342 



SCIENCE' 



[N. S. Vol. XLIV. No. 1132 



a filarial parasite in man, lives in the 

 tissues and at times the female bores out- 

 ward through the epidermis, discharging 

 the embryos which, if they find water, swim 

 about and enter a small crustacean, the 

 waterflea or cyclops, in which they remain 

 for several weeks undergoing certain trans- 

 formations. Finally they may enter the 

 stomach of man through drinking water 

 and then bore through the stomach wall 

 into the tissues again. The liver fluke, 

 common in sheep causing the disease 

 "sheep rot" and occasionally found in 

 man, uses several varieties of snails as 

 hosts passing through certain rather com- 

 plex changes and later leaving the snail to 

 become encysted on grass or weeds which 

 are eaten by sheep. 



Through the recent work in Egypt of 

 Colonel Leiper and his associates in the 

 Royal Army Medical Corps it seems now 

 to be firmly established, contrary to the 

 views of Looss, that the fluke, Schistosoma 

 haematobium, the cause of bilharziosis, after 

 leaving the body in the urine uses the snail 

 as an intermediate host in which it under- 

 goes a metamorphosis before it is capable 

 of infecting another person. Infection 

 with the fluke actually takes place both by 

 mouth and through the skin of the individ- 

 ual. It has been shown that eradication of 

 this very prevalent and serious disease in 

 the Orient will depend upon the destruc- 

 tion of snails, the cooperation of the in- 

 fected individual not being necessary. 



This resume in a general way I think in- 

 cludes the modes of transmission of at least 

 most of the human-animal diseases as we 

 at present know them. 



I wish now to call attention to a num- 

 ber of points which are frequently of great 

 importance in the control of many of these 

 diseases. For convenience of presentation 

 I will mention them under four heads : 



1. A lower animal may be the only 

 agency in the spread of a disease. In ma- 



laria the available evidence indicates that 

 the disease is spread only by the anopheles 

 mosquito, although several varieties of this 

 species harbor the parasite. So also yellow 

 fever is spread, so far as we now know, 

 only by the stegomyia mosquito. 



2. Several different species of lower ani- 

 mals may be concerned in the transmission 

 of the disease. As examples we may cite 

 rabies which is transmitted by dogs, cats, 

 wolves, horses and other animals ; anthrax 

 by sheep, cows, etc. ; bubonic plague by rats 

 and ground squirrels. The question as to 

 whether a given disease is transmitted by 

 one animal only or by several is so impor- 

 tant so far as control measures are con- 

 cerned that I need only pause to mention it. 



3. The lower animal may be a "healthy" 

 carrier. That is, the infectious agent 

 though pei-haps highly virulent to man 

 may not cause the animal to become sick. 

 A striking example of this condition is the 

 Malta fever infection (M. melitensis) in 

 goats. Malta fever, a human disease very 

 common in Mediterranean countries and 

 now prevalent in our southwest, is spread 

 through the milk of goats. In most of 

 these animals there are no symptoms what- 

 ever, the micrococcus being found in the 

 milk or urine or blood of animals perfectly 

 healthy and which remain so. Typhoid 

 bacilli may live in the intestinal canal of 

 flies. Tetanus bacilli may live for months 

 in large numbers in the intestinal canal of 

 horses, certain of these animals becoming 

 virtually tetanus "carriers." 



4. The lower animal may be a diseased 

 "carrier"; that is, the infectious agent 

 may cause the lower animal to become sick. 

 Glanders in horses, rabies in dogs, anthrax 

 in sheep are examples. This matter of de- 

 gree of reaction or the severity of the dis- 

 ease in the animal is of very great impor- 

 tance in the control of the disease. From 

 the standpoint of man's welfare it is often 

 highly advantageous that the infection in 



