Already in the early years of leptospiral research it became clear that different degrees of serological relationship among serovars could be recognized. Borg-Peterson (1944) was apparently the first to introduce the serogroup concept into the classification system in his study of strains related to serovar hebdomadis. During the following years this concept became accepted because it was useful to group together serologically related serovars. This approach had already been taken into account in the first classification system of Wolf and Broom (1954), who devided the 46 known serovars into 20 serogroups.
The term serogroup has not been exactly defined and it has no official taxonomic status. However, for partical purposes it remains a necessary component of the present classification system. In addition, more detailed antigenic studies of a number of the recognized serogroups have shown the possibility of distinguishing well defined sub-serogroups (subroups) within some serogroups by factor analysis (Kmety, 1967; Dikken and Kmety, 1978).
In principle, each serogroup is named after the earliest described serovar within the serogroup. Serogroup, as well as the serovars they contain, are listed in this publication in chronological order. This arrangement reflects the historical aspects of the development of leptospiral research and the justified priorities according to the IC.
Serogroups. The growin number of serovars within certain serogroups became so large that the homogeneity of some groups decreased considerably, reducing their partical impact. This required in some cases a rearrangement into smaller groups with a higher degree of homogeneity. For instance, the original Hebdomadis serogroup contained more than 30 serovars of which some showed very little serological relationship to one another. It was therefore decided to divide the group into three separate serogroups according to their serological affinities viz.: Hebdomadis, Sejroe and Mini. This decision was made at the TSC meeting in Boston in 1982 (Minutes, 1984). Concurrently the Autumalis group was devided into three separate serogroups known as Autumnalis, Djasiman and Lousiana. A third change agreed to at the Manchester meeting in 1986 (Minutes, 1987) concerned the reestablishment of the Sarmin group. The serovar sarmin had been provisionally included in the Icterohaemorrhagiae serogroup in spite of its low grade affinity with that group. After some new serovars were described that were shown to be closely related to sarmin, the reintroduction of a Sarmin serogroup appeared to be justified.
The new list was further extended by another two more serogroups, Ranarum and Manhao. Recently, the introduction of a new serogroup named Kenya has been proposed (Korver and Terpstra, 1990, personal communication) as the serovars kenya, peru and vargonicas show serological relatidness. This suggestion has not yet been discussed by the TSC and therefore this serogroup is not included in the attached list of serovars.
In spite of these rearrangements there still remain, within serogroups, clusters of serovars that are more closely related to one other than to the others. This was supported by the results of studies on the antigenic structure of stains by factor analysis and by the use of monoclonal antibodies. It was suggested therefore that such serogroups should be devided into subgroups (Dikken, Kmety, 1978). However, the decision to do so has been postponed until further information on the subject becomes available.
It also has been found that whereas some serogroups are evidently related antigenically to one other, others show no such relationship. For instance, while members of the Hebdomadis, Sejroe and Mini groups display a clear integroup relationship and a similar serological relationship exists between group Bataviae, Tarrossovi and Shermani, no cross-agglutination occurs between members of many other serogroups. Another well know close relationship observed in the past is that shown by the members of the Icterohaemorrhagie and Canicola serogroups.
This integroup relatedness suggests a possible evolutionary origin.
Serovars. As serovars are considered as infrasubspecific taxa, they are not covered by the IC. For practical purposes however, since serovar is the basic taxon in Leptospira classification, the principles and rules applied by the IC to species, have been observed.
Although a few individual serovars have been described in the past as being the sole member of a serogroup, it was agreed by the TSC meeting in Manschester in 1986 (Minutes, 1987) that such individual serovars should be placed provisionally in the serogroup to which they show the closest serological affinity until other closely related serovars have been described, and found to be fit to form a new serogroup with an existing individual serovar.
See more about chronological list of serogroups.