From March 23 to April 30, 1954, a total of 2,202 fleas, composed of 9 species, was collected during a sylvatic plague episootic among field mice in Northern San Mateo County, California. Of the total number of fleas obtained, the following are the species with their percentage occurrence: Malaraeus telchinum, 70.7; Hystrichopsylla linsdalei, 11.2; Catallagia wymani, 9; Atyphloceras multidentatus, 6.6; Opisodasys keeni nesiotus, 2.1; and 4 other species of minor importance, less than 0.4.
The rates of infection with Pasteurella pestis were determined by bacteriologic cultivation of individuals of all the flea species found. Of the total fleas collected, 111 or 5% harbored plague organisms. H. linsdalei had the highest and the most consistent infection rate throughout the period, with M. telchinum, A. multidentatus, O. k. nesiotus, and C. wymani following in descending order. Other species of fleas were negative for P. pestis infection. Of the total number of fleas found infected, M. telchinum constituted 68%, H. linsdalei made up 25%, and the other 3 species together made up about 7%. All 5 of the above species were found naturally infected with P. pestis for the first time. The 2 main flea vectors of plague during the epizootic appeared to be H. linsdalei and M. telchinum.
Of the 5 major flea species examined, the predominant number of individuals were females. Separate data on infection rates in each of the sexes revealed little difference. A statistical test comparing the mean ratios of sexes of infected and uninfected M. telchinum and H. linsdalei showed no significant differences. Thus the sex of fleas seemed to have no relation to their natural infection rates with P. pestis. On the other hand, the predominance of females in the flea populations sampled suggests that, in this instance, the males may have played a more subordinate role in actual plague transmission to the rodents than the females.
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.