WARNING: Exposure to tropical dengue virus spread by MOSQUITOS may increase ZIKA risk

EXPOSURE to the dengue virus — common in tropical countries including Brazil — may increase the danger from Zika infection, new research has shown.

Scientists believe the discovery may help explain the recent explosive outbreak of Zika in South America and the Caribbean.

Experts have found evidence that previous exposure to the dengue virus, also spread by mosquitoes, may increase the virulence of Zika and make it more of a threat.

The WHO has advised pregnant women not to travel to areas with the Zika virus — which include Rio de Janeiro — where the 2016 Olympics is being held.

The Zika virus causes pregnant mothers infected by the mosquito-borne virus to give birth to babies with abnormally small heads and brain damage, a condition known as microcephaly.

The virus is spread by a certain type of mosquito that transmits the disease when it bites someone infected with Zika, and then goes on to bite another person.

In some cases, Zika virus is transmitted through sex. 

Lead researcher Professor Gavin Screaton, from Imperial College London, said: "Although this work is at a very early stage, it suggests previous exposure to dengue virus may enhance Zika infection.

“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."

However, the same team, writing in the journals, Nature Immunology and Nature, said the links between zika and dengue could potentially aid the development of a vaccine against the Zika virus.

Dengue and Zika both belong to the same viral family, Flaviviridae, and both are transmitted by the Aedes mosquito.

Dengue fever can cause severe headaches, high temperature, pain behind the eyes, nausea and vomiting and swollen glands.

The research showed the due to the similarity between the two viruses, dengue antibodies from the immune system can recognise and target Zika.

Antibodies which fight of viruses and neutralise them, are one of the key weapons the body uses to defend itself against foreign infections. 

But the system can go wrong when an invader appears in different guises — strains that look almost the same, but not quite similar enough.

Experts said that while the immune system recognises them all as the same strain, some antibodies designed specifically to attack the first strain encountered may fail against subsequent strains, leading to infection.

The phenomenon, called antibody-dependent enhancement (ADE), is known to increase the risk of infection by four strains of dengue virus.

It may also make people more susceptible to Zika after they have previously been exposed to dengue, evidence suggests.

Their immune systems roll out dengue antibodies to fight Zika, because the viruses are so similar. But the antibodies are unable to latch onto the Zika virus securely, and merely make it easier for the viral particles to invade immune cells and replicate.

Dr Jeremy Farrar, director of the Wellcome Trust, which funded the research, said: "Zika and dengue come from the same family of viruses and we know they share many similarities in their genetic make-up, transmission pattern and in the immune response they trigger.

"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.

“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."