Virus causing dengue has evolved dramatically in India, finds multi-institutional study

Dengue is a mosquito-borne viral disease that has steadily increased in the last 50 years, predominantly in the South-East Asian countries.  

A multi-institutional study on dengue, led by the Indian Institute of Science (IISc.), shows how the virus causing the disease has evolved dramatically over the last few decades in the Indian sub-continent.

Dengue is a mosquito-borne viral disease that has steadily increased in the last 50 years, predominantly in the South-East Asian countries and there are four broad categories or serotypes of the dengue virus (Dengue 1, 2, 3 and 4).

Very different from original strains

“We were trying to understand how different the Indian variants are, and we found that they are very different from the original strains used to develop the vaccines,” said Rahul Roy, Associate Professor, Department of Chemical Engineering (CE), IISc., and corresponding author of the study published in PLoS Pathogens.

He and his collaborators examined all available (408) genetic sequences of Indian dengue strains from infected patients collected between 1956 and 2018.

Using computational analysis, the team examined how much each of these dengue virus serotypes deviated from their ancestral sequence, from each other, and from other global sequences. The team found that the sequences were changing in a very complex fashion.

Until 2012, the dominant strains in India were Dengue 1 and 3. But in recent years, Dengue 2 has become more dominant, while Dengue 4 – once considered the least infectious – is now making a niche for itself in South India, the researchers found.

Factors that decide

“The team sought to investigate what factors decide which strain is the dominant one at any given time. One possible factor could be Antibody Dependent Enhancement (ADE),” said Suraj Jagtap, PhD student at CE and first author of the study.

Mr. Jagtap said that sometimes, people might be infected first with one serotype and then develop a secondary infection with a different serotype, leading to more severe symptoms.

Scientists believe that if the second serotype is similar to the first, the antibodies in the host’s blood generated after the first infection bind to the new serotype and bind to immune cells called macrophages. This proximity allows the newcomer to infect macrophages, making the infection more severe.

Several types co-exist

At any given time, several strains of each serotype exist in the viral population. The antibodies generated in the human body after a primary infection provide complete protection from all serotypes for about 2-3 years. Over time, the antibody levels begin to drop, and cross-serotype protection is lost.

The researchers propose that if the body is infected around this time by a similar – not identical – viral strain, then ADE kicks in, giving a huge advantage to this new strain, causing it to become the dominant strain in the population. Such an advantage lasts for a few more years, after which the antibody levels become too low to make a difference.