September 8, 1916] 



SCIENCE 



341 



transferred to man from the cow in this 

 way. Tapeworm infections of various 

 kinds are thus transmitted from a number 

 of lower animals. 



V. The infectious agent may be trans- 

 mitted to man through the secretions of 

 the lower animals. Here are included 

 some of the most important and serious of 

 human diseases. Malta fever is trans- 

 mitted largely in this way through the 

 milk and urine of goats infected with 

 Micrococcus melitensis. The malarial par- 

 asite is transmitted to man by the ano- 

 pheles mosquito through its salivary gland. 

 Tuberculosis especially in children is often 

 transmitted from the infected cow through 

 the milk. In this connection the epidemics 

 of streptococcus sore throat are interesting. 

 Over thirty milk-borne epidemics of this in- 

 fection have been reported and from the re- 

 cent work of several investigators it would 

 seem that in some instances virulent strep- 

 tococci of the human type may find their 

 way into the udder of the cow through the 

 contaminated hands of the milker and 

 there multiply and return subsequently in 

 large numbers in the milk and infect the 

 consumer. Some of these epidemics and 

 perhaps many of them have originated 

 thus. Capps and the writer several years 

 ago and more recently Mathers have shown 

 experimentally that virulent human strep- 

 tococci when placed on the abraded teat of 

 a cow will ascend the canal and infect the 

 udder; or when injected directly into the 

 udder will continue to multiply there, 

 causing a mild or even a severe mastitis 

 lasting for several weeks or for months. 

 The streptococci in large numbers will pass 

 out in the milk during this period and will 

 retain their initial virulence for animals. 

 Other kinds of infections may be trans- 

 mitted in this way though perhaps rarely. 

 True diphtheria bacilli, for example, have 

 been isolated by Dean and Todd from the 



ulcerated teats and from the udder of a 

 cow supplying families in which a diph- 

 theria outbreak occurred. 



Milk is such an universal food for both 

 men and bacteria that it has been the ve- 

 hicle for the transmission of many of the 

 infectious diseases. In the outline given I 

 have not mentioned the diseases which may 

 be transmitted through milk, the virus hav- 

 ing entered the milk after leaving the cow. 

 In such infections the animal is not con- 

 cerned directly in the transmission but only 

 indirectly through its product. 



VI. The infectious agent may enter one 

 of the lower animals in which it passes 

 through a regular phase or completes a 

 cycle and then, usually through a bite of 

 the animal, is transmitted to man. This 

 mode of transmission concerns many of the 

 protozoan diseases. "We may group such 

 infections under two heads: (a) those 

 transmitted from man to man by a lower 

 form, examples of which are the malarial 

 parasite in the anopheles mosquito and the 

 yellow fever virus in the stegomyia mos- 

 quito; and (&) those transmitted from an 

 animal to man by one or more of the lower 

 forms, illustrated by the transmission of 

 Trypanosoma gambiense from the antelope 

 or from the dog or the monkey to man 

 through the tse-tse fly. In this case one or 

 more of the lower animals are concerned 

 in the transmission of a second animal, the 

 parasite, to a third animal, man. Both (a) 

 and (&) may occur in the same disease. 

 The principle of host and intermediate host 

 here involved is a very important one and 

 numerous examples might be given. Many 

 of the intestinal parasites, the entozoa, pass 

 a part of their cycle of development in a 

 lower animal. Most interesting relation- 

 ships exist in this connection between some 

 of the nematoda and the trematoda or fluke 

 infections in man and certain small water 

 animals, including crabs, snails and other 

 arthropods. For example the guinea-worm, 



