Incorporation of IgG Depletion in a Neutralization Assay Facilitates Differential Diagnosis of Zika and Dengue in Secondary Flavivirus Infection Cases

Zika virus (ZIKV) has emerged as a major global public health concern due to its link as a causative agent of human birth defects. Laboratory diagnosis of suspected ZIKV infections by serological testing of specimens collected a week or more after symptom onset primarily relies on detection of anti-ZIKV-specific IgM antibodies by enzyme-linked immunosorbent assay coupled with detection of ZIKV-specific neutralizing antibody by neutralization tests. A definitive diagnosis based on serological assays is possible during primary ZIKV infections; however, due to the cross-reactivity of antibodies elicited during flaviviral infections, a definitive diagnosis is not always possible, especially among individuals who have previously been exposed to closely related flaviviruses, such as dengue virus (DENV). Here, we investigated the neutralizing IgM antibody profiles of 33 diagnostic specimens collected from individuals with suspected primary and secondary flaviviral infections acquired when visiting areas experiencing active ZIKV transmission in 2015 and 2016. Specimens collected between 1 day and 3 months postexposure were tested for ZIKV and dengue virus type 1 (DENV1) and type 2 (DENV2) by the plaque reduction neutralization test (PRNT) before and after IgG depletion. We found that IgG depletion prior to neutralization testing had little effect in differentiating samples from individuals with secondary infections taken less than 3 weeks postexposure; however, IgG depletion significantly reduced the cross-reactive neutralizing antibody titers and increased the percentage of cases discernible by PRNT from 15.4% (95% confidence interval [CI], 4.3 to 42.2%) to 76.9% (95% CI, 49.7 to 91.8%) for samples collected between roughly 3 and 12 weeks postexposure. These results highlight the potential of IgG depletion to improve the specificity of PRNT for better confirmation and differential diagnosis of flavivirus infections.