Ecological Studies of Wild Rodent Plague in the San Francisco Bay Area of California

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Abstract
Summary Studies with 1,297 wild rodent fleas, cultured individually and inoculated as 319 pools into animals, showed at least 99 to contain Pasteurella pestis. The efficiency ranges of the two methods were 68 to 87 per cent for culture, and 59 to 98 per cent for animal inoculation but the averages were 79 and 89, respectively. The efficiency of the culturing method appeared to be the same for various concentrations of P. pestis in the flea. Success of the animal inoculation, however, varied with the different densities of plague bacilli; it was about 60 per cent when the number of bacteria per flea was low, and over 90 per cent when the number was very high. Both methods are simple in procedure and equipment, and both are highly sensitive in detecting the presence of plague bacilli in fleas, such as Malaraeus telchinum and Hystrichopsylla linsdalei. The culture method is rapid, it does not require animal facilities, and it allows the processing of large numbers of fleas individually. The animal inoculation of flea pools has few interfering factors, it has a higher degree of efficiency than the culturing method, and it allows the pooling of large numbers of fleas for few injections. Because P. pestis occurs in fleas in nature so rarely, the method of choice for surveillance remains that of animal inoculation. On the other hand, for critical study of epizootic plague among wild rodents and their fleas, individual culturing of fleas provides invaluable epizootiological data.