286 CITY MILK SUPPLY 



to 400 new typhoid fever cases every year. If it be supposed that only 

 2 per cent, of these became chronic carriers and that they remained such 

 for but 15 years there would be 90 to 120 typhoid carriers continually 

 menacing the milk supply of New York City. 



The ability of pasteurization to protect a city from communicable 

 disease is well-illustrated by the experience of Berkeley during the Rich- 

 mond, Cal., outbreak of typhoid fever. Geiger and Kelly report that 

 from a certain dairy that employs 20 .men and keeps 300 cows Richmond 

 receives 90 gal. of milk which was used by 500 people daily and 

 Berkeley 600 gal. which was delivered to 2,000 homes where it was used 

 by at least 6,000 people. In Richmond, where the milk was used raw, 

 12 cases of typhoid fever developed which led to its being promptly 

 withdrawn from the public. 



In Berkeley, where the milk was pasteurized, not a single case ap- 

 peared. The milk was found to have been infected by the head milker 

 who, a little before the source of the contagion was traced out, had been 

 taken to a hospital where he gave a positive Widal reaction. 



It is certain that even those dairymen who are most zealous in their 

 efforts to protect their milk from infection will at times fail. Sometimes 

 the afflicted person may begin spreading disease germs while his malady 

 is still in the prodromal stage; the disease may not be recognized till 

 it is well advanced or it may never be detected, besides which tempo- 

 rary, acute and chronic bacillus carriers may every one of them spread 

 contagion innocently and unobserved. Indeed, to pasteurize milk is 

 the only reliable way to prevent it from sometimes becoming the vehicle 

 of contagion. Communities that use raw milk will inevitably have oc- 

 casional outbreaks of milk-borne infectious disease. 



4. Pasteurization at Low Temperatures Destroys Disease Germs. 

 A fourth factor in convincing people that pasteurization might be prac- 

 tical along right lines was the experimental, chemical and bacteriological 

 studies of several investigators. As has been indicated previously, the 

 chemical changes that take place in pasteurization of milk at low tem- 

 peratures are quite different from those that occur at the higher ones 

 that were in use when the pasteurization of milk was first begun. In- 

 deed, milk is little changed by heating it for 30 to 45 min. at 145F., a 

 temperature and exposure that bacteriological studies have established 

 as sufficient to kill the non-spore-bearing pathogenic organisms that occur 

 in milk. 



Up to 1899, it was thought necessary in order to kill the tubercle 

 bacillus to either heat milk for a moment to high temperatures, as at the 

 175 to 185F. advocated by Bang, or to heat it to 155F. for 20 min. 

 In that year Theobald Smith announced that heating milk to 140F. 

 for 20 min. sufficed to kill the germs, if the milk was stirred or kept 

 covered during heating to prevent the formation of a pellicle on the sur- 



