Prior exposure to the dengue fever virus may increase the severity of Zika virus, a new study says.

Early stage laboratory findings suggest this connection between the two viruses may help explain the current Zika outbreak in Latin American and Caribbean countries, according to the international team of researchers.

Dengue and Zika belong to the Flaviviridae family of viruses and are transmitted by the Aedes aegypti mosquito.

“Although this work is at a very early stage, it suggests previous exposure to dengue virus may enhance Zika infection,” said study senior author Gavin Screaton, dean of the Faculty of Medicine at Imperial College London, in England.

“This may be why the current outbreak has been so severe, and why it has been in areas where dengue is prevalent. We now need further studies to confirm these findings, and to progress towards a vaccine,” Screaton said in a college news release.

In recent decades, cases of dengue fever have increased sharply. The virus is believed to cause about 390 million infections a year, and about half of the world’s population is at risk of dengue, according to the World Health Organization. Like Zika, it is most prevalent in tropical and subtropical climates.

The researchers said that once someone has had dengue exposure, Zika may use the body’s own immune defenses as a “Trojan horse” in order to enter cells undetected. Once inside the cell, Zika replicates rapidly.

“We can’t say yet whether this interaction is playing a role in the current outbreak, but if confirmed it’s likely to have important implications for the control and global spread of Zika, and for the development of any vaccine for the virus,” said Dr. Jeremy Farrar, director of the Wellcome Trust, a medical research charity. The Trust funded the study, published June 23 in Nature Immunology.

Although most people infected by Zika suffer only mild symptoms, in pregnancy it can cause severe birth defects, including brain damage.

In another study, the same research team found that an antibody that’s effective against dengue may also counter Zika. The researchers believe this finding that could aid efforts to develop a vaccine against Zika.

These findings were published June 23 in the journal Nature.

Zika and dengue “share many similarities in their genetic makeup, transmission pattern and in the immune response they trigger,” said Farrar.

“These new studies suggest that prior infection with dengue doesn’t offer any protection against Zika, and may in fact predispose people to a more severe infection,” Farrar said.

For now, there are more questions than answers about Zika and this group of viruses, including dengue, he added.

“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,” Farrar said.

More information

The U.S. Centers for Disease Control and Prevention has more on the Zika virus.

This Q&A will tell you what you need to know about Zika.

To see the CDC list of sites where Zika virus is active and may pose a threat to pregnant women, click here.

Source: HealthDay