XV. Further observations on the transmission of plague by fleas, with special reference to the fate of the plague bacillus in the body of the rat flea (P. cheopis)
1. The average capacity of a rat flea's stomach is approximately 0·5 c.mm. On this basis a flea imbibing the blood of a plague rat showing a good septicaemia might take as many as 5000 germs into its stomach.2. Multiplication of the plague bacillus takes place in the stomach of the rat flea.3. The approximate proportion of fleas in the stomach of which multiplication of plague bacilli takes place has been determined, and it has been shown that this proportion varies with the season of the year, being six times greater in the epidemic season than in the non-epidemic season.4. Plague bacilli are present in the rectum and faeces of fleas taken from plague rats, and such faeces are infective to guinea-pigs both by cutaneous and by subcutaneous inoculation.5. On rare occasions plague bacilli have been found in the oesophagus, but never in any other region of the body, such as the body cavity or salivary glands.6. During the plague season fleas might remain infective for 15 days after imbibing infective blood, but during the non-epidemic season no animal was infective after the 7th day.7. A single rat flea may transmit the disease.8. Both male and female rat fleas can transmit the infection.9. Experimenting with cat fleas (P. felis) and human fleas (P. irritans), 27 experiments with the former were unsuccessful, and out of 37 experiments with the latter three successes were obtained. Two experiments were made with C. fasciatus: both were successful. Multiplication of the plague bacillus takes place in the stomach of the human flea.10. The plague bacillus has never been seen in the body cavity or in the salivary glands of infected fleas.Evidence has been obtained to show that the bite of a healthy flea affords a sufficient avenue for infection by septicaemic blood if it is spread upon the bitten part.No evidence has been obtained in favour of infection by contaminated mouth parts or regurgitation from the stomach, but the possibility of infection by such means cannot be excluded.