Approach

Diagnosis of yellow fever in a patient presenting with a compatible febrile illness after travel to, or living in, an endemic area is based on molecular or serologic testing and the exclusion of alternative diagnoses. It is a difficult disease to diagnose as more severe forms can be mistaken for malaria, leptospirosis, other hemorrhagic fevers, or viral hepatitis. Yellow fever is a notifiable disease in the US.

Clinical presentation

An acute-onset febrile illness in an unimmunized patient living in, or returning from, a country where yellow fever is endemic (Central and South America, the Caribbean, Africa), or in a patient suspected to have been exposed to a biologic warfare agent, should raise the suspicion of yellow fever. Patients often recall a bite from a mosquito. If the history of immunization is distant or unreliable, the patient should be considered to be unimmunized.[15]

The infection is asymptomatic or clinically inapparent in most people. In patients who are symptomatic, the clinical presentation is characterized by 3 phases: infection, remission, and intoxication.

  • After an incubation time of 3 to 6 days, an initial period of infection is seen with an abrupt onset of nonspecific symptoms such as fever (average temperature of 102°F [39°C]), headache, myalgias, dizziness, and malaise lasting 2 to 6 days.[2] Conjunctival injection, relative bradycardia (Faget sign), and leukopenia are characteristic of this period. Most infections do not progress from this initial phase and resolve spontaneously.

  • After a period of remission characterized by defervescence lasting for 24 to 48 hours, 15% to 25% of patients develop a period of intoxication.[2] This period is characterized by a rebound of symptoms, often accompanied by abdominal pain, vomiting, and lethargy. Jaundice, transaminitis, and coagulopathy develop, culminating in shock, hepatorenal failure, and death 7 to 10 days after onset. Hemorrhagic manifestations (i.e., petechiae, ecchymoses, or overt bleeding from the gums, nose, mucosae, or phlebotomy sites) suggest a poor prognosis.

Initial investigations

Initial leukopenia, neutropenia, and elevated liver enzymes is typical. Later, worsening of hepatic transaminitis and elevation of creatinine with albuminuria are common. The degree of liver enzyme elevation correlates with a worsened prognosis.[15][2]

While the preliminary diagnosis is based on the clinical presentation, and dates and locations of travel (if the patient is from a nonendemic area), diagnosis is confirmed by laboratory testing, either by the detection of yellow fever virus RNA by reverse transcription polymerase chain reaction (RT-PCR) or the detection of antibodies to yellow fever virus (serology). Results should be interpreted with caution in the context of the clinical presentation, epidemiologic context, and vaccination history.

Recommendations for diagnostic testing from the Pan American Health Organization/World Health Organization are detailed below.[34]

RT-PCR:

  • RT-PCR is the preferred test for the diagnosis of patients with suspected yellow fever infection.

  • Viral RNA can be detected using RT-PCR within 10 days of symptom onset. Occasionally, viral RNA can be detected for longer than 10 days (detection up to 14 days has been reported), particularly in severe cases.

  • A positive result confirms the diagnosis and no further testing is required. If the result is negative, subsequent serologic testing is recommended.

Serologic testing:

  • Serologic testing using IgM ELISA (or any other immunoassay such as indirect immunofluorescence) is recommended from day 6 onwards in patients with a negative RT-PCR result. Laboratory confirmation requires seroconversion in paired acute and convalescent samples (collected at least one week apart) with a more than 4-fold increase in antibody titers between samples. It is important to note that significant cross-reactivity of yellow fever IgM assays with other flaviviruses can occur, especially in secondary flavivirus infections.

  • A positive IgM result should be followed by differential IgM testing that is appropriate for the epidemiologic situation of the area, particularly in locations where yellow fever virus co-circulates with other flaviviruses such as dengue and Zika. A positive differential test result indicates recent flavivirus infection, but does not rule out yellow fever and further investigation is required. A negative differential test result indicates a probable yellow fever case; however, a positive IgM test in a single sample is not confirmatory and additional clinical and epidemiologic criteria must be used for the final interpretation of the case.

  • A negative IgM result on a sample collected ≥8 days from symptom onset excludes a diagnosis of yellow fever; however, cases should always be investigated and a clinical differential diagnosis performed. A negative IgM result on a sample collected <8 days from symptom onset is considered inconclusive and a second sample should be tested.

  • IgG ELISA and plaque reduction neutralization test (PRNT) may also be used. PRNT offers greater specificity compared with IgM and IgG detection, but requires a specialized laboratory and cross-reactivity with other flaviviruses is still an issue.

  • Results should be interpreted carefully in areas where active vaccination campaigns are ongoing, as detection of vaccine-induced antibodies can occur.

The Centers for Disease Control and Prevention (CDC) recommends that RT-PCR should not be used to rule out a diagnosis of yellow fever, as viral RNA may be undetectable by the time symptoms are recognized.[10] Clinicians should contact their state or local health department, or call the CDC Arboviral Diseases Branch, for assistance with diagnostic testing for yellow fever infections.

Other investigations

Coagulation abnormalities with an elevated PT, thrombocytopenia, and fibrinogen split products define disseminated intravascular coagulation (DIC). ST-T changes on ECG indicate myocardial injury.

Histopathologic analysis on liver sections (and other tissues) is recommended for diagnosis in fatal cases; however, molecular detection performed in fresh tissue samples can also be used.

Direct detection of the virus in tissue cultures (Vero, AP-61, SW-13, BHK-21 cell lines), brain tissue (intracerebral inoculation of suckling mice), or mosquito cells is both sensitive and specific. However, this test is not commercially available and requires specialized referral laboratories.[15][35] While such tests are used in the confirmation of outbreak cases and will likely be useful for the diagnosis of yellow fever in the future, their availability and turnaround time, at present, limits their use to epidemiologic and outbreak investigations.

There have been promising efforts to develop isothermal protocols for yellow fever genome detection, which could be used in resource-limited settings where standard PCR with thermocycling is not feasible.[36]

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