we reviewed leptospirosis case investigation reports by hawaii department of health ( hdoh ) investigators submitted during 19992008 . these reports were ( and still are ) generated for all reported leptospirosis cases in the state . a standardized case investigation form was used , which includes demographic , epidemiologic , clinical , and laboratory information obtained from patient interviews , medical record reviews , and laboratory reports . research for this study was approved by the hdoh institutional review board . for exposure source to be assessed , incubation periods estimated , and exposures classified , patients were asked about high - risk activities that occurred during the 21 days before symptom onset . these included exposure to animals , mud , or potentially contaminated freshwater sources involving occupational activities ( e.g. , farming , ranching ) , recreational activities ( e.g. , freshwater swimming , hiking ) , or habitational activities ( around the home ; e.g. , gardening , trapping rats ) . if exposure was continuous or if persons had been exposed multiple times , the incubation period was considered indeterminate . ascertainment of exposure classification involved placing cases into 3 mutually exclusive exposure categories : occupational , recreational , or habitational . if exposure activities involved > 1 category , the exposure classification was considered indeterminate . outbreaks were defined as > 2 epidemiologically linked cases . a patient with a confirmed case had a clinically compatible illness plus a > 4-fold increase in microscopic agglutination test ( mat ) titer between acute- and convalescent - phase serum specimens or isolation of leptospira spp . from a clinical specimen ( 6,7 ) . all other cases were classified as either probable ( clinically compatible illness with mat titer > 200 in > 1 serum specimens without a 4-fold increase in titer ) or suspected ( clinically compatible illness with less supportive laboratory evidence of infection [ e.g. , mat titer < 200 , positive macroscopic slide agglutination test result , reactive immunoglobulin ( ig ) m elisa , or positive indirect hemagglutination assay results ] ) . only laboratory - confirmed cases in patients whose disease was contracted through exposure within the state of hawaii were included in this analysis . mats were conducted by cdc from january 1999 through november 2004 , and by hdoh from december 2004 through december 2008 . all isolates were sent to cdc for definitive serogroup identification . to determine the presumptive infecting serogroup for serologically confirmed cases , if > 1 serogroup had the same high titer , the identification was labeled indeterminate . to calculate mean annual incidence rates ( overall and by age , sex , and hawaii island on which patient was exposed ) , the numerator was the number of cases for the specified groups over the 10-year observation period divided by 10 . the denominator was the overall or relevant group - specific population estimate from the 2000 us census ( 9 ) . data from our earlier 25-year study period , 19741998 , were used for trend analyses ( 5 ) . we calculated frequencies , tests for trends , and tests for difference using epi info version 3.3.2 ( cdc , atlanta , ga , usa ) ; p values < 0.05 were considered significant . hdoh received 356 leptospirosis case reports ; 345 were related to exposures within the state of hawaii . the 11 cases from exposures occurring out of state included 2 from guam ; 2 from thailand ; and 1 each from panama , the federated states of micronesia , borneo , okinawa , malaysia , singapore , and texas . of the case reports related to in - state exposures , 198 ( 57% ) were laboratory confirmed , 116 ( 34% ) were probable , and 31 ( 9% ) were suspected . the number of confirmed cases reported per year ranged from 11 to 27 ( median 20 ) , and the estimated mean annual incidence rate was 1.63 per 100,000 population . mean monthly reported cases were highest from october through february ( figure 1 ) . the observed seasonal disease occurrence for the recent 10-year study period was significantly different from that of the previously reported 25-year study period ; summer cases predominated in the latter ( p<0.01 ) ( 5 ) . month of onset for 198 laboratory - confirmed leptospirosis cases , hawaii , usa , 19992008 . case - patients were predominately male ( 91% ) , and ages ranged from 3 to 76 years ( median 38 years ) . the highest age - specific rate was among persons 2029 years of age , and the lowest was among children 09 years of age . most cases and the highest incidence rates were related to exposures on the islands of kauai and hawaii ( table 1 ) . in addition , cases were most consistently reported from the northeast , windward sides of the islands : hanalei ( n = 8) and wailua ( n = 12 ) on kauai , waipio valley ( n = 12 ) and hilo ( n = 17 ) on hawaii , and maunawili falls ( n = 13 ) on oahu ( figure 2 ) . case - patients observed over 10 years for the specified category divided by 10 divided by specified subgroup population estimate from 2000 us census data ( 9 ) . exposure locations associated with the greatest number of leptospirosis cases , hawaii , usa , 19992008 . we were able to determine exposure classifications for 177 ( 89% ) of the 198 confirmed cases . recreational exposures accounted for 79 ( 45% ) and were mostly related to freshwater swimming , hiking , and camping . occupational exposures accounted for 78 ( 44% ) , mostly relating to farming , specifically , taro farming . exposures around the home accounted for 20 ( 11% ) , most commonly , gardening . after categorizing cases into 5-year intervals and comparing the results with reports from 1989 through 1998 ( 5 ) , we found that recreational exposures remained relatively stable over the past 20 years ( 19892008 ) , while occupational exposures actually increased , but the difference was not significant ( p = 0.08 ) ( figure 3 ) . after stratification by island , a significant increase in occupational exposures was shown for the island of hawaii ( p = 0.04 ) . trends in exposure classification for laboratory - confirmed leptospirosis cases , hawaii , usa , 19892008 . most cases occurred sporadically . one outbreak ( > 2 epidemiologically linked cases ) , which involved 2 landscapers , occurred on kauai in 1999 ; both cases were laboratory confirmed . another outbreak ( 2 epidemiologically linked cases : 1 confirmed , 1 probable ) was associated with flooding of the university of hawaii campus on october 31 , 2004 , when heavy rains caused an adjacent stream to overflow its banks ( 10 ) . for case - patients with known exposure dates , the median incubation period was 9 days ( range 121 days ) . a total of 118 ( 73% ) of 161 case - patients , for whom treatment information was available , were hospitalized . the most frequent signs and symptoms among patients who sought treatment were fever , myalgias , headache , nausea , and vomiting . abnormal urinalysis results were common ; specimens from 78 ( 73% ) of 107 and 71 ( 68% ) of 105 case - patients showed hematuria and proteinuria , respectively . results of liver function tests were frequently abnormal as well ; laboratory results for 109 ( 74% ) of 147 case - patients showed elevated alanine aminotransferase levels ( > 40 u / l ) , and 85 ( 63% ) of 134 showed elevated total bilirubin levels ( > 1 mg / dl ) . the most common hematologic anomaly was thrombocytopenia ( < 140  10/l ) , which was observed for 97 ( 66% ) of 146 case - patients ( table 2 ) . the most common initial diagnosis was leptospirosis for 114 ( 75% ) of 151 patients . during the 10-year reporting period , 1 death occurred among 198 patients with confirmed cases ( case - fatality rate 0.5% ) . a 23-year - old man who attended college on the mainland had been exposed through recreational activities while at home in hawaii during winter break 2003 . of the 198 patients with confirmed infection , 152 ( 77% ) received a diagnosis on the basis of serologic testing with the mat , 18 ( 9% ) cases were confirmed with culture isolates , and 28 ( 14% ) were confirmed by mat and isolates . forty - three isolates obtained during 20002008 were characterized at cdc by molecular and serologic techniques . isolates were grouped into 4 clades based on mat results and pulsed - field gel electrophoresis : 19 ( 44% ) unknown serovar ( serogroup australis ) , 17 ( 40% ) serovar icterohemorrhagiae ( serogroup icterohemorrhagiae ) , 4 ( 9% ) serovar ballum ( serogroup ballum ) , and 3 ( 7% ) of unknown serovar ( serogroup bataviae ) . cross - agglutination absorption assay identified the unknown serovar from serogroup australis as a new serovar closely related to lora ( 11 ) . the most common infecting serogroups ( identified definitively by isolate or presumptively by mat ) were australis ( n = 50 ) and icterohemorrhagiae ( n = 51 ) . analysis for linear trend , after cases were categorized into 5-year intervals and compared with confirmed cases reported during 19741998 ( 5 ) , showed a significant increase in infections attributed to serogroup australis and a decrease in infections caused by serogroup icterohemorrhagiae ( p<0.0001 for each ) . the most recent 10-year reporting period has demonstrated a statistically significant shift in the seasonal occurrence of leptospirosis from the drier summer months ( 5 ) to the wetter winter months . climatologists have characterized the hawaiian archipelago as having only 2 seasons : summer ( may through september ) and winter ( october through april ) . rainfall and widespread rainstorms are most common during the winter months ( 12 ) . during the earlier reporting period , 19741998 , recreationally associated exposures predominated and increased over time ; therefore , the summer predominance was attributed to the greater likelihood of recreational exposure in the summer . during 19892008 , the frequency of recreational exposures plateaued while frequency of occupational exposures seemed to increase . this observed change in exposure history might allow seasonal climatic effect to have a greater influence on the epidemiology of the disease . in addition , taro farming , a recognized high - risk occupation ( 5 ) , which had been on the decline , has experienced a resurgence relating to renewed interest in the cultural importance to native hawaiians and an awareness of taro s nutritional value ( 13 ) . in 2000 , hawaii produced 7 million pounds of taro , the largest crop yield since 1977 ( 14 ) . the island distribution of leptospirosis cases remains virtually unchanged since our earlier report ( 5 ) . kauai , the island with the highest annual rainfall and second most rural island , had and continues to have the highest incidence rate , followed by hawaii , the most rural island . as in our earlier report , cases were most consistently reported from the wetter , windward , northeast sides of each island . notably , climatic changes have been documented for the hawaiian archipelago with significant trends in increasing temperatures ( 15 ) , decreasing rainfall ( 16 ) , and increasing rain intensity ( 17 ) over the past 30 years . the effects of climate change on ecosystems are complex , but the potential for influencing infectious disease patterns has been well described ( 18,19 ) . temperature and climate changes may affect the host animal s environment , making transmission to humans more likely . increase in rain intensity with resultant flooding is a well - recognized climatic risk factor for transmission of leptospira spp ( 20 ) . flooding was responsible for 1 of the 2 outbreaks during the study period ( 10 ) . the predominance of men among case - patients is well recognized ( 2124 ) and is virtually unchanged from our earlier report ( 5 ) . this predominance has been explained by the tendency of more men to participate in high - risk outdoor exposure activities . the low reported age - specific case rates in children < 10 years of age and highest rates among adults 2050 years of age are also consistently reported ( 2224 ) and similar to our earlier findings ( 5 ) . our findings corroborate other large case series that show that the most common clinical manifestation of leptosporisis are nonspecific signs or symptoms , such as fever , headache , and mylagias ( 5,22,2527 ) . the case - fatality rate ( 0.5% ) is lower than that reported from brazil ( 25 ) , barbados ( 28 ) , guadeloupe ( 22 ) , and the andaman islands ( 26 ) , but similar to the rates found in our earlier study ( 5 ) and in a recent case series from france ( 27 ) . the low case - fatality rate in this series may be explained by early recognition and initiation of supportive therapy and antimicrobial drugs . other case series may be biased toward recognition and inclusion of only the most severely ill , hospitalized patients , which leads to higher case - fatality rates . a recent population - based case - control study from brazil ( 29 ) showed that pulmonary involvement was the strongest independent predictive factor for death caused by severe leptospirosis . pulmonary findings were infrequent among case - patients in this study , the earlier hawaii series ( 5 ) , and the france series ( 27 ) . the changing temporal trend in the infecting serogroup first identified in our earlier study has continued ; most of the current leptospirosis isolates are in the australis serogroup . this documented trend over the past 35 years from the previously predominant serogroup icterohemorrhagiae to the now predominant australis may reflect the influence of different host animals , the effects of climatic and land use changes , or both . serogroup icterohemorrhagiae has been associated with rats ( rattus norwegicus and r. rattus ) , and australis has been associated with swine , including feral swine or wild boars ( sus scrofa ) ( 30,31 ) . recent reports from germany have shown high seroprevalence of australis serogroup ( serovar bratislava ) in urban feral swine ( 32 ) and documented increased size in the feral swine population and habitat changes leading to epidemiologic linkages between leptospirosis occurrence and feral swine exposure ( 33 ) . hawaii has also experienced an increase in the feral swine population , with a concordant sharp increase in the number of feral swine encroaching on urban residential areas ( 34,35 ) . researchers at the university of hawaii are currently investigating the possible influence of feral swine exposure on human disease in hawaii by undertaking a leptospirosis seroprevalence study of feral swine . annual reported leptospirosis incidence rates in the united states ranged from 0.02 to 0.05 per 100,000 population from 1974 through 1994 , the last year leptospirosis was included in the list of nationally notifiable diseases ( 5 ) . if we include probable and suspected cases , as was done nationally , our mean estimated annual incidence rate during this 10-year study period would increase from 1.63 to 2.85 per 100,000 population , 100 greater than that reported nationally . compared with other locales for which annual leptospirosis incidence rates are available , hawaii would be considered in the moderate range category ( 110/100,000 population ) ( 36 ) . countries in this range include cuba ( 2.47/100,000 population ) and costa rica ( 6.72/100,000 population ) ( 3 ) . countries categorized as having high rates ( > 10/100,000 population ) include barbados ( 10.03/100,000 population ) , trinidad and tobago ( 12.04/100,000 population ) , and seychelles ( 43.21/100,000 population ) ( 3 ) . additional countries or regions considered to have high rates ( for which data are not available ) are vietnam and french polynesia ( 36 ) . although leptospirosis is a notifiable disease in hawaii , case reporting is based on passive surveillance and likely underestimates true disease occurrence . during a 1-year period in 1988 and 1989 , an active surveillance study was conducted on the islands of hawaii and kauai , which resulted in a 5-fold increase in case identification ( 37 ) . a recent retrospective analysis of serum obtained from febrile patients during a dengue fever outbreak in hawaii , 20012002 , also identified a substantial number of leptospirosis cases that otherwise would have gone undiagnosed ( 38 ) . future field studies using geographic information system technology to link climatic and environmental phenomena , such as rainfall occurrence and environmental isolates with human and animal infection , could offer valuable insights . given the potential effects of climate and land use changes , public health officials must remain alert to the occurrence and changing epidemiology of emerging and reemerging infectious diseases . without national surveillance , the occurrence of leptospirosis outside of hawaii or other regions that have leptospirosis surveillance may go largely unrecognized , and thus , unmonitored . ongoing surveillance activities , such as ecologic , animal , and laboratory studies are necessary to clarify and track the dynamic epidemiology of this widespread , reemerging zoonotic illness .