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First description of 2015 Zika virus outbreak in Rio de Janiero – implications for diagnosis and possible origin

Since the recent link to severe neurological defects in infants born to mothers infected during pregnancy, Zika virus (ZIKV) has become a public health and research priority. A study published in PLOS Neglected Tropical Diseases reports details from the 2015 Zika outbreak in Rio de Janeiro – the first with a high proportion of cases confirmed by molecular diagnosis – and proposes changes to the current diagnostic criteria for ZIKV disease.

Patrícia Brasil from the Fundação Oswaldo Cruz in Rio de Janeiro, Brazil, and colleagues analyzed data and samples from a total of 364 patients who presented with an acute skin rash from January through July 2015. The first challenge the researchers faced was the accurate diagnosis of ZIKV disease. ZIKV is a relative of both Dengue and Chikungunya virus, and the three infections can cause similar symptoms. Because ZIKV is closely related to DENV, antibody tests also have problems to distinguish between the two viruses.

To make an unambiguous diagnosis, the researchers tested blood samples (available from 262 of the patients), and they were able to detect ZIKV RNA in 119 of them. Of the samples negative for ZIKV, none tested positive for either Dengue or Chikungunya, and disease symptoms were very similar between the confirmed ZIKV cases and the negative samples. This suggests that at least some of the latter were also caused by ZIKV, but that levels of viral RNA in the blood samples were below the detection limit.

Spatial distribution at Rio de Janeiro State
Spatial distribution at Rio de Janeiro State for cases tested positive (black dots), tested negative (dark gray dots) and not tested yet (light gray) for ZIKV between January 1, 2015 and July 31, 2015. The red dot indicates the Instituto Nacional de Infectologia, where patients were seen
Image Credit: Brasil et al.