It is now twelve years since it first became possible to study plague by modern methods, and yet no conclusion on its aetiology has been reached which has met with general acceptance. Two causes, as it seems to me, have chiefly contributed to this disappoinment. One consists in the appeal which has been persistently made to bacteriology for the lights which in fact are not in its power to shed; the other is neglect of the epidemiological method, or at least failure to apply it effectively. In consequence, a field has been occupied by the bacteriologist which could not be profitably laboured by him. The epidemiologist has not seriously disputed this usurpation of his domain, and for his apparent apathy he has not been without excuses of some weight.

Of papers recently published which concern the properties of immune-sera and bear upon the general theory of immunity one of the most important is that of Bordet2 on antisensibilisatrices (anti-immunebodies, anti-amboceptors). This observer obtained an anti-immune-body by injecting the serum of a normal animal (rabbit) into an animal of another species (guinea-pig), and found that there was developed an anti-immune-body which had the property of neutralising the various immune-bodies which might be developed by active immunisation of the first animal. In this way a certain community as regards combining properties was demonstrated among the immune-bodies of a given species of animal. He studied the properties of the anti-immune-body, and in particular showed that it did not combine with the cytophile group of the immune-body, and therefore did not prevent the usual combination of the cell-receptor with the immune-body. This fact he held to be inconsistent with Ehrlich's views regarding the amboceptor constitution of immune-body. His observations on the neutralising effect of the anti-immune-body were carried out by means of haemolytic sera. Ehrlich and Sachs3 confirmed the chief results obtained by Bordet, but so far from admitting the establishment of any objection to the amboceptor theory, claimed that they supplied strong evidence in support of it. They maintained in fact that the anti-immune-body acted by combining with the com piementophile group of the amboceptor, and thus prevented the union of complement, stating also that the results showed that the various immune-bodies from the same species had a similar complementophile apparatus. The theoretical bearings of the facts established will, however, be discussed later.

On reading the excellent paper on the parasites of the Culicidae recently published by Dr Léon Dyé, I was struck by the fact that several of the organisms mentioned by him had been seen and described by me long ago in the course of my original studies on malaria and mosquitoes carried out in India during 1895–1899. My observations were, however, recorded for the most part in Indian medical publications, which are little accessible to European readers, and have been so far forgotten that some of the parasites of mosquitoes seen by me have been recently rediscovered by various observers. It may therefore be of interest—perhaps not alone from the historical point of view—to resuscitate these old records and compare them with more recent and exact work. I may add that attempts were made by me to infect men with two of the organisms which I observed. My writings on the subject were contained in the following publications, which are here numbered for reference:—

A male Pariah-Irish terrier puppy, apparently about three months old, followed me home from the hospital in Krian. This was twelve months ago, so that the dog is now fifteen months old. Of his previous history I know nothing.

In view of the great interest attached to the questions discussed in the preceding paper, and the doubt expressed therein regarding the accuracy of Schaudinn's observations on the relationship between Halteridium and Trypanosoma, it seems expedient to reprint the following short papers by Novy, MacNeal, and Torrey, and Novy and Knapp, which appeared on February 9th, 1906, in Science (vol. 23, pp. 207–208).

In the course of our experiments on haemolytic sera we have met with the following phenomenon which appears of some interest. It consists in the agglutination of the corpuscles of an animal by its own complement through the medium of the corresponding immune-body derived from another animal, and was observed first in the case of ox's corpuscles, the immune-body used being obtained from the rabbit. The fundamental fact is that if a certain amount of immune-body and ox's complement be added to ox's corpuscles, scarcely any lysis of the corpuscles occurs but they become agglutinated into large masses which cannot be dissociated by shaking. The immune-serum from the rabbit contains some agglutinin, but the degree of agglutination produced by this is quite trifling compared with that seen when complement also is added. There is thus no doubt that the agglutination phenomenon depends on the cooperation of two substances in a manner comparable to what obtains in lysis. The following are the chief facts regarding the conditions of occurrence of the agglutination and the nature of the agglutinating substance in the ox's serum.

That out breaks of plague are associated with a more or less simultaneous mortality amongst rats is an observation of great antiquity. After the discovery of the bacillus by Yersin and Kitasato this concomitant disease of rats was identified as plague, and during the last twelve years the association of rat plague and human plague has been shown to be almost invariable. The relationship has been so striking that many observers who have studied the question on the spot, e.g. Yersin, Ogata, Simond, Thompson, Koch, Gaffky and many others, have arrived at the opinion that from an epidemiological point of view one must regard plague as essentially a rat-disease in which human beings may participate.

The parasite referred to in the preceding paper was found by Dr Gerrard in the polymorphonuclear leucocytes of a dog. As is stated by Dr Gerrard, the parasite is evidently similar to that discovered by Bentley and described by him, and afterwards by James, as the Leucocytozoon canis. Dr Gerrard sent some films, stained by him by Leishman's method, to Sir Patrick Manson, and they were handed on to me for description and identification. These films had faded in transit, they were therefore decolorised and restained by various methods. All the staining reactions mentioned in this description refer to these old films.

The cases which from the subject of the following note occurred in the district of Dr Chas. Jackson, M.O.H., of Fulham, and I am greatly indebted to him for so kindly furnishing me with their histories and with samples of the patients' serum.

In a previous paper on Bird Trypanosomes it was pointed out that these organisms grew readily in the test-tube on blood agar and that the resulting forms resembled the flagellates which Schaudinn found in the gut of mosquitoes which had fed on owls infected with Halteridium and with H. Ziemanni. In other words, the position taken was that the flagellates observed in the mosquitoes did not represent stages in the life-history of intracellular parasites but were actually cultures in vivo of trypanosomes present in the blood of the birds used. In confirmation of this position it was desirable to show that trypanosomes could actually grow and multiply in the gut of mosquitoes and that such forms actually did correspond to those which would be obtained in vitro.

In the following experiments, which are a repetition of those of Gauthier and Raybaud (1902, 1903), it is shown that, in the presence of the common Indian rat flea (Pulex cheopis, Rothschild), plague may spread from a plague-infected rat to a healthy rat confined in close proximity, but in such a way as to prevent contact with the body or excreta of the sick rat.

The origin and object of the following paper can be explained in a few words. In April, 1904, the writer published a series of 280 blood examinations (to which were added about 150 more) of natives of southern Angola (Portuguese West Africa) without recording a single instance of the presence either of trypanosomes or of spirochaetes in the circulation. Quite recently in the course of an investigation of the endemic malaria of the same district, 513 examinations made among the same people and with the same technique revealed 3 individuals harbouring trypanosomes and 4 showing spirochaetes in their blood. These observations, together with the fact that a year or so ago the natives reported no cases of Tick Fever, while recently statements are made by them that the Tick Fever of the upper Zambezi has reached Angola, and also the fact that Sleeping Sickness is reported to be advancing southward through the province, have led me strongly to suspect that these diseases have lately begun to spread markedly in my part of the colony, and that one or both of them may not inconceivably assume in the near future epidemic proportions.

The Bacillus cloacae liquefies gelatin very slowly, sometimes taking a month or more to do so. It is in consequence occasionally a source of delay and of great inconvenience to the bacteriologist. In this paper it has been shown that by using appropriate methods the time necessary for demonstrating the liquefaction of gelatin by this organism may be shortened to a week.

The material on which these notes are founded was obtained from moles captured in the neighbourhood of the Lister Institute at Elstree in Hertfordshire from the middle of May to the beginning of July, 1906. In all 14 moles were examined.

In general, it may be said that bacteria once introduced into a culture of trypanosomes tend to outgrow and check the development of the flagellates. In exceptional instances, however, the bacteria thus introduced exert little of no interference and may be even apparently beneficial. While in the former case the trypanosomes soon die out, in the latter instance the mixed cultures may be kept for six months or longer.

While studying the infection of Trypanosoma dimorphon in mice at the Pasteur Institute some five months ago, I noticed in one of the fresh blood preparations made from a brown mouse (A) infected with the trypanosomes, a small and very active spirochaete. At first I was of the opinion that its presence in the blood was due to contamination from the skin, but, as it was constantly present and was constantly present and was found in blood taken from any part of the body, it was evident that it was parasitic in the blood.

Some months ago a bat flew into my house at Grahamstown and was captured. Unfortunately the animal was fatally injured. It was placed in a tobacco-box and on opening the box next day a lively insect like a common bed-bug (Acanthia lectularia) and emiting a similar odour was found in the fur behind the bat's neck. The bug was sent to Mr C. W. Mally, M. Sc., Entomologist to the Eastern Province, for identification and he kindly determined the species as being Acanthia pipistrelli Jenyns, adding that the specimen agreed very well with the description given by Jenyns. Mr Mally wrote, “In the absence of the Northern specimens of this species I cannot express an opinion as to varietal characters. This specimen is of especial importance because there is apparently no record of its occurrence in these parts, nor has the species of bat in connection with which it was first discovered been found at the Cape”. Through the kind intermediation of Mr Mally the bat was forwarded to Professor Duerdin of the Rhodes University, who determined the species to be Vespertilio capensis, “The Cape Serotine Bat”. Prof. Duerdin states it is “apparently one of the commonest of South African bats, but we do not seem to have many”.

The following conclusions appear to us to be justified as a result of the experiments cited above:—

(1) Close contact of plague-infected animals with healthy animals, if fleas are excluded, does not give rise to an epizootic among the latter. As the go-downs were never cleaned out, close contact includes contact with faeces and urine of infected animals, and contact with, and eating of food contaminated with, faeces and urine of infected animals, as well as with pus from open plague ulcers. This conclusion is justified from Experiments I and II and especially from Experiment VI, in which close contact with infected animals was maintained for nearly a month. In Experiment I the animals were inoculated cutaneously, and in many cases had developed ulcers at the site of inoculation.

(2) Close contact of young, even when suckled by plague-infected mothers, did not give the disease to the former.

(3) If fleas are present, then the epizootic, once started, spreads from animal to animal, the rate of progress being in direct proportion to the number of fleas present. Thus, in the experiments during the months of December, January and February, the epizootic was very rapid in those go-downs, namely, Nos. 1 and 2, Experiments IV and V, in which the flea population was abundant and was kept up by a natural supply from the roof: it was much slower in Experiment VII, go-down No. 5, in which the flea supply was kept up artificially; and, finally, it was slowest of all in go-down No. 6, Experiment VI, in which there was no definite natural supply of fleas, and from which the fleas were daily removed for a period of six days, after which removal only a comparatively small number could be caught.

(4) An epizootic of plague may start without direct contact of healthy animal and infected animal. Thus, in the case of Experiment V in go-down No. 1 the healthy guinea-pigs were not put in until the last inoculated guinea-pig had died and been removed.

(5) We have in Part II shown by direct experiment that the rat flea can convey plague from rat to rat. Further experiments (Experiments VI, VII and XI) of a similar nature with the fleas removed from infected go-downs are now recorded.

(6) Infection can take place without any contact with contaminated soil. Thus in go-downs Nos. 1 and 2 (Experiments VIII, IX), guinea-pigs placed in wire cages two inches above the ground developed plague; also the monkey in go-down No. 2 (Experiment XIII) was never in contact with the ground.

(7) Aerial infection is excluded. Thus, guinea-pigs suspended in a cage two feet above the ground did not contract the disease, while in the same go-down those animals allowed to run about and those placed two inches above the floor became infected. Further, the monkey surrounded by tangle-foot was exposed as much to aerial infection as the control animal which contracted the disease.

1. Three fully-established, and two probable, cases of infection with B. paratyphosus B producing a typhoid-like illness have been found in a series of 176 cases of typhoid.

2. Preliminary culture in a fluid medium containing dulcite is of great assistance in isolating paratyphoid organisms from mixtures such as faeces.

3. The diagnosis of paratyphoid fever can generally be made from the agglutinative reactions of the serum.

4. Absorption tests are necessary for the positive or negative diagnosis of mixed infections.

5. The structure of the agglutinated masses of paratyphoid and Gaertner bacilli varies with homologous and heterologus sera, and is of diagnostic import.

6. Gaertner and paratyphoid agglutinins are more thermostabile than typhoid agglutinin by 5°–10° C.

7. The agglutinations of paratyphoid, hog-cholera and Gaertner bacilli with a series of typhiod sera are described, and the circumstances which influence the presence and amount of these secondary agglutinins discussed.