Calcutta’s dengue outbreak seems to be the outcome of the city’s failure to draw lessons from two organisms armed with behavioural traits and genetic tricks that make dengue prevention and control a daunting, yet doable, task.
Biologists say the survival-of-the-fittest axiom is likely to render the long-term war on dengue a formidable challenge, although experience has shown that humans can evade and win battles against localised outbreaks.
Early and efficient preventive action and clinical care can curb the number of infections and deaths, but scientists say the evolution of dengue viruses and emergence of insecticide-resistance in mosquitoes pose new challenges.
Studies in Brazil and Thailand suggest that the Aedes aegypti mosquito that carries the dengue virus has acquired resistance to commonly used insecticides.
“We are trapped in a cycle,” said S.L. Hoti, a senior microbiologist at the Vector Control Research Centre, Pondicherry. “Both organisms (vector and virus) change over time: they adapt, they learn to evade what we throw at them, whether insecticides or our immune system,” he said.
Virologist Cecilia Dayaraj has caught glimpses of how dengue viruses have changed their genetic make-up. In her laboratory, small plastic vials stored in a freezer at minus 80 degrees Celsius contain viruses isolated from dengue patients from across the country since the mid-1960s.
Through a series of genetic studies of some of these viruses, Cecilia and her colleagues at the National Institute of Virology, Pune, have demonstrated that dengue viruses circulating over the past decade are genetically different from the ones that circulated during the 1960s and 1970s.
In a research paper published in the journal Infection, Genetics and Evolution, the team has shown through genetic studies that India and Sri Lanka played a role in the evolution and dispersal of two major genotypes of dengue virus types 3 and 4 that are “more virulent and show higher dissemination potential”.
“Some of these genetic changes may account for the observed increase in disease severity,” Cecilia said. “But in India, we still do not see the levels of severe disease observed in other Southeast Asian countries.”
Sri Lanka, for example, experienced its largest epidemic of dengue fever during 2009-10 with an estimated 70,000 cases and nearly 600 deaths. But a series of control measures reduced the number to 25,000 cases and 169 deaths in 2011.
Hoti, who visited Sri Lanka last year, said important preventive actions included public campaigns on reducing the risk of mosquito breeding, identification of sites where they breed and appropriate clinical management of dengue cases.
Calcutta’s actions contrast with what needs to be done.
Virologists have long underscored the importance of keeping dengue patients in mosquito-proof bed nets to prevent them from being bitten, failing which the virus would spread through the vector-human-vector cycle. At the Infectious Diseases Hospital in Beleghata this year, even doctors didn’t escape dengue.
The early warning signals had come in May when the Calcutta Municipal Corporation’s entomology division detected Aedes aegypti larvae in various parts of the city. But the civic and health authorities ignored the warning.
The CMC has not prepared any geographic information system-based map yet to help determine which areas are more affected by dengue.
While civic officials claim fogging had been undertaken at 350-odd places, residents say they didn’t see any. Mamata Banerjee admitted on September 7 that the civic body had been negligent and reprimanded the mayor a few days later.
In this belated realisation lies the real sting in the dengue tale.