Specificity of Zika virus ELISA: interference with malaria
Van Esbroeck, Marjan
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To the editor: We read the study by Huzly et al.  with interest and agree on the high specificity of the Euroimmun Zika virus (ZIKV) ELISA (Euroimmun, Lübeck, Germany). We evaluated the specificity of this test on convalescent samples of 10 PCR-confirmed dengue patients (n = 3 DENV-1, n = 4 DENV-2, n = 2 DENV-3 and n = 1 DENV-4) with high IgM antibody ratios and a positive (n = 9) or negative (n = 1) result for IgG antibodies. We also tested the assay on 10 samples with high titres of neutralising antibodies against yellow fever virus and on five samples positive for rheumatoid factor. Except for one borderline result (ratio between 0.8 and 1.1) of ZIKV IgM in the convalescent sample from a patient infected with DENV-1 after a stay in Thailand, all results for ZIKV IgM and IgG were negative. However, when we tested samples from malaria patients with a current infection (thick smear and PCR-positive) with Plasmodium falciparum (n = 12), P. falciparum/P. ovale (n = 1), P. vivax (n = 3), P. ovale (n = 5) or P. malariae (n = 5), or a recently treated P. falciparum infection (microscopy-negative, PCR-positive) (n = 8), 14 of these 34 samples tested positive or borderline for ZIKV IgM, IgG or both. Positive or borderline results for both ZIKV IgM and IgG were registered in two of 13 samples from patients with a current infection with P. falciparum (including the patient with the mixed P. falciparum/P. ovale infection) and in one of eight samples from patients with a recently treated P. falciparum infection. Nine samples tested positive or borderline for ZIKV IgM only: four from patients with a current P. falciparum, two each from patients with a P. vivax and recently treated P. falciparum infection, and one from a patient with a P. malariae infection. Finally, one of 13 samples from patients with a current P. falciparum infection and one of five samples from patients with a P. ovale infection tested positive for ZIKV IgG only.