Dengue has four virus types (serotypes) that show distinct variation within themselves. While infection by one serotype results in a lifelong immunity against it, infection by a different one results in an increased infectivity and more severe clinical manifestations. This kind of aggravation is called antibody-dependent enhancement (ADE).
However, in the case of the Zika virus , people infected with any of the four dengue serotypes are said to be at an increased risk of infectivity and clinical manifestations due to ADE.
Two studies published recently have shown that this is not true. The first one, published in Nature Communications , found an enhancement of Zika when tests were carried out on cells taken from four rhesus macaques with dengue immunity. However, when the tests were carried out directly on the macaques, they produced completely different results.
Compared with macaques that were not already infected with dengue, the four did not either show any increase in the amount of Zika virus. Neither did the disease worsen in them.
On the contrary, their previous exposure to dengue led to a reduction in the amount of Zika virus and a faster clearance. There were changes in the immune system’s response but that did not lead to an increased infectivity.
“Results from our work may suggest that pregnant women with previous exposure to dengue virus may have limited Zika viremia [amount of virus] and less tendency to have invasion of the central nervous system,” the authors conclude.
Results of another study carried out among Zika-infected patients with pre-existing dengue infection also did not show an increased infectivity or an enhanced severity of Zika. The results, published in the journal Clinical Infectious Diseases, were based on 65 patients with acute febrile illness (45 patients positive for Zika and 20 patients positive for dengue type-2) between January and June 2016.
The researchers found no difference in Zika viral load between those with previous dengue infection and those without it. All Zika-infected patients recovered after receiving recommended supportive therapy.