[ 371 ] SYLVATIC PLAGUE STUDIES THE VECTOR EFFICIENCY OF NINE SPECIES OF FLEAS COMPARED WITH XENOPSYLLA CHEOPIS BY ALBERT LAWRENCE BURROUGHS*, M.S., PH.D. From the George Williams Hooper Foundation, University of California, San Francisco, California (With Plates 4 and 5) CONTENTS PAOE PAGE Introduction .371 Discussion ....... 388 Historical 372 Mass transmissions ..... 388 Individual transmissions .... 389 Materials and methods 376 Estimation of vector efficiency 390 Selection of fleas 376 Method of determining if the variation in the Source of fleas for study .... 376 vector efficiencies obtained in two experi- Rearing the fleas ...... 376 ments is real or due to chance . 390 Maintenance of pure cultures of fleas . 377 Feeding infected fleas 377 Xenopsylla cheopis 390 Infecting the fleas 378 Diamanus montanus . .391 Echidnophaga gallinacea . 379 Malaraeus telchinum ..... 392 Experimental 379 Orchopeas sexdentatus sexdentatus . 392 Nosopsyllus fasciatus ..... 393 Xenopsylla cheopis . .379 Opisodasys nesiotus 393 Determination of the number of organisms Echidnophaga gallinacea .... 393 present in the regurgitant of a blocked flea. 381 Oropsylla idahoensis . .393 Mass transmissions with Xenopsylla cheopis . 383 Pulex irritans ...... 393 Diamanus montanus . .383 Megabothris abantis ..... 393 Nosopsyllus fasciatus . .383 Malaraeus telchinum ..... 384 Orchopeas sexdentatus sexdentatus . 385 Conclusions ....... 394 Opisodasys nesiotus . .385 Echidnophaga gallinacea . 386 Summary ........ 394 Oropsylla idahoensis 386 Pulex irritans 387 Megabothris abanlis ..... 387 References........ 395 INTRODUCTION Russian workers, as well as American, having The discovery of the existence of a large wild-rodent become aware of the widespread existence of reservoir of plague in the western United States sylvatic plague in the steppes, valleys, foothills and during the last forty years stimulated interest in the mountains encompassing thousands upon thousands study of the vectors infecting these rodent popula- of square miles, undertook a study of the vector tions. Since the time that vector studies were first ability of the flea fauna of various rodents known inaugurated, numerous investigators have proved to be subject to periodic plague epizootics. These that fleas are the only important plague vectors— investigations, correlated with ecological research, that although other arthropods may be capable of have helped to explain the seasonal rise and fall of becoming infected, their role in the transmission of epizootics, the carry-over of plague from one season the disease is incidental. And, further, it has been to another, and have disclosed the animal popula- repeatedly illustrated that while some species of tions most susceptible to plague and the species of fleas are particularly efficient vectors, others possess wild rodent-infesting fleas most hazardous to man. few or none of the characteristics of the good vector. The tendency during the past two decades has The greatest amount of work has been done with been to make comparative studies of different fleas of the genus Xenopsylla, particularly with the species of wild-rodent fleas in relation to their species cheopis. ability to transmit plague. Similar work has been * Now with the Division of Entomology and Economic Zoology, University of Minnesota, St Paul, Minnesota. Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.8, on 28 Sep 2021 at 23:41:12, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0022172400014042 372 Sylvatic plague studies done with rat fleas. Several species of fleas native rats free from parasites were unable to transmit the to North America are now known to possess varying disease to healthy rats kept in the same cage. abilities as vectors. However, if fleas were present, transmission from American •workers have recently presented a sick to healthy rats usually did occur. Simond method which enables one to obtain the vector thought that flea faeces, which he had found to efficiency of a given species (with regard to bio- contain plague organisms, might be a means of logical transmission in contrast to mechanical or percutaneous infection. His early observations faecal transmission) and compare it with the vector even extended to a possible explanation of the efficiency of other species. carry-over of plague from one year to another, for A definite aim has been made in the present he found that fleas could remain infected for a studies to determine the practicability of using this considerable period. method for a comparative study of the different Simond's excellent work was repeatedly confirmed species of fleas which occur in western North in the next few years by various investigators, America. The primary purpose of the studies is an although his theories did not fail to arouse criticism. attempt to cast further light on the epizootiology Thompson (1903) supported Simond, as did Gauthier of sylvatic plague. It is hoped that this purpose & Raybaud (1902—3), who obtained several positive has been realized. results in mass-transmission experiments with rats and rat fleas. HISTORICAL To the problems already presented by the The etiological agent of plague was discovered existence of plague in rats were added questions during an outbreak of the disease at Hongkong at brought about by the discovery of a large wild- the close of the nineteenth century by Yersin (1894), rodent reservoir of plague in Asia, North America, a French scientist, who gave a good description of South America and Africa. Numerous species of the organism. Kitasato (1894), a Japanese worker, wild rodents were known to be involved, and each also announced the discovery of an organism in species was frequently found to harbour one or plague cases, but presented an erroneous descrip- more species of fleas. The ability of the various tion. According to Lagrange (1926), Kitasato is to species of fleas to serve as vectors of plague pre- be honoured for having publicly stated that Yersin sented an important problem. Clemow (1900) alone was the discoverer of the plague bacillus. reported some of the earliest isolations of rodent Roux (1897) first expressed the view that plague plague in animals other than domestic rats. He was primarily a disease of rats. About the same stated that bacteriological proof existed of the time, Ogata (1897) concluded, on epidemiological occurrence of spontaneous plague in monkeys, grounds, that the rat is the principal host and the bandicoots, mice and grey-striped squirrels. The flea the means of dissemination of the causal following corrected list of animals found naturally organism in the rat population. He succeeded in infected with plague was taken from a table producing plague in laboratory animals by inocu- presented by Wu & Pollitzer (1932): lating them with triturated fleas taken from dead Tarabagan (Arctomys [Marmota] sibirica),* Trans- rats. Ogata was suspicious of fleas, as they were baikalia and Mongolia, 1895. frequently the most numerous ectoparasites of rats, Squirrel (Sdurus [Funamhulus] palmarum), and were known to leave dead rats in search of a Ceylon, India, 1898. blood meal. Mole rate (Gunomys [Bandicota] bengalensis), Simond (1898), after having made successful Ceylon, India, 1906. mass-transmission experiments, reported the trans- Striped mouse (Rhabdomyspumilio), South Africa, emission of plague from rat to rat by fleas. He showed 1906. that the cannibalistic and necrophilic habits of rats Hamster rat (Cricetomys gambianus), South rarely resulted in their infection, and that enormous Africa, 1906. numbers of bacteria had to be ingested before such California ground squirrel (Citellus beecheyi), infections could occur. He concluded that the California, 1908. pathology then differed from that of animals Dusky-footed wood rat (Neotomu fuscipes), infected by the cutaneous route. California, 1910. Simond, furthermore, attached significance to the Tree rat (Epimys [Rattus] dolichurus),\ East local reaction which appears in about 5 % of human Africa, 1913. plague cases. This local reaction manifests itself as Small suslik (Citellus pygmaeus), South-east a blister-like eruption and contains many plague Russia, 1913. bacilli. He logically concluded that fleas, which * Wu & Pollitzer listed this as Arctomys bobac. This frequently attack the lower extremities where the species occurs in Poland, while sibirica occurs in reaction was found, were probably responsible. Transbaikalia. Experimentally, Simond found that plague-infected f No reference to this species could be found. Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.8, on 28 Sep 2021 at 23:41:12, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0022172400014042 ALBERT LAWRENCE BURROUGHS 373 Large suslik (Citellus fulvus), South-east Russia, known to have resulted from contact with wild 1917. animals, usually rodents. De la Barrera (1939) Multimammate mouse (Rattus concha), South reported an outbreak of plague in rural Argentina Africa, 1921. in which the ' cuis' or fence rabbit was incriminated The world-wide sylvatic reservoir of plague as the rodent reservoir. Meyer (1943) states that proved to be of epidemiological significance. Wu domestic rodents are the animals primarily respon- (1934) described an outbreak first reported in 1907 sible for bringing the disease into contact with in south-east Russian Turkestan.