Part 5

The most striking difference between natural and artificial plague in rats is the presence of a reaction at the site of inoculation in the majority of cases where the organism is introduced subcutaneously, and in about a third of the cases where the infectious material is rubbed on the shaven skin (cutaneous inoculation). The local reaction may exist only as a yellowish-brown crust, overlying a granulating surface, and associated with a trifling thickening of the skin and subcutaneous tissue. It may appear as one or more firm papules 3 or 4 millimeters in diameter. The most frequent appearance is a brawny œdematous and blood-stained reaction which extends over an area perhaps an inch in diameter; at times purulent change may be well advanced. Very rarely one finds so extensive an œdema as to cause the lesion to somewhat resemble the widespread gelatinous reaction seen so commonly in the guinea pig. On one or two occasions we have seen an extensive slough at the site of inoculation.

BUBO.

It is very exceptional that one finds in cases of induced plague the typical, firm, caseous bubo surrounded by an infiltrated area, as is so commonly seen in natural infection in rats. The glands are sometimes enlarged and injected without other changes. The commonest lesion, however, is a markedly enlarged gland which upon close inspection is seen to have a number of yellowish points just under the capsule. These points are especially well seen when a section is made through the gland. The gland may be squeezed out of the capsule and it breaks down readily enough when pressure is made upon it; but the uniform necrotic process that one sees so often in natural rat plague is absent.

LIVER.

Granular lesions precisely like those found in natural infections are very common. If the rat has died on the sixth day or later, the ordinary lesions are apt to be replaced by necrotic foci that may be as much as 2 millimeters in diameter.

SPLEEN.

This organ is found mottled more frequently than in natural plague infection, and large granules are much more common.

The subcutaneous injection is rarely so well marked as it is in natural infections.

Pleural effusion of the same nature as that found in natural plague is common. Hemorrhagic foci are not rare in the lungs, and occasionally the organs are partly consolidated.

CHRONIC PLAGUE DUE TO ARTIFICIAL INOCULATION.

Occasionally a rat that has been inoculated but has survived a week or longer, will show, when killed, only an abscess at the site of the injection. Stained smear preparations may show a large variety of bacterial forms. We have not been able to demonstrate the presence of _B. pestis_ in these lesions, yet there is no doubt but that the lesion is the result of the inoculation.

A lesion more frequently found is a caseous or a purulent lymphatic gland. If the inoculated rat has been killed about ten days after the inoculation, in some cases one or more of the peripheral lymph glands will be found to be surrounded by an infiltration, and the gland itself will be purulent or less frequently caseous. Such lesions are occasionally met with in rats in which there is no suspicion of plague infection; but they are seen so frequently among rats that have survived artificial inoculation with _B. pestis_, there is no doubt but that in these cases they are the result of the inoculation. In several such cases pest-like organisms have been demonstrated in smears, and acute plague has been produced in guinea pigs by inoculation with the pus found in these lesions. Not infrequently in these cases the spleen will be found enlarged and looking very much like the organ in acute plague, but cultures from this organ in such cases have in my experience remained sterile.

In other cases the only lesions will be found in the spleen. The organ is enlarged and contains a number of caseous nodules. These nodules vary in number from four or five to thirty or forty and in size from the head of a pin to a lesion 0.3 centimeter in diameter. In a number of such cases the nature of the lesion has been demonstrated by animal inoculation. For example, in a series of experiments carried out to determine the susceptibility of San Francisco rats to plague infection a large _Mus rattus_ died on the eleventh day after inoculation. The post-mortem examination showed nothing except an enlarged spleen which contained about a dozen caseous nodules, the largest of which was not over 2 millimeters in diameter. The nodules were very firm and the capsule smooth, so that they were held with difficulty with dressing forceps. Cultures from the liver and the spleen remained sterile, but a piece of the spleen was placed beneath the skin of a guinea pig. This animal died of acute plague, and a pure culture of _B. pestis_ was isolated from its liver. In some of these cases the liver will show large, distinct, whitish caseous foci. In another case a small _Mus norvegicus_ was killed on the twelfth day after a cutaneous inoculation from an artificially infected squirrel. No lesion was found except in the spleen which was not materially enlarged, but which presented two small whitish caseous granules on the surface, neither being over 1 millimeter in diameter. A piece of the spleen containing one of these granules was put under the skin of the belly of a guinea pig. The guinea pig died on the fourth day with the usual lesions of acute plague. Occasionally in these cases of chronic plague punctate hemorrhages or even areas of consolidation are found in the lungs.

THE HISTOLOGY OF RAT PLAGUE.

The most recent and satisfactory work on this subject is that of Ledingham [14], who has studied the lesions of both natural and induced plague in rats. The following is a very brief abstract of his work. The reader is referred to the original for a full study of the subject.

NATURAL RAT PLAGUE.

Two groups of cases are distinguished, first, those in which a large number of _B. pestis_ are found in the liver and in the spleen. In the spleen this is accompanied by hemorrhages and congestion of the pulp sinuses and in the liver with congestion of the capillaries. These are early cases.

In the second group, or the later cases, there are extensive reaction changes in the tissues. In the spleen this leads at times to distinct abscess formation, but more frequently to a walling off of the foci of necrosis. In the liver more or less focal necrosis is found; sometimes the areas of “necrosis” may be so extensive that little healthy liver tissue remains. Bacilli are usually to be demonstrated in these areas of necrosis. Giant cells of the Langhans type may be found in the neighborhood of these foci.

The granular appearance of the liver is attributed to “hemorrhages and the focal necroses, together with the fatty changes in the liver cells. It must be understood, however, that a peculiar honeycomb-like vacuolar degeneration of the liver cell protoplasm was far more frequent than any actual, coarse, fatty infiltration. The granular appearance of the spleen is due partly to endothelial catarrh and partly to subcapsular changes.”

In experimental rat plague Ledingham found the lesions to resemble those of the first group of cases referred to above. There is usually marked bacteraemia; focal necroses of the liver are scanty.

In a chronic case, minute abscesses were found scattered through the spleen. In the center of the abscesses were found clumps of degenerated bacilli. The areas were walled off by epithelioid and spindle cells and numerous giant cells of the tubercular type.

IMMUNITY OF RATS.

Contrary to the general impression the wild rat is not an animal especially susceptible to plague infection. The Indian Plague Commission [19] found that when rats are inoculated by the cutaneous method from the spleen of infected rats 59 per cent are immune to infection. A series of experiments conducted in the federal laboratory in San Francisco showed that when inoculated with highly virulent cultures of _B. pestis_ there is an immunity which is, however, more frequent among the large rats. When inoculated cutaneously with tissue containing large numbers of _B. pestis_ from plague infected human beings, rats, or squirrels, about 15 per cent of small rats and about 50 per cent of large ones were found to be immune. There is no good reason for believing that this immunity of San Francisco rats was due to a previous attack of the disease. Indeed, it was known beyond a doubt that some of the immune rats had never had an opportunity of becoming infected with plague in nature and thereby establishing an acquired immunity. We may mention here the fact that has been observed by many workers, and which we have amply confirmed, that rats are readily immunized by antiplague serum.

The subject of the transfer of infection directly from rat to rat by cutaneous or subcutaneous inoculation through a series of the animals is one that is evidently intimately associated with the preceding subject, as it is quite evident that an immune rat or several of them might terminate a series without any actual diminution in the virulence of the organism transferred. It is quite plain that the success of such an experiment would depend largely upon the number of rats used in each transfer. The Indian Plague Commission [19] had no difficulty in carrying infection through twenty-six transfers, using from six to fifty rats in each transfer.

Pound [7] in a series of eight experiments, was never able to convey the infection successfully beyond the sixth rat, using but one rat for each transfer. There was no apparent lessening of the virulence of the organism and each series appears to have been terminated abruptly by encountering an immune rat.

Baxter-Tyrie [15] says:

It is probable that under certain natural circumstances a reduction in the virulence of the organism is effected and a comparative immunity is conferred on the rats. The infection of immigrant rats is, however, severe, and their arrival is heralded by a heavy mortality. In the same manner an infected rat imported into a fresh locality produces a similar result. This attenuation of virulence is responsible for the condition known as chronic rat plague.

Several experiments conducted in San Francisco to determine this point have given results that I regard as showing merely the presence of a considerable percentage of immunity among the rats. It was observed that in each case certain of the rats died of acute plague even in the last transfer. It was very evident that had certain combinations of immune rats been encountered the experiment might have terminated at any point. On the other hand, by being especially fortunate in using nonimmune rats, the experiments might have given a much higher percentage of cases of acute plague. Unfortunately it was necessary to terminate these experiments in each instance before they could be regarded as completed.

The reason for the natural subsidence of plague among rats in any community is a point about which much more evidence must be obtained before we can speak with any degree of certainty. It may be due to the lack of susceptible material, possibly to a loss of virulence of the organism; but it seems more probable that it is due to a change in the number or relations of the ectoparasites of the rat.

Adequate measures of rat extermination, while they may never bring about the ideal condition of a community that is free from rats, are, as is shown by the recent experience in San Francisco, of the utmost value in shortening the epizootic.

REFERENCES.

Endnote 1:

Skschivan (Centralblatt für Bacter., etc., 1903, Vol. XXXIII, No. 4, p. 260).

Endnote 2:

Kister & Schumacher (Zeit. für Hyg. u. inf. Krank., Vol. LI, 1905).

Endnote 3:

Indian Plague Commission (Journal of Hygiene, 1907, Vol. VII, No. 3).

Endnote 4:

White (Medical Record, vol. 67, No. 4, Jan. 28, 1905).

Endnote 5:

Kitasato (Philippine Journal of Science, Vol. I, No. 5, 1906).

Endnote 6:

Wherry, Walker & Howell (Journal Am. Med. Assn., April 11, 1908, Vol. L, No. 15).

Endnote 7:

Pound (1907, Report on Plague in Queensland, B. B. Ham, p. 134).

Endnote 8:

Dunbar & Kister (1904, Centralblatt für Bact., etc., Vol. XXXVI, No. 1, p. 127).

Endnote 9:

Kolle & Martini (Deut. Med. Woch., Jan. 2, 1902, Vol. XXVIII).

Endnote 10:

Kister (Centralblatt für Bact., etc., July 24, 1906, Vol. XLI, No. 7).

Endnote 11:

Neumans (1903, Zeit. f. Hyg. u. inf. Krank., Vol. XLV, No. 3, p. 451).

Endnote 12:

Kister & Schmidt (1904, Centralblatt für Bact., etc., Vol. XXXVI, No. 3, p. 454).

Endnote 13:

Aujeszky (1904, Centralblatt für Bact., etc., Vol. XXXVI, No. 5, p. 603).

Endnote 14:

Ledingham (Journal of Hygiene, 1907, Vol. VII, No. 3).

Endnote 15:

Baxter-Tyrie (Journ. of Hygiene, Vol. V, 1905, p. 315).

Endnote 16:

Wherry (The Journal of Infect. Diseases, Dec. 18, 1908, Vol. V, No. 5).

Endnote 17:

Klein (The Bacteriology and Etiology of Oriental Plague, MacMillan and Co., London, 1906).

Endnote 18:

Indian Plague Commission (Journal of Hygiene, 1907, Vol. VII, No. 6, p. 761).

Endnote 19:

Indian Plague Commission (Journal of Hygiene, 1906, Vol. VI, No. 4).

RAT RECORD CARD.

[Legend: O = Ordinary; W = White belly; R = Red; Go. = Gopher rat; S = Small; M = Medium; L = Large; M. R. = Mus rattus; M. N. = Mus norvegicus.]

───┬────────┬───────────────┬─────┬────────┬────────────┬─────┬───────── No.│ Date. │District No. 6.│Sex. │ Size. │ M. N. │M. R.│Pregnant. ───┼────────┼───────────────┼──┬──┼──┬──┬──┼──┬──┬──┬───┼──┬──┼───────── „ │ „ │ „ │M.│F.│S.│M.│L.│O.│W.│R.│Go.│O.│W.│ „ ───┼────────┼───────────────┼──┼──┼──┼──┼──┼──┼──┼──┼───┼──┼──┼───────── 19 │Dec. 10,│ 401 Fillmore │1 │ │ │1 │ │ │ │1 │ │ │ │ │ 1908 │ street │ │ │ │ │ │ │ │ │ │ │ │ 20 │ do │ do │ │1 │ │ │1 │1 │ │ │ │ │ │ [S]7 ───┴────────┴───────────────┴──┴──┴──┴──┴──┴──┴──┴──┴───┴──┴──┴─────────

Footnote S:

Number of fœtuses.

PLAGUE RAT CARD.

PLAGUE RAT NO. 50.

Date: June 20, 1908. Species: _M. norvegicus_. From District No. 6, sewer, Haight and Steiner Streets. Condition: Badly injured by trap; thorax crushed. Subcutaneous injection: General, marked. Lymphatic glands, bubo or other lesions: Right inguinal bubo, caseous. Liver: Typical whitish granules. Spleen: Large, dark, firm. Pleural effusion: Unable to say. Purulent or caseous foci: Diagnosis from gross lesions: Plague. Diagnosis from smears: Plague (spleen and bubo). Cultures: _B. pestis_ recovered from liver culture. Inoculation, guinea pig No. 50 A, +6.25.08. Vaccination, guinea pig No. 50 B, +6.26.08. Date suspicious: Date positive: June 20, 1908. Date negative:

[Illustration:

A HOEN & CO BALTIMORE.

NECROPSY APPEARANCE OF PLAGUE-INFECTED RAT ]

[Illustration:

A HOEN & CO BALTIMORE

NECROPSY APPEARANCE OF NORMAL RAT ]

RAT LEPROSY.

By WALTER R. BRINCKERHOFF, S. B., M. D.,

_Assistant Director Leprosy Investigation Station, United States Public Health and Marine-Hospital Service, Honolulu, Hawaii_.

INTRODUCTION.

The leprosy-like disease of the rat is of great interest to leprologists because of its close similarity to the disease leprosy in man. Its practical importance to those engaged in the study of the human disease is increased by the fact that it can be artificially propagated under laboratory conditions from animal to animal and, still more important, can be transferred from the species in which it occurs naturally (_Mus norvegicus_) to a more tractable laboratory animal (_Mus albus_). The brief description of the affection which follows is intended to assist in its recognition and to stimulate the interest of investigators in the disease, which presents problems replete with interest to the study of pathology or bacteriology and of great promise to those engaged in the investigation of human leprosy. It is earnestly hoped that the investigation of this disease will be undertaken in general medical research laboratories, as it is extremely probable that certain of the most difficult problems presented by leprosy in man can be studied in this disease of the rat, and if solved there the information gained can be directly applied to the solution of the analogous problems in the human disease.

REVIEW OF LITERATURE.

The first publication on rat leprosy was made by Stefansky (1903), who observed the disease in Odessa during an antiplague campaign against rats.

Rabinowitch (1903) found the disease among rats in Berlin and confirmed the work of Stefansky.

Dean (1903) discovered the disease independently in London, and in a later publication (1905) reports success in transferring the disease by artificial inoculation.

Tidswell (1906) reports a case of the disease in a rat caught in Sydney, New South Wales, Australia.

The English Plague Commission observed the disease in India in 1907 (Wherry).

Wherry (1908) and McCoy (1908) report upon the finding of the disease in rats caught in San Francisco, Cal.

Mezincescu (1908) has studied the disease and attempted to determine its relationship to known human lepra by complement fixation tests.

DESCRIPTION OF DISEASE.

_Geographical occurrence._—It would be premature at present to make didactic statements as to the geographical distribution of the disease, for its discovery has usually depended upon antiplague measures, which are not world-wide in their scope. In spite of this it seems profitable to briefly review the known occurrence of the disease in relation to that of human leprosy. When such a comparison is made we note that the disease is present among the rats of Berlin, a city which is practically free from human lepra. On the other hand in Honolulu, which is an endemic focus of human leprosy, in the examination of 16,000 rats, during an antiplague campaign, no case of rat leprosy was encountered. In addition to the scrutiny of the rats examined for plague in Honolulu, an attempt was made to obtain leper rats by offering a reward for a rat, dead or alive, infected with the disease. This offer was given wide publicity in the Territory, but brought no results.

_Occurrence of the disease._—The proportion of rats infected with the disease in different localities varies greatly, as will be seen in the following table:

TABLE 1.—_Proportion of leper rats to the total rats examined._

───────────────────────────┬───────────────────────────┬─────────────── Place. │ Observer. │ Proportion. ───────────────────────────┼───────────────────────────┼─────────────── │ │ _Per cent._ Odessa │Stefansky [301] │ 4–5.000 Sydney │Tidswell [305] │ .001 San Francisco │Wherry [308] │ .210 Do │McCoy [310] │ .160 Honolulu │Currie[T] │ .000 ───────────────────────────┴───────────────────────────┴───────────────

Footnote T:

Personal communication.

Rats in the late stage of the disease are easily recognized by the presence of a patchy alopecia associated with cutaneous and subcutaneous nodules, which may or may not be the site of open ulcers. The diagnosis can be readily confirmed by a microscopic examination of a smear from an ulcer or a nodule, which will show the specific bacillus of the disease in enormous numbers.

Stefansky[301] describes two clinical types of the disease, the one localized particularly in the lymph nodes, the other in the skin and muscles. The glandular type was the more common. Dean[304] thinks that no line of demarcation can be drawn between these clinical types.

Dean[304] and Wherry[307] both mention that attention was attracted to the diseased animals by the fact that they were seen abroad during daylight in an obviously sick condition.

The skin, in a well-developed case of the disease, presents a patchy alopecia coincident with thickening and nodule formation, which is situated in the subcutaneous tissue. The cut surface of the nodules or thickenings is light yellow in color, is clean, dry, and cheese-like. In the region of the nodules the skin is atrophic, and ulcers often form on the prominent parts of the affected area. The subcutaneous fat tissue is diminished in amount. Histologically the process is seen to be practically confined to the subcutaneous tissue and to consist essentially in the presence of cells rich in protoplasm, with vesicular nuclei, whose cell body is more or less completely filled with slender acid-fast bacilli. The subcutaneous fat is replaced by such a tissue. All investigators who have studied the disease agree in emphasizing the similarity of the histology of the lesion to that in leprosy in man.

When the musculature is involved the muscle fibers atrophy and the fibers are infiltrated with the specific bacilli. The affected muscle is friable, and macroscopically grayish white in color.

The peripheral lymph nodes are commonly involved, though McCoy[310] reports a case in which only the pelvic and mesenteric nodes were diseased, and in the Tidswell case[305] the peripheral nodes were not enlarged. The typically affected nodes are enlarged, sometimes measuring as much as 3 centimeters in the greatest extent, firm, and, on section, opaque pale yellow-white in color. In the experimental disease the writer has frequently found the characteristic bacilli of the disease in peripheral lymph nodes which were very slightly enlarged and presented no macroscopic lesion. Dean[304] has observed invasion of the submaxillary or salivary glands by extension from infected cervical lymph nodes. Wherry[308] notes that in his cases he did not find the submaxillary or cervical glands involved, which fact he contrasts with two early cases in which the skin and adjacent axillary or inguinal nodes were involved.

Microscopically the lymph nodes show large numbers of cells in the sinuses similar to those in the skin lesions. Multinuclear giant cells are frequently observed which may measure as much as 70 to 80 microns[304]. The protoplasm of the cells is loaded with the specific bacilli of the disease. The lymph follicles, trabeculæ, and capsule of the glands are also invaded by the bacilli.

The internal organs are relatively slightly affected in the natural disease. Small foci have been found in the liver by Dean[304] and in the liver and spleen by McCoy[310]. Wherry[307] reports finding the bacilli in smears from both the liver and spleen. The writer has found microscopic lesions containing the characteristic bacilli in the liver in a case of the experimental disease.

Lesions have been observed in the bone marrow by Dean[304], and the same author states that the nerves are invaded by the bacilli of the disease. McCoy[310] found the bacilli in the urinary bladder in one case.

With a disease showing such a striking similarity to human leprosy, attention has naturally been directed to the bacteriological examination of the nasal mucus. Dean[304] and Wherry[307] have both found the characteristic bacilli in the nasal mucus, while McCoy[310] has failed to do so. The writer’s experience has been confined to the experimental disease, and in his animals the nasal examinations have been negative.

ETIOLOGY.

The accepted etiological factor in the disease is an acid-fast bacillus 3 to 5 microns in length and 0.5 micron wide. The bacilli resemble very closely the lepra bacillus of man, but seem to have somewhat greater power to hold carbol-fuchsin stain against mineral acids. The bacilli often have rounded ends and may be curved. The beaded appearance so often seen in lepra bacilli is common. The bacilli show the same tendency to form bundles that is such a marked characteristic of _Bacillus lepræ_. To one familiar with the microscopic appearance of smears from the discharges and lesions of human leprosy the picture presented by similar preparations from the disease of the rat is most striking.

The organism does not grow on the usual culture media—Stefansky [301], Rabinowitch[302], Dean[304], Tidswell[305]—or on certain special media—Dean[304].

The organism is not pathogenic for the guinea pigs—Dean[304], Tidswell[305]—rabbit, mouse, monkey—Dean[304]. The disease can be transmitted to black and white rats—Dean[304], Wherry.

SUMMARY.

In the leprosy-like disease of rats we have an affection which closely resembles, both in its etiological factor and in its pathology, the disease leprosy in man. The fact that the disease is readily propagated in a laboratory animal permits of its investigation in any laboratory. It is earnestly hoped that the study of this disease will be taken up by bacteriologists and pathologists, as in this way valuable information may be gained which will be applicable to the problems presented by leprosy in man.

BIBLIOGRAPHY.

Endnote 301: