A 2014 study found that India’s National Vector Borne Disease Control Programme captures only 0.35% of the annual number of clinically diagnosed dengue cases.
Sandhya Sekar is a freelance science writer interested in evolution, ecology and wildlife.
“All it takes is an empty coconut shell, left out in the rain,” says Dr George Varghese, from the Christian Medical College (CMC), Vellore, while discussing the recent dengue epidemics in India. The shell can act as a breeding ground for mosquitoes, which transmit the dengue virus between humans with when they bite. The mosquito Aedes aegypti, the primary carrier of the dengue virus, bites mostly during the day – which means unless you’re in an air-conditioned cocoon without any ventilation, you are in danger of the pesky insects and the disease they carry.
The coconut shell is just an example. Stagnant water along roads, temporary rain puddles, not-so-temporary rubbish-ridden rivers, a garbage dump waterlogged in the rain – all make excellent breeding grounds for mosquitoes.
Dengue is one of the main mosquito-borne diseases in India, along with malaria, chikungunya and Japanese encephalitis. What sets it apart (or not) is the generality of its symptoms: high fever, headaches and joint pain, indicative of any viral infection. The dengue virus DENV also causes pain behind the eyes, itching and severe joint pain – a triad of symptoms used to profile dengue clinically, according to Cecilia Dayaraj from the National Institute of Virology in her 2014 review of dengue in India.
Only one in four dengue infections manifest symptoms, but asymptomatic cases act as carriers of infection, a silent reservoir that can aid the spread of dengue epidemics. The current notion is that dengue can cause a “continuous spectrum of clinical disease”. It may not give rise to symptoms at all or just cause a mild fever. On the other end of the spectrum, it can cause the fatal dengue hemorrhagic fever (DHF). DHF is a ‘vascular leak syndrome’, where fluid oozes out of blood vessels, leaving behind thick blood that cannot flow. Severe DHF paralyses the circulatory system and leads to death.
Another fallout of dengue being so generic in it symptoms is that it is grossly underreported. A 2014 study found that India’s National Vector Borne Disease Control Programme (NVBDCP) captures only 0.35% of the annual number of clinically diagnosed dengue cases. Using data from a case study in Madurai and extrapolations suggested by an expert panel, the study found that India could have had an annual average of 5,778,406 clinically diagnosed dengue cases between 2006 and 2012. That’s 282-times the number reported every year (average).
“The government numbers are just the tip of the iceberg,” says Soumya Swaminathan, the outgoing director general of the Indian Council of Medical Research (ICMR) and soon to be deputy chief of the World Health Organisation, in an interview with PTI. Bearing this in mind, it is important to note that the NVBDCP estimated close to 130,000 cases and 250 deaths in 2016. As of August 2017, about 36,000 cases have already been reported in India.
A virus-mosquito match that clicked
Dengue is caused by viruses of the genus Flavivirus. The story of dengue evolution started in the forests of Southeast Asia, where the ancient Flavivirus was probably a mosquito- or tick-borne virus, which diverged around 2,000 years ago by adapting to vertebrate hosts like birds, bats and forest-dwelling primates. The virus then adapted to survive in mosquitoes, using them as vectors: agents that can carry the virus and transmit the disease between hosts. The virus first evolved with Aedes albopictus, a day-biting mosquito common near forest areas, and later also began using Ae. aegypti.
The fantastically versatile Ae. aegypti soon became the primary vector for dengue. The mosquito is highly adapted to the urban environment. It can breed in small collections of water. Even a small plastic cup would suffice. As the Aedes species spread all over the world with increased movement of humans and goods, dengue went with them. The 18th, 19th and 20th centuries saw many dengue pandemics, and the two World Wars accelerated the spread of the disease.
“The main reason [for dengue’s spread] is a combination of global trends that have all emerged and converged in the past few decades,” wrote Duane Gubler, an internationally renowned dengue expert from the Duke-NUS Medical School, to The Wire in an email. “Population growth, unprecedented urban growth that provides the ideal ecology for viruses and mosquitoes, globalisation and modern transportation that provides the ideal mechanism for transporting the viruses and mosquitoes among population centres; and, deterioration of public health infrastructure and lack of effective mosquito control in tropical cities.”
The dengue virus today has four serotypes. A serotype is a group within a species of microorganisms that has its own set of distinctive surface structures. One serotype is usually responsible for one epidemic, but there have been cases where more than one serotype circulates during an epidemic.
“When a person already infected with one serotype is infected by a second, the body cannot fight both serotypes at the same time,” CMC Vellore’s Varghese told The Wire. The dengue virus has found a way to make its host’s body turn against itself. Antibodies produced against one dengue serotype can enhance infection by a second serotype, a phenomenon called antibody-dependent enhancement of infection. “This could be one reason for the increased severity of dengue infection in some cases,” according to Varghese.
A recent genetic analysis by the National Institute of Virology found that the prevalent dengue strains in India have been changing temporally. When dengue epidemics started in India in the 1950s and 1960s, all four serotypes were present in a mild form. Today, per the study, all four have been replaced by lineages with greater virulence and transmissibility. Genetic analyses have shown that the DENV-4 serotype, and a particular genotype of DENV-3, originated in India.
“Increased transmission increases the rate of mutation in the virus, increasing the probability of new virus strains emerging that have greater fitness, epidemic potential and virulence”, said Gubler.
The Indian dengue epidemics have a very distinct trend. The first epidemics are in the south and then the disease spreads north. Mosquitoes breed better in warm, humid environments and they follow the monsoons northwards. “The rainy season, with increased humidity, increases the survival of the mosquito, thus increasing the probability that an infected mosquito will live long enough to transmit the virus to other persons,” Gubler added.
Combating dengue: getting rid of the mosquito
Fighting the disease by targeting the mosquito has been the favoured method to combat dengue thus far. Killing off adult mosquitoes by aerial spraying has been the go-to solution but its effectiveness has been questionable. Ae. aegypti dwells in sheltered areas where the spray doth not go. The chemicals need repeated application; the mosquitoes can develop resistance; and their environmental effects are numerous (remember DDT?). Studies have found that targeting the larval stage with chemicals like temefos is not a sure-shot method either. Using fish, like guppies, that eat up mosquito larvae has been discussed as an option but it could get expensive in the large scale. Using genetically modified mosquitoes carrying a lethal gene that can induce sterility in wild mosquitoes is under trial.
Bacteria of the genus Wolbachia naturally infect many insects, though not mosquitoes that cause dengue. The bacteria can inhibit the growth of other microbes in its hosts and shorten their lifespan. Wolbachia is also compatible with a range of insects: a strain collected from one insect species can be used to infect another, even if the insect hosts are not related. A 2011 study by Scott O’Neill and team from Monash University, Australia, found that the bacterium Wolbachia pipientis can be used to infect Ae. aegypti, where it can prevent the dengue virus from replicating in the mosquito.
When introduced into an uninfected species, Wolbachia spreads rapidly. The idea is for Wolbachia-infected mosquitoes – which cannot carry dengue – to replace wild mosquitoes, inhibiting the virus’s spread. During a field trial in Australia, Ae. aegypti mosquitoes infected with Wolbachia were able to transform a population of wild mosquitoes. Moreover, after three months, the incidence of dengue in the mosquito population remained low. Based on the success of further field trials, Monash University launched a nonprofit called the World Mosquito Program, until recently called ‘Eliminate Dengue’. With an international team of funding agencies, the program has expanded to countries in Southeast Asia, South America, the Pacific islands and, most recently, to India.
“From the first discovery that Wolbachia can block the replication of arboviruses in the mosquito, to the establishment of Wolbachia in five countries with one million people protected by Wolbachia, has been an astonishing journey in a short space of time,” said Cameron Simmons, the director of impact assessment at the World Mosquito Program.
“Our approach is not inconsistent with traditional mosquito control programs. In fact, we encourage communities and government to continue these efforts during and after we’ve deployed Wolbachia,” he added. “However, the reality is that traditional mosquito control programs are underfunded and imperfectly delivered. Indeed decades of evidence tells us that they have failed to stop disease transmission in endemic countries. It’s time to try novel approaches that can deliver long-term, cost-effective reductions in disease incidence.”
Using Wolbachia to combat mosquito-borne diseases has recently received approval from the WHO. Simmons and others at the World Mosquito Program are also confident that further evidence of reduced disease transmission, and improved public health in areas where Wolbachia has been introduced, will bring in more funding and widespread adoption of this technique. What will happen in India, however, remains to be seen.
Combating dengue by tackling the virus
When a mosquito carrying dengue bites an unwary human being, the virus pops out of the mosquito’s salivary glands onto human skin. The skin cell folds around the virus, forming a little pouch that draws the virus into the cell. Once safely inside, the virus disintegrates to reveal its incredibly simple genetic material: one strand of ribonucleic acid (RNA), which is similar to our DNA. The virus then shows its true colours: it uses the host cell’s machinery to make copies of itself. Different parts of baby viruses are made, assembled and finally released into the blood. The viruses then go on to infect other cells.
The human body does not remain idle once it encounters a virus. It mounts an immune response, secreting white blood cells that seek out the virus and attempt to engulf it. But the virus has a devious trick up its sleeve: it infects the very cells sent out to kill it. As the infected white blood cells travel around the body, so does the virus, spreading all over and resulting in viremia – the presence of viruses in the bloodstream. Infected cells secrete chemicals called interferons, which activate further immune defences. Some types of white blood cells secrete antibodies that recognise and neutralise virus particles. The infected person now suffers from a fever lasting for about a week.
Since fever is the first manifestation of the disease, it becomes difficult to differentiate dengue from a simple flu. The National Institute of Virology has a kit that can detect dengue, and other commercial tests are also available. An example is Dengue Day 1, a test that can detect dengue even before the onset of fever. Developed by Navin Khanna’s research group from the International Centre for Genetic Engineering and Biotechnology, the test is rapid and sensitive, capable of detecting dengue antibodies in very dilute samples.
“The test can even tell you if you have had dengue in the past,” Khanna told The Wire. “This kind of information is critical to the doctor. If it’s secondary dengue, it is much more severe, and you can lose the patient in a day.” Secondary dengue means a person who has had dengue before and has been infected again.
“Even as recently as five years ago, tests to detect dengue were very expensive”, said Khanna. “Now, the costs have plummeted.”
Since dengue has been around for many hundreds of years, traditional medical practices have evolved their own ways of dealing with the disease. Vaccinating against dengue has been part of the plan to combat the disease – but vaccine candidates are still not in a position to hit the market. Global pharmaceutical giant Sanofi has developed a vaccine against dengue but India has not approved it for use. “I salute Sanofi for spending 20 years, and 1.2 billion dollars, on a tropical disease that is not their problem,” said Khanna. “The Sanofi vaccine does not work on patients who have never had dengue before; it is a booster vaccine. There has also been health issues post vaccination, especially in children”.
The WHO has said, “a dengue vaccine would represent a major advance in the control of the disease”, and many teams are working on a vaccine that can provide cover against all four dengue serotypes.
The Global Burden of Disease Study estimated 58.4 million dengue cases in 2013 and about 10,000 deaths worldwide. These numbers translate into an economic burden of almost $9 billion in 2013 alone. Though the number of deaths due to dengue is relatively small, the pressure on healthcare systems and the reduction in workforce strength due to the illness take a heavy toll.
The fight against dengue thus needs to be at many levels: measures to control Aedes mosquito populations, vaccinating against the mosquito using techniques like Wolbachia infections, rapid diagnosis and effective treatment of the disease and, finally, developing a vaccine that can fight against all four serotypes.