By reason of claims that the rat flea would not bite man these convincing experiments were in a measure disregarded. The complete confirmation of the correctness of this view, as to transmission of bubonic plague, was brought about by the Indian Plague Commission. In a large number of experiments it was shown that when healthy and plague-infected guinea pigs were confined together in spaces where there were no fleas, there were no plague infections of any of these well animals.
Fig. 65.—1 and 2, male and female Xenopsylla cheopis. 3, Head of Ceratophyllus. 4 and 5, male and egg-distended female of Sarcopsylla penetrans.
Xenopsylla cheopis is the most important plague transmitter of the flea family as it is the common rat flea of India where there is so much plague.
Sarcopsylla or Dermatophilus penetrans is an important factor in a disabling skin disease, especially of the feet, in many parts of the tropics.
On the other hand in 35 experiments, when fleas had access to the spaces, plague infections were the rule. Again, guinea pigs in cages which were suspended only two inches above a plague flea infected floor, became infected, but other animals, which were suspended so high that the fleas could not jump up to them, remained well. Two cages, each containing a monkey, were placed in a plague flea infected room. One was surrounded with a protecting zone of 6 inches of “tanglefoot” fly paper, this being the limit of the distance a flea can jump, while the other cage was not so protected. The monkey in the cage without the sticky paper contracted plague while the second monkey remained well. It is only when there is a great incidence of plague among rats that we have outbreaks of bubonic plague in man, and it has been noted that the greater the epizootic, the more heavily infected was the blood of the sick rats with the plague bacilli. A flea with a stomach capacity of about ½ c.mm. could take in several thousand plague bacilli in a feeding on a rat whose blood was teeming with bacilli. The blood of a rat dying with plague may contain as many as fifty million bacilli to the cc. Human blood rarely contains more than a million to the cc. There is a multiplication of the organisms in the flea, so that when it defecates, thousands of plague bacilli are deposited near the puncture wound made by the flea when subsequently feeding on a man. The infected faeces are rubbed into the wound by the man in scratching the site of the bite, so that we have here an instance of a contaminative method of infection as contrasted with the inoculative method by the mosquito in malaria.
Fig. 66.—1, Ctenocephalus felis. 2, Ceratophyllus fasciatus. 3, Hoplopsyllus anomalus. 4, Ctenopsylla musculi. 5, Xenopsylla cheopis. 6, Pulex irritans. 7, Internal anatomy of flea. (After Fox.) (a) Maxillary palpus; (a-1) epipharynx; (a-2) mandible; (a-3) labial palpi; (a-4) maxillae; (a-5) basal elements of rostrum and mandibles; (b) salivary pump; (c) hypopharynx; (d) aspiratory pharynx; (e) muscles of the aspiratory pharynx; (f) eye; (g) oesophageal ganglia (brain); (h) thoracic ganglia; (i) oesophagus; (j) salivary duct; (k) gizzard; (l) salivary gland; (m) stomach; (n) aorta; (o) ovaries; (p) malpighian tubules; (q) pygidium; (r) rectum showing rectal glands; (s) anus; (t) intestines; (u) bursa copulatrix; (u-1) ductus obturatorius (blind duct); (v) receptaculi seminis or spermatheca; (w) ducts of spermatheca; (x) vagina; (y) uterus; (z) abdominal ganglia.