with the outbreak came to the conclusion that certain houses
were centres of infection, it was noticed that these houses were
most infective at night, and that they might actually be centres
of infection although uninhabited; indeed the infection seemed
to spread to houses between which and the infected house there
appeared to be no intercommunication of any kind. This
seemed to be inexplicable except on the assumption that the
infective agent, the Bacillus pest is, was, in some way or other,
carried by animals. It had already been noted that rats
disappeared from plague-stricken houses, many dying before the
appearance of the plague in the human population. Simond,
noting these conditions, suggested that the plague bacillus might
be transmitted by the flea from rat to rat and from rat to man.
Although he was not able to demonstrate this connexion he
indicated a line of research to other observers, who, as knowledge
accumulated, were able to complete each Unk in the chain
of infection. The plague bacillus having been found in the rat,
the next step was to demonstrate its presence in the flea, and
living plague germs were found in the stomachs of fleas inhabiting
plague-infected houses. Several species seem to be able to
transmit this germ, but in none of them does the plague baciUus
appear to undergo any special development —alternation of
generations or the like — as in the case of the protozoan of malaria
in its passage from and to the mosquito and the human subject—it
simply passes unchanged through the alimentary canal
of the flea, is excreted in the faeces, and is carried into the wound
made by the epipharynx-mandibles of the flea.
At least three species of animals, two rats and the human subject, and three species of fleas are involved in this chain. The rat fleas are Pulex cheopis found in India, and Ceratophyllus fasciatus, the rat flea of northern Europe, and Pulex irritans, the common flea, all of which have the power of transmitting the disease. In India of course the Pulex cheopis, usually solely associated with rats, seems to play the most prominent part. The two rats involved are the Mus decumanus, or brown rat, which is found in the sewers and develops the plague first, and the Mus rattus, the common black house-rat. From the sewer-rat the house-rat is infected, and from the house-rat man. Under ordinary conditions rat fleas do not attack the human subject, but, as the rats are attacked by plague and die, the infected fleas, starve out as it were, leave them and transfer their attentions to other animals and the human subject, infecting many of those they bite. Colonel Bannerman maintains that this infection takes place in the majority of cases, by this chain of transmission, and that there is no evidence that the excreta of these rats infect food or contaminate the soil. Colonel Lamb, summarizing the experimental evidence on this question, writes:-
“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 godowns (experimental huts) 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; (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; (4) an epizootic of plague may start without direct contact of healthy animal and infected animal; (5) the rat flea can convey plague from rat to rat; (6) infection can take place without any contact with contaminated soil; (7) aerial infection is excluded.”
The experiments lead to the conclusion that fleas and fleas alone, are the transmitting agents of infection. Bannerman gives in concise form similar evidence in relation to naturally infected native houses. Infection is carried from place to place by fleas, usually on the body or in the clothing of the human being. Such fleas, fed on infected blood, may remain alive for three weeks, and of this period, we are told, may remain infective for fifteen days. At the first opportunity these fleas forsake the human host and return to their natural host the rat. In most of the epidemics there is a definite sequence of events. First the brown rats are attacked, then the black rats, then the human subject, and Colonel Lamb suggests that after the rat disappears the Ilea starves for about three days and then attacks the human subject. Then comes the incubation period of plague, three days. Following this is the period of average duration of the disease, five or six days. This time-table, he says, corresponds to the period—when the epidemics are at their height—that intervenes between the maximum death-rate in rats and the maximum death-rate in man, about (en to fourteen days. This history of the connexion between the flea, the rat, and the human subject reads almost like a fairy tale, but it is now one of the well-authenticated and sober facts of modern medicine.
In India, where the notions of cleanliness are somewhat different from those recognized in Great Britain, most of the conditions favourable to the spread of the plague bacillus are of the most perfect character. This organism may pass into the sofl with faeces; it may there remain for some time, and then be taken into the body of one of the lower animals, or of man, and give rise to a fresh outbreak. Kitasato and Yersin were both able to prove that soil and dust from infected houses contain the baciUus, that such bacillus is capable of inducing an attack of plague in the lower animals, and that flies fed on the dejecta or other bacillus-containing material, die, and in turn contain bacilli which are capable of setting up infection. Hankin claims that ants may carry the plague to and from rats, and so to the human being. It has already been mentioned that the organism rapidly loses its virulence when cultivated outside the body; on the other hand, on being passed through a series of animals its virulence gradually increases. Thus may be explained the fact that in most outbreaks of plague there is an early period during which the death-rate is very low; after a time the percentage mortaUty is enormously increased, the virulence of the disease being very great and its course rapid. There seem to be notable differences in the degree of susceptibility of different races and different individuals, and those who have passed safely through an attack appear to have acquired a marked degree of immunity.
Two methods of treatment, both of which seem to have been attended with a certain degree of success, are now being tried. Haffkine, who was the first to produce a vaccine for the treatment of cholera, prepared a vaccine of a somewhat similar type for the treatment of plague. For this the Bacillus peslis is cultivated in flasks of bouillon; to this small drops or particles of ghee (Indian butter) are added; these form centres around which the organisms may develop. As the organisms multiply they grow down into the broth, but gradually becoming fewer in number as the floating mass on the surface is left, they fine down to a point and so come to resemble stalactites. These are broken off, from time to time, by shaking, others immediately beginning to form in their place. This may go on for six weeks. The flask with its contents is then well shaken and heated in a water bath to 70° C. for from one to three hours. On testing by culture the fluid should now be sterile, i.e. no bacilli should remain alive, and the fluid, ready for use, may be injected into the subcutaneous tissues of the arm in a dose of from 3 cc. for a man and 2 to 212 cc. for a woman, children receiving relatively small amounts. A rise of temperature, followed by malaise and headache, which pass off in about 24 hours, is soon noted, and some local swelling and redness appear at the seat of injection. The Indian Plague Commission were satisfied that the use of this vaccine diminishes the incidence of attacks of plague, and that, although it does not confer a complete immunity against the disease, the case mortality is lowered. They are of opinion also that protection is not conferred at once, but Lieut.-Colonel Bannerman states that the protection is immediate and lasts for six or even twelve months. In the official report (Annual Report of the Sanitarj Commissioner with the Government of India) for 1904 occurs the following: “That its value is great is certain, not only does it largely diminish the danger of plague being contracted, but, if it fails to prevent the attack, the probability of a fatal event is reduced by one-half.”
This method of treatment, however, is of no avail in the case of patients already attacked; for such cases Yersin’s serum treatment must be called in. Various other vaccines have been described, but all consist of some form of killed or attenuated bacilli, and the results attained do not var>' very greatly. Yersin, who first demonstrated the plague bacillus also devised the method of preparing an “antipest serum.” A horse was inoculated repeatedly, at intervals, and with gradually increasing doses of living plague bacilli. It was afterwards found that cultures sterilized by heat served equally
well for this inoculation of the horse and of course were much more