SCIENCE 



[N. S. Vol. XXXVI. No. 914 



nizable form in the immune animal. The 

 condition is analogous to that observed in 

 the chronic carriers of typhoid fever and 

 other bacterial diseases; it implies that a 

 reciprocal immunity has been established, 

 an armed truce so to speak, between the 

 host and invader. 



In trypanosomal disease chronic carriers 

 are equally in evidence. When recovery 

 occurs, as it does in some animals, the or- 

 ganism, though present in the blood, is 

 practically unrecognizable by means other 

 than inoculation of animals or by artificial 

 cultivation. The culture method had re- 

 vealed the presence of such trypanosomes 

 in a large percentage of birds, and more 

 recently the same procedure has demon- 

 strated the existence of similar parasites in 

 the cattle of various countries. 



The disease caused by sub-microscopic or 

 invisible organisms may show this same per- 

 sistence of the infective agent long after 

 recovery has taken place. A striking ex- 

 ample of this fact has but recently been 

 determined in connection with infantile 

 paralysis where the virus has been found 

 to persist in the naso-pharynx for many 

 months. The existence of the chronic car- 

 rier being recognized, it is no longer sur- 

 prising to learn of sudden outbreaks of this 

 dangerous disease, apparently spontaneous 

 in character, in a locality where no previ- 

 ous case was known to exist. 



INVERTEBRATE CAEEIERS 



Important as the vertebrate carrier may 

 be, it is quite overshadowed by the enor- 

 mous importance of the invertebrate car- 

 rier. It may be asserted without fear of 

 contradiction that the most valuable re- 

 sults which have been accomplished in pre- 

 ventive medicine in recent years have come 

 from the recognition of the exclusive role 

 played by these carriers in the transmission 

 of many diseases. Texas fever, malaria, 



yellow fever, sleeping sickness, not to men- 

 tion a score of other infections, find their 

 natural transmission in the agency of 

 insects, ticks and other sanguivorous or- 

 ganisms. 



It has been customary for some years to 

 speak of insects as passive and as active 

 carriers, which terms convey certain well- 

 defined conceptions. The passive carrier is 

 an accidental conveyer rather than a nat- 

 ural host for the germ. A fly feeding upon 

 typhoid excreta may soil its feet or pro- 

 boscis and on alighting elsewhere may de- 

 posit such mechanically adhering particles. 

 The part played by the passive carrier is 

 merely one of indirect contamination — a 

 purely mechanical transmission of the in- 

 fective agent from one place to another. 

 This transference of disease organisms to 

 articles of food, or even into wounds by 

 flying insects may lead to infection, and, in 

 fact, it is generally recognized that certain 

 bacterial and even protozoal diseases may 

 thus be spread. Cholera, dysentery, tu- 

 berculosis and typhoid fever are most often 

 mentioned in this connection. 



The active carrier, on the other hand, is 

 essentially a diseased individual and corre- 

 sponds in a way to the chronic vertebrate 

 carrier already discussed. Some, however, 

 can be compared more correctly to the 

 healthy carriers. The insect, tick, leech, 

 and the like, which feed upon an infected 

 animal may become a suitable soil for the 

 disease organism, which either multiplies 

 directly, or else passes through a develop- 

 mental cycle in its new host. It has been 

 supposed by some that active carriers can 

 harbor only animal parasites, such as the 

 pathogenic protozoa and filaria, an assump- 

 tion which is quite erroneous. It is un- 

 doubtedly true that many of the known 

 active carriers do transmit animal para- 

 sites, but that fact is not sufficient to ex- 

 clude a like transmission of bacteria, nor 



