264 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE. 



One of the most important branches of this work has been con- 

 cerned "svith the production of immunity in cattle. It is very evident 

 that the power to immunize cattle safely against all tuberculous 

 attacks would afford the greatest possible assistance wherever at- 

 tempts were made to eradicate the disease, as all of the healthy 

 animals could receive immunizing treatment and the diseased por- 

 tion of the herd could be destroyed or quarantined. The experiments 

 necessary for the establishment of any satisfactory conclusions in a 

 study of this character must extend over a long period of time, and 

 tests must be made upon many animals of different ages and condi- 

 tions before any definite conclusions can be reached. AVliile no 

 method of vaccination has yet been devised whereby cattle may be 

 fortified against attacks of tubercular infection under all circum- 

 stances, still it has been shown by the cooperative investigations of the 

 Experiment Station and the Pathological Division that their resist- 

 ance may be materially increased by means of suitable inoculations. 



In some of the rare cases in which cattle giving a reaction to tuber- 

 culin did not disclose any macroscopic lesions of tuberculosis when 

 submitted to post-mortem examination, glands from the cattle in 

 question have been sent to this laboratory and tested by microscopic 

 examinations or animal inoculations so as to determine the presence 

 or absence of tubercle bacilli. In more than half of such examina- 

 tions tubercle bacilli have been positively identified when the tissues 

 have been closely examined by laboratory methods, thus showing 

 that tuberculosis was really present in these carcasses, but in the 

 most incipient form. This also shows that the diagnoses given by 

 tuberculin are in realit}^ even more accurate than is indicated by 

 ordinary post-mortem examinations of reacting animals. 



EXAMINATION OF MILK. 



The question of the presence of tubercle bacilli in market milk has 

 also received careful attention. Milk from the cities of Chicago, 

 Philadelphia, and Washington has been examined at the laboratories 

 of the Bureau in each of the above-named cities for the purpose of 

 determining what percentage of the samples tested contained viru- 

 lent tubercle bacilli. The general bacterial content of these samples 

 was also ascertained. 



In Chicago the work of testing milk has been carried on in co- 

 operation with the city department of health. Up to the end of the 

 fiscal year 173 samples of raw milk, 44 samples of pasteurized milk, 

 and 10 samples of certified milk had been submitted to examination. 

 No deductions have yet been made relative to the presence of tubercle 

 bacilli in the pasteurized or certified samples, as the inoculation tests 

 have not j^et been concluded. 



Two hundred laboratorj^ animals were inoculated with raw milk, 

 one animal being used for each sample of milk. A mixture of cream 

 and sediment from the bottom of the centrifugalizing tubes was in- 

 oculated subcutaneously in each case. One case of tuberculosis was 

 found, and this sample was traced to the farm where the milk was 

 produced. Twenty-six cows were furnishing the milk from this 

 place. By consent of the owner the tuberculin test was applied to all 

 of these cows, and 3 of them gave a positive reaction for tuberculosis. 



