The three species of Rattus (R. norvegicus, R. rattus and R. exulans) from two geographically distinct areas of the State of Hawaii have been examined to determine their comparative degree of resistance to plague. Rodents from the enzootic Hamakua District were compared to like numbers from “plague-free” Oahu, in tests with a virulent strain of Pasteurella pestis originally isolated in Hawaii.
After a small sample of 60 rodents was used to establish baselines for the main trials, two samples of 180 rats each (30 rats of each species from each island) were tested. Results of these procedures indicated that the R. norvegicus and R. exulans from the Hamakua District were significantly more resistant to plague than were their Oahu counterparts. No such difference was seen in the R. rattus samples. The observed differences were apparently due to geographical factors associated with the presence or absence of enzootic plague, as the resistances could not be correlated to the sex or age of the animals or to the season of collection.
San Francisco Field Station, Communicable Disease Center, Public Health Service, Department of Health, Education, and Welfare, San Francisco, California 94118.
Department of Health, State of Hawaii, Honolulu, Hawaii.