Leptospirosis is a zoonotic infection caused by pathogenic members of the genus Leptospira
. Human disease is usually acquired following environmental exposure to Leptospira
shed in the urine of an infected animal 
. Infection is acquired during occupational or recreational exposure to contaminated soil and water, organisms gaining entry to the accidental human host via abrasions or less commonly the conjunctiva 
. Disease may also be acquired through direct contact with infected animals, and occurs in farmers, veterinarians and abattoir workers 
. The disease has a worldwide distribution but is most common in tropical regions where incidence peaks during the rainy season 
. Clinical manifestations are broad ranging and follow a biphasic pattern in which a septicemic phase lasting around one week is followed by an immune phase during which antibodies are raised and organisms localize in tissues and appear in urine. Much disease is sub-clinical or mild, but patients reaching medical attention usually have an acute febrile illness associated with one or more of chills, headache, myalgia, conjunctival suffusion, and abdominal symptoms which can include nausea, vomiting and diarrhea 
. Leptospirosis has been described as anicteric or icteric; the former represents 85–90% of cases and is associated with a good prognosis, while the latter may be associated with multisystem disease involving particularly the kidneys, lung and heart, with a reported mortality rate of 5–15% 
Leptospirosis is an emerging infectious disease in Thailand 
. Before 1996, the number of cases reported to the Department of Disease Control (DDC) was approximately 200 per year. Leptospirosis was sporadic and reported mainly from central and southern regions. A marked change occurred in the subsequent decade, with a year-on-year rise from 398 cases in 1996 to a peak of 14,285 cases in 2000. This was followed by a continuous decline with 2,868 cases reported during 2005 
. Reporting in Thailand is voluntary and probably represents a small proportion of true cases. There was also a shift in the geographical distribution, with the majority of cases being reported in the northeast. One explanation for the outbreak is that it was related to the emergence of a biologically successful clone of Leptospira
. This possibility is supported by a study of 44 leptospiral strains obtained from humans during three outbreaks in Brazilian urban centers, in which typing using arbitrarily primed PCR demonstrated that 43 isolates exhibited very similar fingerprints suggestive of a clonal population of L. interrogans
. In addition, during a large urban outbreak in Brazil, L. interrogans
serovar Copenhageni was isolated from 87% of cases with positive blood cultures 
. Although it is currently unclear to what extent genetic relatedness can be informed by serotype alone, this observation is consistent with the majority of cases being caused by the expansion of a single outbreak clone.
The aim of this study was to define the molecular epidemiology of Leptospira strains isolated from humans during the Thai outbreak, and to relate this to the maintenance animal host. To achieve this, an MLST scheme was developed for L. interrogans, the major cause of human disease. This approach has the advantage over existing typing schemes in that the data generated are amenable to detailed evolutionary analysis. MLST data are also readily comparable via the internet, and establishment of an MLST scheme therefore paves the way for future studies. Our results confirm the emergence of a dominant clone of L. interrogans serovar Autumnalis; this was the major cause of human disease, and was found in a maintenance host which was defined as the bandicoot rat.