Zika infection unlikely to get worse due to prior dengue

People who are infected by Zika virus after having dengue fever do not appear to become more severely ill than people with Zika who have never had dengue, new research has found. Previous research using only cells and rodents suggested prior dengue infection would intensify Zika disease by facilitating replication of the virus.

"Our results show this aggravation doesn't occur, or occurs only very rarely and can't be detected by a study such as this," said virologist Maurício Lacerda Nogueira, Professor at the Sao Jose do Rio Preto Medical School (FAMERP) and principal investigator for the study.

The findings, published in the journal Clinical Infectious Diseases, is the first to show that prior dengue infection in human beings infected by Zika does not necessarily lead to a worse illness.

During the period when the Zika epidemic was at its most intense, between January and July 2016, Nogueira's team collected blood samples from 65 people who presented with fever and symptoms of dengue or Zika (similar and easily confused) at the emergency unit of the reference hospital in Sao Jose do Rio Preto, a healthcare hub for northern and northwestern Sao Paulo.

Analysis of the viral genetic material found in these blood samples showed 45 patients had been infected by Zika and 20 by dengue. The tests also showed 78 per cent of those with Zika and 70 per cent of those with dengue had been infected previously by dengue virus.

Shortly after the Zika epidemic emerged, it began to be suspected that prior infection by dengue could lead to more severe clinical manifestations of Zika, similar to those of dengue hemorrhagic fever, such as bleeding under the skin, a large decrease in blood pressure and even shock in particularly severe cases.

The team measured the numbers of Zika virus copies in the blood of patients previously infected by dengue and compared them with the numbers found in the blood of patients who had never been exposed to dengue.

If prior dengue infection facilitated the multiplication of Zika, the number should be much higher in the former group, but the researchers found both groups had similar viral loads.