METHODS AND CHANNELS OF INFECTION 667 



been acquired from the eating of vegetables which have been washed in 

 water containing the pathogens. Oysters and various shell fish have 

 been known to carry the microbic agents of typhoid fever and Asiatic 

 cholera. Three infections coming from meat and certain other foods 

 sometimes occur, namely, botulism, enteritis and occasionally para- 

 typhoid fever. In these instances the causal microorganisms are in the 

 meat. Another type of infection known as ptomain poisoning also 

 occurs from the eating of meat or fish which has been acted upon by 

 saprophytic bacteria and the proteins split up into toxic substances. 



Animal Carriers of Infection. Animals may communicate disease 

 microorganisms to one another and to man in three ways, namely, 

 first, by direct or indirect contact; second, by serving as mechanical 

 carriers from one individual to another; and third, by serving as inter- 

 mediate hosts for the microbic agent and then subsequently com- 

 municating it to another. As examples of the first proposition, the 

 fact that tuberculosis has been communicated from cattle to man, that 

 glanders has been communicated from horses to man, and that 

 anthrax has been communicated from sheep to man by contact may 

 be mentioned. It has also been stated that the cat, while not suffering 

 from true diphtheria, seems to be able to transmit this infection and 

 the dog may also transmit rabies to the man. In the second method 

 of transfer, the mechanical carrying of an infection, the insects are 

 principally concerned. It is well demonstrated that common flies 

 frequently carry B. typhosus on their feet from the infected patient or 

 the excreta and deposit them on the food materials thus causing 

 infection when the food is eaten. The various suctorial insects also 

 may suck up the blood of one individual and carry the infectious 

 agent to the normal individual. Notable examples of this are found 

 in the transmission of the various trypanosomiases by the tsetse and 

 other tropical flies, and of Rocky Mountain spotted fever by the wood 

 tick. The same is true of Bad. pestis of plague which is carried by the 

 flea, of Texas fever by the cattle tick, and it has been shown recently 

 that the louse may be one of the agents in the transmission of typhus 

 fever. As an example of the third method, the serving as an inter- 

 mediate host and the carrying of the causal agent, the mosquitoes 

 which serve as the only means of transmission of the causal micro- 

 organisms of malaria and yellow fever and in which these parasites 

 pass a certain cycle of their existence, may be mentioned. 



