El Niño plays significant role in outbreak of haemorrhagic fevers

The dengue virus affects 390 million people globally every year, and fears are that early 2016 will see an epidemic, particularly in South-East Asia, due to the predicted extreme intensity of El Niño. A new study published in the Proceedings of the National Academy of Sciences has revealed the significant role that this monster climatic phenomenon plays in the outbreak of haemorrhagic fevers.

Reviewing health reports from eight South-East Asian countries, spanning a period of 18 years, the research group in question developed models for dengue fever transmission across the entire region and highlighted the correlation between the most serious epidemic waves and the abnormally high atmospheric temperatures associated with intense El Niño events.

The number of cases of dengue fever throughout the world is rising rapidly, propelling it to the status of a 're-emerging disease'. Many countries in the intertropical zone see the number of cases of haemorrhagic fever peak during the rainy season each year. But when and why do these annual peaks turn into real outbreaks, spreading beyond borders, like the viruses rife in Latin America and the Caribbean in 2010 and 2013? Up to now, it has been difficult to understand and predict these major epidemic waves.

• Eighteen years of epidemiological data scrutinised

To lift the veil on this question, an international research group collected and analysed 18 years of monthly dengue fever monitoring reports from eight countries in South-East Asia. This enabled them to identify very pronounced trends in the transmission of the virus across the entire region.

• Monster El Niño phenomenon to blame

Of particular note, the researchers observed a period of high incidence throughout South-East Asia from 1997 to 1998. This coincided with the most intense El Niño event of the 20th century. For them, the link was unmistakeable: this climatic phenomenon, originating from the Pacific Ocean around every five years, plays a key role in the development of major dengue fever epidemics, as it brings abnormally high atmospheric temperatures. These temperatures allow the vector mosquitoes to reproduce more quickly and proliferate, increasing the spread of the virus.

• Higher risk in built-up areas

This study also retraced the spread of the main epidemic waves to hit South-East Asia over a period of almost 20 years. These waves had successively emerged from western Thailand, central Laos and the southern Philippines. Finally, the findings from this work highlighted the increased risk of an epidemic in large urban areas, such as Bangkok, Singapore, Phnom Penh or Kuala Lumpur, due to the constant influx of new residents never previously exposed to the virus, making them particularly vulnerable to infection.

Now that climatologists have announced the most intense El Niño since 1997-98, these findings have led to fears of a major new dengue fever epidemic in Asia in early 2016. However, by sounding the warning bell in advance, this work will enable the introduction of immediate health measures.