In previous papers it has been demonstrated that ground-dwelling rats are the principal reservoir of leptospirosis in Malaya. The present paper considers the distribution of infection by sex and weight in the ten principal rat species. There appears to be a general tendency for females to be infected more frequently than males, but significant differences were demonstrated only in R. sabanus (more than three times as many females as males infected) and R. whiteheadi. In Malaya, where seasonal changes are minimal, weights can be used as a good index of age in rats. In rat species with a low incidence of infection the incidence appeared to rise steadily with age. In species with a medium incidence the infection rate rose at first with age, fell in the 6-8-month age-group, and then rose again. In high-incidence species the rate rose rapidly from the second month.There appear to be three types of enzootic infection; (1) intensive transmission of a single serogroup in a crowded population of rats of a single species (transmission probably being through urinary contamination of damp soil); (2) low-intensity transmission of several serogroups among ground-rats frequenting wet places (probably with urinary transmission); and (3) low-intensity transmission of several serogroups among ground-rats in dry places (the transmission may be venereal).
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