The magnitude and duration of titres of leptospiral agglutinins in human sera.

Sixty-nine meat inspectors with titres of leptospiral agglutinins ranging from 1:768 to 1:24, were re-bled and reexamined periodically over a period of 53 months. Some individuals maintained titres of 1:384 and 1:192 for at least 30 months and others with initial titres of 1:48 and 1:24 maintained such titres for 53 months. Only six, of 63 initially seronegative (less than 1:24) meat inspectors, seroconverted during the period of study. The magnitudes of titres at a single sampling from each of 162 people, with previous histories of medically confirmed leptospirosis, were analysed in relation to time elapsed since initial diagnosis. Some individuals had titres of 1:192 seven years after infection, while others had lower titres after more than 20 years. Conversely, 12% of the population was seronegative two years after infection. These results indicate that it is not possible, from the results of a leptospiral agglutination test, to estimate retrospectively the time at which infection may have occurred. In a small proportion of individuals recently affected by leptospirosis, it will not be possible to demonstrate a change in agglutinating titre.