Unique Challenges in Arbovirus Surveillance in Florida: Finding Dengue Cases in a Year of West Nile

BACKGROUND:  Florida  has one of the highest number imported cases of dengue fever reported in the US and  has reported local introductions of dengue every year since 2009. In addition, since 2001 Florida has had seasonal transmission of the West Nile virus (WNV) resulting in an average of 25 cases per year (range 0-92). Dengue fever and non-neuroinvasive WNV illness can demonstrate a similar clinical presentation, and significant antibody cross-reaction can occur on immunologic tests. This poses a challenge to health care providers as well as local and state public health agencies. In addition, persons with repeat dengue infections may mount a transient IgM response that can be missed. The misclassification of dengue fever as non-neuroinvasive WNV illness or other febrile illness can result in suboptimal patient treatment, cause delays in identification of local dengue introductions, and result in ineffective vector surveillance and control measures.  A nationwide WNV outbreak in 2012, resulted in media and public attention focusing on WNV activity while a Caribbean dengue epidemic received little attention, highlighting the need for public health surveillance and rapid confirmatory testing for endemic arboviruses as well as dengue.

METHODS:  Dengue and WNV illness are both reportable conditions in Florida.  Confirmatory testing at the state public health laboratory is required for all WNV illness cases and is recommended for all acute dengue fever samples.  A dengue polymerase chain reaction (PCR) test developed by Centers of Disease Control and Prevention was routinely used to test all suspect acute dengue and WNV fever samples.  Dengue IgM enzyme-linked immunoassay (EIA) was routinely performed on all suspect dengue and WNV fever samples.

RESULTS:  Implementation of dengue testing for suspected WNV fever cases resulted in identification of the first local dengue case in Florida in 2012 as well as misidentified imported dengue cases.  Routine use of dengue PCR resulted in definitive identification and serotype of a second local dengue case with IgM negative test results.  Approximately half of all probable and confirmed dengue cases identified in Florida in 2012 had positive dengue PCR results.   

CONCLUSIONS:  In areas where cases of both WNV illness and dengue fever may be found, it is essential that public health officials have access to tests that can rapidly and accurately identify both, and should craft standard surveillance procedures to target cases most likely to be misidentified.