Dengue, West Nile, and Other Mosquito-Borne Arboviruses

Mosquito-borne arboviruses are transmitted to humans through the bite of an infected mosquito, and infections typically occur during the summer months when insects are most active. In the United States, the most common mosquito-borne illness is West Nile virus (WNV), a neurotropic human pathogen capable of spreading to the brain via hematogenous dissemination. Other neurotropic mosquito-borne arboviruses, including St. Louis encephalitis (SLE) virus and Japanese encephalitis virus, are less common but may cause similar symptoms.

Arboviruses—including dengue, Zika, and chikungunya—have similar epidemiologies, transmission cycles, and clinical symptoms at onset (although complications vary substantially). Laboratory testing options for mosquito-borne arboviruses include serology and nucleic acid amplification testing (NAAT). The appropriate methodology depends on the suspected infection and duration of symptoms.

Quick Answers for Clinicians

Why is it important to test for related viruses in persons suspected of having a mosquito-borne illness?

Many mosquito-borne viruses cocirculate in certain areas of the world and can cause similar clinical presentations. Moreover, serology testing can be complicated by cross-reactivity with related arboviruses. Therefore, adequate history of travel and vaccinations, especially if an individual has been in areas where viruses are cocirculating or where immunizations (eg, for yellow fever) are routine, can help determine the best strategy for further testing to identify the infecting virus. Plaque-reduction neutralization testing (PRNT) is recommended for confirmation because it can resolve nonspecific reactivity and, in some cases, identify the infecting virus.

Where do mosquito-borne arboviruses circulate?

Refer to the following sites for national and global data and maps of mosquito-borne virus circulation trends:

CDC ArboNET: up-to-date provisional reporting of arboviruses circulating in the United States, including West Nile virus (WNV), St. Louis encephalitis (SLE) virus, Eastern equine encephalitis (EEE) virus, and dengue, chikungunya, and Zika viruses
Zika virus travel information: global map of countries and territories with Zika virus cases
DengueMap: global map of dengue virus risk throughout the world
Chikungunya geographic distribution: map of countries and territories where chikungunya virus cases have been reported

Where can I find the most up-to-date testing guidance for mosquito-borne viruses?

Refer to the following disease-specific CDC sites for current testing information:

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.

Summary of Laboratory Testing Recommendations for Pregnant Women and Infants
Population	Patient Characteristic(s)	Recommendation(s)
Pregnant women	Symptomatic	Perform dengue and Zika virus NAAT on a serum specimen and Zika NAAT on a urine specimen Perform dengue IgM testing Refer to the CDC’s Zika and Dengue Testing Guidance (updated September 2022) for more information
	Asymptomatic, living in or with a history of recent travel to the U.S. and its territories	Routine Zika testing is NOT recommended
	Asymptomatic, living in or with a history of recent travel to an area with a risk of Zika virus outside of the U.S. and its territories	Routine Zika testing is NOT recommended, but NAAT may still be considered
	Prenatal ultrasound findings consistent with congenital Zika virus infection; residence in or a history of travel to areas with a risk of Zika virus during pregnancy	Perform Zika virus NAAT and IgM testing on maternal serum and NAAT on maternal urine If Zika NAAT is negative and IgM is positive, confirmatory PRNT should be performed against Zika and dengue If amniocentesis is performed as part of clinical care, Zika virus NAAT of amniocentesis specimens should also be performed; refer to the CDC’s Zika and Dengue Testing Guidance (updated September 2022) for more information
Infants	Birth defects consistent with congenital Zika syndrome and born to mothers with possible Zika virus exposure during pregnancy, regardless of the mother’s Zika virus testing result	Testing recommended Perform serum and urine NAAT and serum IgM serology concurrently If CSF is obtained for other purposes, perform NAAT and IgM antibody testing on CSF Cord testing is not recommended; refer to the CDC testing algorithm for infants with possible congenital Zika virus infection for more information
	No birth defects consistent with congenital Zika syndrome but born to mothers with laboratory evidence of possible Zika virus infection during pregnancy	Testing recommended Perform serum and urine NAAT and serum IgM serology concurrently If CSF is obtained for other purposes, perform NAAT and IgM antibody testing on CSF Cord testing is not recommended; refer to the CDC testing algorithm for infants with possible congenital Zika virus infection for more information
	No birth defects consistent with congenital Zika syndrome and born to mothers without laboratory evidence of possible Zika virus infection during pregnancy	No testing recommended
Source: CDC, 2022; CDC, 2022

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.

Zika IgM Serology Testing
Result Interpretation
Presumptive Zika	Possible Zika	Flavivirus Positive	Negative for Zika
Zika IgM antibody detected Confirmatory PRNT by CDC or CDC-designated laboratory required Testing for related viruses with cocirculation should also be considered	Zika IgM antibody detected, suggesting possible recent infection Other arbovirus IgM antibodies with Zika virus cross-reactivity may be present Confirmatory PRNT by CDC or CDC-designated laboratory required Testing for related viruses with cocirculation should also be considered	Presumptive positive for other flaviviruses Perform dengue and WNV IgM testing	No evidence of IgM antibodies to Zika virus Perform NAAT to exclude false-negative IgM result for specimens collected <14 days after symptom onset No further testing required for specimens collected ≥14 days after symptom onset
Source: CDC, 2022

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.

Dengue Virus by PCR Testing
Result Interpretation
Detected	Not Detected
Indicative of current infection No further testing indicated	Does not exclude dengue virus infection If test was performed during the first 5 days of illness, follow with dengue IgM serology
PCR, polymerase chain reaction Source: CDC, 2019

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.

Dengue Virus IgM and IgG Serology Testing
Result Interpretation
IgM	IgG	Interpretation
Positive	Positive	Presumptive current or recent dengue virus infection
Negative	Negative	No evidence of dengue virus infection
Positive	Negative	Current or recent dengue virus infection; consider repeat testing to demonstrate seroconversion
Negative	Positive	Past dengue virus infection
Source: CDC, 2019

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.

Chikungunya Virus by PCR Testing
Result Interpretation
Detected	Not Detected
Indicative of current infection No further testing indicated	Does not exclude chikungunya virus infection If test was performed during the first 5 days of illness, follow with IgM serology
Source: Johnson, 2016

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.

Chikungunya Virus IgM and IgG Serology Testing
Result Interpretation
IgM	IgG	Interpretation
Positive	Positive	Current or recent chikungunya virus infection
Negative	Negative	No evidence of chikungunya virus infection
Positive	Negative	Current or recent chikungunya virus infection; consider repeat testing to demonstrate seroconversion
Negative	Positive	Past chikungunya virus infection
Source: Johnson, 2016