According to the recent Dengue Epidemiology Forecast, laboratory-confirmed cases of dengue reported by the national surveillance systems across Brazil, India, Mexico, Singapore and Thailand (the 5MM) will rise on average 1.13% each year for the next decade. The report also notes that long-dormant or new dengue serotypes are a significant driver in disease transmission. We decided to investigate their impact.

Long-dormant serotypes

Highlighting the reemergence of the DENV-3 serotype in South Pacific after nearly 20 years, the World Health Organization explains how infection from one of the four different dengue serotypes provides protection from that particular serotype for life. This means that an outbreak could eventually reach a level where a significant proportion of the population is immune and the serotype then simply lies dormant.

However, infection with one serotype only provides protection from the other three for a very short period – at most, months. So, while one serotype takes a backseat, one of the other three may instead take hold. Worse still, it would seem that immunity to one serotype could make you more susceptible to infection with severe dengue from another.

Dengue serotypes can reemerge in a region after an absence of 15 to 20 years, having slept while the next generation is born. The proportion of the population with no immunity will eventually increase above a certain threshold and the serotype can take hold again – with the potential to cause a major outbreak.

This tendency to lie dormant has contributed to dengue’s neglect by public health experts, asserts an article in the Nature online journal. It suggests that health authorities may mistakenly give dengue a low priority while the region’s endemic serotypes are just awaiting the next opportunity to explode into a severe outbreak.

New serotypes

The forecast also refers to the impact of new serotypes on disease transmission. Fortunately, the only new dengue serotype identified in the last 50 years is one that circulates primarily in non-human primates. Science Magazine reports how the DENV-5 serotype was discovered within samples from an outbreak in Malaysia in 2007, but this is the only human outbreak ever recorded.

Currently, it is not certain what impact this new finding may have, though it may assist us in understanding the origins of dengue and how it is evolving.

One thing is clear, however: even when the battle seems to be won, we should not let our guard down against dengue. Those combatting dengue today must fight to increase the focus on dengue and help turn the forecast into reduced cases.