BACTERIAL DISEASES OF THE RAT, OTHER THAN PLAGUE AND RAT LEPROSY.

By Donald H. Currie,

Passed Assistant Surgeon, United States Public Health and Marine-Hospital Service.

So far as is known, the several species of rats that are found about the habitations of man—Mus norvegicus, Mus rattus, Mus alexandrinus, and Mus musculus—are naturally subject to but few bacterial diseases as compared to some other animals. Interest in this matter has only recently been aroused, owing to the rôle played by the rat in the spread of bubonic plague. When we consider the immense number of rats that have been examined in connection with antiplague work by trained investigators in recent years, and that to many investigators the thought must have come that the discovery of some rat destroying bacterium would be of the greatest utility, it appears more than probable that few such natural diseases exist.

Plague is the one natural bacterial disease that has demonstrated its power to destroy these rodents in numbers sufficiently large to attract general attention; scientific investigation has only been able to add a few other bacterial diseases, and these are probably for the most part rare ones, causing the death of a very small percentage of the total rat population.

Of the “natural” diseases (i. e., spontaneous, in distinction to diseases that can only be produced artificially, under laboratory conditions) the following are the more important ones:

Rat plague and rat leprosy, which are made the subject of special chapters in this publication, must be mentioned as the most important diseases observed among rat populations.

DANYSZ’S BACILLUS OR BACILLUS TYPHI MURIUM OF LOEFFLER.

These are probably identical organisms, differing only in their degree of virulence, at least their pathogenicity alone distinguishes them in the laboratory. They are both members of the paracolon group. They produce a diffuse cloudiness in broth, ferment glucose but not lactose or saccharose, do not liquefy gelatin nor coagulate milk.

56B. typhi murium (Loeffler) is fatal to mice (Mus musculus), but not to rats. M. Danysz isolated a bacillus during an epidemic of field mice which was indistinguishable from the above, except that its virulence was capable of being raised to a point where it would destroy a relatively large percentage of rats inoculated with it by feeding. We see from this that, strictly speaking, it is not a natural disease among rats, still there are cases where its virulence has for a time remained high enough to infect a considerable per cent of rats exposed to those that have sickened of it. Not only is this true in cage experiments, but probably it sometimes occurs in nature after the virus is once thoroughly introduced (an article by M. Danysz; also experience of this service in plague in San Francisco, 1903 to 1905), and may therefore be grouped under the list of “natural” infections. This bacillus is unfortunately of a very unstable nature, in so far as its virulence is concerned; some cultures appearing to be avirulent, while others cause an all but absolute mortality among the rodents eating it.

The duration of the disease is variable and appears to depend somewhat on the size of the dose received, as well as virulence of the culture. We have seen death in thirty-six hours or less following ingestion. On the other hand, it may occur in two weeks. Usually it occurs in from six to twelve days. In a typical case when the animal has lived ten or twelve days it is much emaciated, its tissues are dry, and intestinal hemorrhages are sometimes met with. When the disease is much prolonged a pustular eruption may be present over the skin. The organism can often be isolated from the heart blood by plating, such isolation alone affording means of diagnosis. The only present interest this organism has is as a means of destroying the rat. It was believed to be harmless to man, but more recently cases of human illness have been reported that were believed to have been caused by infection with this bacillus.

PNEUMONIA.

We have recently seen a case of lobar pneumonia in a rat in which a diplococcus was present in pure culture. Possibly connected with this is a condition of abscess of lung, which is not very uncommon. The cavity is filled with a creamy or cheesy matter composed of broken-down cells. Often these cavities break into the pleura. Several morphological types of organisms are found, but from their variation this laboratory has regarded them as secondary or accidental, especially as we have failed to demonstrate that this material was infectious.

STAPHLOCOCCUS ABSCESSES.

These are rather common and may occur subcutaneously or in the superficial muscles of any part of the body.

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BACILLUS PSEUDO-TUBERCULOSIS RODENTIUM (PFEIFFER).

This organism that infects rats is of interest from its close resemblance to the plague bacillus. It is difficult to distinguish the two organisms by ordinary cultural or animal tests. The earlier writers claimed that B. pseudo-tuberculosis rodentium could be differentiated by its power of coagulating milk, but more recently this difference has been found to be an inconstant one.

TOYAMA’S BACILLUS.

Toyama has described an organism which he states is pathogenic for Mus rattus, field and house mice (Mus musculus), but not pathogenic for Mus norvegicus.

It causes congestion of lungs, enlargement of lymph nodes, especially in the neck, and enlargement of the spleen. It was isolated from a natural epizootic among Mus rattus. It is a nonspore-bearing bacillus, without capsule, stains without showing bipolarity, and grows upon ordinary media.

Among other bacteria that have been described as causing diseases in rats may be mentioned:

Von Schilling’s bacillus, allied to Danysz’s organism.

Bacillus “Eris,” a member of the colon group.

Bacillus muris, a member of the B. diphtheria group.

Of the bacteria that show virulence for rats under laboratory conditions, but, so far as is known, cause no spontaneous outbreaks, the following are the best-known examples:

Bacillus bovisepticus produces a fatal disease bacillus of swine erysipelas (especially for albino rats), and the bacillus of tetanus.

Of the higher fungi (not strictly bacterial) we have:

Streptothrix maduræ produces local swellings when inoculated artificially.

It has been stated that rats occasionally suffer from a disease similar or identical to the affliction in man known as favus (Achorion Schönleinii).

INFECTIONS OF MICE (MUS MUSCULUS).

This species of Mus is very susceptible to a large number of bacterial diseases when inoculated under laboratory conditions. The following are some of the best-known examples:

B. murisepticus, Staphlococcus pyogenes, Streptococci, Diplococcus pneumoniæ, B. pneumoniæ (Friedlander), Diplococcus of pleuro-pneumonia of horses, B. Typhi murium, B. anthracis, B. of malignant edema, B. tetani, B. mallei, B. diphtheriæ vitulorum, B. bovisepticus, B. suisepticus, the bacillus of Mereshkowsky, and many others. The last-named organism has been utilized to a limited extent for the destruction of mice about dwellings.

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