Having utilized the technic devised by Teague, I have had no difficulty in performing the agglutination test in plague. The emulsion of plague bacilli, to be used for the test, was prepared by suspending young cultures of virulent plague bacilli, grown at 30° C., in salt solution and filtering the suspension through filter paper. No antiseptic was added nor heat applied. Serial dilutions of unheated patient's serum were mixed with equal amounts of bacterial suspension in small test tubes. Incubation at 35° C. followed. Controls, consisting of serial dilutions of normal human serum as well as bacterial suspensions without serum, excluded any possible error which might have been caused by spontaneous sedimentation of the bacterial suspension; while a parallel test with highly agglutinant serum facilitated the reading of positive results.
Altogether, 22 tests were performed on 15 patients, 11 of whom were fatal cases and 4 of whom recovered. In the negative reactions, the duration of the disease at the time of examination ranges from two to six days. The non-fatal cases showed slight agglutination from the sixth day on. From that day, the agglutination titer of the serum was found to rise, and the agglutinins persisted in the blood of convalescents up to the seventh week of the disease.[11]
[11] It is hoped that it will be possible to examine some of the survivors for agglutination from time to time.
It must be borne in mind that the patients, who showed positive agglutination, had been vigorously treated with antiplague serum. Nevertheless, in consideration of the low titer of the curative serum (dilution 1:32, agglutination positive; dilution 1:64, agglutination negative), the rise of the agglutinant power of the patient's serum in dilutions higher than 1:16 cannot be explained as wholly due to passive immunity, but rather to active immunity arrived at on the principle of simultaneous immunization.
From the preceding observations the following conclusions are drawn:
- The importance of blood cultures as a diagnostic means is evident from the fact that positive blood culture was obtained in practically every case that was examined in the febrile stage of the disease, even when buboes or signs of pulmonary involvement had not been detected clinically.
- It is also evident that Bacillus pestis may be found in the circulating blood of the patients even in cases which subsequently recover.
- The period of time during which Bacillus pestis circulates in the blood is evidently short and irregular.
- Mixed infection may be encountered in plague septicæmia (Streptococcus, Pneumococcus).
- The agglutination test is of no value for the diagnosis of plague, as it was found positive only in convalescents.
- Phagocytosis of plague bacilli in the bubo was noticed only in patients who recovered after being vigorously treated with curative serum.
- The presence of numerous plague bacilli in comparatively insignificant skin lesions during the life of the patient points to the possibility of direct transmission, while the fact that a patient without any apparent bubo, who is not so sick as to be detained from his daily occupation, may expectorate large numbers of plague bacilli, are facts of great importance with regard to the communication of the disease. It is obvious that the last-mentioned condition might, and very likely does, give rise to an epidemic of pneumonic plague if the atmospheric and sanitary conditions are favorable.
[Table III.]—Insects Found to Contain Bacillus Pestis
| Author | Insect | Source of infection | Experimental transmission |
|---|---|---|---|
| Yersin | Flies | Laboratory infection | |
| Nuttal | Flies | Experimental infection | |
| Nuttal | Bedbugs | Experimental infection | Negative by bite. |
| Nuttal | Flea | Experimental infection | Negative. |
| Hankin | Ant's fæces | Fed on plague material | |
| Hankin | Bedbugs | Plague hospital | |
| Ogata | Flea | Plague rats | |
| Simond | Flea | Plague rats, experimental | Positive. |
| Tindswell, 1900 | Flea | Plague rats | Negative. |
| Tindswell, 1903 | Flea | Plague rats | Negative. |
| Kolle | Flea | Experimental infection | Negative. |
| Gauthier and Raybaud | Flea | Experimental infection | Positive. |
| Liston | Flea | Epidemic among pigs; harbored fleas; dead rats found | Positive. |
| Zirolia | Flea | Retained Bacillus pestis, 7–8 days | |
| British Commission | Flea | Repeated experiments | Positive. |
| Verbijtski | Flea and bedbug | Experimental infection | Positive. |
| La Bonadière and Xanthopulides | Fly | ||
| Herzog | Pediculus capitis | Dead body of a plague case |
II. Observations on the Transmission of Plague By Blood-sucking Insects
Judging from the data which have been collected from the literature[12] on the transmission of plague ([Table III]), Simond seems to have been the first to call attention to the important part which blood-sucking insects, particularly fleas, play in the transmission of plague. Although many investigators have been successful in demonstrating the presence of Bacillus pestis in the digestive system of blood-sucking insects, it was not until the experiments of Gauthier and Raybaud that the actual transmission of plague infection by fleas was convincingly proved. Ever since the exhaustive and conclusive experiments, which were carried out both under natural and artificial conditions by the British Plague Commission, and the work of Verbijtski, which antedates the British Commission, were presented, there has been no doubt that the transmission of plague by blood-sucking insects, particularly by the fleas, is one, although not the only, mode of spreading this disease. It is obvious, as Herzog correctly remarks, that the factors which are responsible for the spreading of plague must be considered individually in each epidemic and in various parts of the world as well. There is no doubt that the importance of any insect in the transmission of plague depends on its habits as well as on those of the host, be it either animal or man.