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

IV. The Fluctuation and Intensity of Natural Infection with Pasteurella pestis in Fleas During an Epizootic

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Observations were made of the intensity of Pasteurella pestis infection in fleas collected during an epizootic of wild rodent plague. The number of plague bacilli in infected fleas had a wide range of from a few to millions. The degree of infection in the fleas during 6 weeks of the epizotic showed that whereas fleas with high and low numbers of bacilli were present during the first 3 weeks, those found infected during the last 3 weeks had massive infections exclusively. Thus in certain species, like Malaraeus telchinum and Hystrichopsylla linsdalei, intense infections during a period of epizootic decline may represent a reservoir mechanism.

There was a generally poor correlation between infection rates of individual fleas and their fecal droppings. Likewise, visual criteria in regard to the presence of “typical” plague masses in the intestinal tract of fleas were but poorly borne out by subsequent culture of the same individuals. These findings confirmed former experimental results.

Observations of the fluctuation of P. pestis infection in fleas suggested that the epizootic lasted for approximately 8 weeks with vigorous activity during the middle 4 weeks. The peak period occurred during the third week, March 26, 1954, and the infection rate declined to low levels in the next 3 weeks and finally terminated on about April 30. It appeared probable that a limiting factor in the epizootic was the availability of rodents susceptible to plague.

Author Notes

U. S. Department of Health, Education, and Welfare, Public Health Service, Bureau of State Services, Communicable Disease Center, San Francisco Field Station, San Francisco, California.