This being so, it becomes important to know all about the fleas of rats. Quite unexpected facts have been discovered in regard to them. In Europe a very large flea is found on the grey and the black rat. This kind has not, I believe, ever been found on human beings or been known to bite them. But in India, in the Philippines, and in the ports of the Mediterranean, this northern rat-flea is rare, and its place is taken by a smaller and more actively vagrant flea, which Mr. Charles Rothschild (who is the great authority on fleas) found upon several different kinds of small animals in Egypt. He named it “Pulex cheopis.” This is the flea (and not our big northern rat-flea) which acts as the carrier of plague-germs from rats to man in India. It appears from experiments that the common flea of man (Pulex irritans) and the cat-and-dog flea (Pulex felis), as well as the big northern rat-flea (Ceratophyllus fasciatus), can harbour the plague-bacillus if fed on plague-stricken animals, but there are no observations to show (as there are about the “Cheops flea”) that they pass habitually from man to rats and rats to men.
It is happily so long (200 years) since we had a real outbreak of plague in Europe that we are still in doubt as to whether the Grey rat or the Black rat is the more susceptible to the disease--and what flea, if any, acts, or has acted, as the carrier from rat to man in this part of the world. The suggestion has been made that the Grey Norwegian rat takes plague less easily than the Black rat, or than the Indian Mole-rat (Nesokia), and that the multiplication of the Grey rat in England and France and consequent decrease in Black rats, is, therefore, an advantage, so far as plague is concerned. Possibly with the Grey rat has come the big rat-flea, which does not attack man as does the Cheops flea. The disappearance of plague in Western Europe seems to correspond in date with the arrival of the Grey rat. But, on the other hand, an alteration in the character of our houses and their greater “accommodation” for the new rat rather than the old black species may account both for the increase of the latter and for the absence of dirt and vermin in the dwelling-rooms and bed-chambers which formerly enabled the plague-bacillus to flourish amongst us, and to reach the human population--as it does now in India and China. All this shows how necessary it is to have accurate true knowledge of such despised creatures as rats and fleas, if we are to live in great crowded cities closely packed together. And it should also make us try to gain further knowledge as to these creatures, so that we may form a reasonable anticipation of the consequences we are bringing down on our heads when we set about exterminating this or that race of animals. We are not yet sure that the Norwegian Grey rat is not a blessing in disguise.