again exalted virulence.
Usually these endemic outbreaks show in their local configuration, a secondary type of wave. That this is not always the case we have already indicated. The epidemic of 1732–1733 was a recurrence of that of 1729–1730. The epidemic of 1782 had as its source the epidemic of the years 1780–1781. The epidemic of 1788 recurred until 1800, and was quite possibly associated with those of 1802, 1803 and 1805–1806. That of 1830 recurred in 1831–1832. Next we have in 1833 the true pandemic originating in Russia. Recurrences of the epidemic of 1836–1837 were found in 1838 and in 1841. Those spreads which occurred in 1847 and 1848 found successors in the year 1851. In 1890 the influenza outbreaks were as a rule single or isolated and occurred in only a few places of Europe, particularly in Lisbon, Nürnberg, Paris, Copenhagen, Edinburgh, Riga, London, etc. It is reported that there was an unusually severe local outbreak in Japan in August, 1890. In 1891 no general direction of spread was manifested, yet in heavily populated areas, or states rich in lines of communication, especially those of Europe and North America, one could frequently trace some definite direction followed by the disease within these relatively small territories.
A. Netter made the following observation at that time: “La Grippe a fait des explosions simultanées ou successives, et on n’a pu en aucune façon subordonner ces différents foyers comme cela avait été possible en 1889–90. Il parait y avoir eu des reveils de l’épidémie sur divers points.”
Leichtenstern describes the subsequent spread of the disease: “The transfer of the disease by ships which played such an important role in the first epidemic appeared to be insignificant in 1891, in spite of the fact that influenza was present in many of the English colonies. The third real epidemic spread of influenza was a true pandemic which began in the autumn (October) of 1891 and lasted through the whole winter until the spring of 1892. It involved all of Europe and North America and spread to all other lands, but here again the geographic distribution followed no rule. There was no spread of influenza from a central point, no continuous spread following lines of communication, and there was no longer an early predominance in the cities lying on the lines of communication or in the larger cities and commercial centers, as had been the case in the first epidemic. In England in 1891 the first outbreaks occurred frequently in country districts. The epidemic raged nearly four months in the northern part before it finally reached London in May. The same was true of Australia.
“One peculiarity of the recurrent epidemic lay in the much more contagious character of the disease and the remarkably greater mortality. In Sheffield the mortality in the recurrent epidemic was greater than in the pandemic, even though the epidemic picture was that of a primary wave.”
By way of summary of our knowledge of the primary and secondary spread in general up to the epidemic of 1918, we may enumerate the more important characteristics:
1. Occurrence of true pandemics at wide intervals, primarily intervals of several decades.