Dengue infection does not increase Zika severity

Posted By News On July 15, 2017 - 2:05pm

 A study finds that in and around São José do Rio Preto (São Paulo State, Brazil), where dengue is endemic and there was a rapid outbreak of Zika during the 2016 epidemic, prior dengue infection did not lead to a worse illness for those who got Zika.

Previous research using only cells and rodents suggested prior dengue infection would intensify Zika disease by facilitating replication of the virus. Some physicians and virologists suspected this possible viral amplification could explain the concentration of Zika-associated microcephaly cases in the Northeast of Brazil, where dengue is more prevalent than in other regions of the country.

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 São José do Rio Preto, a healthcare hub for northern and northwestern São 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% of those with Zika (35 people) and 70% 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. About 90% of patients with dengue hemorrhagic fever have previously had dengue and are infected by a different subtype (there are four subtypes of dengue virus).

The problem is that the antibodies produced by the immune system against one subtype do not always effectively neutralize the other subtypes, leading to only partial immunity.

According to a hypothesis called antibody-dependent enhancement (ADE), incomplete immunization appears to help the virus enter defense system cells, where it reproduces, increasing the number of copies of itself in the organism and intensifying the severity of the infection. Because dengue and Zika are both flaviviruses and genetically similar, it was believed that the partial immunization observed after dengue infection might also occur in Zika-infected individuals with prior dengue infection.

This suspicion was strengthened in mid-2016, when research first showed that antibodies against dengue virus also protect individuals against Zika virus but do not neutralize it completely. In March 2017, US researchers found partial immunization to be the explanation for multiplication of Zika in a study using mice with weakened immune systems.

The study affirms what the American Council on Science and Health has long said about studies of cells cultured in vitro and in laboratory mice - they are not little people.