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Although infrequently diagnosed in the United States,
leptospirosis is a notable reemerging infectious disease
throughout developing countries. Until 1995, when the dis-
ease was eliminated from the US list of nationally notifi able
diseases, Hawaii led the nation in reported annual incidence
rates. Leptospirosis remains a notifi able disease in Hawaii.
To ascertain the status of leptospirosis in Hawaii since the
most recent US report in 2002, we reviewed 1999–2008
data obtained from case investigation reports by the Hawaii
State Department of Health. Of the 345 case reports related
to in-state exposures, 198 (57%) were laboratory confi rmed.
Our fi ndings indicate a change in seasonal disease occur-
rence from summer to winter and in the infective serogroup
from Icterohemorrhagiae to Australis. Also, during the past
20 years, recreational exposures have plateaued, while oc-
cupational exposures have increased. Ongoing surveillance
is needed to clarify and track the dynamic epidemiology of
this widespread zoonosis.
L
eptospirosis is considered the most globally widespread
zoonotic illness; it has been classifi ed as an emerging
or reemerging infectious disease by the World Health Or-
ganization (1) and the US Centers for Disease Control and
Prevention (CDC) (2). Most frequently recognized as a dis-
ease of the developing world (3), leptospirosis was removed
from the US list of nationally reportable infectious diseases
in 1995 (4). Before the disease’s removal from national
surveillance, Hawaii consistently led the nation in reported
annual incidence rates (5). The state of Hawaii continues
to include leptospirosis as a notifi able illness. The last pub-
lished US population-based surveillance report was from
Hawaii and covered data obtained during 1974–1998 (5).
This study serves as an update for leptospirosis in Hawaii
during 1999–2008.
Methods
We reviewed leptospirosis case investigation reports
by Hawaii Department of Health (HDOH) investigators
submitted during 1999–2008. 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 Re-
view Board.
For exposure source to be assessed, incubation periods
estimated, and exposures classifi ed, patients were asked
about high-risk activities that occurred during the 21 days
before symptom onset. These included exposure to ani-
mals, 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., garden-
ing, trapping rats). If exposure was continuous or if persons
had been exposed multiple times, the incubation period was
considered indeterminate. Ascertainment of exposure clas-
sifi cation involved placing cases into 3 mutually exclusive
exposure categories: occupational, recreational, or habi-
tational. If exposure activities involved >1 category, the
exposure classifi cation was considered indeterminate. Out-
breaks were defi ned as >2 epidemiologically linked cases.
A patient with a confi rmed case had a clinically com-
patible illness plus a >4-fold increase in microscopic agglu-
tination 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 classifi ed
as either probable (clinically compatible illness with MAT
titer >200 in >1 serum specimens without a 4-fold increase
Leptospirosis in Hawaii, USA,
1999–2008
Alan R. Katz, Arlene E. Buchholz, Kialani Hinson, Sarah Y. Park, and Paul V. Effl er
Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 17, No. 2, February 2011 221
Author affi liations: University of Hawaii, Honolulu, Hawaii, USA
(A.R. Katz, K. Hinson); and Hawaii State Department of Health,
Honolulu (A.E. Buchholz, S.Y. Park, P.V. Effl er)
DOI: 10.3201/eid1412.080470
RESEARCH
in titer [8]) or suspected (clinically compatible illness with
less supportive laboratory evidence of infection [e.g., MAT
titer <200, positive macroscopic slide agglutination test re-
sult, reactive immunoglobulin (Ig) M ELISA, or positive
indirect hemagglutination assay results]). Only laboratory-
confi rmed 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 Janu-
ary 1999 through November 2004, and by HDOH from De-
cember 2004 through December 2008.
All isolates were sent to CDC for defi nitive serogroup
identifi cation. To determine the presumptive infecting se-
rogroup for serologically confi rmed cases, MAT titers were
examined. The highest and most recent titer was presumed
to be the infecting serogroup. If >1 serogroup had the same
high titer, the identifi cation was labeled indeterminate.
To calculate mean annual incidence rates (overall and
by age, sex, and Hawaii island on which patient was ex-
posed), the numerator was the number of cases for the spec-
ifi ed groups over the 10-year observation period divided
by 10. The denominator was the overall or relevant group-
specifi c population estimate from the 2000 US Census (9).
Data from our earlier 25-year study period, 1974–1998,
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 signifi cant. All
statistical tests were 2-tailed.
Results
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, Malay-
sia, Singapore, and Texas. Of the case reports related to in-
state exposures, 198 (57%) were laboratory confi rmed, 116
(34%) were probable, and 31 (9%) were suspected.
The number of confi rmed 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 dis-
ease occurrence for the recent 10-year study period was
signifi cantly different from that of the previously reported
25-year study period; summer cases predominated in the
latter (p<0.01) (5).
Case-patients were predominately male (91%), and
ages ranged from 3 to 76 years (median 38 years). The
highest age-specifi c rate was among persons 20–29 years
of age, and the lowest was among children 0–9 years of
age. Most cases and the highest incidence rates were re-
lated to exposures on the islands of Kauai and Hawaii
(Table 1). In addition, cases were most consistently report-
ed from the northeast, windward sides of the islands: Hana-
lei (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).
We were able to determine exposure classifi cations for
177 (89%) of the 198 confi rmed cases. Recreational expo-
sures accounted for 79 (45%) and were mostly related to
freshwater swimming, hiking, and camping. Occupational
exposures accounted for 78 (44%), mostly relating to farm-
ing, specifi cally, taro farming. Exposures around the home
accounted for 20 (11%), most commonly, gardening. After
222 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 17, No. 2, February 2011
20
25
s
Hawaiian "summer"
5
10
15
20
25
No. cases
Hawaiian "summer"
0
5
10
15
20
25
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
No. cases
Month of onset
Hawaiian "summer"
0
5
10
15
20
25
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
No. cases
Month of onset
Hawaiian "summer"
Figure 1. Month of onset for 198 laboratory-confi rmed leptospirosis
cases, Hawaii, USA, 1999–2008.
Table 1. Sex, age, and island of exposure for 198 case-patients
with laboratory-confirmed leptospirosis, Hawaii, USA, 1999–2008
Variable
No. (%)
case-patients
Estimated mean annual
incidence rate*
Sex
M 181 (91) 2.97
F 17 (9) 0.28
Age group, y
0–9 1 (1) 0.06
10–19 24 (12) 1.46
20–29 48 (24) 2.87
30–39 31 (16) 1.69
40–49 38 (19) 2.05
50–59 36 (18) 2.55
60–69 15 (8) 1.68
70–79 5 (3) 0.64
Island
Hawaii 98 (49) 6.59
Kauai 47 (24) 8.06
Oahu 44 (23) 0.50
Maui 4 (2) 0.34
Molokai 1 (1) 1.38
Unknown 4 (2)
*No. cases/100,000 population. Rate = no. 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).
Leptospirosis in Hawaii
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 (1989–2008), while occupational expo-
sures actually increased, but the difference was not signifi -
cant (p = 0.08) (Figure 3). After stratifi cation by island, a
signifi cant increase in occupational exposures was shown
for the island of Hawaii (p = 0.04). No other trends for ex-
posure classifi cation were signifi cant.
Most cases occurred sporadically. One outbreak (>2
epidemiologically linked cases), which involved 2 land-
scapers, occurred on Kauai in 1999; both cases were labo-
ratory confi rmed. Another outbreak (2 epidemiologically
linked cases: 1 confi rmed, 1 probable) was associated with
fl ooding of the University of Hawaii campus on October
31, 2004, when heavy rains caused an adjacent stream to
overfl ow its banks (10).
For case-patients with known exposure dates, the me-
dian incubation period was 9 days (range 1–21 days). The
median duration of illness was 14 days (range 3–90 days).
A total of 118 (73%) of 161 case-patients, for whom treat-
ment information was available, were hospitalized.
The most frequent signs and symptoms among patients
who sought treatment were fever, myalgias, headache, nau-
sea, and vomiting. Abnormal urinalysis results were com-
mon; specimens from 78 (73%) of 107 and 71 (68%) of
105 case-patients showed hematuria and proteinuria, re-
spectively. 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 he-
matologic anomaly was thrombocytopenia (<140 × 10
9
/L),
which was observed for 97 (66%) of 146 case-patients
(Table 2). Initial clinical impression was recorded for 151
(76%) of 198 patients. 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 confi rmed 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. Symp-
toms developed after he returned to school, and he died in
January 2004.
Of the 198 patients with confi rmed infection, 152 (77%)
received a diagnosis on the basis of serologic testing with
the MAT, 18 (9%) cases were confi rmed with culture iso-
lates, and 28 (14%) were confi rmed by MAT and isolates.
Forty-three isolates obtained during 2000–2008 were char-
acterized at CDC by molecular and serologic techniques.
Isolates were grouped into 4 clades based on MAT results
and pulsed-fi eld gel electrophoresis: 19 (44%) unknown
serovar (serogroup Australis), 17 (40%) serovar Icterohe-
morrhagiae (serogroup Icterohemorrhagiae), 4 (9%) sero-
var Ballum (serogroup Ballum), and 3 (7%) of unknown
serovar (serogroup Bataviae). Cross-agglutination absorp-
tion assay identifi ed the unknown serovar from serogroup
Australis as a new serovar closely related to Lora (11).
The most common infecting serogroups (identifi ed de-
fi nitively by isolate or presumptively by MAT) were Aus-
tralis (n = 50) and Icterohemorrhagiae (n = 51). Analysis
for linear trend, after cases were categorized into 5-year in-
tervals and compared with confi rmed cases reported during
1974–1998 (5), showed a signifi cant increase in infections
attributed to serogroup Australis and a decrease in infec-
tions caused by serogroup Icterohemorrhagiae (p<0.0001
for each).
Discussion
The most recent 10-year reporting period has demon-
strated a statistically signifi cant shift in the seasonal occur-
rence of leptospirosis from the drier summer months (5) to
Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 17, No. 2, February 2011 223
Figure 2. Exposure locations associated with the greatest number
of leptospirosis cases, Hawaii, USA, 1999–2008.
40
50
60
s
es
Recreational Occupational Habitational
20
30
40
50
60
% Total cases
Recreational Occupational Habitational
0
10
20
30
40
50
60
1989–1993 1994–1998 1999–2003 2004–2008
% Total cases
Recreational Occupational Habitational
Figure 3. Trends in exposure classifi cation for laboratory-confi rmed
leptospirosis cases, Hawaii, USA, 1989–2008
.
RESEARCH
the wetter winter months. Climatologists have character-
ized 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, 1974–1998, rec-
reationally associated exposures predominated and in-
creased over time; therefore, the summer predominance
was attributed to the greater likelihood of recreational ex-
posure in the summer. During 1989–2008, 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 infl uence 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 aware-
ness of taro’s nutritional value (13). In 2000, Hawaii pro-
duced 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 inci-
dence 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 signifi cant 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 poten-
tial for infl uencing 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 re-
sultant fl ooding is a well-recognized climatic risk factor for
transmission of Leptospira spp. (20). Flooding was respon-
sible for 1 of the 2 outbreaks during the study period (10).
The predominance of men among case-patients is well
recognized (21–24) 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-specifi c
case rates in children <10 years of age and highest rates
among adults 20–50 years of age are also consistently re-
ported (22–24) and similar to our earlier fi ndings (5).
Our fi ndings corroborate other large case series that
show that the most common clinical manifestation of
leptosporisis are nonspecifi c signs or symptoms, such as
fever, headache, and mylagias (5,22,25–27). The case-fa-
224 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 17, No. 2, February 2011
Table 2. Clinical findings for 198 case-patients with laboratory-confirmed leptospirosis, Hawaii, USA, 1999–2008
Sign, symptoms, and laboratory result No. (%) patients affected No. patients with data available
Sign or symptom
Fever 187 (98) 191
Myalgia 162 (88) 185
Headache 156 (87) 179
Nausea 117 (68) 173
Vomiting 101 (59) 172
Arthralgia 73 (46) 157
Diarrhea 79 (46) 171
Backache 50 (34) 146
Jaundice 53 (33) 163
Oliguria or anuria 32 (21) 152
Conjunctival suffusion 30 (19) 156
Nuchal rigidity 23 (14) 159
Pneumonia 13 (8) 154
Hepatosplenomegaly 8 (6) 140
Laboratory results
Renal
Hematuria 78 (73) 107
Proteinuria 71 (68) 105
Elevated creatinine (>1.5 mg/dL) 60 (51) 118
Elevated blood urea nitrogen (>20 mg/dL) 68 (50) 136
Hepatic
Elevated alanine aminotransferase (>40 U/L) 109 (74) 147
Elevated total bilirubin (>1 mg/dL) 85 (63) 134
Hematologic
Thrombocytopenia (<140 u 10
9
/L)
97 (66) 146
Elevated leukocyte count (>10 u 10
9
cells/L)
74 (48) 155
Decreased hematocrit (<34%) 56 (38) 146
Leptospirosis in Hawaii
tality 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 to-
ward 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 fi ndings 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 sero-
group fi rst identifi ed in our earlier study has continued;
most of the current leptospirosis isolates are in the Austra-
lis serogroup. This documented trend over the past 35 years
from the previously predominant serogroup Icterohemor-
rhagiae to the now predominant Australis may refl ect the
infl uence of different host animals, the effects of climatic
and land use changes, or both. Serogroup Icterohemorrha-
giae 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 serop-
revalence 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 experi-
enced 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). Research-
ers at the University of Hawaii are currently investigating
the possible infl uence of feral swine exposure on human
disease in Hawaii by undertaking a leptospirosis serop-
revalence study of feral swine.
Annual reported leptospirosis incidence rates in the
United States ranged from 0.02 to 0.05 per 100,000 popu-
lation from 1974 through 1994, the last year leptospirosis
was included in the list of nationally notifi able diseases (5).
If we include probable and suspected cases, as was done
nationally, our mean estimated annual incidence rate dur-
ing this 10-year study period would increase from 1.63 to
2.85 per 100,000 population, ≈100× greater than that re-
ported nationally. Compared with other locales for which
annual leptospirosis incidence rates are available, Ha-
waii would be considered in the moderate range category
(1–10/100,000 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 Bar-
bados (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 notifi able 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 identifi cation (37). A recent retrospective analysis of
serum obtained from febrile patients during a dengue fe-
ver outbreak in Hawaii, 2001–2002, also identifi ed a sub-
stantial number of leptospirosis cases that otherwise would
have gone undiagnosed (38).
Conclusions
Future fi eld studies using geographic information
system technology to link climatic and environmental
phenomena, such as rainfall occurrence and environmen-
tal isolates with human and animal infection, could offer
valuable insights. Given the potential effects of climate and
land use changes, public health offi cials must remain alert
to the occurrence and changing epidemiology of emerging
and reemerging infectious diseases. Without national sur-
veillance, 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 labo-
ratory studies are necessary to clarify and track the dynam-
ic epidemiology of this widespread, reemerging zoonotic
illness.
Acknowledgments
We gratefully acknowledge A. Christian Whelen, Norman
O’Connor, Harry Domen, Jan Ishibashi, and Renee Galloway for
laboratory support; and Joe Elm, Mayee Wong, and Erick Cremer
for case investigation.
Dr Katz is a professor of epidemiology at the University of
Hawaii. His primary research interests are leptospirosis and sexu-
ally transmitted infections.
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Address for correspondence: Alan R. Katz, Department of Public Health
Sciences, John A. Burns School of Medicine, University of Hawaii,
Biomedical Sciences Building, Rm D104M, 1960 East-West Rd, Honolulu,
HI 96822, USA; email: katz@hawaii.edu
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