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BACKGROUND: Dengue is a re-emerging vector borne disease that is endemic throughout the tropics including northern Mexico. Sporadic outbreaks were first reported in southern Texas in 1980, where less than 60 locally acquired dengue cases have since been identified. In June 2013, the incidence of dengue in northern Mexico began to rise, and peaked in August. Between July and October, 17 suspected dengue cases were reported in southern Texas and six had no recent travel outside of Texas. The Texas Department of State Health Services and CDC Dengue Branch developed enhanced surveillance to characterize the extent of the outbreak.
METHODS: Dengue cases that occurred in southern Texas during July–December were identified through reporting to health departments and retrospective record review at eight hospitals in Hidalgo and Cameron counties. All available serum specimens from suspected dengue cases previously tested at two commercial laboratories by anti-dengue virus (DENV) IgM ELISA were further tested by RT-PCR at CDC. Laboratory-positive dengue case-patients were contacted, and they and their household members were offered dengue diagnostic testing by RT-PCR and anti-DENV IgM ELISA.
RESULTS: As of December 31, 2013, 46 laboratory-positive dengue cases were identified. Of these, 30 (65%) had detectable anti-DENV IgM, 11 (24%) had DENV detected by RT-PCR, and 5 (11%) were positive by both. DENV-1 and 3 were detected in 13 and 3 cases, respectively. Case-patients resided in Cameron (36), Hidalgo (9) and Willacy (1) counties; over half (59%) lived in the City of Brownsville. Peak month of illness onset was November when 19 cases occurred. Males and females were equally affected, and median age was 31 years (range: 1–85). Travel to Mexico was reported by 17 (37%) cases; however, 13 (28%) cases reported no recent travel. From 18 dengue case-patients’ households, 37 household members provided a serum specimen and six (16%) had detectable anti-DENV IgM, of which five (83%) had no recent travel history.
CONCLUSIONS: Enhanced surveillance during a dengue outbreak in southern Texas enabled identification of 46 cases. To date, 13 locally acquired dengue cases have been identified, the largest number reported during a single year in southern Texas since 1980, and the investigation is on-going. Sixteen percent of household contacts had evidence of recent DENV infection and most were locally acquired. Because both travel-associated and locally acquired dengue cases are expected to continue to occur in southern Texas, dengue surveillance and laboratory capacity should continue to be strengthened.