196 



SCIENCE. 



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



reality more complicated relations. It is 

 of the greatest importance that these rela- 

 tions be definitely established, for on them 

 depends the introduction of a rational hy- 

 giene, and yet even the merely mechanical 

 function of the fly in, the dissemination of 

 disease calls for strict measures to abate 

 this nuisance. Any one may convince 

 himself, even by superficial observation, 

 that both individuals and communities 

 through carelessness allow and produce 

 conditions which breed enormous numbers 

 of unnecessary flies. Rational hygiene 

 calls for the removal of these conditions 

 and the extermination of flies. Fortu- 

 nately to-day one does not need to empha- 

 size in civilized countries the undesirable 

 character of bed-bugs, cockroaches and 

 other vermin, which doubtless play a part 

 in the mechanical transfer of disease germs, 

 and probably are also associated more inti- 

 mately with certain maladies, as will ap- 

 pear in the succeeding section. 



Animals are also breeders of disease as 

 well as carriers in a mechanical sense ; and 

 the part they play as breeders of disease 

 may be either purely facultative or, on the 

 other hand, essential to the spread of the 

 malady. Regarding the facultative role 

 of animals in breeding disease compara- 

 tively little exact evidence is at hand. It 

 is somewhat generally maintained that 

 various human diseases afflict certain ani- 

 mals, and the domesticated animals which 

 stand in such close relations to man have 

 been those against which, up to the present 

 time, such charges have been most fre- 

 quently made. The evidence is scanty, in- 

 conclusive and in some cases of no value 

 at all ; and yet one can not doubt that some 

 of the germs which infect man do live also 

 in other animals. Even among the higher 

 animal parasites but few species are con- 

 fined exclusively to the human host, and 

 some, like Echinococcus bladders or Trichi- 



nella, may occur in a wide range of hosts. 

 It is an important duty for the students of 

 comparative medicine to determine to what 

 extent disease-producing organisms may 

 parasitize other hosts than man, for in this 

 possibility lies the secret of the transmis- 

 sion and appearance at isolated points of 

 new disease foci in some of the cases hith- 

 erto unexplained. It should be noted dis- 

 tinctly that when animals are facultative 

 breeders of disease they merely atford a 

 suitable ground in which the disease germs 

 may multiply and an agency by which 

 they may be distributed. Such animals 

 are not in any way necessary to the exist- 

 ence or development of the germs ; they 

 only serve to increase the percentage of 

 infection or the area of distribution char- 

 acteristic of the disease. It is thus an 

 important, but not an essential, role. With- 

 out question it plays some part, but how 

 weighty its influence may be or in just 

 what directions it may be exerted we are 

 at present entirely unable to measure or 

 estimate. This is unquestionably a most 

 important and fruitful field for investiga-. 

 tion. 



In another sense also animals are breed- 

 ers of disease, as when some part of the 

 life history of the disease-producing germs 

 is passed within the animal before that 

 stage is reached in which the germ may 

 infect a new human host. Here the rela- 

 tion is an essential one, and the interme- 

 diate host is a condicio sine qua non for 

 the further spread of the disease. Such a 

 relation is very widely known among ani- 

 mal parasites. The embryo of the sheep 

 liver fluke, for instance, must undergo cer- 

 tain phases of development and reproduc- 

 tion within a snail before it reaches that 

 form M'hich can reinfect the sheep. The 

 embryo of the unarmed human tapeworm 

 must enter another host, the beve, and grow 

 to a bladder worm, and this alone can 



