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Rat Ectoparasite Surveys in Puerto Rico with Toxicological Studies on Xenopsylla Cheopis^*

Rat ectoparasite surveys made in San Juan, Puerto Rico, from 1960 to 1964 showed no increase in Xenopsylla cheopis in one section of the city, Santuree, where each year less than 5 percent of the rats were infested, whereas in another section, La Perla, there was a marked increase, the infestation rate reaching 44 percent. Laboratory colonies of a La Perla strain and of a San Francisco strain were established and tested for tolerance of DDT and dieldrin with the standard WHO technique for adults and with original techniques for larvae and pupae.

Adults of the La Perla strain were more resistant to DDT than were those of the San Francisco strain (LC₅₀, 1.27% compared with LC₅₀, 0.82%), and were also more resistant to dieldrin (LC₅₀, 0.34% compared with LC₅₀, 0.12%). As larvae and adults were found to have different toxicological characteristics, these had to be tested separately. Emergence of adults from cocoons was not suppressed by concentrations of DDT or dieldrin sufficient to kill adults and larvae. Laboratory colonies started from a single pair produced thousands of fleas in 3 months' time, indicating that a control program which fails to kill all stages of the flea may be ineffective. The flea population is made up chiefly of pupae in cocoons and larvae; adult fleas are less numerous, most being found off the rat in the breeding medium.

^* This investigation was supported by Research Grants AI-02887 and AI-03821, and Graduate Research Training Grant 5-T1 AI 15 from the National Institute of Allergy and Infectious Diseases, National Institutes of Health, U. S. Public Health Service.