Dengue, West Nile, and Other Mosquito-Borne Arboviruses

Neurotropic Mosquito-Borne Viruses

West Nile Virus

WNV can range in severity from asymptomatic disease to severe meningitis and encephalitis. The milder version of WNV, referred to as West Nile fever, can be characterized by an abrupt onset of fever, headache, and in some cases, rash, among other symptoms. The more severe neuroinvasive disease affects one in 150 infected individuals and presents with meningitis, encephalitis, or flaccid paralysis syndrome. 

Indications for Testing

Testing for WNV is indicated for patients who either live in or have traveled to areas where WNV is circulating during mosquito season and who present with acute, unexplained febrile illness, rash, and/or meningitis/encephalitis. 

Diagnostic Laboratory Testing

Serology

Acute WNV infection is diagnosed by testing serum or cerebrospinal fluid (CSF) for WNV-specific immunoglobulin M (IgM) antibodies. WNV IgM antibodies are detectable 3 to 8 days after illness onset, but in some cases can persist for 30 to 90 days. IgG is detectable approximately 3 weeks postinfection and persists for years; the presence of IgG only indicates previous exposure. False-positive results can occur due to cross-reactivity with other arboviruses (eg, Eastern equine encephalitis [EEE] virus, Western equine encephalitis [WEE] virus, SLE virus) and dengue virus. Plaque-reduction neutralization testing (PRNT) may be required to determine the specific infecting virus. 

Nucleic Acid Amplification Testing

NAAT is not recommended for diagnosis and is typically reserved for immunocompromised patients or for clarifying equivocal serology results. 

Other Neurotropic Arboviruses

Although less common than WNV, the following arboviruses are endemic in the U.S. and can cause meningitis and encephalitis:

• SLE virus 
• EEE virus 
• California encephalitis virus
• WEE virus

Japanese encephalitis virus, endemic in Asia, can also cause meningitis and encephalitis. 

These viruses are diagnosed in a similar manner as WNV.

Nonneurotropic Mosquito-Borne Viruses

Zika Virus

Zika virus is transmitted to humans by mosquitos and is also transmissible through sexual contact, blood product transfusions, and pregnancy (from mother to fetus). Notably, infection with Zika virus during pregnancy has been linked to congenital microcephaly and other serious brain defects in fetuses and infants. Human-to-human transmission apart from sexual transmission has been described, but the mechanism of transmission is uncertain.  The majority of individuals infected with Zika virus are asymptomatic. In symptomatic individuals, infection is usually mild and self-limiting and may last for several days to a week. Zika viral infection symptoms include fever, headache, retro-orbital pain, conjunctivitis, maculopapular rash, myalgia, and arthralgia. These symptoms are not unique to Zika virus and may be seen in patients infected with other arboviruses, such as dengue and chikungunya.

Diagnostic tests for Zika virus include NAAT and IgM serology. The test of choice depends on when the patient presents in relation to symptom onset or last possible exposure to Zika virus. Due to cocirculation and cross-reactivity, additional testing for dengue and chikungunya viruses should be considered.

Indications for Testing

Zika testing is not recommended for nonpregnant individuals; symptomatic nonpregnant individuals are advised to consider dengue virus testing.   In the event that a country reports an outbreak of Zika virus, the CDC’s Dengue and Zika Virus Diagnostic Testing for Patients with a Clinically Compatible Illness and Risk for Infection with Both Viruses  should be followed. The CDC’s current Zika virus testing guidance by population is detailed in the table below.

Diagnostic Laboratory Testing

Nucleic Acid Amplification Testing

The CDC recommends molecular testing by NAAT on serum and urine in symptomatic pregnant women who meet the criteria in the Summary of Laboratory Testing Recommendations for Pregnant Women and Infants table during the acute phase of the disease (<14 days since symptom onset).  Refer to testing guidance for dengue virus if patients are symptomatic and not pregnant. Zika testing is not currently recommended for this group based on the current epidemiology of the virus. 

NAAT tests may not detect Zika virus RNA in an infant who had a Zika virus infection in utero or in a child if the period of viremia has passed. 

Serology

Zika virus-specific IgM and neutralizing antibodies typically develop toward the end of the first week of illness. IgM levels are variable but are generally positive beginning about 4 days after symptom onset and typically persist for up to 12 weeks after symptom onset or exposure (but may persist longer). False-positive Zika virus IgM serology results may occur in patients with a recent closely related arboviral infection (eg, dengue virus) or with recent vaccinations to related arboviruses (eg, Japanese encephalitis or yellow fever). Therefore, concurrent evaluation for other possible cocirculating arboviruses, especially dengue and chikungunya, should be considered.     The CDC currently does not advise the use of Zika virus-specific serology testing in nonpregnant individuals. 

Clinical decisions regarding patient management cannot be based on a positive IgM result alone. Confirmatory PRNT for Zika virus neutralizing antibodies is performed by the CDC or a CDC-designated laboratory on all presumptive positive Zika IgM results.  

Dengue Virus

Dengue virus and Zika virus are closely related arboviruses with similar symptoms and distribution patterns throughout the tropics.  Dengue virus has four distinct serotypes (1, 2, 3, and 4) and is endemic throughout the tropics and subtropics. Although transmission most commonly occurs from the bite of an infected Aedes spp mosquito, dengue virus can also be transmitted during childbirth and through blood and breast milk. Clinical manifestations range from the more common self-limited dengue fever to dengue hemorrhagic fever with shock syndrome, the most serious manifestation. The risk of progression to severe disease is much higher in secondary dengue infection than in primary infection. 

Diagnostic tests for dengue virus include serology, NS1 antigen testing, and NAAT. The test of choice depends on when the patient presents in relation to symptom onset or last possible exposure to dengue virus. Additional testing for other arboviruses, particularly Zika virus, is recommended if an outbreak is occurring or has occurred.  

Indications for Testing

Testing for dengue virus should be considered in patients with appropriate clinical manifestations (eg, fever, headache, hemorrhage, retro-orbital pain) and recent travel to or residence in an area with known dengue transmission.  

Diagnostic Laboratory Testing

Nucleic Acid Amplification Testing

NAAT or NS1 antigen and IgM antibody testing are recommended during the first week of illness.   NAAT is the preferred method for diagnosis because it can confirm infection and identify the specific virus, distinguishing between dengue and Zika viruses that often cocirculate, for example. A negative NAAT or NS1 result should be followed with an IgM antibody test.  Refer to the CDC for more information about differentiating between dengue and Zika virus. 

Serology

Although generally the preferred method of diagnosis, NAAT can only detect dengue virus infection for a short period of time. Therefore, serology is usually used to diagnose dengue virus infection. After 7 days of illness, only serology testing is recommended. In primary dengue infection, IgM antibodies are reliably detectable after 7 days from symptom onset and can remain detectable for 3 months or longer.  Although IgM antibody testing is important for diagnosis, false-positive dengue enzyme-linked immunosorbent assay (ELISA) results can occur due to cross-reactivity in patients infected with other flaviviruses such as Zika virus, WNV, and SLE virus.   Therefore, concurrent evaluation for other possible cocirculating arboviruses, such as Zika virus, should be considered, and PRNT is recommended.    Detailed vaccine, travel, and possible exposure history is key for determining the need for additional laboratory testing to identify the infecting virus. A patient with a positive dengue IgM-only result (negative for NAAT and NS1) is considered to have presumptive dengue infection only if no other arboviruses are circulating in the areas where the patient was exposed. 

Chikungunya Virus

Chikungunya virus is primarily transmitted to humans by Aedes spp mosquitos. Bloodborne transmission is possible, and rare in utero and intrapartum transmissions have been documented.  Unlike individuals with dengue and other mosquito-borne arboviruses, most individuals infected with chikungunya virus become symptomatic. Acute onset of fever and polyarthralgia are the most common clinical symptoms associated with chikungunya virus. Joint pain can be severe and may be persistent or relapsing in the months following acute illness. 

Indications for Testing

Testing for chikungunya should be considered in patients with acute onset of fever and polyarthralgia and recent travel to or residence in an area with known chikungunya virus transmission. 

Diagnostic Laboratory Testing

Nucleic Acid Amplification Testing

NAAT is appropriate for all individuals suspected of having chikungunya virus who present in the early stages of disease. A positive result indicates current infection,  and no further testing is indicated. A negative PCR result does not exclude infection and should be followed by virus-specific IgM serology. 

Serology

IgM serology is appropriate for individuals with clinical suspicion for chikungunya virus who present later in the course of illness (beyond the first week of illness). Virus-specific chikungunya IgM antibodies are detectable toward the end of the first week of illness and remain detectable for 3-4 months after infection, whereas IgG antibodies to chikungunya virus remain detectable for years.

As with Zika virus, false-positive chikungunya virus IgM serology results may occur in patients with closely related infections or with recent vaccinations to related viruses. Therefore, concurrent evaluation for other possible cocirculating viruses, especially Zika and dengue, should be considered.