Indirect immunofluorescence (IF) was performed on C6/36 cells with DENV-1, -2, -3, and -4 serotype–specific monoclonal antibodies (kindly provided by Nick Karabatsos, Centers for Disease Control and Prevention, Fort Collins, Colorado); on mouse hyperimmune ascitic fluids for West Nile virus and Bunyamwera virus; and on grouping fluids for alphaviruses, phleboviruses, and California serogroup viruses. After isolation attempts, remaining sera were used for serologic studies but not for DENV-3–specific reverse transcription–polymerase chain reaction (RT-PCR) because too little serum was available after serologic tests. RNA Preparation and cDNA Synthesis Viral RNA was extracted from 200-µ L aliquots of infected cells supernatants by using the High Pure Viral RNA kit (Roche Diagnostics, Meylan, France) following manufacturer’s protocol. Two overlapping viral cDNA fragments were generated by RT with Superscript II reverse transcriptase (Invitrogen/Life Technologies, Cergy Pontoise, France) according to the manufacturer protocols with D3/C/863/18 and D3/C/1419/18 primers (Table). Specific primers (Table) designed from the nucleotide sequence of the reference DENV-3 H87 (GenBank acces- sion no. L11423) (13) were used for PCR amplification by using AmpliTaq DNA Gold (Applera, Courtaboeuf, France). DNA Sequencing PCR products were purified from 1.5 % agarose gels, by using the QIAquick gel extraction kit (Qiagen, Courtaboeuf, France) and directly sequenced with the direct and reverse primers (Table) and the Big Dye Sequencing kit (Applera). Sequencing was carried out with an automatic sequence analyzer (ABI PRISM 3100, Applera) following the manufacturer’s protocol. Sequences of the Saint Martin DENV isolates were compared to GenBank database DENV sequences (com- plete list in Online Appendix). Alignments of nucleotide and amino acid sequences of the prM/M and partial E nucleotide sequences (nucleotide numbers 437–1144 by reference to the sequence of the D3H87 strain) were per- formed with ClustalW1.7 software (14). Phylograms were constructed with the MEGA 2 program (15), and tree drawing used the Jukes-Cantor algorithm for genetic dis- tance determination and the Neighbor Joining method. The robustness of the resulting tree was tested by 1,000 boot- strap replications. Results and Discussion From October 2003 to April 2004, DENV-3 circulated on the island, and 108 cases of dengue fever were serolog- ically or virologically confirmed in both the Lepers Dengue Type 3 Virus, Saint Martin, 2003–2004 Laboratory and the IMTSSA laboratory, and 12 persons were hospitalized (10). If one takes into account the 180 suspected cases observed during this period, the incidence of dengue fever during the epidemic was ≈0.62% (180/29,000) on the French part of the island; this value was consistent with values already observed in the Caribbean region (http://www.carec.org/data/dengue/ 1998/table2.html). No data were available from the Dutch half. During this period, an additional 26 blood samples from patients with dengue-like syndromes at early stage of the infection were received in IMTSSA Tropical Virology Laboratory in Marseille. Among them, 13 were positive for dengue infection by presence of IgM, elevation (4-fold) of specific IgG, or both. Virus was isolated from 6 other patients, and DENV-3 was identified by IF using serotype- specific monoclonal antibodies. No fluorescence was observed when DENV-1, DENV-2, or DENV-4 monoclonal antibodies were used. Moreover, IF tests were negative when antibody to West Nile virus, Toscana virus, Bunyam- wera virus, alphaviruses, phleboviruses and California serogroup viruses was used. These cases were considered confirmed dengue fever; all the patients recovered fully. DENV-3 was genetically identified in the 6 IF positive isolates (D3StMart1–D3StMart6, referred to as GenBank accession no. AY750713–AY750718) by using BLAST- NCBI software. Comparison of partial sequences showed a high degree of identity between the strains isolated from patients on Saint Martin: paired identity at the nucleotide level ranged from 99.3% to 100%. When compared to 00PuertoR1 strain of the Latin American cluster defined by Messer (5), the values were 99.5%–99.6%. Nucleotide comparison gave the following values: 98.3%–98.4% for East African cluster (5), 98.8%–98.9% for Group B (5), and 96.8%–96.9% for Group A (5). This comparison indi- cated a close relationship between Saint Martin and Latin American isolates. The strains from Saint Martin shared an 11-nucleotide insertion between position 10,275 and position 10,276 in the 3′UTR (AGTGAAAAAGA). The same insertion was found in isolates from Martinique 2 years earlier (6) and from DENV-3 Sri Lanka (6), indicating a probable com- Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 11, No. 5, May 2005 759