Dengue and chikungunya sicken far more people in India than officially recorded, show lab tests of a little over 1,000 people across 50 neighbourhoods in Chennai.
Results of blood tests of 1,010 Chennai residents show that nearly all have had dengue and close to half (44%) have been infected by chikungunya, report researchers from the John Hopkins Bloomberg School of Public Health in PLOS Neglected Tropical Diseases.
However, almost all of the people with dengue antibodies do not report it, either because they are not diagnosed or because the disease is mild and they do not show symptoms.
Based on the data, researchers estimate that on an average, 23% of those who have not had dengue yet get infected every year, corresponding to roughly 228,000 infections per year in Chennai alone.
This infection rate is almost three times higher than in areas of Brazil and Thailand where transmission is thought to be high.Despite Delhi health minister Satyendra Jain’s belief that dengue is a rich person’s disease – the aedes egypti mosquito breeds in clean water – data from Chennai shows the rate of infection being similar in both Chennai's affluent and poor neighborhoods.
Awareness about symptoms is low, finds the study.
“We asked participants if they had ever been ill with dengue and only one percent of them said yes, when in fact 93% had been infected by it," says the study's leader Isabel Rodriguez-Barraquer, a research associate in the Department of Epidemiology at the Bloomberg School of Public Health.
Dengue is endemic in many parts of India, with Andhra Pradesh, Karnataka, Kerala, Tamil Nadu, Gujarat, Maharashtra, Odisha and West Bengal reporting the maximum cases.
"Our results show that the extent of the problem has been vastly underestimated," says Rodriguez-Barraquer.
"States report all confirmed cases to the Centre. The infection is often mild and self-limiting, and in such cases, people don’t get tested and over time, build immnity,” says an official from National Vector Borne Disease Control Programme, which tracks infection across India’s 35 states and UTs.
The US research is the first to systematically measure dengue and chikungunya infection rates in India. "If you don't understand the extent of the problem, you can't address it," said Rodriguez-Barraquer.