Since the large explosive outbreak of an-icteric leptospirosis occurred in Wenjiang District of Sichuan Province in 1958, extensive epidemiological surveys and intensive researches relating to the disease have been pursued with nation-wide interest.
With the exception of the north-western part, it is now known that the disease is wide-spread over the whole country, especially very often seen in the areas along the Yangtze River Valley and southern provinces where rice is grown.
The distribution of the disease over China can be broadly divided into 3 categories according the main clinical features and mode of spread. The first category covers the areas where the disease appears as simple flu-like syndrome without jaundice. This an-icteric form is often caused by inundation either due to flood or after a heavy rainfall, and hence the disease may be scattered all over the low-lying lands or plains of the country. The infecting organism is mostly of the pomona serogroup of leptospires and the reservoir host can usually be traced to pigs or occasionally to dogs. The infections in Yunnan may also be included in this category as the rice field fever seen in this province is clinically very similar to the simple flu-like form of the disease although differs widely in respect of the infecting serovars and reservoir hosts. The second category includes the areas where the clinical manifestations, aside from the usual features typical for the rice field fever seen in Yunnan, are marked by presence of pronounced pulmonary haemorrhage. Such infections are mainly distributed over the rice-growing areas along Yangtze River Valley. The principal reservoir host is Apodemus agrarius and a new serovar lai of the icterohaemorrhagiae serogroup accounts for most of the infections. The third category consists of the areas where both icteric and an-icteric forms co-exist or, at some places, the icteric form prevails while at other places, the anicteric form outweighs the icteric form. Four provinces to the south of the estuary of Yangtze River and along the coast of East China Sea and South China Sea belong to this category. The reservoir hosts vary in different localities and the infections are broadly distributed among a variety of serogroups.
The situation in Sichuan and Yunnan is of particular interest as the infections in these two provinces each has its own special and distinguishing features. The disease in Sichuan differs not only clinically from the usual rice field fever seen elsewhere in China and has a wider area distribution but also the main serovar responsible for the infections is Leptospira lai of the icterohaemorrhagiae serogroup, while in Yunnan, the infection is noted for its multiplicity of the serovars existing there. As a matter of fact, Yunnan is an ideal place for studying the ecology of leptospires and also it is a very valuable 'repositorium' for the micro-organisms of this genus.