Handbook of Medical Entomology

by William A. Riley; O. A. Johannsen

(a) Many fleas were caught in the tangle-foot, a certain proportion of which were found on dissection to contain in their stomachs abundant bacilli microscopically identical with plague bacilli. Out of eighty-five human fleas dissected only one contained these bacilli, while out of seventy-seven rat fleas twenty-three were found thus infected.

(b) The animals surrounded with tangle-foot in no instance developed plague, while several (24 per cent) of the non-protected animals died of the disease.

Thus, the experimental evidence that fleas transmit the plague from rat to rat, from rats to guinea pigs, and from rats to monkeys is indisputable. There is lacking direct experimental proof of its transfer from rodents to man but the whole chain of indirect evidence is so complete that there can be no doubt that such a transfer does occur so commonly that in the case of bubonic plague it must be regarded as the normal method.

Rats are not the only animals naturally attacked by the plague but as already suggested, it occurs in various other rodents. In California the disease has spread from rats to ground squirrels (Otospermophilus beecheyi), a condition readily arising from the frequency of association of rats with the squirrels in the neighborhood of towns, and from the fact that the two species of fleas found on them are also found on rats. While the danger of the disease being conveyed from squirrels to man is comparatively slight, the menace in the situation is that the squirrels may become a more or less permanent reservoir of the disease and infect rats, which may come into more frequent contact with man.

The tarbagan (Arctomys bobac), is a rodent found in North Manchuria, which is much prized for its fur. It is claimed that this animal is extremely susceptible to the plague and there is evidence to indicate that it was the primary source of the great outbreak of pneumonic plague which occurred in Manchuria and North China during the winter of 1910-11.

Of fleas, any species which attacks both rodents and man may be an agent in the transmission of the plague. We have seen that in India the species most commonly implicated is the rat flea, Xenopsylla cheopis, (= Lœmopsylla or Pulex cheopis) ([fig. 89]). This species has also been found commonly on rats in San Francisco. The cat flea, Ctenocephalus felis, the dog flea, Ctenocephalus canis, the human flea, Pulex irritans, the rat fleas, Ceratophyllus fasciatus and Ctenopsyllus musculi have all been shown to meet the conditions.

But, however clear the evidence that fleas are the most important agent in the transfer of plague, it is a mistake fraught with danger to assume that they are the only factor in the spread of the disease. The causative organism is a bacillus and is not dependent upon any insect for the completion of its development.

Therefore, any blood-sucking insect which feeds upon a plague infected man or animal and then passes to a healthy individual, conceivably might transfer the bacilli. Verjbitski (1908) has shown experimentally that bed-bugs may thus convey the disease. Hertzog found the bacilli in a head-louse, Pediculus humanus, taken from a child which had died from the plague, and McCoy found them in a louse taken from a plague-infected squirrel. On account of their stationary habits, the latter insects could be of little significance in spreading the disease.

Contaminated food may also be a source of danger. While this source, formerly supposed to be the principal one, is now regarded as unimportant, there is abundant experimental evidence to show that it cannot be disregarded. It is believed that infection in this way can occur only when there is some lesion in the alimentary canal.

Still more important is the proof that in pneumonic plague the patient is directly infective and that the disease is spread from man to man without any intermediary. Especially conclusive is the evidence obtained by Drs. Strong and Teague during the Manchurian epidemic of 1910-11. They found that during coughing, in pneumonic plague cases, even when sputum visible to the naked eye is not expelled, plague bacilli in large numbers may become widely disseminated into the surrounding air. By exposing sterile plates before patients who coughed a single time, very numerous colonies of the bacillus were obtained.