Encouraged by air travel and urban breeding grounds, a virus borne by the tiger mosquito is evolving in deadly fashion
Among the many random markers for globalization are Pizza, Pepsi (and Coke), McDonald’s, money markets, mobile phones, air travel and the Internet. For every positive indicator of humanity’s attempt to cohere, there is a negative, which is usually not discussed as much as it should be. Dengue (pronounced den-gay) fever is one of those deleterious effects—in this case, of air travel and urbanization.
In Bangalore, where I grew up and now live, no one ever discussed dengue fever until well after the turn of the century. Now, schools issue regular warnings. The timelines vary for Indian cities, but dengue fever and its more virulent cousins, dengue haemorrhage fever (DHF) and dengue shock syndrome, are of recent origins.
“During 1960-80 dengue disease profile in India was mild," notes a paper by scientists at Pune’s National Institute of Virology (NIV) in the December 2012 issue of the journal Infection, Genetics and Evolution.
The reported incidence of dengue fever nationwide has risen nearly 600% over the last five years, according to the latest data (as of 5 November 2012) collated by the National Vector Borne Disease Control Programme. I say “reported" because it is likely most cases of dengue never make it to official records. The majority of those infected develop no symptoms. Many who contract the disease—characterized by joint pains, fever, headache and a rash akin to measles—visit doctors who usually do not report the disease.
Dengue has four major strains. The most virulent is Dengue-4, or DENV-4. Borne primarily on international air travel, these strains are hopping continents. There is no telling where they will emerge and how they will evolve. There is no vaccine against dengue and no specific drug to treat it.
“Dengue…constitutes a public health emergency of international concern," says a 2011 NIV paper published in the Virology Journal. Investigating the detection of the Dengue-4 virus, after 30 years in Pune, the paper remarked on its “increasing diversity", suggesting it was mutating in response to a changing world and a transforming India.
“It is likely that Ae.aegypti (the mosquito that spreads dengue) is continually responding or adapting to environmental change," says the website of the Centers for Disease Control and Prevention in the US. More than 50 million cases of dengue are reported every year, says the centre, and one-third of humanity is at risk. This figure is predicted to double by 2085, I noted in my last column, as the world warms and allows new areas for dengue to flourish.
India is living that trend.
“Since the 1990s the occurrence of DHF has increased. The increase has been attributed to changes in virus lineage," says the NIV paper, referring to its growing diversity, encouraged by favourable breeding conditions.
Dengue fever is caused by a virus and spread by infected mosquitoes, in particular the female of the species Aedes Aegypti, easily recognized by white stripes on its legs, hence, the popular name, the tiger mosquito. Unlike its more well-known cousin, the Anopheles, which spreads malaria and is active at dusk, the tiger operates by day. It has adapted to human civilization, breeding in water that collects in old tyres, toilet tanks, flower vases, even shower floors.
In 1996, I was witness to the tiger’s ability to panic people. I lived then in New Delhi, ground zero of the largest reported Indian outbreak of DHF and shock syndrome. A few hundreds had died after bleeding from their nose, ear and other orifices. “(The outbreak) indicates a serious resurgence of dengue virus infection in this country," wrote researchers at the All-India Institute of Medical Sciences in a 1999 paper.
By dint of being the science reporter, I found myself in the position of unofficial mosquito consultant. For instance, every day, a colleague brought me a match box and opened it to reveal dead mosquitoes, which she had meticulously collected from her house.
“Are these tiger mosquitoes," she asked, in the manner of someone awaiting a death sentence.
They usually were not. One day I noticed the stripes.
“That is definitely a tiger mosquito," I said, with some reluctance, aware of her proclivity to panic. “But that doesn’t mean it was carrying the virus or that you are infected."
My colleague’s face turned ashen, and as I recall, she got herself repeatedly tested.
With the recent death of Bollywood director Yash Chopra from dengue fever, the fear will rise, as it has in my own mind.
Every time I get my toddler ready for play school, I lather her with repellent and make sure she is wearing long sleeves and pants. I am no paranoid father, but when two girls recently died of dengue fever at my former school, I knew it was not a good idea to play Russian roulette with the tiger mosquito and its deadly guest.
Samar Halarnkar is a Bangalore-based journalist. This is a fortnightly column that explores the cutting edge of science and technology.