In previous reports (vol. vi. p. 470 and vol. vii. p. 421), we described several series of experiments the results of which went to show that fleas and fleas alone were the transmitting agents of the infection of plague. It will be remembered that these experiments were carried out in a series of small godowns, the construction of which has been described in detail. The animals employed in the majority of previous experiments were guinea-pigs, though a limited number of experiments were carried out with monkeys. Gottschlich (Kolle and Wassermann's Handbuch der Pathogenen Mikroorganismen, Suppl. vol. ii., 1907, p. 52) considers that the results obtained with guineapigs cannot be applied to rats as the latter animals, unlike guineapigs, feed on the carcases of their dead companions. Gottschlich while admitting that fleas may transmit the disease from rat to rat considers this method of spread to be of only subsidiary importance. He is of opinion that, among rats, plague is chiefly spread by the healthy animals feeding on the carcases of the plague infected. In order to examine the validity of these criticisms we decided to repeat the godown experiments using wild Bombay rats instead of guinea-pigs.

1. The abscesses in the viscera, especially the spleen, and in the peripheral lymphatic glands, some of which contain living plague bacilli, which have been previously described as “chronic plague in rats” are shown to be plague lesions in the process of healing. The name “resolving plague” is more appropriate.

2. These lesions are found most frequently towards the close of, and immediately after, the acute plague epidemic.

3. They have been found frequently in rats in Belgaum and Poona, and a re-examination of Bombay rats shows that they also occur there. We had previously only found them in the Punjab villages.

It is well established that plague, once it has gained a foothold in a place, tends to recur every year at the same season and that the plague season varies in different places. It has been demonstrated that in Bombay City the periods between the epidemics are bridged over by cases of acute plague amongst the rats accompanied by a few cases in man. Knowing, then, the factors which determine the rise and fall of the epizootic amongst the rats, once the infection is present, we have a fairly complete conception of the seasonal prevalence of human plague in a city such as Bombay. In the case, however, of a large province, such as the Punjab, with scattered villages, the question of the annual recrudescence in the several villages is not so simply answered. For, it will be remembered that in the two Punjab villages of Dhand and Kasel, which were under close observation by the Commission for a whole year, no acute plague was found amongst either man or rats in the long interval between the epidemics. While this is so, a certain number of rats were caught alive, which, although apparently in good health, harboured living and virulent plague bacilli in chronic abscesses (see above, p. 335).

(a) General description and geographical position. Belgaum is a small town and military cantonment situated in the southern division of the Bombay Presidency, 75 miles inland from the western sea coast of India. It is situated 15° 51′ N. and 74° 31′ E. It lies on a plateau 2500 feet above the sea level on the northern slope of a water-course. The town is built on laterite which lies upon Deccan trap. The country round is fertile and well wooded.

Poona, the most important city of the Deccan, is situated 75 miles S.E. of Bombay on the eastern watershed of the range of Western Ghats, 1850 feet above the sea level. It lies on the Basalt Rock, with a surface layer of Deccan Trap and Black-Cotton Soil.

The primary source from which all vaccines are derived is a culture of the organism against infection by which it is desired to protect the animal. The culture may be used as a vaccine either alive or dead.- We are concerned here only with the latter. Such a killed culture comprises the organism itself and the products of its metabolism, together with part or the whole of the culture medium in which it has grown. This mixture may be called a whole vaccine. By appropriate treatment a “whole vaccine,” may be deprived of some or all of such constituents as are not concerned in bringing about, when inoculated into animals, those changes which constitute protection. Such a vaccine may be called a derived vaccine in the case when all the immaterial constituents are not removed, the term antigen being reserved to designate that substance which alone is concerned in conferring protection. The main object of this investigation is the preparation of a derived vaccine with the hope that the antigen may ultimately be isolated.

The following statement embodies the chief conclusions which have been provisionally reached by the Advisory Committee as the result of the investigations made under their direction from 1905—1909 into the mode of spread of plague in India.