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Journal of Clinical Microbiology, May 2002, p. 1840-1844, Vol. 40, No. 5
0095-1137/02/$04.00+0 DOI: 10.1128/JCM.40.5.1840-1844.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Division of Research and Diagnosis, Center for Disease Control, Department of Health,1 Co-Wealth Medical Science and Biotechnology, Taipei,3 Department of Emergency Medicine, Medical Center, Tzu Chi University, Hualien, Taiwan, Republic of China2
Received 16 August 2001/ Returned for modification 20 October 2001/ Accepted 17 February 2002
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There have been a number of historical dengue epidemics (either regional or island-wide) over the last century (1915, 1931, 1942-1943, 1981, 1987-1988, 1991, 1994, 1995, 1998, and 2000) in Taiwan (19, 20). Among these, the 1915, 1931, and 1942-1943 outbreaks were large island-wide epidemics (Table 1). Dengue hemorrhagic fever- and dengue shock syndrome-like syndromes were described during the 1931 and 1942-1943 outbreaks. No dengue outbreaks were reported after 1945 until a dengue fever epidemic occurred in Liuchiu Hsiang, Pingtung County, an islet about 15 km southwest of Taiwan, in the summer of 1981. It was estimated that approximately 80% of the inhabitants were infected and the serotype was identified as DEN-2 (9, 19). During the winter of 1987-1988, a small outbreak circulated in southern Taiwan, affecting Kaohsiung City, Kaohsiung County, and Pingtung County (7, 12). The serotype was identified as DEN-1. Since then, small regional epidemics have been reported almost every year in southern Taiwan, with the exception of a 1995 DEN-1 outbreak that occurred in Chungho, Taipei County, in northern Taiwan. To better understand the present status of dengue virus infection in Taiwan, we have recently initiated several seroprevalence surveys in various areas of Taiwan.
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TABLE 1. Major dengue epidemics in Taiwan between 1901 and 1988
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Recently, we reported the development of an NS1-specific indirect ELISA to detect and differentiate JE virus and dengue virus infections using monoclonal antibody and NS1 antigens secreted in the culture supernatants of Vero cells infected with DEN or JE virus (16, 17). Specificity analysis showed that NS1-specific antibodies induced by JE virus and dengue virus infections do not cross-react with each other (17). Huang et al. (10) also reported a DEN NS1-specific antibody response using recombinant DEN NS1 protein. When recombinant DEN NS1 was evaluated for various flavivirus infections, relatively low cross-reactivity was found from sera of JE and yellow fever patients. More recently, we have reclassified the criteria of primary dengue virus infections, using the E- and M-specific capture IgM/IgG ratio (
1.2) instead of hemagglutination inhibition titer (
1,280), and analyzed more than 100 convalescent-phase sera from primary dengue patients covering all four serotypes (reference 11 and unpublished data). The results showed that more than 90% of the sera tested could be correctly serotyped using NS1 serotype-specific IgG ELISA. Comparison between NS1-specific IgG serotyping and PRNT showed good correlation (unpublished data).
The serum samples used in this study were obtained from volunteers of residents in Liuchiu Hsiang, Pingtung County, in southern Taiwan during 1997-1998. These sera were collected for seroepidemiologic study in an attempt to understand the age-specific seroprevalence of JE virus and DEN in Taiwan. A total of 1,317 serum samples were collected and analyzed for this report.
D2/8-1 is an NS1-specific monoclonal antibody generated and analyzed as previously described (2). It recognizes a linear epitope on the NS1 antigen common to JE virus and four DEN types. Monoclonal antibodies were purified from ascitic fluid by protein A-Sepharose 4B Fast Flow affinity chromatography (Pharmacia Biotech) as described previously (3). NS1-specific indirect ELISA was performed as previously described (16). This assay can be used to analyze isotype- and serotype-specific antibody responses to NS1 antigens. In this study, serum samples were first screened for DEN NS1-specific IgG antibodies using pooled NS1 antigens from culture supernatants of DEN-1-, DEN-2-, DEN-3-, or DEN-4-infected Vero cells. DEN NS1 serotype-specific IgG antibodies were then analyzed using culture supernatant from each of the DEN-1-, DEN-2-, DEN-3-, or DEN-4-infected Vero cells.
The PRNTs were performed in BHK-21 cells as previously described (15). The prototype strains of DEN-1, DEN-2, DEN3, and DEN-4 (Hawaii, New Guinea C, H87, and H241, respectively) were used in the assay. The titer of a serum was considered to be the reciprocal of the highest dilution which neutralized
50% of the average number of plaques present in control wells.
The optical densities (OD) read from culture supernatants of Vero cells with or without dengue virus infection were assigned as the test absorbance and negative control, respectively, for each sample in the NS1-specific indirect ELISA. Positivity was determined by comparison to individual negative controls. A positive sample was defined as having a test absorbance/negative control ratio of
2.0 and a negative sample was defined as having a ratio of <2.0. Serotype specificity was defined as positive if the test absorbance ratio of homologous virus (the highest OD value) to heterologous virus (the second highest OD value) was
120%. All serum specimens were tested at least twice for reproducibility. Kappa statistics were used to evaluate the correlation between NS1 serotype-specific IgG and PRNT results (8).
In this report, we show the results of serum samples collected from residents of Liuchiu Hsiang, Pingtung County, an isolated island in southern Taiwan, during 1997-1998. DEN antibody-positive sera were first screened by DEN NS1-specific IgG ELISA. Positive sera were then analyzed by DEN NS1 serotype-specific IgG ELISA and DEN PRNT. Table 2 shows the results of NS1-specific IgG ELISA in arbitrarily specified age groups in order to fit the recorded dengue outbreaks. The results show an age-dependent increase in seroprevalence, in that 93.6, 87.5, 73.1, and 24.6% of serum samples were positive from individuals born before 1931 and in 1932-1941, 1944-1980, and 1982-1986, respectively. These data correlated very well with the known epidemics in Taiwan since the 1931 and 1942-1943 outbreaks were indeed dengue epidemics and the 1981 and 1987-1988 outbreaks occurred in Liuchiu Hsiung, affecting a large percentage of residents.
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TABLE 2. Age-dependent seroprevalence of dengue virus infection in Liuchiu Hsiang, Pingtung County, analyzed by NS1-specific IgG ELISA
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FIG. 1. Serotype analysis of DEN-positive sera from residents of Liuchiu Hsiang, Pingtung County, born between 1982 and 1986. Serum samples from 52 randomly selected DEN-positive individuals were analyzed and representative data are shown. (A) NS1 serotype-specific IgG ELISA was used to analyze the NS1-specific IgG antibodies to various DEN serotypes. (B) PRNT was used to measure neutralizing antibody titers to various DEN serotypes.
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FIG. 2. Serotype analysis of DEN-positive sera from residents of Liuchiu Hsiang, Pingtung County, born between 1944 and 1980. Serum samples from 50 randomly selected DEN-positive individuals were analyzed and representative data demonstrating primary and secondary or multiple infections are shown. (A) NS1 serotype-specific IgG ELISA was used to analyze the NS1-specific IgG antibodies to various DEN serotypes. (B) PRNT was used to measure neutralizing antibody titers to various DEN serotypes.
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Kappa statistics were applied to evaluate the correlation between NS1 serotype-specific IgG ELISA and PRNT. Among 117 serum samples available for analysis, 73 sera showed primary dengue virus infection and 32 sera showed secondary dengue virus infection, agreeing with both assays. Twelve sera were found to have discordant results. Among these, six sera showed primary infection in the NS1 assay but were classified as secondary by PRNT, and the other six showed the opposite pattern. The results suggested that good correlation existed for DEN NS1 serotype-specific IgG ELISA and DEN PRNT (
= 0.766).
Liuchiu Hsiang has been used as a good model in the study of dengue epidemics and control in Taiwan (13, 18, 19). This small island is located about 15 km southwest of Taiwan with a total population of around 16,000. It experienced several major dengue epidemics during the past century (Table 1). Among these, the 1931, 1942-1943, 1981, and 1987-1988 outbreaks were well documented. In fact, the 1981 Liuchiu Hsiang outbreak was the first dengue epidemic in Taiwan since World War II. A total of 21 virus strains were isolated for the first time in Taiwan and the serotype was identified as DEN-2. It was estimated that approximately 80% of the inhabitants were infected during this epidemic that lasted for 5 months. The 1987-1988 DEN-1 epidemic also affected Liuchiu Hsiang, although this outbreak mainly circulated in Kaohsiung City, with more than 10,000 reported cases. Although it was generally believed that those earlier epidemics (1931 and 1942-1943) were caused by DEN, direct evidence was lacking and the serotypes involved were not known. In this study, we addressed these questions by using NS1 serotype-specific IgG ELISA and comparing it with PRNT. The results correlated very well with the known epidemics in that an age-dependent increase in seroprevalence was observed. Particularly, the DEN-positive rate was found to be 73.1% for residents born between 1944 and 1980. This is close to a previous report estimating that approximately 80% of the inhabitants were infected (19). In addition, the DEN-positive rate was found to be 24.6% for residents born between 1982 and 1986. This suggested that the infection rate of the 1987-1988 epidemic in Liuchiu Hsiang was higher than was previously recognized. The age-dependent increase in NS1-specific IgG-positive sera from older individuals provided strong support that the 1942-1943 and 1931 outbreaks were indeed dengue infections. It is interesting to find that none of the 158 serum samples from residents born after 1989 was DEN positive. This is very encouraging and suggests that integrated control measures against Aedes aegypti were very successful during the last 10 years in Liuchiu Hsiang.
DEN-positive sera were analyzed by NS1 serotype-specific IgG ELISA and PRNT. The DEN serotypes identified for each age group match correctly with the known serotypes of corresponding epidemics. For instance, all of the 52 randomly selected DEN-positive sera from residents born between 1982 and 1986 showed significantly higher DEN-1 NS1-specific IgG antibody responses than the other serotypes. This suggested that all these residents had been infected with DEN-1 during the 1987-1988 outbreak. For those individuals born between 1944 and 1980, the situation was more complicated, since they could have been infected during the 1987-1988 DEN-1 and/or 1981 DEN-2 outbreaks. Figure 2 demonstrates the different patterns of primary and secondary dengue virus infections. The results showed that NS1 serotyping correlated very well with PRNT in that each of these DEN-1 and DEN-2 sera had their corresponding serotype-specific neutralizing antibodies detected. In addition, many individuals had strong and complex NS1-specific IgG antibodies to at least two DEN serotypes. It is interesting to find that some of these individuals had neutralizing antibodies to DEN-3 in addition to DEN-1 or DEN-2. It is tempting to speculate that these individuals had been infected with DEN-3 before, since many residents are fishermen and go fishing in the northern area of the Philippines. For those individuals born during 1932-1941 and before 1931, the situation was even more complicated, since they could be infected and/or stimulated several times during the 1987-1988 DEN-1, 1981 DEN-2, 1942-1943, and 1931 outbreaks. The results did show that most of the sera tested showed strong and complex NS1 serotype-specific IgG responses.
In conclusion, we have shown the potential application of NS1-specific IgG ELISA in the study of the seroprevalence of dengue virus infection. The serotype of each dengue epidemic can be correctly identified from primarily infected sera using NS1 serotype-specific IgG ELISA. Comparison between NS1 serotype-specific IgG ELISA and PRNT demonstrated high correlation. Due to the high sensitivity, high specificity, and simplicity of NS1 serotype-specific IgG ELISA, we believe it can replace PRNT for seroepidemiologic study and DEN serotyping.
This work was in part supported by grants NSC 88-2318-B-043B-001-M51 and NSC 89-2318-B-043B-001-M51 from the National Science Council, Taiwan, Republic of China.
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