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.