Scientists studying the Zika outbreak in Brazil say previous exposure to another mosquito-borne virus, dengue, may exacerbate the potency of Zika infection.
Early-stage laboratory findings by researchers in Britain France and Thailand suggest Zika uses the body's own defences as a "Trojan horse", allowing it to enter a human cell undetected. Once inside the cell, it replicates rapidly.
The scientists said their results, published in the journal Nature Immunology, suggested that some dengue antibodies can recognise and bind to Zika due to the similarities between the two viruses, but that these antibodies may also amplify Zika infection in a phenomenon called antibody-dependent enhancement.
This effect is already known with dengue, they said, and is thought to explain why, when a person gets dengue fever a second time, the infection is often more serious than the first.
"Although this work is at a very early stage, it suggests previous exposure to dengue virus may enhance Zika infection," said Gavin Screaton, a professor at Britain's Imperial College who led the research.
"This may be why the current outbreak has been so severe, and why it has been in areas where dengue is prevalent."
Dengue infections have increased dramatically over recent decades. The virus causes around 390 million infections a year globally - with 40 percent of the world's population living in areas of risk. Dengue is common in Brazil, and the health ministry there reported a leap in cases this year.
Zika is spread by the same mosquitoes and has been causing alarm throughout the Americas since cases of the birth defect microcephaly were reported in Brazil, the country hardest hit by the current outbreak.
The rare birth defect is marked by unusually small head size and potentially severe developmental problems. Brazilian authorities in Brazil have confirmed more than 1,400 cases of microcephaly in babies whose mothers were exposed to Zika during pregnancy.
People from all over the world are already beginning to converge on Brazil for the Olympics in Rio in August.
Antibodies are large proteins that latch onto invading bacteria or viruses, neutralising them and enabling the immune system to destroy the pathogens. The antibodies are then primed to recognise the same invaders should another attack occur.
The studies found that existing dengue virus antibodies latch onto Zika when it invades. But because the two viruses are not exactly the same, they cannot latch on securely, and instead the antibodies actually help Zika to get into the human immune cells. Here, the Zika virus replicates and causes disease.
Jeremy Farrar, an infectious disease specialist and director of the Wellcome Trust global health charity which part-funded the research, said the results offered potential clues about the current outbreak and about how to make progress on vaccines.
"We know that Zika has been present in Southeast Asia and Africa for many years and yet has not taken off there as it has in South America. This is what the international research effort needs to work out, and quickly," he said.
In a second study by the same team and published in a sister journal, Nature, the scientists found that one of several antibodies that work against dengue can also neutralise Zika - providing a potential target for a vaccine.
"We now need further studies to confirm these findings, and to progress towards a vaccine," Screaton said.