We pass now from rats to fleas. That fleas might be connected with the spreading of plague was suggested in the year 1897 when Ogata first found bacilli in fleas. He obtained fleas from plague-sick rats. These he crushed, and injected the liquid into a couple of mice. One of these died of plague in three days. The German Plague Commission in Bombay found plague bacilli in fleas, but, for various reasons, did not consider that the bite of the flea was the means by which the disease was transmitted.
The real credit is due to Simond, a Frenchman, who worked during the Indian epidemics. He took fleas from infected animals and observed in their stomachs bacilli identical with B. pestis. He suggested that the bacillus was carried from rats to men; and he brought forward some evidence tending to show that infected fleas could transmit infection by biting. But Simond was not able to bring forward conclusive proof. He pointed out a line of research to others which has proved exceedingly fruitful. In the same year (1898) Hankin suggested that some biting insect might be the means of transmission from rats to man. The bacillus of plague has now been identified in ants, bugs, and flies as well as fleas. It seems likely that any suctorial insect which feeds on a plague-stricken rat will take numbers of the bacilli into its stomach.
The points which Simond wished to establish were that plague-stricken rats with fleas are exceedingly infective, that they cease to be infective when they have been deserted by their fleas, and that fleas which infest rats will transfer themselves to man. Since 1905 an elaborate series of observations and experiments have been carried out. Post-mortems have been made of countless rats. Numberless fleas have been collected and dissected. But this summary would be very incomplete if it did not mention the work of Verjbitski, a Russian doctor at Cronstadt, whose labours remained almost unnoticed although he made his experiments as long ago as 1902-1903. His thesis, written in Russian, was not published in any scientific journal. But his ingenious and careful experiments showed that fleas could transmit plague from animal to animal. He found that the commonest flea captured off rats at Cronstadt was Leptopsylla musculi, the usual host of which in other places is the mouse. Now this flea does not, except very rarely, bite human beings, and the real significance of the facts discovered was not appreciated.
The common rat-flea in most parts of Europe is Ceratophyllus fasciatus and in India and sub-tropical countries Xenopsylla cheopis. This last species has acquired the title of “the plague flea,” or, more accurately, the oriental rat-flea.
During the plague investigations in India many careful experiments have been made proving beyond doubt that the disease may be transferred from rat to rat by the transference of fleas from a septicæmic to a healthy animal. It was first shown that when fleas were present the plague could be transferred from rat to rat, kept in proximity, but carefully screened so as to avoid any possibility of contact. Next, fleas were collected from rats dead or dying of septicæmic plague and transferred to healthy rats living in flea-proof cages. More than half of the healthy rats contracted plague. It was shown that if fleas are present, the disease once started spreads from animal to animal; and it would seem that the rate of progress was in direct proportion to the number of fleas present.
The blood of a plague-infected rat may contain an enormous number of plague bacilli. Although such figures do not convey any very clear idea of numbers, as many as a hundred million bacilli have been found in a cubic centimetre of rat’s blood. A rat-flea, with a stomach of average size, might receive therefore as many as 5000 germs into its stomach; and it is clear that fleas feeding on a large proportion of plague-infected rats just before death would be almost certain to imbibe at least some plague bacilli. There is, moreover, good evidence for believing that multiplication of the plague bacilli may take place in the flea’s stomach. Nor does the blood imbibed by the flea cease to be infective when it passes from the stomach. Both the contents of the rectum and the excrements of fleas taken from plague rats often contain abundant and actively virulent plague bacilli. A number of infected fleas are put into a test-tube: the mouth of the tube is covered with a glass slide, and the mouth is turned upside down. The fleas are then seen to run over the slide, and, in a short time, they deposit an appreciable amount of fæcal matter on the surface. This under the microscope is seen to be covered with plague bacilli; and a large percentage of guinea-pigs, who have an emulsion of the fæcal matter injected into them, contract plague.
It is remarkable that, so far as we know at present, the plague bacillus is confined to the flea’s alimentary canal. On rare occasions it is found in the gullet when fleas have been killed immediately after feeding on septicæmic blood. But no plague bacillus has been found in the body-cavity or in the salivary glands.
In the stomach of the flea, plague bacilli have been found in vast numbers twelve and even twenty days after the insect has imbibed septicæmic blood. It is naturally of great practical importance to know how long fleas taken from plague-infected rats remain infective: that is to say, are capable of transmitting the infection to healthy animals. Two series of careful experiments, made during the epidemic plague season in India, have shown that fleas could remain infective for as long as fifteen days. In a third series of experiments, made during the non-epidemic season, it was found that the fleas remained infective for only seven days.
It has been ascertained that both the male and the female oriental rat (X. cheopis) flea can transmit plague.