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Underdiagnosis of Dengue --- Laredo, Texas, 1999

Dengue outbreaks have been reported in communities along the Mexico--U.S. border since 1980 (1); however, during 1987--July 1999, no cases were reported from Laredo, Texas (1999 population: 162,000). During January--July 1999, approximately 300--325 dengue cases were reported from Nuevo Laredo, Tamaulipas, Mexico (1999 population: 274,000), a city across the Rio Grande from Laredo. To determine whether undiagnosed or unreported dengue cases had occurred in Laredo, the Texas Department of Health (TDH) reviewed medical records from five Laredo health facilities (the two city hospitals and the three largest of five community clinics). This report summarizes the findings of the review, which indicated that during July 23--August 20, 1999, 50% of suspected case-patients had undiagnosed dengue infection. Recognition of the diagnosis of dengue can be improved through heightened surveillance, professional and public education, and prompt reporting of cases by the health-care providers to local or state health departments.

Medical records were reviewed for all patients who presented to one of the five facilities with fever, arthralgias, myalgias, rash, or headache during July 23--August 20. A case of dengue was suspected in a person aged >5 years with a temperature of >101 F (>38.3 C) and rash of any duration or fever for >3 days without cough or diarrhea. During August 20--October 31, blood was drawn from suspected dengue case-patients and serum samples were tested for antidengue IgG and dengue IgM antibodies at the TDH laboratory. A confirmed case of recent dengue was defined as a positive IgM test or a fourfold or greater increase in the IgG antibody titer between acute- and convalescent-phase serum samples.

Forty-nine suspected dengue case-patients were identified from 494 records; 24 (49%) were located and interviewed. Of these, 22 (92%) agreed to provide a serum sample. Eleven case-patients had serologic evidence of recent dengue infection; 10 (91%) of the 11 tested positive for both IgM and IgG antibodies. One case-patient was negative for IgM antibodies but had a fourfold increase in IgG antibody titers over a 3-month period. Symptoms reported by the 11 confirmed case-patients included fever (100%), arthralgias (73%), headache (64%), malaise (64%), and rash (45%). Discharge diagnoses of "viral syndrome" or "viral fever" were given to nine (82%) and "flu-like illness" were given to two (18%). Nine case-patients reported a history of travel to Mexico within 2 weeks of illness onset; two had not been outside Texas.

Reported by: G Peña, City of Laredo Health Dept, Laredo; E Svenkerud, MD, Bur of Communicable Disease Control; B Liszka, Bur of Laboratories; K Hendricks, MD, J Rawlings, MPH, Infectious Diseases Epidemiology and Surveillance Div, Texas Dept of Health. Div of Applied Public Health Training, Epidemiology Program Office; Dengue Br, Div of Vectorborne Infectious Diseases, National Center for Infectious Diseases; and an EIS Officer, CDC.

Editorial Note: 

Dengue is an arboviral illness of tropical and subtropical areas commonly transmitted by Aedes aegypti mosquitoes (2,3). Approximately 2.5 billion persons live in regions where dengue is endemic and 50--100 million infections occur annually (2,4). Although infection may result in lifelong homotypic immunity, cross-protective immunity does not occur among the four dengue virus serotypes. Infection with any dengue serotype can be asymptomatic or can cause dengue, dengue hemorrhagic fever (DHF), or dengue shock syndrome (DSS). DHF and DSS are life-threatening conditions (5). Since the 1970s, outbreaks of dengue, DHF, and DSS have increased in frequency and severity in the Americas and the Caribbean (2,6).

Dengue may present as an undifferentiated febrile illness and unless physicians retain a high level of suspicion, a dengue diagnosis may be missed easily in areas where the virus is not endemic. Laboratory testing is necessary for diagnostic confirmation. Acute- and convalescent-phase serum samples should be obtained for diagnosis and sent for confirmation to state or territorial health department laboratories. Serum samples should be accompanied by a summary of clinical and epidemiologic information, including onset date, sample collection date, and a travel history for the 3 weeks before illness onset.

An estimated two million crossings occur each month between Laredo and Nuevo Laredo, and Ae. aegypti is found in both cities. Movement of infected persons can introduce the virus into dengue-free areas. Travelers to regions where dengue is endemic should avoid exposure to mosquito bites by using repellents and protective clothing and by staying in well-screened or air-conditioned quarters. Residents of areas where dengue is endemic and Mexico-U.S. border communities can reduce the Ae. aegypti population in and around homes by changing water in bird baths or flower vases daily, tightly covering stored water receptacles, and eliminating old tires, containers, tree holes, and other potential mosquito breeding sites.

Following identification of dengue cases, the Laredo Health Department implemented mosquito reduction activities (e.g., aggressive refuse and tire disposal campaigns and insecticide fogging). Dengue alerts were sent to health-care providers, and mosquito reduction and personal protection information was distributed through health fairs and schools. Information exchange increased substantially between health officials from Laredo and Nuevo Laredo. Although no suspected cases were reported before the alerts were issued, 161 suspected dengue cases were reported during mid-August--December 1999; 18 cases tested positive for dengue. No positive cases were reported from Laredo in 2000.

When a case of dengue is confirmed in a community, the public health response should include education of health-care providers and the public, intensified surveillance, and enhanced vector-control activities. Additional information about dengue is available on the World-Wide Web,


  1. Rawlings JA, Hendricks KA, Burgess CR, et al. Dengue surveillance in Texas, 1995. Am J Trop Med Hyg 1998;59:95--9.
  2. Gubler JG. Dengue and dengue hemorrhagic fever: its history and resurgence as a global public health problem. In: Gubler DJ, Kuno G, eds. Dengue and dengue hemorrhagic fever. New York, New York: Cab International, 1997:1--22.
  3. Pan American Health Organization. Dengue and dengue hemorrhagic fever in the Americas: guidelines for prevention and control. Washington, DC: Pan American Health Organization, 1994 (Scientific publication no. 548).
  4. Halstead SB. Pathogenesis of dengue: challenges to molecular biology. Science 1988;239:476--81.
  5. Henchal EA, Putnak JR. Dengue viruses. Clin Microbiol Rev 1990;3:376--96.
  6. Pan American Health Organization. Reemergence of dengue in the Americas. Epidemiological Bulletin of the Pan American Health Organization 1997;18:1--10.

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