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

V. The Distribution of Naturally Infected Fleas During an Epizootic in Relation to their Infection Rates

Observations were made for 6 weeks on the distribution of infected fleas during a plague epizootic among wild rodents. The area harboring the greatest concentration of infected fleas was about 120,000 square feet of coastal brush vegetative cover on a loam-type soil. Infected fleas were also found in a contiguous area to the west and northwest of the main epizootic focus. The much lower activity of the infection in this adjacent area suggested that it had been initiated by spread from the epizootic focus.

The evidence from the distribution of infected fleas and their infection rates suggests that the plague epizootic among the wild rodents was strongly localized. No instance of spread of the infection at a distance was noted. The extent of the area in which infected fleas were found contracted steadily from a maximum of over 62,000 square feet during the 1st week of observations to about 5,000 square feet in the 6th week. This shrinkage was accompanied by a decline in total trap sites and in the number of new trap sites yielding infected fleas each week, and by a concomitant decline in flea infection rates.

The presence of commensal rats in a plot contiguous to the epizootic focus, and the finding of a Rattus norvegicus dead from plague, suggested that the avenue of possible major spread was into the rat population. This eventuality appeared to have been precluded by a prompt rat control campaign by local health authorities.

^* 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.