Why second dengue infection can be life-threatening

A second dengue infection can be life-threatening, new research shows.

This is because, for decades, scientists have thought that there are four genetically-distinct types of the deadly dengue virus. Now, a global consortium of laboratories has shown that far more important are the differences in the virus' anti-genic properties - the "coats" that the viruses wear that help our immune systems identify them.

The discovery can now potentially answer the question about how the dengue virus evolves and, most importantly, why a first dengue infection is often mild while many second infections are life-threatening.

For the study, researchers from the Dengue Antigenic Cartography Consortium analysed 47 strains of dengue virus with 148 samples taken from both humans and primates to see whether they indeed fit into four distinct types.

The researchers found a significant amount of antigenic difference within each dengue serotype.

This implies that an individual infected with one type may not be protected against anti-genically different viruses of the same type.

In some cases, the individual may be protected against some antigenically similar strains of a different type.

"We were surprised at how much variation we saw not only between the existing four known types of dengue, but also within each type. This means that hypotheses that put antigenic differences at the centre of dengue epidemiology are now back on the table," explained Leah Katzelnick, researcher from University of Cambridge who began studying dengue after herself contracting the disease.

One of the unusual aspects of dengue is that in some cases when an individual becomes infected for a second time, rather than being immune to infection, the disease can be much more severe.

One hypothesis to explain this is that the antibodies produced in response to infection with one strain of the virus somehow allow viruses of a different strain to enter undetected into cells, implying that antigenic differences between the serotypes are important.

According to senior study author professor Derek Smith, this discovery is in many ways similar to when researchers first began using the microscope.

"Characterising the global variation of dengue viruses will be important for understanding where current vaccines will be protective," he added.

In the future, it may assist us in determining which strain to include in vaccination programmes and to follow the virus as it evolves, the researchers pointed out.

Dengue virus infects up to 390 million people each year.

Around a quarter of these people will experience fever, headaches and joint pains but approximately 500,000 people will experience potentially life-threatening complications, including haemorrhage and shock, where dangerously low blood pressure occurs.

There are currently no vaccines against infection with dengue virus.

The research was detailed in the journal Science.