Search for blood-sucking insects which would feed on both rodents and man, and which might therefore be implicated, indicated that the fleas most nearly met the conditions. At first it was urged that rat fleas would not feed upon man and that the fleas ordinarily attacking man would not feed upon rats. More critical study of the habits of fleas soon showed that these objections were not well-founded. Especially important was the evidence that soon after the death of their host, rat fleas deserted its body and might then become a pest in houses where they had not been noticed before.
Attention was directed to the fact that while feeding, fleas are in the habit of squirting blood from the anus and that in the case of those which had fed upon rats and mice dying of the plague, virulent plague bacilli were to be found in such blood. Liston (1905) even found, and subsequent investigations confirmed, that the plague bacilli multiply in the stomach of the insect and that thus the blood ejected was richer in the organisms than was that of the diseased animal. It was found that a film of this infected blood spread out under the body of the flea and that thus the bacilli might be inoculated by the bite of the insect and by scratching.
Very recently, Bacot and Martin (1914) have paid especial attention to the question of the mechanism of the transmission of the plague bacilli by fleas. They believe that plague infested fleas regurgitate blood through the mouth, and that under conditions precluding the possibility of infection by dejecta, the disease may be thus transmitted. The evidence does not seem sufficient to establish that this is the chief method of transmission.
Conclusive experimental proof that fleas transmit the disease is further available from a number of sources. The most extensive series of experiments is that of the English Plague Commission in India, which reported in 1906 that:
On thirty occasions a healthy rat contracted plague in sequence of living in the neighborhood of a plague infected rat under circumstances which prevented the healthy rat coming in contact with either the body or excreta of the diseased animal.
In twenty-one experiments out of thirty-eight, healthy rats living in flea-proof cages contracted plague when exposed to rat fleas (Xenopsylla cheopis), collected from rats dead or dying of septicæmic plague.
Close contact of plague-infected with healthy animals, if fleas are excluded, does not give rise to an epizootic among the latter. As the huts were never cleaned out, close contact included contact with feces and urine of infected animals, and contact with, and eating of food contaminated with feces and urine of infected animals, as well as pus from open plague ulcers. Close contact of young, even when suckled by plague-infected mothers, did not give the disease to the former.
If fleas are present, then the epizootic, once started, spreads from animal to animal, the rate of progress being in direct proportion to the number of fleas.
Aerial infection was excluded. Thus guinea-pigs suspended in a cage two feet above the ground did not contract the disease, while in the same hut those animals allowed to run about and those placed two inches above the floor became infected. It had previously been found that a rat flea could not hop farther than about five inches.
Guinea pigs and monkeys were placed in plague houses in pairs, both protected from soil contact infection and both equally exposed to aerial infection, but one surrounded with a layer of tangle-foot paper and the other surrounded with a layer of sand. The following observations were made: