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SCIENCE. 



[N. S. Vol. XXII. No. 555. 



the animal becomes a carrier of disease 

 germs. This is a merely mechanical func- 

 tion and such disease-producing organisms 

 as may be adherent to the body of the 

 carrier are transported unwittingly from 

 one point to a new environment where 

 similar chance causes them to be deposited. 

 In this way such germs may be distributed 

 widely from an originally small focus and 

 may be brought into inappropriate and 

 unfortunate relations with members of the 

 human species. A very large number of 

 isolated cases might be cited to demonstrate 

 such mechanical transport by animal car- 

 riers. One of the best known is the trans- 

 port of typhoid germs by means of flies. 

 The bacilli which are found in excreta ad- 

 here to the feet and hairs of the fly walking 

 over such material, only to be carried by 

 the next flight of the insect to a dish of 

 food or pan of milk left standing on a 

 table in the house or a bench at the door. 

 In this new environment the germs may 

 multiply and with it gain entrance to the 

 new human host with disastrous results. 

 Veeder has given a most vivid description 

 of the unsanitary conditions which actually 

 existed in our army camps in the Spanish 

 American War and which demonstrated on 

 a large scale this mechanical transfer of 

 typhoid germs. Moreover, it can not be 

 doubted that the bacilli actually are carried 

 in such fashion, for in experiments re- 

 ported in the transactions of this society, 

 Maddox demonstrated that when such flies 

 as have visited cultures of disease germs, 

 walk over sterile gelatine plates they leave 

 foci from which develop new colonies of 

 bacteria of the specific sort. Experimental 

 evidence is wanting which shall determine 

 the actual extent of this infection, the dis- 

 tance to which such germs may be carried, 

 and the length of time in which they re- 

 main alive and capable of producing an 

 infection, as well as the other factors which. 



control the importance of such mechanical 

 transport. That it does play an important 

 role can hardly be doubted, for to the nu- 

 merous instances cited in 1899 in Nuttall's 

 splendid monograph the intervening years 

 have added both numbers and weight. To 

 the instance already discussed where such 

 transport was active must be added the 

 passive transfer as of typhoid germs in 

 oysters, which is well established. 



It should not be inferred from the pre- 

 ceding that only typhoid bacilli are trans- 

 ported actively or passively by animal 

 carriers. The germs of cholera, anthrax, 

 septicemia, pyemia, erysipelas, tuberculosis 

 and bubonic plague are said to have been 

 transferred from one host to another in 

 the same way. In some cases the evidence 

 seems clear; in others a verdict of 'not 

 proven' must be entered; and yet the ob- 

 servations already on record call for a most 

 thorough investigation and extended ex- 

 perimentation in order to reach a final 

 conclusion as to how widely the mechanical 

 transfer of disease-producing bacteria may 

 extend. In many cases it is doubtless 

 purely accidental — casual — as in the hos- 

 pital cases infected by flies which Leidy 

 records in Philadelphia, or by mosquitoes 

 as Giles notes in India. In such transfer 

 of disease germs not only are flies the car- 

 riers, but also mosquitoes, bed-bugs, fleas, 

 and other blood-sucking insects, though to 

 a less extent, if present evidence represents 

 fairly the actual conditions. Probably 

 such carriers of disease will be confined 

 chiefly to the insects and the passive agents 

 will be exceedingly rare. 



It has been observed, however, that such 

 agents may transmit disease germs in other 

 manner than merely adherent to the ex- 

 ternal parts of the body. Many experi- 

 ments have demonstrated that various ba- 

 cilli may pass unharmed through the in- 

 testine of the fly and be distributed with 



