July 5, 1912] 



SCIENCE 



does it justify the assumption, so freely 

 resorted to, that an unseen and unrecog- 

 nizable germ which is transmitted by an 

 active carrier is ipso facto a protozoon. 



The best example of an insect bacillus 

 carrier is seen in bubonic plague, which is 

 spread almost wholly by the bites of fleas 

 which come from naturally infected rats or 

 other rodents. The plague bacillus is pres- 

 ent in the blood of the diseased animal, 

 usually some hours before death, and the 

 flea which sucks up such blood into its 

 stomach becomes infected with a variable 

 number of the organisms. These then un- 

 dergo multiplication in the digestive tube, 

 and persist therein for four to ten to 

 twenty-one days, depending upon the tem- 

 perature, method of feeding, and perhaps 

 on factors inherent in the flea itself. There 

 is no evidence that the flea is affected in 

 any way by the presence of the bacillus, 

 which leaves the body chiefly, if not en- 

 tirely, along with the feces. The contact 

 of this excretal matter with the wound is 

 perhaps the chief means by which infection 

 occurs. It is clear, therefore, that the flea 

 is not a mere passive carrier, but an actual 

 host (a healthy carrier), even though the 

 parasite is unable to maintain a very pro- 

 longed existence within its digestive tube. 



The common rat flea is also one of the 

 agents which brings about the transmission 

 of a protozoal disease among rats, namely, 

 Trypanosoma Leivisi. Opinions have dif- 

 fered as to the part played by the flea in 

 the transmission of this infection; as to 

 whether it was a mere passive, or a real 

 active carrier. But recent studies have 

 shown clearly that passive transmission, in 

 this as in other trypanosomatic diseases, 

 though possible, is of no special conse- 

 quence ; and, that the flea is a true host, an 

 active carrier. The work of Minchin and 

 Thomson indicates the existence of an in- 

 tracellular multiplication of the parasite in 



the epithelial cells of the mid-gut, as a 

 result of which the trypanosome breaks up 

 into a number, possibly eight, daughter 

 trypanosomes, which then become free and 

 undergo further development and multipli- 

 cation. The period of incubation is six 

 to seven days or more, but the insect once 

 infected remains so for an indefinite pe- 

 riod. Regurgitation of the ripe infective 

 form from the stomach of the flea into the 

 wound made by the proboscis of the flea is, 

 according to these investigators, the normal 

 method of transmission. 



Incidentally it may be stated that evi- 

 dence has been presented, though it can not 

 be said to be conclusive, which goes to show 

 that the dog flea is responsible for the 

 spread of the disease known as infantile 

 kala-azar. Further investigation is needed 

 to determine the mode of transmission of 

 this deadly infection. 



The tick family is an extremely impor- 

 tant group as regards the transmission of 

 disease of man and animals. The infective 

 agent may be a bacillus, a spirochete ; or a 

 typical protozoon such as a piroplasm or a 

 trypanosome. For example, we know from 

 the splendid studies of Eicketts on the so- 

 called Rocky Mountain fever, also known 

 as "spotted" or "tick" fever, that this dis- 

 ease is due in all probability to a bacillus, 

 and, furthermore, that this is invariably 

 transmitted by the bite of a tick, or of its 

 offspring. 



The spirochetes which many workers, 

 following the lead of Schaudinn, believed 

 to be protozoa are now rarely classed as such 

 even by zoologists. Instead, they have either 

 been turned back among the spiral bacteria, 

 or have been given a separate position, 

 intermediate between them and the proto- 

 zoa. This fact is of interest because most 

 of the spirochetal diseases are transmitted 

 through the agency of active carriers, such 

 as the louse and especially ticks. The best 



