By extending her investigations a number of rats and mice that were caught in the birds' quarters of the Berlin gardens were found in several instances to have contracted tuberculosis. Conversely it was learned that the birds would eat the affected carcasses of the rats and mice and contract tuberculosis by that means, and in this manner the rodents served to spread the infection materially.

In 1908 Dr. Oluf Bang reported the results of a number of tests made for the purpose of determining the susceptibility of birds for mammalian tubercle bacilli and also the extent to which mammals could be infected with tubercle bacilli of the avian type. Bang tested 18 mammalian cultures carefully in regard to their pathogenicity for fowls, and found 12 that were capable of causing tuberculous lesions in hens, while 6 were harmless by any of the means commonly used for inoculation.

The results obtained in attempts to infect mammals by using avian tubercle bacilli were also successful in proving that young kids, calves, and foals possess considerable susceptibility for these organisms. He also found that continued retention of mammalian tubercle bacilli in the tissues of birds altered their pathogenicity for guinea pigs, as they gradually became less virulent for these animals.

The present writers in 1906 reported the successful inoculation of guinea pigs, rabbits, and a cat with avian tubercle bacilli, and also noted a profound alteration in the pathogenicity and cultural characteristics of the culture used, until at last it would cause welldefined tubercular necroses within the viscera of the guinea pigs, instead of merely a general congestion of the visceral organs, as was the case at first. Instead of spreading uniformly over the surface of the serum in a moist white layer, as at the beginning of the experiment, the recovered cultures appeared to increase in growths of small whitish clumps, somewhat similar to the manner of growth common to tubercle bacilli of the human type, except that the clumps were slightly flatter.

Since the publication of the experiments just mentioned, some criticism of the culture which was used in the tests has been noted. The objection has been made that the avian culture used in our experiments was not examined to see if it was pathogenic for fowls. The writers made the statement that this culture "is well known to bacteriologists and is accepted by most of them as meeting all the requirements of tubercle bacilli of the avian type." This statement obviously includes its pathogenicity for fowls, and this fact was fully proven by the inoculation of chickens, which resulted in the production of well-marked lesions of tuberculosis, one of the birds inoculated developing a very typical tuberculous tumor upon one of its joints in addition to the lesions of the abdominal viscera.

RELATIONSHIP OF DIFFERENT TYPES OF TUBERCLE BACILLI.

The greatest interest attaches to the relationship of tubercle bacilli as found in their various hosts. This relationship between the tubercle bacilli recovered from the various species of mammals, birds, fishes, and reptiles is certainly very intimate. It may be that the slight differences which may now be demonstrated between the different types of tubercle bacilli have been of slow development, and due to their environment and to differences in the temperature at which they have been forced to live, or to differences in the amount and quality of nutrition with which they have been supplied. Whatever the variations between the types, they are not great enough to prevent the successful interchange of tubercle bacilli by means of inoculations between representative hosts of the several types.

The slow but gradual transformation of certain growths of tubercle bacilli should not be considered such an impossibility. Other forms of bacterial life yield to the peculiar influences of their environment, and why should not the tubercle bacillus be equally susceptible to change? The attenuation of the Bacillus anthracis by submitting it to elevated temperatures, and the prompt recovery of its primary virulence by passage through a white mouse, affords a notable instance of the manner in which one of the most dangerous and active pathogenic micro-organisms may be transformed. The diphtheria bacillus becomes promptly attenuated by the addition of a small amount of iodin trichlorid to the nutrient medium in which it is growing. Swine erysipelas becomes much less virulent by repeated passage through rabbits. The Streptococcus pyogenes and the bacillus of Asiatic cholera rapidly become affected while growing artificially, through the action of the products of their own growth, and unless removed frequently to fresh nutriment they will gradually weaken and die.

Since, then, it must be admitted that there are many forms of pathogenic micro-organisms that can be materially altered by increase of heat to their surroundings, by the application of the direct rays of sunlight, by increasing or decreasing the acidity or the alkalinity of their nutriment, by the influences of the products of their own growth, and by passage through animals either susceptible or resistant to their action, is it too much to suppose that tubercle bacilli may also be altered in form, virulence, and in vigor of growth when cultivated under unusual conditions? Experiments have shown that some tubercle bacilli may be transformed in form, pathogenicity, and cultural characteristics as well. More than this, cultures which seem incapable of attacking certain species of test animals with the degree

of severity which one would expect in view of their average virulence for animals of other species, may be brought to change their peculiar affinities until they will prove virulent for a species of animal formerly resistant.

Until a few years ago tubercle bacilli were grouped for all practical purposes into two classes-mammalian and avian; the former were found in man and other mammals, while the latter were isolated from birds.

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The experiments which were made by the writers in a comparative study of tubercle bacilli from various sources have demonstrated that the mammalian bacilli are distinguished from other forms of bacilli chiefly by a higher degree of virulence as well as by certain less important morphological and cultural characteristics; but it was likewise found that there are also tubercle bacilli of avian origin which can not be distinguished from those derived from mammals. Furthermore, these differences are not constant, for transition forms are observed in different hosts which are not typical of the forms usually peculiar to that host. Nor is the virulence of the bovine, human, and avian bacillus always the same, but it varies within a wide range. In fact, the virulence of certain bacilli from both mammals and birds has been found to become accentuated in consequence of passage through a series of animals, and, on the other hand, the pathogenesis has been diminished by long development on artificial culture media. Therefore tubercle bacilli should be regarded as polymorphic organisms of a single species of bacteria which have become differentiated by their environment in the different hosts, and the variations which afterwards form among individual varieties are transitory forms which have not had sufficient time or proper nutriment to become transformed into the typical varieties. .

The fact that tubercle bacilli of one species may be transmitted to an animal of a different species, or to man, makes it apparent that any preventive methods for controlling tuberculosis, to be successful, must take into consideration all species of animals which are susceptible to this infection.

SUMMARY AND CONCLUSIONS.

An outbreak of tuberculosis among the fowls on a large ranch in Oregon that seemed to extend to the swine of the same farm through feeding the hogs upon the carcasses of fowls that succumbed to the disease led to the inauguration of a series of experiments designed to

"Bulletin 96, Bureau of Animal Industry. Twenty-third Annual Report, Bureau of Animal Industry, 1906, page 113.

ascertain if the bacilli of avian tuberculosis may be transmitted to mammals under suitable conditions.

Four living hens were obtained from the infected ranch. Two of these were fed to a pair of tuberculin-tested pigs between 2 and 3 months of age. After a period of about fifteen weeks the pigs were killed and carefully examined, when it was found that both of them had developed marked cases of tuberculosis. In both instances the submaxillary and mesenteric glands were visibly affected, while one of the animals had numerous tubercle bacilli in the spleen and liver.

The affected tissues of these two experiment pigs were at once used in the execution of other experiments. They were fed to healthy fowls and were administered in various ways to guinea pigs and rabbits to test their virulence for mammalia. Cultures were quickly obtained from these inoculations, all of which proved to be avian in their biological characteristics.

Interesting investigations of eggs from the diseased hens led to very unexpected results. During the five months immediately following the arrival of the hens at the laboratory only one egg was laid, probably because of the unthrifty condition of the birds. A second egg was laid subsequently. Microscopic examination of these eggs failed to disclose the presence of tubercle bacilli, but the inoculation of guinea pigs proved their presence. Several guinea pigs were inoculated with the white of the egg and others with the yolk. Those which received inoculations of the yolk failed to develop tuberculosis, but those which were injected with the white, with one exception, developed very characteristic lesions within a short period.

By means of this series of experiments and others that have been previously made with avian tubercle bacilli it has been proved that the micro-organisms of naturally acquired tuberculosis in fowls can be made to lodge and multiply within the tissues of swine, cats, rabbits, and guinea pigs. They will cause progressive wasting and death in guinea pigs without producing characteristic necrotic foci in the organs, although recourse to the microscope reveals the presence of vast numbers of tubercle bacilli in the lungs, spleen, liver, or kidneys. Repeated passage of such avian tubercle bacilli in large numbers from animal to animal will result in the final development of a type of tubercle bacilli which will produce typical lesions of tuberculosis in mammals.

The carcasses of tuberculous fowls should never be fed to swine, as the latter may contract tuberculosis from such food. Eggs produced by tuberculous fowls should be considered dangerous until cooked, as they may harbor tubercle bacilli in important numbers.

Thriving cultures of avian tubercle bacilli were readily obtained directly from the affected tissues of the fowls by planting upon glycerin agar and egg media as well as after passage through laboratory animals.

Tubercle bacilli of one species may be transmitted to an animal of a different species, which fact makes it apparent that, to be successful, any preventive methods for controlling tuberculosis must take into consideration all species of animals that are susceptible to this infection.