Dengue Epidemic in Belem, Para, Brazil, 1996-97 - Statistical Data Included

Emerging Infectious Diseases, May, 2000 by Amelia P.A. Travassos da Rosa, Pedro F.C. Vasconcelos, Elizabeth S. Travassos da Rosa, Sueli G. Rodrigues, Bernard Mondet, Ana C.R. Cruz, Maria R. Sousa, Jorge F.S. Travassos da Rosa

We describe clinical and epidemiologic findings during the first epidemic of dengue fever in Belem, Para. State, Brazil, in 1996-97. Of 40,237 serum samples, 17,440 (43%) were positive for dengue by virus isolation or serologic testing. No hemorrhagic cases or deaths were reported.

Dengue fever (DF) and dengue hemorrhagic fever (DHF) are caused by infection with one of the four serotypes of dengue virus (DEN-1, DEN-2, DEN-3, and DEN-4), transmitted by Aedes aegypti mosquitoes. In Brazil, DF epidemics reported in the 1980s and 1990s involved more than a million cases. However, only 671 DHF cases were diagnosed, with 26 deaths (1).

Ae. aegypti was reintroduced in Para State in 1992, and the first dengue cases were reported in 1995 in the southeast region (Redencao and Rondon do Para). In October 1996, eight cases of febrile denguelike illness were reported in Belem (population 1,300,000), a city in the Brazilian Amazon region at the confluence of the Amazon River and the Atlantic Ocean. In early November, DEN-1 virus was isolated and identified (2-4). DEN-2 virus was identified in October 1997, and since then, both serotypes have been responsible for illness in Belem. This was the first time dengue virus transmission occurred in Belem during the last 70 years and the third time the disease occurred in the Brazilian Amazon region. Previous outbreaks had been reported in 1981-82 in Boa Vista, Roraima (5), and in 1991 in Araguaina, Tocantins State (6).

We describe cases of denguelike illness diagnosed at Instituto Evandro Chagas. A case of dengue was defined as illness with the following symptoms: acute onset of high fever, headache, myalgia, arthralgia, dizziness, and other symptoms and signs suggestive of denguelike illness in a patient with a positive IgM by IgM-capture enzyme-linked immunosorbent assay (MAC ELISA), virus isolation or serologic conversion in paired serum samples, and an increase of at least fourfold in titer in the convalescent-phase serum sample (7-9).

The Study

From January to December 1997, 40,237 serum samples were drawn from febrile patients in Belem, 20,038 (49.8%) of whom were male. The patients were all residents of the municipalities of Belem (31,506 samples) and Ananindeua (8,731 samples). Most were ambulatory patients seen in the Arbovirus Department, Evandro Chagas Institute; some patients were referred by public health centers, private physicians, and clinics. At the Institute, all patients were examined clinically and had blood samples drawn. A questionnaire was administered that included information about clinical symptoms and signs and demographic data. Patients who had been ill [is less than] 10 days and whose serologic tests were negative for dengue were requested to provide a second blood sample 7 to 14 days later. For patients who had at least one hemorrhage and either dehydration or hemoconcentration, a leukogram, platelet count, and hematocrit were performed.

Serum samples were tested for dengue antibodies by MAC ELISA (4) and hemagglutination-inhibition test (2). The antigens for both tests were prepared by using infected mouse brain extracted by the sucrose acetone method. The criteria used for establishing primary and secondary infection were those recommended by the World Health Organization (7,8).

To isolate dengue virus, 0.1-ml aliquots of whole blood from patients with clinical symptoms lasting [is less than] 5 days and negative serologic results were injected into cultures of C6/36 cells (10). The cultures were visually examined daily, and cells were tested on days 7 and 14 by immunofluorescence (3).

Isolated virus strains were initially screened by direct immunofluorescence against a flavivirus standard hyperimmune fluid prepared at the Institute. Strains that reacted were identified to serotype by using an indirect immunofluorescence test with monoclonal antibodies against the four dengue viruses provided by the Centers for Disease Control and Prevention.

The epidemic distribution was accompanied early in 1997 by a seasonal increase in rainfall typical of the Brazilian Amazon. However, the highest dengue positivity rates were reported in the dry months of September to December. Coincidentally, DEN-2 virus was isolated in October. At first, cases were reported only in Belem and Ananindeua, but in December 1997, at least 15 other municipalities reported transmission cycles involving either DEN-1 or DEN-2 or both.

Of all sera collected, 17,525 (43.5%) were positive by serologic testing or DEN virus isolation: 13,805 (78.8%) from Belem and 3,720 (21.2%) cases from Ananindeua (Figure). Both primary and secondary serologic responses were found. Among the positive samples, 9,469 (54.25%) were among female and 7,971 (45.75%) among male patients (p [is less than] 0.0001); 45.95% of the patients were 25 to 44 years of age (Table 1). Paired serum samples were obtained from 3,558 patients, 2,997 (84.2%) of whom had serologic conversions.

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Table 1. Distribution of patients with serologic tests positive for dengue