Influenza

by Warren T. Vaughan

The spread of influenza is usually not limited to a single community. Almost invariably it will travel on to another locality, carried thither by human intercourse, and will there build again a local epidemiologic picture more or less modified by changes in the environment and changes in the virulence of the virus itself.

Spread, in primary waves.—Reference to the table of epidemics in history will show that in many of the epidemics and in most of the widespread epidemics and pandemics there appears to have been a definite, clearcut, direction of spread from one locality to others. In the recent literature there has appeared considerable discussion concerning the site of origin, the endemic focus of pandemic influenza. Briefly the question raised is as to whether there are single or multiple foci. We will for the time ignore this perplexing question. In either case, after the influenza virus has once attained such communicability as to produce a pandemic it does follow a direct course over countries and continents. This may be followed in resumé in our table.

The disease does not at any time spread more rapidly than the available speed of human communication between the areas affected. If influenza does appear simultaneously in two widely separated communities without having been brought there from a common source it must be that it arose spontaneously from simultaneous increase in virulence of the virus in those localities.

Influenza was prevalent in Turkestan, Western Asia, in May of 1889. It spread first to Tomsk in Siberia and did not appear in Petrograd until the end of October. By the middle of November it had reached Berlin and Paris, and one month later it was epidemic in New York and Boston. Four months had been required for the disease to reach Petrograd from Bokhara in Turkestan, while within two months thereafter it had traveled from Russia to the United States. In both cases the rapidity of spread corresponded to the rapidity of the means of communication of the locality; the caravan in Turkestan and the transatlantic liner to America. North America was widely infected in January of 1890. So, also, Honolulu, Mexico, Hong Kong, Japan. Ceylon first experienced the epidemic early in February, India at the end of the month, Borneo and Australia on the first of March, Mandalay towards the first of May, China and Iceland in July, Central Africa in August and Abyssinia in November of 1890.

It should be noted that influenza was reported to have been prevalent in Greenland at about the same time that it was in Bokhara. There appears to have been no relationship between these two outbreaks.

The spread of the pandemic may be followed also by recording the period of greatest mortality in the various cities. This period at Stockholm followed that at Petrograd by three weeks, and that of Berlin by another week. The period for Paris was a week later than for Berlin, that for London another week later, and that for Dublin three weeks later than that for London. The week of highest mortality in Dublin was later than that for New York or Boston.

The earlier epidemics progressed more slowly. That of 1762 prevailed in Germany in February, in London in April, in France in July, and in America in October. In 1782 it attacked London in May, Exeter two weeks later and Edinburgh early in June. In 1830–1832 the spread from Moscow and Petrograd through Germany required no less than eight months to cover the latter country.

In 1872 the time required for spread from Leipzig to Amsterdam was eighteen days, the same time that was required for a merchant in the latter town to reach Leipzig.

There are many instances on record in which influenza has passed by small towns in its onward course to attack a larger city and only at some later date has the small town, not on the main line of communication, been affected. Not only is the speed of transportation between two communities of importance, but also the volume of the transportation undoubtedly plays a part in the rapidity of development in a second locality. When the disease is carried by a vessel the first places to be attacked are the seaports and the coast towns, be the land a continent or an island. From there it spreads inland either rapidly or slowly according to the transportation facilities. Formerly the question was raised whether influenza spread in continuous lines or radiated in circles. Naturally it follows the direct lines of communication, most of which are radially distributed around large centers.

Leichtenstern calls attention to the fact that in the 1898 epidemic, as in the previous one, the general direction of spread was from East to West across Europe. This was also true of the epidemics of 1729, 1732, 1742, 1781, 1788, 1799, 1833, and 1889.