Typing leptospira from the perspective of a reference laboratory.

Leptospirosis is caused by different leptospiral variants. Analysis by cross agglutination absorption tests (CAAT) led to the definition of entities called serovars to distinguish between leptospires on sub-species level, and to the designation of reference strains representing serovars. For decades CAAT has been used to classify leptospires and now approximately 200 serovars have been recognized. In the last few years, it has become increasingly more clear that the serovar concept is no longer fully satisfactory as it may fail to adequately define epidemiologically important entities. In addition, CAAT is too cumbersome and time-consuming for routine typing. Various methods have been developed based on antigenic or genetic analysis with the purpose to supplement or to replace the CAAT. Most of these methods are still in an experimental state. It is to be expected that a typing method based on genomic analysis will eventually become most important. Such a new method should have considerable advantages in order to be acceptable for the development of a new classification system replacing the system based on serovars, which is widely accepted and in many respects still satisfactory. From the new methods, analysis of leptospiral DNA fragment length after digestion with restriction enzymes (REA) has been widely used and proven to be useful for typing. Pending the development of new typing methods that have clear advantages and may lead to a new classification system, it is suggested that the classification system based on serovars is maintained and that REA is added to each description of a new serovar.