Leptospiral agglutinins in the Cayo Santiago macaques.

The colony of free-ranging rhesus monkeys (Macaca mulatta) on Cayo Santiago, Puerto Rico, was surveyed for leptospiral agglutinins. Only 5 (3%) of 169 monkeys (25% of the population) were seropositive (titers of ⩾ 1:100). An additional 29 animals (17%) had titers ⩽1:50. The proportions of seropositive males and females were 22% and 18%, respectively, and twice as many adult as immature animals (32% vs. 15%) were positive. In most seropositive monkeys a single serogroup predominated, Icterohaemorrhagiae being recorded in nearly 60% of these animals. In a follow-up survey conducted a year later, 4% of 158 of the original monkeys were seropositive, with titers of from 1:100 to 1:200. None of 22 Rattus rattus captured on Cayo Santiago had agglutinins to pathogenic serovars. Despite contact with rats and ingestion of stagnant water, the serological evidence, the excellent clinical condition, low mortality, and high reproductive rates of the Cayo Santiago macaques indicate that leptospirosis is not a health problem in this free ranging monkey colony.