The Horace Dobell lectures on insect porters of bacterial infection : delivered before the Royal College of Physicians Lecture II: The transmission of plague by fleas / by C.J. Martin.

  • Martin, Charles J. (Charles James), Sir, 1866-1955.
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Credit: The Horace Dobell lectures on insect porters of bacterial infection : delivered before the Royal College of Physicians Lecture II: The transmission of plague by fleas / by C.J. Martin. Source: Wellcome Collection.

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    —_—_—_—_—<_$_$$_—_—_—— : Verjbitski (1904) carried out an extensive scrics of ex- eriments on_ the flea-transmission of plague in St. petersburg. He showed that th» plague bacilli could be recovered from the stomach contcm's of fleas six days after they liad fed on plague rats, and that the faeces of such fleas contained the B. pestis for about the same time. He succeeded in infecting rats with fleas from diseased rats fifteen times in seventy-six experiments. In cach experiment ten possibly infected fleas were allowed to tecd on the rat. Verjbitski also made experinents similar to those of Gauthicr and Raybaud described above, and out of forty experiments, using ten fleas apicce, infection was conveyed in four. Liston (1905) pointed out that the common flea infesting vats in India was not Ceratophyllus Fascratus or Typilo- psyllam usculi,as in Europe, but a non-pectinated flea identi- fied by Rothschild as Xenopsylla cheopis. Liston observed multiplication of the plague bacillus in the stomach of this fica. Although his experiments on transmission, which were made by allowing fleas to bite first an animal suffering from plague and subsequently a healthy animal, were not successful, he brought forward much interesting and valuable circumstantial evidence in favour of the view that plague is epidemiologically thus spread. He showed that X. cheopis takes readily to another host, for example, uinea-pigs and man, when rats are not available. “The possibility of the rat-flea carrying plague from one rat to another was demonstrated in a considerable series of experiments by the Commission for Investigation of Plague in India (Report, 1906, pp. 435-450). A glass case was used containing two wire cages side by side, each ~standing in a tin tray which collected the urine. Both trays were filled with sand in order to provide dry- ness ard shelter for the fleas. Each cage was furnished with a hd through which the rats were introduced and food and water given to them, and the whole apparatus was covered in with fine muslin to prevent the escape of the fleas. A rat placed in one of these wire cages could not come in contact with a rat in the other cage, nor with its urine or faeces. . The method of experiment was as follows: A plague- infected rat and a number of rat-fleas were placed in one of the wire cages. After the rat died, a fresh healthy rat was put into the other cage, the ecrpse of the infected rat being left in for twenty-four hours loncer. Sixty-six experiments with English white rats and with Bombay wild rats were done in this way, with the result that 30 healthy rats contracted plaguc; fleas formed the only apparent mcans of transmission of the bacilli from tat to rat. In order to exclude aérial infection a second series of experiments was carried out, in which fleas were taken from a rat which had died of plague and placcd on a fresh rat ina clean flea-proof cage of similar construction to that already described, but containing only one wire cage. Out of 38 experiments 21 successful transmissions were obtained in this way. The Importance of Flea-Transmission in Epizootics. Lhe following experiments, which had as their object the determination of the relative importance of the Indian tat-flea, X. cheopis, and of actual close contact in the tbsence of fleas in the dissemination of plague from imimal to animal, were carried out by the Commission (Reports. 1906, pp. 450-467; 1907, pp. 421-436; 1910, Pp. 515-335) in a series of small cabins, which were built especially for this purpose. In these animal houses plague-infected animals, rats or guinca-pigs, were kept in Close contact with healthy animals. They ran about together in a confined space and ate out of the same dishes. Lhe plague-infected animals were allowed to dic, and te Corpses were not removed until some time after death. Some instances the concentration of infection was very &reat, in one case 21 infected animals being at onc time in eputact with 25 healthy ones. The cabins were never Cleaned out, so that the animals ate food contaminated with the facces and urine of their infected companions. nthe experiments with rats, the healthy frequently ate oa farcasses of the infected ones introduced. Reni series of experiments a carried an ae Catena : ma A 70 animals. ; ah expor gra die eae were contro! experimen ioe fas were introduced. In all the control experiments a i Tut Nntrisn not one of the healthy animals contracted plague, whercas in those cases where fleas were present an epizootic occurred, varying in extent and rate of spread according to the number of fleas present. Some interesting experiments (Reports, 1906, p. 461) were also carricd out in the animal houses that lad been used for some of the previous experiments. Guinea-pigs and monkeys were introduced into them for one night. Some of the animals were in cages placed on thie floor, and others in cages covered by fine muslin, gauze, or ‘sur- rounded by a layer of sticky fly-paper, 6 in. wide—that is, the simplest precautions were taken to exclude fleas. Of 13 unprotected monkeys, 6 died; of protected, none. Of 24 unprotected guinea-pigs, 18 died; of protected, none. The animal houses remained infective three weeks after the last animal had died of plague. This period corre- sponds with the maximum time fleas fed on a plague rat have been shown to remain infective when placed on a series of healthy animals. ELerperiments in Plague Houses. The Commission subsequently (Reports, 1906, pp. 467- 482, and 1907, pp. 436-446) repeated similar experiments in plague houses in Bombay. The infectivity of 142 houses where cases had occurred or rats had died was tested by placing a guinea-pig there overnight. The yuinea-pig was subsequently segregated, and in 31 cases it died of plague. ; : Ninety-two experiments were made in which two similar animals—monkeys, guinea-pigs, or rats, the one protected by gauze or sticky paper, the other not pro- tected—were placed in suspected houses; 15 unprotected animals died; none of the protected. On nincty-six occasions fleas taken off guinea-pigs which had spent the night in a suspected house were transferred to a healthy animal. In 26 cases the second animal died of plague. although not infrequently: the guinea-pig from which they had been removed escaped infection. The conclusion that in a plague-infected house the infection is due to infected rat-fleas, and not to an infection of the soil or the air, seems to me abundantly justified. Fleas as the Agents of Transmission from Rat to Man. In considering how far the results just detailed can be applied to man, we enter at once upon less secure territory, because it is Impossibie to put conclusions to the test of experiment. The only measure of the correctness of such an interpretation is its adequacy to interpret all the known epidemiological facts concerning the spread of plague. The justice of applying the results of these animal experiments has been severely criticized in several quarters, and particularly by Galli-Valerio (1907), on the ground that rat-fleas do not bite man. It is true thiat, speaking generally, different animals harbour specific parasites; but the more we learn upon this subject thie clearer it becomes that such specificity is not so sharply defined as was imagined, and that a particular fica, although commonly confined to a few hosts of allied specics, may frequeutly be found in considerable numbers upon quite different animals. Morcover, it has become abundantly clear that a flea, although exhibiting a decided preference for one species, will, if hungry, betake himsclf to animals of widely different type in the absence of its proper host. The common flea found on rats in India and other warm parts of the world* is X. cheopis. This flea is a non- combed species, and not unlike the human flea, Puler arritans, In appearance. That X. eheopis readily fecds on man was observed by Tidswell (1903), Gauthier and taybaud (1903), and Liston (1905). The Commission for the Investigation of Plague in India kept X. cheopis alive for four wecks upon an exclusively human dict, and my colleague Mr. Bacot has bred them for years on the same regimen. | With regard to the readiness with which Ceratophyllus fasciatus, the common rat-flea in Europe, attacks man, considerable divergence of opinion has hitherto existed. According to Wagner (quoted by Tiraboschi, 1904, p. 180), Tiraboschi (1904, p. 266), and Galli-Valerio (1907), this of rat-fleas bas recently been dealt with by Chick and myself in the Journal of Hygiene, vol, ii, wv. 129 (1911), * The distribution Dr.
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