INTRODUCTION
Dengue is a mosquito-borne viral infection caused by four dengue viruses (DENV-1, DENV-2, DENV-3, and DENV-4) that are antigenically distinct.1 Infection with one DENV confers lifelong type-specific immunity and short-term cross protection against another DENV. All DENVs are transmitted to humans through the bite of infected Aedes mosquitoes, primarily Ae. aegypti and Ae. albopictus. Many dengue infections, particularly among children, cause no symptoms or a mild, nonspecific febrile illness, but each DENV is capable of causing classic dengue fever (DF) or dengue hemorrhagic fever (DHF). Classic DF is characterized by sudden onset of fever plus two or more of the following: retro-orbital pain, myalgias, arthralgias, rash, hemorrhagic manifestation, and inflammation.2 Dengue hemorrhagic fever, a less common, more severe manifestation of the infection, is defined by fever, thrombocy-topenia, hemorrhagic manifestations, and increased vascular permeability. There is no effective vaccine or specific pharmacotherapy for dengue. Prompt and judicious administration of intravenous fluids to patients with DHF can improve outcomes.
An estimated 40% of the world’s population lives in more than 100 dengue-endemic countries in the tropical and subtropical areas where DENV can be transmitted.3 The World Health Organization (WHO) estimates that 50 million dengue infections and 500,000 cases of DHF occur annually, with children bearing much of the disease burden.4
Dengue fever and DHF are reportable by law in Puerto Rico, a Commonwealth territory of the United States in the Caribbean. Dengue virus was first isolated in Puerto Rico during a large outbreak in 1963–1964 (only DENV-3 was isolated), 5,6 although outbreaks attributed to dengue were reported in 19157 and 1945. This confirmation was followed by large, island-wide outbreaks in 1969 (DENV-2),8 1977 (DENV-2 most commonly isolated, followed by DENV-3, and DENV-1),9 1978 (DENV-1),10 1981 (DENV-4 and DENV-1), 11 1982 (DENV-4), 10–12 and 1985–1986 (DENV-4, DENV-1, and DENV-2). 13 The first case of DHF was documented in 1975, although multiple cases of DHF did not occur until 1986. Two large island-wide outbreaks occurred in the 1990s, one in 1994–1995 (DENV-2, DENV-4, and DENV-1) 14 and the other in 1998 (DENV-4, DENV-1, DENV-2, and DENV-3). 15 The 1998 outbreak was notable for the re-introduction of DENV-3 after a 20-year absence and the first time that the simultaneous transmission of all four DENVs was documented in Puerto Rico.
In this report, we describe a large, island-wide dengue outbreak that occurred in Puerto Rico in 2007. The outbreak was important because it was the largest outbreak in Puerto Rico in nearly a decade and only the second outbreak to involve the simultaneous transmission of all four DENVs. We also compare the 2007 epidemic with the last two large, island-wide dengue outbreaks in 1994–1995 and 1998, specifically assessing the severity of reported cases in the two earlier outbreaks versus the 2007 outbreak.
METHODS
Study population.
Puerto Rico is a U.S. unincorporated territory made up of a main island and a number of keys and smaller islands, two of which, Vieques and Culebra, are inhabited. Puerto Rico is approximately 3,514 square miles (i.e., the size of Connecticut) and is divided into 78 municipalities, the administrative equivalent of a county in the United States. According to the 2000 U.S. Census, Puerto Rico has a population of 3,808,610, 16 which is similar demographically to that of the United States, with a median age of 32.1 years and 48.1% of the population being male (compared with 35.3% and 49.1%, respectively, for the United States). The median household income (in 1999 dollars) is $14,412 (versus $41,994 for the United States). Puerto Rico has 69 hospitals, of which 61 are general hospitals that treat dengue patients, and approximately 150 outpatient clinics. The mosquito Aedes aegypti is the only vector for dengue in Puerto Rico, and it is found throughout the island.
Data sources.
We used data from the island-wide, laboratory-based passive dengue surveillance system (PDSS) that has been maintained by the Dengue Branch of the Centers for Disease Control and Prevention (CDC) in San Juan, Puerto Rico, and the Puerto Rico Department of Health (PRDH) for more than 30 years. Dengue fever and DHF are reportable diseases by law in Puerto Rico. The PDSS relies on healthcare provider–initiated requests for diagnostic testing, and submission of serum samples accompanied by a Dengue Case Investigation Report (DCIR) from suspected dengue case-patients to the CDC Dengue Branch Laboratories. The specimens are processed and tested in accordance with an algorithm, and results are mailed and faxed to the submitting provider. Data from the DCIR, which include patient demographic and clinical information, are entered by CDC into an electronic data system. A weekly summary of PDSS data is disseminated by CDC and used by the PRDH to plan public health interventions and coordinate its vector control activities.
Data from all 2007 death certificates having the word dengue included in the description of the cause of death and suspected cases reported to the PDSS that were identified as fatal cases were obtained from the Demographic Registry of Puerto Rico, and medical records from these cases were reviewed. In addition, when available, data from an independent, hospital-based infection control nurse (ICN) dengue surveillance system 17 were used. In 2007, the hospital-based ICN system received 405 case reports, of which 337 (83.2%) cases were also reported to the PDSS. Data from these ICN case reports were used to improve data quality on the DCIR when a case was reported to both systems.
Laboratory diagnosis.
Serum or plasma samples were processed and tested according to our algorithm. Less than 5% of all serum specimens submitted to the CDC Dengue Branch Laboratories were not processed because of inadequate volume or information. Specimens are categorized into acute phase (those collected five or less days after the onset of symptoms) and convalescent phase (those collected six or more days after onset). Acute-phase specimens were tested by serotype-specific real-time reverse transcription–polymerase chain reaction (RT-PCR) 18,19 adapted for high throughput using MDX-10 Universal and M48 systems (Qiagen, Valencia, CA). Selected positive specimens were later cultured for virus isolation using cell culture. 11,20 Convalescent-phase specimens were tested for IgM antibodies to dengue with an IgM antibody-capture enzyme-linked immunosorbent assay (MAC-ELISA). 21 Specimens with borderline results were retested against standard negative serum. A quantitative IgG ELISA was performed to detect IgG antibodies for all fatal cases. 22,23 Because of the large outbreak and limited amount of dengue antigen in 2007, IgG testing of outbreak specimens was done only for a randomly selected subset of RT-PCR–positive specimens to determine the proportion of primary versus secondary cases by serotype. Autopsy tissue was sent to CDC in Atlanta, Georgia, for identification of dengue viral antigen by immunofluorescent or immunohistochemical (IHC) analysis.
Definitions.
A suspected dengue case is a dengue-like illness in a person whose health-care provider submits a DCIR and serum sample to the Dengue Branch at CDC for diagnostic testing.
A laboratory-positive dengue case is a suspected dengue case meeting any one of the following criteria for having either a current acute dengue infection (i.e., criteria 1–4) or a probable recent dengue infection (criteria 5): 1) isolation of dengue virus from serum or autopsy tissue specimens; 2) detection of DENV RNA in serum, cerebrospinal fluid, or autopsy tissue specimens by RT-PCR; 3) identification of dengue viral antigen or RNA in autopsy tissue specimens by IHC analysis; 4) seroconversion from negative to positive IgM antibody to dengue or demonstration of a four-fold or greater increase in IgG antibody titers in paired serum specimens; and 5) a single positive MAC-ELISA result in an acute-phase or convalescent-phase specimen (i.e., IgM antibodies ≥ 2.0 positive/ negative ratio). 22
A laboratory-negative dengue case is a suspected dengue case meeting the following criteria: no dengue IgM antibodies detected in a convalescent-phase sample, and either 1) no acute sample submitted for diagnostic testing or 2) the acute-phase sample was negative.
A laboratory-indeterminate dengue case is a suspected dengue case meeting all of the following criteria: 1) dengue virus or IgM antibodies to dengue is not detected in the acute-phase sample, 2) dengue viral antigen is not identified in an autopsy tissue sample, and 3) no convalescent-phase sample was submitted for diagnostic testing.
A hemorrhagic manifestation is defined by the presence of a positive tourniquet test result, or report of any one or more of the following: petechiae, echymosis, epistaxis, bleeding gums, hematuria, hematemesis, melena, or menorrhagia.
Dengue hemorrhagic fever is any suspected dengue case that meets the following four WHO criteria2: 1) fever or recent history of fever, 2) any hemorrhagic manifestation, 3) thrombo-cytopenia (platelet count ≤ 100,000/mm3), and 4) evidence of increased vascular permeability. Evidence of plasma leakage caused by increased vascular permeability consists of at least one of the following: an increased hematocrit ≥ 20% above the U.S. population mean hematocrit for age and sex, 24–26 a decrease in hematocrit after volume-replacement treatment ≥ 20% of the baseline hematocrit, presence of pleural effusion or ascites detected by radiography or other imaging method, or hypoproteinemia or hypoalbuminemia as determined by laboratory test. 27–29
Dengue shock syndrome is any case that meets the four criteria for DHF (as outlined above) and has evidence of circulatory failure manifested by 1) rapid and weak pulse, and narrow pulse pressure (≤ 20 mm of Hg [2.7 kPa]) or 2) hypotension for age, 25 and cold, clammy skin and restlessness.
A primary dengue infection (first case of dengue infection) is a laboratory-positive case in which the IgG antibody titer against dengue was < 1:160 in the acute-phase sample. 22
A secondary dengue infection (a second, third or fourth case of dengue infection) is defined as laboratory-positive case in which the IgG antibody titer against dengue was ≥ 1:160 in the acute-phase sample. 22
Statistical analysis.
A descriptive analysis was performed by calculating frequencies of clinical, demographic, and laboratory features of all suspected dengue cases reported to the PDSS in 2007 with an illness onset date within the calendar year. Rates of suspected and laboratory-positive dengue per 10,000 population were calculated by age, sex, and municipality of residence using 2000 U.S. Census data for Puerto Rico. Rates of DENV-3–positive samples and laboratory-positive dengue infections were ranked by quintile. Incidence rate ratios for sex and age groups were calculated by dividing the incidence rate in one group by the incidence rate in the comparison group. Statistical differences in proportions were tested by applying the chi-square test and Fisher’s exact test when applicable. A comparison of the median age between diagnostic groups was conducted by using the nonparametric equality-of-medians test. All data analyses were conducted using STATA (Stata Corporation, College Station, TX) and SAS (SAS Institute Inc., Cary, NC) software.
RESULTS
There were 10,508 suspected dengue cases reported to the PDSS that had onset of symptoms in 2007 (Figure 1). Cases were reported every week throughout the year. On average, 202 suspected cases (range = 46–871) were reported per week. However, reporting of suspected cases and detection of laboratory-positive cases varied greatly over time. Weekly case reporting surpassed our historical epidemic threshold (i.e., two standard deviations above the mean number of weekly reported cases during non-outbreak years) for two consecutive weeks during week 22 (May 27–June 2, 2007), and an outbreak was declared. Case reporting markedly increased after week 31 (August 5–11, 2007) and peaked during week 40 (October 1–7, 2007). Case reporting remained above the epidemic threshold until week 51 (December 16–22, 2007).
Of the 10,508 suspected dengue cases, paired specimens (i.e., one acute-phase and one convalescent-phase specimen) were available for 595 (5.7%). Specimens were processed for 9,983 (95.0%) of the suspected cases; 3,293 (33.0%) were laboratory positive, 1,206 (12.1%) were laboratory negative, and 5,484 (54.9%) were indeterminate. Of the 3,293 laboratory-positive cases, 2,175 (66.0%) were RT-PCR positive for DENV, one was confirmed by detection of viral antigen in a tissue sample, and the remaining 1,117 (33.9%) cases were confirmed by serologic analysis. Among the 2,175 RT-PCR–positive viral identifications, DENV-3 (1342, 61.7%) and DENV-2 (677, 31.1%) were detected more frequently than either DENV-1 (132, 6.1%) or DENV-4 (24, 1.1%). On average, 63 cases per week were laboratory positive (range = 2–292). The highest percentage of laboratory-positive cases (40.4%) was detected during week 37 (September 10–16, 2007), three weeks before the peak of the outbreak. The average percentage of laboratory-positive cases during the epidemic was 30.2%, compared with 25.2% during non-epidemic weeks (P < 0.001, by Z-ratio test).
The median age of patients with suspected dengue infections was 21.1 years; 52.9% were male (Table 1). Patients with laboratory-positive dengue infection tended to be younger than those who were laboratory negative (median age 20.7 versus 26.9 years), whereas patients who were laboratory indeterminate were of similar age (median age = 19.9 years). The proportion of patients who were male did not vary by laboratory result.
Half (52.5%) of the 10,508 persons with reported suspected dengue were hospitalized, and nearly one-third (31.8%) reportedly had a hemorrhagic manifestation (Table 1). However, only 227 (2.2%) of all suspected cases met WHO criteria for DHF, and there were 40 (0.4%) deaths among the reported cases. Persons with laboratory-positive and indeterminate cases were less likely to be hospitalized than were laboratory-negative patients (53.9% and 51.2% versus 59.0%, respectively). A similar proportion of laboratory-positive and laboratory-negative case-patients reported having at least one hemorrhagic manifestation (34.2% and 35.1%, respectively) and met criteria for DHF (2.6% and 3.1%, respectively), and case-patients with indeterminate laboratory results were less likely to report a hemorrhagic manifestation or meet criteria for DHF. A higher proportion of laboratory-negative case-patients died compared with laboratory-positive or indeterminate case patients (1.0% versus 0.3%, respectively).
Among the 227 patients who met criteria for DHF, 37.0% had laboratory positive cases: 16.3%, laboratory negative, 44.1% laboratory indeterminate, and 2.6% did not have a specimen processed. Among the 84 persons with laboratory-positive DHF, nearly all (73, 86.9%) were hospitalized, and 7 (8.3%) died. Half (45, 51.1%) of the laboratory-positive DHF cases were positive by RT-PCR. Most (23, 54.8%) of these persons were infected with DENV-3, and 28.6% were infected with DENV-2, 11.9% with DENV-1, and 4.8% with DENV-4. No patient during this outbreak met the WHO criteria for DSS because the variables rapid, weak pulse and cold, clammy skin were not well reported on the DCIR. Only nine (10.7%) persons with laboratory-positive DHF reported having hypotension or a narrow pulse pressure.
The overall incidence of dengue in Puerto Rico in 2007 was 27.6 suspected dengue cases and 8.6 laboratory-positive dengue cases per 10,000 population. The incidence of suspected and laboratory-positive dengue varied by age (Figure 2) and by sex. The incidence of laboratory-positive dengue was highest among 10–14-year-old persons (19.0 laboratory-positive dengue infections per 10,000 10–14-year-old persons), followed by 15–19-year-old persons (17.9), infants (10.9), 20–24-year-old persons (9.1), and 5–9-year-old persons (8.2) (Figure 2). The lowest incidence of laboratory-positive dengue (3.2) was among those 1–4 years of age. Interestingly, this group of children had the highest percentage of indeterminate results (i.e., 73.5% for males and 70.2% for females compared with 43.9% to 60.8% for all other age groups).
Dengue virus type 3 was the predominant serotype identified in all age groups except infants and 50–59-year-old persons. In these age groups, DENV-2 was more commonly identified as the infecting serotype. Dengue virus type 1 was most commonly identified among those 5–24 years of age. During 2007, DENV-3 was identified in specimens from 71 (92.2%) of the 77 municipalities reporting to PDSS (Figure 3), DENV-2 in 59 (76.6%), DENV-1 in 34 (44.2%), and DENV-4 in 7 (9.1%). Since 2000, DENV-2 and DENV-3 have accounted for most viral isolates in Puerto Rico (CDC, unpublished data). However during the 2007 outbreak, rates of infection with DENV-3 were highest in municipalities on the east and west coasts, and the south central part of the island, whereas in the past, 15 rates have been highest in the north central part of the island, in the San Juan metropolitan area.
Rates of suspected and laboratory-positive dengue infections were 1.2 times higher among males than females (incidence rate ratio [IRR] = 1.21; 95% confidence interval [CI] = 1.17–1.26, P < 0.0001 versus IRR = 1.19, 95% CI = 1.11–1.28, P < 0.0001). The incidence of suspected dengue and laboratory-positive dengue for males in Puerto Rico was 30.3 and 9.4 cases per 10,000 males, respectively. For females, these rates were 24.9 and 7.9, respectively. Differences in rates were greatest among infants and those ≥ 40 years of age. Rates of laboratory-positive dengue were 1.5 times higher (IRR = 1.52, 95% CI = 0.89–2.65, P = 0.0534) among male infants than female infants (12.8 versus 8.4, respectively). Similarly, rates were 1.5 times higher (IRR = 1.48, 95% CI = 1.28–1.70, P < 0.0001) among males ≥ 40 years of age than their female counterparts (6.5 versus 4.4, respectively).
In addition, rates of hospitalization were slightly higher (IRR = 1.20, 95% CI = 1.20–1.14, P < 0.0001) among males than females (15.8 versus 13.2 per 10,000, respectively). The rate of hospitalization was 1.5 times higher for men with laboratory-positive dengue who were ≥ 40 years of age than for their female counterparts (IRR = 1.47, 95% CI = 1.22–1.77, P < 0.0001). Rates of hospitalization were 1.6 times higher for male infants with laboratory-positive dengue than for infant females (9.8 versus 6.0 per 10,000). However, because of the small number of hospitalized, laboratory-positive cases in this age group (i.e., 29 male infant cases versus 17 female infant cases), this difference was not statistically significant (IRR = 1.64, 95% CI = 0.87–3.18, P = 0.0523). Nearly twice as many deaths occurred among dengue laboratory-positive males versus females (7 versus 4 deaths, respectively).
The overall incidence of suspected dengue and laboratory-positive dengue differed by municipality of residence(Figure 4). One of the 78 municipalities, Vieques, did not report any cases of suspected dengue in 2007, and another municipality, Patillas, has a separate, enhanced dengue surveillance system and does not report to PDSS. The incidence of suspected dengue in the remaining 76 municipalities ranged from 52.7 cases per 10,000 population in Salinas to 2.8 in Barranquitas. The incidence of laboratory-positive dengue ranged from 19.9 in Coamo to 0.7 in Barranquitas. Rates of laboratory-positive dengue were highest (i.e., ≥ 95th percentile) in Coamo, Arroyo, Corozal, Sabana Grande, Ponce, Lares, Moca, Guánica, Canóvanas, Mayagüez, Aguada, and Salinas. Half of these 12 municipalities are located in south central Puerto Rico, and 4 others are located in west central Puerto Rico (Figure 4). In the top 12 municipalities, 982 cases were laboratory positive, representing approximately 30% of all 2007 PDSS laboratory-positive cases. Of these cases, 689 were RT-PCR positive, 59.8% were DENV-3, and 37.2% DENV-2, whereas only 2.6% were DENV-1 and 0.4% DENV-4.
Two hundred twenty-three specimens, or approximately 10% of RT-PCR–positive specimens for DENV-1, DENV-2, and DENV-3, were randomly selected and evaluated for primary versus secondary infection. All 24 DENV-4 specimens were included. Overall, 23.8% or 53 of the 223 samples tested were primary infections. The rates of primary infection among the serotypes were 43.8%, 1.5%, 36.4%, and 5.3% for DENV-1 through DENV-4, respectively. The median age for patients with primary infection was 14 years compared with 26 years for those with secondary infections. This sample was not adequately powered to measure significant differences in reported severity of disease.
Forty fatal cases of suspected dengue were detected by the PDSS. Four additional deaths that were attributed to dengue on the death certificate were reported to CDC and the PRDH by the Demographic Registry of Puerto Rico. These four deaths had not been reported to the PDSS. Of the 40 fatal cases for whom we had a serum or tissue sample, 11 were laboratory positive. The median age of the patients was 26.2 years (range = 5.5 months to 78 years), and seven were male. Five of the laboratory-positive persons who died had an RT-PCR-positive serum sample (one DENV-1, one DENV-2, three DENV-3), one had a PCR-positive tissue sample (DENV-2), one seroconverted, two had positive tissue sample IHC result for dengue, and two had positive IgM results in a single serum samples. Seven persons with laboratory-positive fatal cases had submitted an acute-phase serum specimen: five had secondary infections, and two had primary infections. Fourteen of the fatal cases were laboratory negative. Of these cases, eight were laboratory positive for leptospirosis, and one was positive for group A Streptococcus septicemia. The remaining 15 samples were indeterminate.
DISCUSSION
The dengue outbreak of 2007 was the largest outbreak in nearly a decade, and only the second outbreak in Puerto Rico involving the simultaneous transmission of all four DENVs and the second with a predominance of DENV-3 transmission (Figure 3). In fact, DENV-3 has been identified in only three other outbreaks, in 1963–1964, 1977, and 1998. The 1998 outbreak, during which transmission of DENV-1 and DENV-4 predominated, was notable for being the first time simultaneous transmission of all four DENV was documented in Puerto Rico and for the reappearance of DENV-3 after a 20-year absence. 15 During the 1998 outbreak, DENV-3 accounted for only 6% of all isolates and in contrast to the 2007 outbreak, rates of DENV-3 were highest in north central Puerto Rico, in the San Juan metropolitan area (Figure 3). 15 During five non-outbreak years that followed (1999–2003), DENV-3 accounted for most isolates, which was likely caused by introduction of a new genotype III DENV-3 into the Americas in 1998 30 and the lack of immunity to DENV-3 among Puerto Rico residents. During 2004–2006, DENV-2 almost completely displaced DENV-3 as the predominant serotype in Puerto Rico, accounting for 58%, 87%, and 77% of all PCR-positive specimens in 2004, 2005, and 2006, respectively (CDC, unpublished data). Interestingly, this predominance shift from DENV-3 to DENV-2 (and back to DENV-3) was also described in Paraguay during 2005–2007. 31 Sequence analysis will establish if the DENV-2 and DENV-3 strains involved in the 2007 epidemic are different from strains previously circulating in Puerto Rico. We plan to conduct additional biological analyses depending on the differences observed.
DENV-1 and DENV-4 reappeared in 2007 after nearly a decade of absence. DENV-4 was limited to the San Juan Metropolitan area, and DENV-1 was identified in only 34 of 78 municipalities. This finding warrants close monitoring because aside from DENV-4 involvement in the outbreaks of 1981, 1982, 1986, and 1998, DENV-4 has constituted only a minority of viral isolates in Puerto Rico. Moreover, a CDC study conducted in 2001 found that only 39% of adults (> 19 years of age) had neutralizing antibodies to DENV-4, compared with 78% for DENV-1, 67% for DENV-2, and 50% for DENV-3 (Vorndam AV, unpublished data). Several factors may have limited DENV-1 and DENV-4 transmission, including high concurrent endemicity of DENV-2 and DENV-3, unfavorable genetic structure of the viruses, 32 the late timing of their reemergence (end of July 2007), and herd immunity against DENV-1 after the large 1998 outbreak (when DENV-1 and DENV-4 predominated). In addition, DENV-1 and DENV-4 infections may have occurred primarily among children born after 1998 (susceptible population), resulting in higher proportion of primary DENV-1 and DENV-4 infections not detected by our surveillance system. The low number of cases of detected DENV-4 disease, of which 5% were primary disease, may suggest primary DENV-4 disease is more likely asymptomatic than primary DENV-1 disease.
We found that rates of laboratory-positive dengue were highest among 10–19-year-old persons, a finding has been consistent for more than three decades. 9,14,15 In contrast, in many other dengue-endemic countries, young children bear the greatest burden of the disease. 33,34 In a recent hospital-based study in Nicaragua, rates of laboratory-confirmed dengue were highest among children 5–9 years of age, who accounted for 58% of all confirmed dengue cases in the study population. 33 However, in some dengue-endemic countries, a gradual shift in peak attack rate towards older age groups has been noted. 35–38 For example, the median age of hospitalized DHF patients in Bangkok, Thailand, increased progressively from three years and ten months in the 1960s to five years and seven months in the 1970s, to seven years and five months in the 1980s, and to a mean of eight years in the 1990s. 39,40 In a 2001 hospital-based study of serologically confirmed dengue patients in southeastern Thailand, the median age of DHF patients was 10 years, and the most frequent age groups affected were persons 10–14 years of age (37%), 4–9 years of age (36%) and 15–24 years of age (15%). 41
Rates of suspected dengue infections and laboratory-positive dengue infections were slightly higher among males than females, a finding that has not been previously reported in Puerto Rico 13–15 and has not been consistently supported by findings from published studies and surveillance reports. 38–42 Differences were especially pronounced among infants and persons ≥ 40 years of age, age groups whose rates of laboratory-positive dengue infections were 1.5 times higher among males than among females. Sex differences in infection rates among adults may be associated with differential exposure to dengue because of sex-specific behaviors and lifestyles as well as differential access to care. 43 However, our data also suggest a sex difference in the severity of infection.
Several experimental, clinical, and epidemiologic studies have found that in general, males have poorer outcomes than females after infectious challenge. 43–46 One of the most consistent explanations for this difference is a sex-related modulation of the innate immune response. Differences in monocyte cytokine production have been demonstrated such that males tend to have a more intense inflammatory response to microbial stimulation than females, resulting in higher levels of pro-inflammatory cytokines (tumor necrosis factor [αTNF-α], interleukin-1β [IL-1β], IL-6) 44,47–51 and possibly lower levels of anti-inflammatory cytokines (IL-10). 51 Although the pathogenesis of dengue is not fully understood, increased levels of vaso-active cytokines such as TNF-α and IL-6 as well as interferon-γ and IL-2 have been associated with increased vascular permeability and plasma leakage in DHF. 52,53 Cytokine-mediated immunopathogenesis may help explain sex differences in severity of dengue infections. In contrast, previous studies finding that dengue infections may be more severe among females have proposed that non-neutralizing antibody enhancement of viral replication may be greater in females because females tend to have a more intense humoral immune response to antigen than males. 44,54 Future studies should be sufficiently powered to evaluate biological and social factors associated with sex differences.
Compared with the 2007 outbreak, more cases of suspected dengue were reported to the PDSS during the 1994–1995(June1, 1994 to May 31, 1995) and 1998 outbreaks, when 24,700 and 17,000 cases, respectively, were reported. 15,16 However, there is some evidence that disease was more severe during the 2007 outbreak, suggesting either a change in reporting to the PDSS or a true increase in severity of dengue infections in Puerto Rico. In 2007, the number and proportion of reported hospitalizations among persons with suspected dengue were greater than during either of these previous outbreaks (5,517 hospitalizations [52.5% of all reported suspected cases] in 2007 versus 4,662 [18.9%] in 1994–1995 and 4,607 [27.1%] in 1998). 14,15 Moreover, this 2007 figure does not include the additional hospitalizations reported to the ICN surveillance system that were not also reported to the PDSS, as had been done in the past (e.g., 2,004 hospitalizations in 1994–1995 and 1,580 in 1998). In fact, a nearly steady increase has occurred in the proportion of hospitalizations among persons with suspected dengue in Puerto Rico since 1988, 17 despite a major privatization in the healthcare system since 1994. A second indicator of disease severity is that a higher proportion of suspected dengue cases (3,340, 31.8%) were reported as having a hemorrhagic manifestation in 2007 than in 1994 (5,687, 23.0%) or in 1998 (4,693, 27.6%). In addition, more cases of DHF and laboratory-positive DHF were reported in 2007 (227 DHF, 84 laboratory-positive DHF) compared with 1994–1995 (152, 50) and 1998 (174, 59).
The main limitation of our findings is that the PDSS is a passive surveillance system. As such, it relies on healthcare provider–initiated reporting of cases and submission of diagnostic specimens and is subject to underreporting of suspected cases and a high proportion of indeterminate cases. For example, in 2007, 1,872 suspected dengue cases were reported to the PRDH (by the PRDH Category I Infectious Disease Case Reporting Form), and 68 hospitalized, suspected dengue cases reported to the ICN surveillance system that were not included in this analysis because they were not reported to the PDSS (i.e., we did not receive the requisite serum sample and DCIR). In addition, the PDSS is based on signs and symptoms at time of presentation and does not provide follow-up data on ultimate disease severity. Thus, it underestimates true severity of cases reported. Past studies estimated that for every case of dengue fever reported to the PDSS, between 10 and 27 symptomatic cases of dengue fever would not be reported, and that for every case of DHF reported, about 3 cases of DHF would not be reported. 17,55,56 The PDSS itself, which has been in operation since 1975, may have changed by the privatization of the healthcare system, whereby providers are not compensated for public health functions. The PRDH has worked since 2003 to improve dengue reporting by making DHF a reportable disease in Puerto Rico and targeting their efforts toward hospital staff. However, since 2003, the proportion of reported cases that fit WHO criteria for DHF has remained relatively constant.
The dengue outbreak of 2007 in Puerto Rico was the largest epidemic in the United States in nearly a decade, and marked the second time for simultaneous transmission of all four DENVs. Having a PDSS in place for more than three decades enabled monitoring of trends in the transmission of dengue island-wide. Epidemiologic evidence suggests that the severity of dengue infections may be increasing and the disease is seen more commonly in males. The reasons for such epidemiologic trends are complex, and active population surveillance accounting for severity of disease is needed to determine the true burden of disease. Efforts to reduce indeterminate results, including implementation of more sensitive tests and education of providers on the importance of early testing and submission of paired specimens, are under way. Vector control and public health measures have been ineffective in preventing dengue from becoming endemic, and perhaps now hyper-endemic, in Puerto Rico.
Characteristics of all reported cases of suspected dengue infection, overall and by laboratory result, laboratory-based, passive dengue surveillance system, Puerto Rico, 2007*
Number of reported cases of suspected dengue infections by final laboratory diagnosis and week of symptom onset, laboratory-based, passive dengue surveillance system, Puerto Rico, 2007. This figure appears in color at www.ajtmh.org.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 81, 3; 10.4269/ajtmh.2009.81.467
Number of reported cases of suspected dengue infections by final laboratory diagnosis and week of symptom onset, laboratory-based, passive dengue surveillance system, Puerto Rico, 2007. This figure appears in color at www.ajtmh.org.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 81, 3; 10.4269/ajtmh.2009.81.467
Number of reported cases of suspected dengue infections by final laboratory diagnosis and week of symptom onset, laboratory-based, passive dengue surveillance system, Puerto Rico, 2007. This figure appears in color at www.ajtmh.org.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 81, 3; 10.4269/ajtmh.2009.81.467
Number and rate of laboratory-positive dengue infections by patient age group, laboratory-based, dengue surveillance system, Puerto Rico, 2007. This figure appears in color at www.ajtmh.org.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 81, 3; 10.4269/ajtmh.2009.81.467
Number and rate of laboratory-positive dengue infections by patient age group, laboratory-based, dengue surveillance system, Puerto Rico, 2007. This figure appears in color at www.ajtmh.org.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 81, 3; 10.4269/ajtmh.2009.81.467
Number and rate of laboratory-positive dengue infections by patient age group, laboratory-based, dengue surveillance system, Puerto Rico, 2007. This figure appears in color at www.ajtmh.org.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 81, 3; 10.4269/ajtmh.2009.81.467
Rates of dengue virus type 3–positive isolates by municipality of residence ranked by quintile, laboratory-based dengue surveillance system, Puerto Rico, 2007. This figure appears in color at www.ajtmh.org.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 81, 3; 10.4269/ajtmh.2009.81.467
Rates of dengue virus type 3–positive isolates by municipality of residence ranked by quintile, laboratory-based dengue surveillance system, Puerto Rico, 2007. This figure appears in color at www.ajtmh.org.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 81, 3; 10.4269/ajtmh.2009.81.467
Rates of dengue virus type 3–positive isolates by municipality of residence ranked by quintile, laboratory-based dengue surveillance system, Puerto Rico, 2007. This figure appears in color at www.ajtmh.org.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 81, 3; 10.4269/ajtmh.2009.81.467
Rates of laboratory-positive dengue infections by municipality of residence ranked by quintile, laboratory-based dengue surveillance system, Puerto Rico, 2007. This figure appears in color at www.ajtmh.org.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 81, 3; 10.4269/ajtmh.2009.81.467
Rates of laboratory-positive dengue infections by municipality of residence ranked by quintile, laboratory-based dengue surveillance system, Puerto Rico, 2007. This figure appears in color at www.ajtmh.org.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 81, 3; 10.4269/ajtmh.2009.81.467
Rates of laboratory-positive dengue infections by municipality of residence ranked by quintile, laboratory-based dengue surveillance system, Puerto Rico, 2007. This figure appears in color at www.ajtmh.org.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 81, 3; 10.4269/ajtmh.2009.81.467
Address correspondence to Kay M. Tomashek, Dengue Branch, Division of Vector-Borne Infectious Diseases, National Center for Zoonotic, Vector-Borne, and Enteric Diseases, Centers for Disease Control and Prevention, 1324 Calle Cañada, San Juan, PR 00920. E-mail: ktomashek@cdc.gov
Authors’ addresses: Kay M. Tomashek, Aidsa Rivera, Jorge L. Muñoz-Jordan, Elizabeth Hunsperger, Luis Santiago, Oscar Padro, and Wellington Sun, Dengue Branch, Division of Vector-Borne Infectious Disease, Centers for Disease Control and Prevention, San Juan, PR 00920. Enid Garcia, Puerto Rico Department of Health, San Juan, PR 00911.
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