Page:Popular Science Monthly Volume 69.djvu/248

244 of laboratory experiment, it can not be concluded that they will be equally refractory when exposed to the natural sources and modes of infection. In the laboratory the virulent infectious agent is brought into the animal by injection, under the skin, into the serous cavities or into the circulation, which are avenues through which in the natural disease infection rarely if ever takes place. And while this mode of introduction of the virulent bacilli into the body may, theoretically, be more severe than their introduction into the lungs with inhaled air, or into the stomach through infected stalls and food, yet the profound differences in the defenses of the body with which the bacilli come into conflict, under these different circumstances, may, after all, determine the issue in a manner quite contrary to our expectations. It is, therefore, of the highest interest to learn that in their later tests Behring and his co-workers exposed vaccinated cattle to stalls and herds which were known to be badly infected, with the result that at the time of the report, they had apparently escaped infection. I am enabled through the courtesy of a private communication from Dr. Pearson to state that cattle vaccinated by himself and Gilliland which were kept for two years under natural conditions of infection have not contracted tuberculosis, while the control animals, exposed to the same conditions, have all developed the disease, some dying spontaneously by reason of the severity of the infection. Dr. Pearson also informs me that their experiments indicate that the degree of resistance bears a rather definite relation to the number of vaccinations given the cattle. No cattle vaccinated three times with their standard vaccine—a living culture of tubercle bacilli of human origin—have developed tuberculous lesions even after two years' severe exposure. In their experience, two injections of Behring's vaccine do not always suffice for such heavy exposure as they employed.

As regards the question of duration of the protection, it may be said that Behring, basing his views on results of vaccination made three years before, expressed the belief in 1901 that it would endure during the life of the animal. As young healthy cattle are vaccinated before they fall victims to infected stalls and herds, it would seem as if infected herds might therefore gradually be replaced by healthy ones. The gain, this being true, would be almost incalculable to agriculture.

I am in the fortunate position of being able to bring before you a critical summary of the subjects just presented by one wholly conversant with its practical as well as its theoretical aspects. Through the courtesy of Dr. Leonard Pearson I have been enabled to read the advance sheets of a review on immunization in tuberculosis which will soon be issued from the Phipp's Institute. Dr. Pearson concludes that there appears to be no doubt that different cultures of human bacilli have different immunizing values. Some can not be used at all because