V. Evaluation of Cross-Immunity against Type 1 Dengue Fever in Human Subjects Convalescent from Subclinical Natural Japanese Encephalitis Virus Infection and Vaccinated with 17D Strain Yellow Fever Vaccine
Weekly collections of fleas from live trapped rodents were made for 1 year in a defined area of commingling wild and domestic rodents. Eleven species of fleas were taken from nine species of mammals; in order of predominance the commonest were: fleas, Malaraeus telchinus, Catallagia wymani, Hystrichopsylla linsdalei, Atyphloceras multidentatus multidentatus, Opisodasys keeni nesiotus, and Nosopsyllus fasciatus; and rodents, Microtus californicus, Peromyscus maniculatus, Reithrodontomys megalotis, Rattus norvegicus and Mus musculus.
Seasonal fluctuations of the indices of fleas were positively correlated with the abundance of their usual or preferred hosts. In addition, Catallagia wymani was most abundant during the wet season. Malaraeus telchinus, the most abundant flea, was most numerous during dry weather. Nosopsyllus fasciatus was spatially limited by the presence of its host.
Increased numbers of wild rodent fleas were found on rats when they invaded an area occupied by wild rodents. These rats originated from a hog farm, and it is suggested that these fleas were acquired from the invaded territory. If actual exchange of fleas between hosts took place in the area, the data indicated that transfer occurred principally from wild rodents to rats, and the fleas seemingly transferred more frequently to Rattus than to Peromyscus or Reithrodontomys. The likelihood and the rate of exchange of a given flea species was probably influenced by its host preference and available hosts. Two factors in host densities were noted to create a response to flea occurrence: (1) the relative abundance of the preferred host and (2) the presence and abundance of other hosts.
The data suggest that the magnitude of a flea population is a function of the relative densities of hosts present. However, the host preference of a given flea or its tolerance to environmental factors (as wet or dry seasons) may impose a limit on the population.
San Francisco Field Station, Technology Branch, Communicable Disease Center, Public Health Service, U. S. Department of Health, Education, and Welfare, San Francisco, California.