Dengue, the world’s most rapidly spreading mosquito-borne viral disease, is taking a far bigger human toll than was believed to be the case. As many as 390 million people across the globe could be falling victim to the virus each year, according to a multinational study published by Nature on Sunday.

India emerges in the analysis as the country with the world’s highest dengue burden, with about 34 per cent of all such cases occurring here.

According to the World Health Organisation (WHO), incidence of dengue has shot up 30 fold in the past 50 years. Its estimate has been that globally there were 50-100 million dengue infections taking place annually.

For their study, Samir Bhatt at the University of Oxford and his colleagues used a map-based approach to model how many dengue cases were occurring in various parts of the world, thereby capturing its global distribution.

They estimated that worldwide, 96 million people suffered each year from ‘apparent infections’ where the disease was severe enough to disrupt an individual’s regular routine. In addition, there were 294 million asymptomatic infections.

With “large swathes of densely populated regions coinciding with very high suitability for disease transmission,” Asia bore 70 per cent of the apparent infections that took place, the scientists pointed out in the paper.

Africa contributed about 16 per cent of the global dengue infections and the Americas 14 per cent.

“I consider it to be the most comprehensive study of dengue disease burden to date,” said Duane J. Gubler, an internationally known expert on the disease, when asked for his views on the Nature paper.

The study’s estimate of 390 million infections was “much closer to the actual figure than the 50 million WHO is still using,” observed Professor Gubler, who is now with the Duke-NUS Graduate Medical School in Singapore.

“Considering that mosquito control has failed in all dengue-endemic countries, that over half of the world’s population now lives in urban areas, and that dengue is an urban disease, even that number may be too low,” he said in an e-mail.

The study estimated that India had the largest number of dengue cases, with about 33 million apparent and another 100 million asymptomatic infections occurring annually.

However “these are estimates and there are many gaps which we now need to fill,” cautioned Jeremy Farrar, a senior author of the study, in an e-mail. “But it would not surprise me that India was home to the most dengue [patients] globally.”

The model used in the study could help provide a framework to estimate the burden of disease. Inevitably, there were gaps in the data and one needed to extrapolate from other areas. Better data collection should be encouraged so that the estimates were as accurate as possible, said Professor Farrar, who is director of the Wellcome Trust Vietnam Research Programme and Oxford University Clinical Research Unit Hospital for Tropical Diseases in Vietnam.

“We have a tremendous problem of dengue all over India,” said Umesh C. Chaturvedi, agreeing with the finding of the paper. A virologist who has studied the disease, he is a scientific consultant to the Indian Council of Medical Research.