Before proceeding to a discussion of the experiments upon cold-blooded animals, it is necessary to review briefly some of the work recently done with the bacillus of leprosy. The appearance of the bacillus in man and its behavior under artificial cultivation, and in the tissues of lower animals, should be considered in order that comparisons may be drawn.

In their studies with the organism under cultivation, Duval and Gurd pointed out that the long, slender, and beaded appearance of the leprosy bacillus described by Hansen, in 1872, is lost when removed for several generations from the parent stem, and under artificial cultivation the organism becomes unbeaded, short, and coccoid. Duval also noted that these changes in morphology were always followed by rapid multiplication of the organism. Duval argues, a priori, that the bacillus is not in a favorable environment in the human tissues. If these deductions are correct, the morphology of the leprosy bacillus should vary according to the resistance offered by the tissues of different animals.

The resistance of the human host to the leprosy bacillus becomes more evident in the light of the clinical aspect of the disease. The long period of incubation, the duration of the disease, and the disappearance of the bacilli preceding the healing of the infected foci show that the resistance offered to the bacillus by the human tissues is not to be overestimated. This opinion is confirmed when the behavior of the leprosy bacillus under cultivation and in the tissues of various mammals is compared.

When cats, rabbits, bats, guinea pigs, and rats are inoculated either below the skin or into the peritoneal cavity with large quantities of Bacillus lepræ, a slight local reaction follows within twenty-four to forty-eight hours, but no definite lesions are produced and the bacilli soon disappear. The resistance of some animals to Bacillus lepræ is well illustrated by two cats which were inoculated subcutaneously and intraperitoneally with a heavy suspension of Bacillus lepræ. These animals were killed and examined three days later, but the bacilli were not demonstrable from the regions about the sites of inoculation.

Pigeons are likewise refractory. It is impossible to cause a local reaction in these birds, and the injected bacilli disappear rapidly. Hence, probably no multiplication takes place in them.

Goats, young pigs, and white and dancing mice are in a degree susceptible to injections, and though undoubted lesions are produced, and multiplication of the bacilli occurs, the lesions and bacilli disappear after a limited time. Acid-fast bacilli which are recovered from the lesions are long, slim, and beaded, though the organisms used in the inoculations were short, unbeaded, and coccoid.

Monkeys inoculated with cultures of the short unbeaded forms react promptly. The lesions resulting, though confined in most instances to the site of inoculation, occasionally appear at distant points. The number of bacilli present in the nodules and their arrangement within typical lepra cells show that multiplication has taken place. The organism has, however, changed from the short coccoid form to the long, slender, beaded form. Though the lesions induced and the bacilli present are in every way similar to those found in man, their tendency to disappear gradually after a quiescent stage clearly denotes that the tissues of the monkey, although less refractory than the tissues of the animals previously mentioned, still offer resistance to invasion.

While mammals react but poorly to inoculations of the leprosy bacillus, this reaction manifests itself in various ways in different species. For example, while multiplication of the organism with the production of lesions occurs in some species, in others that are more refractory, the injected bacilli assume the involuted or beaded forms and do not multiply or produce lesions; in others, still more resistant to the action of the leprosy bacillus, the organisms quickly undergo granular metamorphosis and disappear. Furthermore, in some species the lesions are, in most instances, limited to the site of inoculation, and though presenting all the characteristics of the lesion in man, the nodules and the bacilli disappear after a variable time. This behavior of the leprosy bacillus can be accounted for only by the degree of resistance offered by the tissues of the individual host.

Since the morphology of the organism invariably changes from the short coccoid to the large beaded form when placed in insusceptible animals, and conversely, from the long beaded forms to the short coccoid forms when placed in susceptible animals, the deduction can be drawn that the organism varies in morphology and rapidity of growth according to the susceptibility of the host.

Examples of similar behavior of Bacillus lepræ in the human subject are known to all investigators of leprosy. Ulcers and nodular areas often heal, and the bacilli disappear with little or no treatment. It is true that while older lesions are healing, new ones are constantly appearing, yet the duration of the disease and its undoubted tendency towards healing shows that conditions in the human subject are variable, and suggests that the organism has its natural habitat in some other host.

The experiments presented here serve to show that the bacillus of leprosy meets but little or no resistance in the tissues of cold-blooded animals, multiplies in their tissues, and may be harbored by them without apparent discomfort or external evidence of the disease.

That no appreciable resistance is offered to the multiplication of the leprosy bacillus by many species of cold-blooded animals is shown by the fact that aside from the trauma produced by the inoculation and the slight initial reaction of the tissues, the organism continues to grow profusely, and to invade the tissues without further reaction. Quite the opposite condition occurs in mammals: in some of these the leprosy bacillus degenerates into a granular mass shortly after inoculation; in others that are less refractory, typical lesions appear, but they seldom extend from the point of inoculation; and while the bacilli multiply slowly, they do not infiltrate the tissues, but disappear after a short time, the lesions healing.

That multiplication of Bacillus lepræ occurs in the tissues of cold-blooded animals is shown by the fact that while animals examined a few days after inoculation show but a few scattered organisms, those killed at longer intervals show a proportional increase in the number of bacilli. Furthermore, the few bacilli found at the early-period are extracellular and scattered, while after longer periods they tend to be massed and enclosed in large lepra cells.

The supposition that these lepra cells are phagocytes has naturally arisen. Duval holds that they are not phagocytes in the true sense of the term, that the bacilli penetrate the cells rather than that the cells engulf them, after which, finding conditions for growth favorable, they multiply without causing serious injury to the cell. The size of the cell depends upon the size of the colony within. The experimental work bears out this view since the decrease in number of the organisms observed in animals killed shortly after inoculation depends not upon phagocytic action nor upon cells which appear later when active lesions are established. In early lesions, the lepra cells are smaller, barely measuring twenty to thirty microns in diameter, and contain but few bacilli; whereas in older ones, they attain a diameter of 100 microns or even more, and contain enormous numbers of bacilli. Were this increase in size due to phagocytic action, some cells would be found in which the limit of their capacity had been reached; and they would either contain a mass of dead and disintegrated bacteria or would themselves show evidence of disintegration. On the contrary, the bacilli, though they occupy most of the cell, show no signs of disintegration, and the nucleus and the cytoplasm of the cell retain normal staining properties. That the invasion and multiplication of the bacilli cause an irritation is evident by the amitotic divisions of the nucleus which occur in the larger cells.

The absence of external evidence of invasion by Bacillus lepræ in cold-blooded animals, and the apparent lack of discomfort caused by the presence of the organism within their tissues, are points which should be remembered in considering the sources from which leprosy may be transmitted. In not a single instance in the numerous experiments presented here would it have been possible, from any external sign, to suspect that the animals were harboring multitudes of leprosy bacilli.

While the evidence in support of the opinion that leprosy may be transmitted from man to man appears sufficiently strong to warrant this belief, the number of cases in which infection can be actually traced to this source is small. Since leprosy is known to be prevalent where fish and sea-food are plentiful, and since the experiments here recorded prove that fish can be infected by being fed cultures of Bacillus lepræ, or nodules from human lepers, or bits of fish previously infected with the leprosy organism, account should be taken of the possibility that leprosy, in certain localities, may arise from this source of infection.

The question as to how and from what source leprosy bacilli enter the human body may be still regarded as an open one. Isolated examples of direct infection of healthy human beings from lepers have been reported by Arning and Nonne, by Manson, and others. The notion that the agency of infection is already infected human beings, that is lepers, is at the foundation of the modern practice of the isolation and segregation of lepers, which would seem to have brought about a definite decrease in the prevalence of the disease. It is an acknowledged fact, however, that the lepers confined in institutions practically never cause infection of nurses, etc. Some other factor than the human agency may therefore be considered as affecting this issue.

It is well known that Jonathan Hutchinson has brought forward the idea that fish are the source of the infection, basing the view on the high prevalence of the disease along the coast countries of Norway and Sweden, and in the Pacific Islands, and in the countries bordering the Mediterranean and Black Seas, in all of which fish furnish the chief food material. No convincing proof was ever adduced in support of this contention. But now that it has been shown that the leprosy bacillus survives and multiplies in cold-blooded animals, at least at room temperature in a warm climate, and since methods have been devised for cultivating and identifying the leprosy bacillus, the question has been opened up to accurate investigation.

Duval has shown that the leprosy bacillus in cultures grows better at room temperature than at 37° C., so that growth in cold-blooded animals kept at room temperature is perhaps in some way connected with this phenomenon. What must now be ascertained, in order to test the Hutchinsonian theory more accurately is whether such growth takes place at a temperature corresponding with the average mean temperature of such a body of water as the North Sea and that of the fiords of Norway. Since cold-blooded animals possess the same temperature as their surroundings, they would be suitable media for the cultivation of leprosy bacilli at those temperatures. For the waters of the Mediterranean Sea and the tropical Pacific Ocean, this consideration would count less. But the theory will stand or fall according as it can account for the whole, and not only for a part, of the phenomena to be explained.

As the length and shape of the bacilli and the number of chromatin masses are constant for a given species of cold- or warm-blooded animals, which features are governed by the resistance of the individual species, the following conclusions seem justified: that the morphology and rapid multiplication of the leprosy bacillus in cultures and in some species of cold-blooded animals indicate that Bacillus lepræ under natural conditions is short, coccoid, and un-beaded, and that the long, slender, beaded variety which occurs in the mammalian species is atypical and the product of an unfavorable environment.

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