In the following paper an attempt will be made to give a description of certain characteristic features which we have observed on postmortem examination of a large number of plague rats in the course of the daily routine rat investigation in Bombay. An account of the diagnosis of rat plague may appear at first sight to cover well-known ground and to be consequently unnecessary. Having regard however to its importance the subject seems to have received remarkably little attention, judging from the scattered references to it in the literature.

The pathological histology of plague in man and experimental plague in animals has already been the subject of several researches, notably those of Aoyama (1896), Yamagiwa (1897), Albrecht and Gohn (1897), Dürck (1904) and Hamdi (1904) in the case of human plague, and of van der Stricht (1897), Honl (1897), Lustig and Zardo (1897), Babes and Livadite (1897), Albrecht and Gohn (1900) and Sata (1900) in the case of experimental plague.

1. It is possible to infect wild rats of Bombay with plague by feeding them with the viscera of dead plague rats, 21·4% being susceptible to this method of infection. Bombay rats show a greater immunity to infection by feeding than rats of the same species, which have not been subjected to a plague epizootic.

A series of experiments were also done with Mus rattus caught in the Punjab. Of these rats 67·8% were susceptible. In this series a considerably larger dose of infected material was given.

We have infected a large number (38%) of wild Bombay rats by feeding them on the whole carcases of their plague-infected comrades. No difference as regards the post-mortem appearances or the distribution of the primary bubo was found between rats infected in this way and rats infected by feeding on soft viscera.

2. The general pathological lesions found in all rats infected by feeding are, in the main, the same as those found in rats naturally infected. There are, however, two striking differences:—

(a) The distribution of the primary bubo is different. The common site in naturally infected plague rats is in the neck, no mesenteric bubo having been seen out of 5000 post-mortems. In the case of fed rats the common site is the mesentery.

(b) In the case of naturally infected rats the stomach and intestines show no marked pathological change. In the case of fed rats well marked pathological lesions are found in the intestines.

3. It would appear that in nature intestinal infection rarely or never takes place, and that in consequence rats do not become infected by eating the carcases of their comrades.

4. A large series of rats were fed on the urine of plague cases. None of these contracted the disease.

Cervical buboes preponderate, on the one hand, in naturally infected rats and in guinea-pigs infected by being placed in plague-infected houses and also in rats and guinea-pigs artificially infected with fleas. In rats artificially infected by feeding mesenteric buboes are the most frequent, whereas in upwards of 5000 naturally infected rats in not a single case was a mesenteric bubo present. It may, therefore, be concluded that rats in nature are not infected by feeding on plague-infected material, but probably by the agency of fleas.

1. The average capacity of a rat flea's stomach is approximately 0·5 c.mm. On this basis a flea imbibing the blood of a plague rat showing a good septicaemia might take as many as 5000 germs into its stomach.

2. Multiplication of the plague bacillus takes place in the stomach of the rat flea.

3. The approximate proportion of fleas in the stomach of which multiplication of plague bacilli takes place has been determined, and it has been shown that this proportion varies with the season of the year, being six times greater in the epidemic season than in the non-epidemic season.

4. Plague bacilli are present in the rectum and faeces of fleas taken from plague rats, and such faeces are infective to guinea-pigs both by cutaneous and by subcutaneous inoculation.

5. On rare occasions plague bacilli have been found in the oesophagus, but never in any other region of the body, such as the body cavity or salivary glands.

6. During the plague season fleas might remain infective for 15 days after imbibing infective blood, but during the non-epidemic season no animal was infective after the 7th day.

7. A single rat flea may transmit the disease.

8. Both male and female rat fleas can transmit the infection.

9. Experimenting with cat fleas (P. felis) and human fleas (P. irritans), 27 experiments with the former were unsuccessful, and out of 37 experiments with the latter three successes were obtained. Two experiments were made with C. fasciatus: both were successful. Multiplication of the plague bacillus takes place in the stomach of the human flea.

10. The plague bacillus has never been seen in the body cavity or in the salivary glands of infected fleas.

Evidence has been obtained to show that the bite of a healthy flea affords a sufficient avenue for infection by septicaemic blood if it is spread upon the bitten part.

No evidence has been obtained in favour of infection by contaminated mouth parts or regurgitation from the stomach, but the possibility of infection by such means cannot be excluded.

In the first report (vol. vi. p. 450) we described several series of experiments which had as their object the determination of the relative importance of the Indian rat flea, Palex cheopis, and of actual close contact in the absence of fleas in the dissemination of plague from animal to animal. These observations were carried out in a series of small godowns or cabins, which were built especially for the purpose. The experiments which we have now to put on record are a continuation of those already published. They were carried out during the non-plague season of 1906 and the plague season of 1907.

1. In 19 out of 100 experiments guinea-pigs allowed to run free in houses which were presumably plague infected developed the disease and died.

2. On three occasions, namely, on every occasion on which the experiment was made, fleas transferred from plague-infected rats found in houses infected fresh guinea-pigs in the laboratory.

3. Rat fleas, caught on guinea-pigs in plague houses and transferred to fresh guinea-pigs, transmitted the disease in 35% of the cases.

4. A census of rat fleas in houses in Bombay, which were proved plague infected, indicated that rat fleas were 12 times as numerous as in control houses; and that in presumably infected houses rat fleas were four times as numerous as in control houses.

5. In 41 out of 130 fleas taken on guinea-pigs in plague-infected houses, bacilli microscopically indistinguishable from plague were found in the stomach.

6. In the case of 24 of the 27 houses definitely proved to be plague infected, dead rats had been found shortly before the experiments were made.

In a previous paper (vol. vi. p. 486) the physiological anatomy of the mouth parts and alimentary canal of P. cheopis has been described. In the present paper we propose to give a detailed description of the external morphology.

For those who are not familiar with entomological nomenclature it is necessary briefly to detail the general structure of a flea.

(1) The characteristic feature of chronic rat plague as described in the foregoing account is the presence of circumscribed abscesses containing plague bacilli in rats caught alive, the animals usually showing no other lesions nor signs of ill-health. No bacilli were seen on microscopical examination of the heart-blood and of the spleen tissue in any of the rats. The bacilli in the great majority of the cases were virulent.

(2) We have grouped the 45 rats conforming to this description which were found during the year's investigation in the Punjab into two classes, one group including those in which the lesions were situated in the abdominal viscera, and the other group including those in which the abscesses were found in regions occupied by peripheral lymphatic glands.

(3) Lesions of the viscera were found principally in the spleen and in the mesentery, while the submaxillary group are most frequently affected among the lymphatic glands.

(4) The peripheral type was observed chiefly during the decline of the epizootic, while the visceral type predominated in the off-season.

(5) In Bombay, only one chronic plague rat was met with out of 17,000 plague-infected rats. In Kasel, 9% of all the rats which were proved plague infected had the chronic disease, while in Dhand the proportion was as high as 28%. With our present knowledge we can advance no adequate explanation of these facts.

(6) We have no direct evidence that chronic plague, as it occurs in the Punjab villages, possesses any significance in the seasonal recurrence amongst the rats of the infection in an acute form, nor is any evidence available which excludes this possibility.

We have shown that in the laboratory the rat flea, P. cheopis, will readily bite man. When very numerous it will bite man even in the presence of its natural host. We have been able to keep this species of flea alive for more than three weeks by feeding it on man alone.

In the course of some experiments in godowns which were infested with P. cheopis alone, we have often taken fleas in considerable numbers on the legs of men who have entered the godowns for a short time.

In a building in Bombay, in which there had been a severe rat mortality, proved to be due to plague, we have taken rat fleas in large numbers on the legs of men who entered some of the rooms in this building even for a short time.

We can conclude, therefore, that the rat flea, P. cheopis, under certain circumstances, is attracted by man, and will readily bite and feed on him.