540 
THE VIRULENCE A HE VITALITY 0 E TUBERCLE BACILLI IN DAIRY 
PRODUCTS. 
Less than ten years ago tubercle bacilli were grouped for all 
practical pin-poses in two classes, mammalian and avian, or those 
which affect man and other mammals and those which affect birds. 
Xo one doubted openly that bacilli from cattle, in meat and dairy 
products, were as injurious for man as those derived from persons. 
Pulmonary tuberculosis, or consumption of the lungs, was then, as 
now. the commonest form in which the disease manifested itself, 
and this was explained by the assumption that the bacilli entered the 
body more frequently with the breath than in any other wav, and 
that the greatest danger of infection was through dried and pulver- 
ized tuberculous material that floated in the air as fine dust. 
The beginning of the present century brought a change of views. 
Attention was called to the fact that the inhalation theory to ac- 
count for the frequent presence of tuberculosis in the pulmonary 
tissues had not been proven and that living tubercle bacilli in dust 
were difficult to find or could not be found at all. The infectious- 
ness of bacilli from animals for man was cpiestioned and the inves- 
tigation of tuberculosis generally was given a fresh impetus through 
which many new facts and theories came to light. 
As tubercle bacilli in dairy products are mainly derived from 
bovine sources and enter the body in a moist state, to understand 
the true significance they have for public health we must give some 
attention to the infectiousness of tubercle bacilli from bovine sources 
for man and to the ways in which tubercle bacilli enter the bodies 
of those who become affected with tuberculosis. 
Since Theobald Smith ° published his studies on different varieties 
of tubercle bacilli, the evidence in favor of two distinct types virulent 
for mammals— the one found more commonly in bovine and the 
other in human lesions — has grown stronger. But different varieties 
or types do not necessarily mean different species or even subspecies. 
As Smith himself stated. ” Varieties have been found among nearly 
all of those specific forms of pathogenic bacteria which have received 
a considerable amoimt of attention ." 6 The term “ varieties 77 is 
here clearly used to designate differences of a kind to be expected 
among the individuals of a large and widespread species, such differ- 
ences as we know occur among higher organisms than bacteria with 
a wide geographic distribution. There is a distinct parallelism be- 
tween a vide geographic distribution of higher plants and animals 
a Twelfth and Thirteenth Annual Reports. Bureau of Animal Industry. 1895 
and 1896. Jour, of Experimental Medicine, toL 3, New York, 1898. 
1 Twelfth and Thirteenth Annual Reports. Bureau of Animal Industry, 1895 
and 1896, p. 119. 
