Editor's note: The article originally appeared in the February 11, 1911 issue of Scientific American.
ALTHOUGH the recent epidemics of bubonic plague in China, India, and other parts of the world have been always associated with outbreaks of the same disease among rats, the historical study of plague throughout the world reveals the singular fact that previous to 1800 very few references to a coincident mortality among rats have been put on record. Many excellent accounts of the older outbreaks, notably of the Black Death in Europe in 1347, and the Great Plague of London in 1665, are in existence, but careful research into these documents by modern historiographers — Haeser, Hirsch, Abel, and Sticker — has shown that for reasons difficult to discover very scanty mention of associated rat mortality has been made.
The earliest recorded instance is perhaps that given in the Bible in the account of the pestilence among the Philistines, which they ascribed apparently to “the mice that marred the land.” Avicenna refers to the association between rats and plague in his description of the epidemic in Mesopotamia about the year 1000 A.D. Nicephorus Gregoras, writing of the Great Plague of 1348, which entered Europe by way of Constantinople, makes a similar reference. Rats are mentioned in connection with the plague in Yunnan about 1757, and later in 1871-3. In India an association between rats and plague is noted in the “Bhagavata Purana,” by the Emperor Jehangir in the plague epidemic of 1615, and in a report of the Pali plague in Rajputana in 1836. Lastly, Orreus refers definitely to rat mortality in his account of the epidemic of 1771 in Moscow.
The identity of the disease in rats with that affecting man was established by the discovery in 1894 of Y. pestis by Yersin and Kitasato.
Within the next few years the relationship between rat and human plague was investigated in many parts of the world — by Thompson and Tidswell in Sydney, Clark and Hunter in Hongkong, Snow, Weir, Hankin and James in India, and by Kitasato in Japan. In 1905 the Plague Research Commission was appointed to investigate plague in India, and the reports of this commission represent the results of the most exhaustive inquiry into the subject that has yet been carried out.
The commission early turned its attention to the relationship of rat plague and human plague, and instituted an extensive examination of the rats in Bombay and elsewhere for the presence of plague infection. The maps and charts, representing graphically the results of this examination, clearly show the correlation between the epizootic and the epidemic — the rat epizootic preceding the epidemic by an interval of ten to fourteen days. Every outbreak of bubonic plague, when adequately investigated, was found to be associated with the disease among rats. The conclusion must be drawn that every epidemic of bubonic plague is caused by the concomitant rat plague.
In Bombay the rat population is an enormous one, Mus decumanus (the brown or gray rat) [later named Rattus norvegicus ] swarming in the sewers, gullies, and outhouses in the city, and Mus rattus [later named Rattus rattus ] (the black rat) living in countless numbers in the houses of the people. The latter species is of especial importance in plague epidemics, because it is essentially a house rat; it may almost be said to be a domesticated animal. The severity of the epizootics in the two species will be appreciated when it is stated that during one year the examination of 70,789 M. decumanus, taken from all parts of Bombay city, proved that 13,277 were plague-infected = 18.8 percent, and that out of 46,302 M. rattus examined 4,381 were plague-infected = 9.4 per cent. The heavier incidence of plague in M. decumanus in explicable by the circumstance that the flea infestation of this species is more than twice that of M. rattus.
Some interesting observations on the distribution of different species of rats in India have been made recently by Capt. R. E. Lloyd, I.M.S. The most common rats in India are M. rattus, M. decumanus, and Gunomys (Nesokia bengalensis). M. decumanus is common both in Bombay and Calcutta, but is absent from the city of Madras. It is significant that Madras is the one port in India which has never been seriously infected with plague. M. rattus appears to be universally distributed in India, whereas M. decumanus does not seem to occur in India except in seaports. Nesokia bengalensis is found in every part of India.
The question of the transportation of plague by ship rats is an extremely important one, but has not so far been thoroughly worked out. It would appear that M. decumanus is the species most commonly infesting ships, although M. rattus is also found.
Sticker, in his history of plague epidemics, quotes the statement that M. decumanus got into Europe from Persia about the year 1725. In England M. rattus was displaced by the invasion of M. decumanus about this time. At the present day the predominating species in this country is undoubtedly M. decumanus; M. rattus is, however, becoming increasingly common in the seaports.
An important question in plague epidemiology is the mode of conveyance of the infective organism from the plague rat to man. It is impossible even to summarize here the numerous, experiments and observations on this subject, but it may be said that from many sides, and especially from experiments in the laboratory and in actual plague-infected houses, a mass of evidence has been raised which incriminates and indeed convicts the rat flea as the transmitting agent of the infection.
In India the rat flea, Loemopsylla cheopis, which closely resembles the human flea, Pulex irritans, in appearance, is by far the most commonly found species. In England the common rat flea is Ceratophyllus fasciatus ; a single specimen only of L. cheopis has been found up to the present time.
L. cheopis, especially if hungry, will bite man; C. fasciatus does not take to man with any readiness, but will undoubtedly bite on occasion. This difference in the appetite of the two species for human blood may be of significance in determining the likelihood of the spread of rat plague to human beings. — Nature.