Diagnosis of Zika virus infection is based on clinical suspicion along with molecular and serological testing. Although the majority of infected people are asymptomatic, physicians should have a high index of suspicion for patients who present with fever, a maculopapular (sometimes morbilliform) rash, arthralgia/myalgia, and conjunctivitis in the correct epidemiological context (i.e., residence in/travel from an area where there is a current outbreak or the potential for transmission).
The clinical spectrum of disease overlaps with that caused by other arbovirus infections. As a consequence, the differential diagnosis is broad and includes dengue and chikungunya infection. It is important to differentiate between Zika, dengue, and chikungunya virus infection as the 3 diseases can produce similar symptoms, particularly during the acute phase. The World Health Organization (WHO) has produced a tool to help physicians differentiate between these 3 diseases.  Molecular or serological testing is required to confirm the diagnosis.
There is strong scientific consensus that Zika virus is a cause of microcephaly and other congenital abnormalities. The range of abnormalities seen and the likely causal link to Zika virus suggest a new congenital syndrome which WHO is in the process of defining.  Preliminary results from a case-control study have confirmed that Zika virus causes microcephaly. 
Guillain-Barre syndrome and other neurological disorders are strongly associated with and suspected to be caused by Zika virus infection but the link is unproven and studies are ongoing, including to elucidate a possible mechanism.    
Standard precautions (e.g., hand hygiene, use of personal protective equipment, respiratory hygiene and cough etiquette, safe injection practices, safe handling of potentially contaminated equipment or surfaces) are recommended for the protection of healthcare professionals and patients in healthcare settings and labour and delivery settings. These precautions are recommended regardless of whether the infection is suspected or confirmed. 
Diagnosis should be suspected in patients who have resided in/travelled from an area where there is a current outbreak (or where the Aedes mosquito is present) in the 2 weeks prior to symptom onset. Non-vector transmission events (e.g., perinatal,  in utero,   sexual,    and transfusion transmission     ) have also been reported.  Sperm donation is a theoretical concern; however, there have been no reports as yet.
The incubation period after transmission is between 3 and 14 days.  Approximately 80% of patients do not develop symptoms.  In those who do, characteristic clinical findings include fever, an itchy maculopapular (sometimes morbilliform) rash, arthralgia, and non-purulent conjunctivitis. The characteristic rash is one of the most distinctive symptoms. 
Other commonly reported symptoms include myalgia, malaise, and headache.   Less common symptoms include vomiting/diarrhoea, abdominal pain, anorexia, oedema of the lower limbs, and retro-orbital pain.  No differences in clinical presentation have been described between pregnant women and non-pregnant patients, or between adults and children. Most children have a rash, and more than half have a fever and rash.  Symptoms generally develop within 1 week of infection in 50% of patients, and within 2 weeks of infection in 99% of patients. 
Clinical illness is usually self-limited with mild symptoms lasting 2 to 7 days.   Severe disease requiring hospitalisation is uncommon and the case fatality is low.  Immunosuppressed patients may experience more severe complications; however, data from a small number of case reports suggest that people with HIV infection are not at an increased risk for severe illness. 
Neurological examination should be performed on all patients with suspected GBS. Key diagnostic factors include paraesthesias (usually of the hands and feet), muscle weakness, pain (usually starts in the back and legs), and paralysis. Oropharyngeal, facial, and extraocular weakness may also occur. The WHO recommends using the Brighton criteria for the case definition of GBS.  The Pan American Health Organization (PAHO) has also published a case definition for Zika-related GBS.  Physicians should be vigilant for early signs and symptoms of GBS as it may progress faster than usual in patients with Zika virus infection.  Neurological consultation is recommended in patients with suspected GBS. 
Congenital Zika syndrome is a recognised pattern of congenital anomalies (i.e., microcephaly, intracranial calcifications or other brain anomalies, or eye anomalies, among others) in infants associated with Zika virus infection during pregnancy.      Infants may present with microcephaly or other manifestations including spasticity, seizures, craniofacial disproportion, brainstem dysfunction, ocular abnormalities, hearing loss, findings on neuro-imaging (e.g., cortical disorders, calcifications, ventriculomegaly), arthrogryposis (e.g., congenital joint contractures), irritability, dysphagia,  and feeding difficulties. Other presentations include ocular abnormalities in infants without microcephaly or other brain abnormalities, postnatal-onset microcephaly in infants born with a normal head circumference, postnatal-onset hydrocephalus in infants born with microcephaly, sleep electroencephalogram (EEG) abnormalities, and diaphragmatic paralysis in infants born with microcephaly and arthrogryposis.  Features consistent with fetal immobility (e.g., dimples, feet malpositions, distal hand/finger contractures) may also be present.  There have been reports of these abnormalities in infants who have a normal head circumference and with mothers who do not report having a rash during pregnancy.      The syndrome does not appear to be associated with maternal disease severity.  Signs of congenital brain injury due to Zika virus infection acquired during the third trimester of pregnancy have been reported.  Poor head growth with microcephaly developing after birth has been reported in a small number of patients in Brazil.  Eye abnormalities may be the only initial finding; therefore, it is recommended that all infants with potential Zika virus exposure should undergo an eye examination regardless of the presence or absence of other symptoms.  Other infectious causes of microcephaly should be ruled out. 
Case definitions have been published by WHO, Centers for Disease Control and Prevention (CDC), and PAHO:
Case definitions vary and their sensitivity has been questioned, particularly in areas where other arboviruses, such as chikungunya and dengue, circulate. One study found that the symptoms most strongly associated with Zika virus infection included rash, pruritus, conjunctival hyperaemia, absence of fever (axillary temperature <37.5°C), no petechiae, and no anorexia. 
Diagnosis can be confirmed with molecular or serological testing, which includes reverse transcriptase-polymerase chain reaction (RT-PCR) for viral RNA, and IgM ELISA plus plaque-reduction neutralisation testing (PRNT) for Zika virus antibodies. PRNT can be performed to measure virus-specific neutralising antibodies and discriminate between cross-reacting antibodies in primary flavivirus infections (e.g., dengue and yellow fever viruses, yellow fever vaccine recipients);  however, there are currently few laboratories that perform this test. Availability of commercial tests depends on location.
Antenatal ultrasound and amniocentesis may be recommended in some pregnant women. Head circumference measurement, CT/MRI of the head, cranial ultrasound, hearing screen, ophthalmological screen, and neurological examination may be recommended in infants with suspected congenital Zika syndrome.
Nerve conduction studies/electromyography and CSF examination should be performed in patients with suspected GBS if available; however, these investigations are not needed to make a clinical diagnosis and should not delay treatment.  Interpretation of these studies and definitive diagnosis requires consultation with a neurologist.
The duration of typical IgM detectability is about 12 weeks.  However, data suggest that Zika virus IgM can persist beyond 12 weeks in some people; therefore, a positive IgM result may not always indicate recent infection. Therefore, IgM test results cannot always reliably distinguish between infection that occurred before or during the current pregnancy, particularly in women with possible Zika exposure before the current pregnancy. Additionally, as the prevalence of Zika virus disease declines, the likelihood of false-positive test results increases.  Before testing, limitations should be discussed with the patient.
Sensitivity and negative predictive value for all Zika investigations has not been fully established, especially for asymptomatic people. Consequently, transmission prevention precautions remain important even when a test is negative. Additionally, it is important to exclude differential diagnoses that may resemble Zika virus infection, dengue fever, and chikungunya. Other conditions such as malaria or leptospirosis may require specific and urgent treatment.
Testing is recommended in all symptomatic patients with possible exposure. The CDC recommends RT-PCR on serum or patient-matched serum and urine for specimens collected <14 days after symptom onset. If RT-PCR is positive, infection is confirmed. If RT-PCR is negative, or the specimen is collected ≥14 days after symptom onset, serological testing is recommended. If serological testing is negative, infection can be ruled out and no further testing is required. If positive (or equivocal), PRNT is recommended to confirm diagnosis (except in Puerto Rico). A PRNT <10 rules out infection. Plasma, whole blood, CSF, and amniotic fluid may also be used as test specimens for RT-PCR if necessary. Dengue and chikungunya testing (RT-PCR or serology) should also be performed in patients at risk of exposure with clinically compatible illness. 
Testing recommendations may differ between guidelines and locations with WHO and PAHO offering different recommendations:
All pregnant women should be asked about possible Zika virus exposure before and during the current pregnancy, at every antenatal care visit. The CDC recommendations for testing symptomatic and asymptomatic pregnant women are detailed below. 
Symptomatic pregnant women
Testing is recommended in all symptomatic pregnant women with possible Zika virus exposure (i.e., travel to, or residence in, an area with risk for mosquito-borne Zika virus transmission, or sex with a partner who has travelled to or resides in an area with risk for mosquito-borne Zika virus transmission).
Concurrent RT-PCR (paired serum and urine specimens) and serological testing (serum) is recommended in pregnant women as soon as possible up to 12 weeks after symptom onset. If RT-PCR is positive, infection is confirmed, although further testing may be required if the IgM result is negative. If RT-PCR and serology are both negative, diagnosis is excluded. If RT-PCR is negative and serology is either positive or equivocal, PRNT is recommended. A PRNT <10 rules out diagnosis.
Serological testing may be considered >12 weeks after symptom onset; however, a negative result does not rule out infection during pregnancy as IgM levels decrease over time. A positive result should be interpreted in the context of the known limitations of serological testing.
Dengue virus IgM antibody testing is also recommended in symptomatic pregnant women.
Asymptomatic pregnant women
Asymptomatic pregnant women with ongoing possible exposure: RT-PCR (serum and urine) is recommended 3 times during pregnancy (e.g., at the initial antenatal care visit, and then at 2 non-consecutive antenatal visits). If the result is positive, infection is confirmed. Additional testing is not recommended in women who have a positive test any time before or during the current pregnancy. IgM testing is no longer routinely recommended.
Asymptomatic pregnant women with recent possible exposure but no ongoing possible exposure (e.g., travel or sexual exposure): testing is not routinely recommended but may be considered on a case-by-case basis.
Pregnant women with possible exposure to Zika virus who have a fetus with antenatal ultrasound findings consistent with congenital Zika virus infection should be tested following the same recommendations for symptomatic pregnant women. If amniocentesis is performed as part of routine clinical care, RT-PCR should be performed on amniotic fluid as a positive result may indicate fetal infection. Testing placental and fetal tissues is not routinely recommended, but may be performed in certain situations (e.g., a woman without laboratory-confirmed infection who has a fetus or infant with possible Zika virus-associated abnormalities). 
Pregnant women with laboratory evidence of possible Zika virus infection should have serial ultrasounds every 3 to 4 weeks to monitor fetal anatomy and growth, and be referred to a specialist.  Intrauterine diagnosis of microcephaly is made when the head circumference is ≥2 standard deviations below the mean for sex and gestational age. 
The CDC has produced an algorithm to assist in clinical decision making about testing based on current recommendations:
Congenital Zika syndrome is a recognised pattern of congenital anomalies (i.e., microcephaly, intracranial calcifications or other brain anomalies, or eye anomalies, among others) in infants associated with Zika virus infection during pregnancy.      Other presentations include ocular abnormalities in infants without microcephaly or other brain abnormalities, postnatal-onset microcephaly in infants born with a normal head circumference, postnatal-onset hydrocephalus in infants born with microcephaly, sleep electroencephalogram (EEG) abnormalities, and diaphragmatic paralysis in infants born with microcephaly and arthrogryposis.  Suspected cases should be referred to a paediatrician.
Both molecular and serological testing are recommended in: 
Infants with clinical findings consistent with congenital Zika syndrome born to mothers with possible Zika virus exposure in pregnancy, regardless of maternal testing results
Infants without clinical findings consistent with congenital Zika virus syndrome who were born to mothers with laboratory evidence of possible Zika virus infection.
Laboratory testing is not routinely recommended for infants without clinical findings consistent with congenital Zika syndrome born to mothers with possible Zika exposure during pregnancy but without laboratory evidence of maternal infection.
Initial samples for testing (i.e., infant serum and urine; whole blood; cord blood is no longer recommended) should be collected directly from the infant in the first 2 days of life for simultaneous RT-PCR (on serum and urine) and IgM ELISA testing (note: specimens collected within the first few weeks to months after birth still may be useful, especially in infants born in areas without risk of Zika). CSF testing can be considered if CSF is obtained for other purposes.  A positive RT-PCR on serum or urine confirms congenital infection. A negative RT-PCR result with a positive IgM result suggests probable congenital infection.  PRNT can be used to help identify false-positive results, and confirm or rule out congenital Zika virus infection in children aged ≥18 months.
A standard evaluation is recommended in all infants born to mothers with possible or confirmed infection, regardless of whether the infant has clinical findings consistent with congenital Zika syndrome. The evaluation should include: 
Comprehensive physical examination including growth parameters
Developmental monitoring and screening using validated tools
Newborn hearing screen at birth, with automated auditory brainstem response (ABR) if possible.
In addition to this, infants with clinical findings consistent with congenital Zika syndrome, and infants without clinical findings born to mothers with laboratory evidence of possible Zika virus infection should have the following investigations: 
Comprehensive ophthalmological examination by 1 month of age
Automated ABR by 1 month of age (if not already done as part of standard newborn evaluation).
Infants with clinical findings consistent with congenital Zika virus syndrome should also have the following: 
Referral to a development specialist, early intervention service programmes, and family support services
Consultation with infectious disease, clinical genetics, and neurology specialists, as well as any other clinical specialists based on the infant’s clinical findings (e.g., endocrinologist, lactation specialist, gastroenterologist, speech or occupational therapist, orthopaedist, physiotherapist, pulmonologist).
Infants who do not have clinical findings consistent with congenital Zika virus syndrome should be referred to an appropriate specialist if any clinical findings are noted at follow-up visits.
Follow-up care requires a multidisciplinary team to facilitate coordination of care and will depend on the clinical findings in the infant. Healthcare providers should be vigilant for other clinical findings (e.g., difficulty swallowing, hydrocephaly) or new clinical findings. A standard evaluation should be performed at subsequent well-child visits in all infants, along with routine paediatric care. Automated ABR testing is no longer recommended in infants at 4 to 6 months of age if they passed the initial hearing screen with ABR. 
The WHO offers specific guidance for the screening, assessment, and management of neonates and infants with congenital Zika infection.
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