Bitten by the Bug
Sanjana Salunkhe and her family moved from Mumbai’s crowded Andheri suburb to an upscale ground-floor apartment in Richmond Town, Bengaluru, shortly after the Covid-19 pandemic. The 30-year-old freelance content strategist was drawn to their new domicile’s open spaces and gardens, which she thought would be an ideal place for her four-year-old son, Armaan, to grow up in. However, when the monsoons hit, their home was swarming with mosquitoes.
Despite all efforts to prevent breeding such as removing stagnant water, maintaining cleanliness around the house and staying indoors most of the time, Armaan developed a 102°F fever. He was diagnosed with dengue. “He was sick for 10 days, and although he recovered, he fell ill again soon after returning to school.
The doctor explained that dengue had compromised his immune system, making him more vulnerable to other illnesses,” she says. With the mosquito problem persisting in the locality and no fumigation or intervention from local authorities, Salunkhe and her family have decided to move to another area of Bengaluru with fewer dengue cases.
Their experience is part of a larger dengue issue in Karnataka. On August 3, the southern state had recorded 19,313 dengue cases and 10 deaths, surpassing the total number of cases reported last year; and it is only August. The State Health Department’s data revealed that among the afflicted are some of the most vulnerable: 360 infants aged under one and 6,863 children under 18.
In India’s bustling IT capital, the number of cases skyrocketed nearly six-fold in a month, surging from 1,563 on July 1 to a staggering 8,800 by August 3. The city, usually synonymous with innovation and progress, found itself at the epicentre of a public health crisis, with neighbourhoods turning into breeding grounds for mosquitoes and hospitals filling with dengue patients.
The situation was serious enough to prompt the Union Health Ministry to convene a high-level review meeting, chaired by Union Health Secretary Apurva Chandra, with representatives from nine states and 18 municipalities, all facing high dengue numbers. Mosquitoes do not differentiate between their victims; last week Tribal Affairs Union minister Jual Oram’s 58-year-old wife Jhingia died of dengue.
In Delhi, Haryana, Odisha, Maharashtra, Kerala, Telangana Maharashtra, Tamil Nadu, Andhra Pradesh and Punjab, dengue has arrived with the rains. Not just dengue: symptoms of chikungunya and Zika have been detected in some patients. As the world shrinks because of easier travel, insect-borne and animal-borne diseases are here to stay, as the deadly disaster of Covid-19 showed. A month ago, Kerala issued a Nipah virus alert; health experts believe 40-75 per cent of people infected with Nipah will die from the virus.
PEST SEASON IS FAST AND FURIOUS
A single infected mosquito can transmit the dengue virus to up to eight people, leading to clusters of illness within communities. Although many cases are asymptomatic, those who do exhibit symptoms often suffer from a debilitating combination of high fever, severe headache, intense body aches, nausea and a rash.
Most patients recover within 1-2 weeks, but severe cases can necessitate hospitalisation and, in extreme situations, may be fatal. All four types of the dengue virus, DENV-1, DENV-2, DENV-3, and DENV-4, share similar symptoms and follow the same line of treatment. Of these, DENV-2 is considered the most severe and deadly. It is, however, yet to be ascertained as to which strain spreads faster.
The most alarming aspect of these diseases is their infection speed. Over half of current dengue cases were reported in just two weeks, from July 20 to August 3. India enters the most dangerous part of the year for dengue: August to November, when the monsoon’s aftermath leaves behind pools of stagnant water, which make fecund breeding grounds for mosquitoes.
Although dengue cases generally peak in October, this year’s trend shows that, as of July 31, the number of cases is already almost 50 per cent higher than at the same time last year. Dr B Reddya Naik, a professor of zoology and head of the medical entomology laboratory at Osmania University, Hyderabad, reveals a critical reason behind this alarming trend.
“Over the past decade, India has seen a significant increase in its mosquito population, particularly species like Aedes aegypti, a primary vector for dengue, Zika virus and chikungunya. This surge is directly linked to the more frequent and severe dengue outbreaks we’re witnessing,” he says. This brings into sharp focus the main antagonist in this unfolding health crisis, the mosquito. The rapid rise and global spread of the blood-sucking species, particularly in India, is increasingly concerning.
WHY BLOODSUCKERS ARE THRIVING
Dengue is primarily spread by the Aedes aegypti mosquitoes, notorious for their day-biting habits. Aedes albopictus also contributes to the spread, though to a lesser extent. These mosquitoes are typically associated with hot, humid climates, thriving in environments where stagnant water is abundant. However, new research published in 2018 by NIH uncovered a surprisingly alarmist twist: even dry conditions can fuel mosquito breeding now. In dry weather, mosquitoes become desperate for moisture and are driven to feed on blood more frequently.
This means that even in arid conditions, the threat of dengue can persist or even worsen. India’s status as the world’s most populous country makes it mosquito heaven. “Over recent decades, population growth, particularly in highly urbanised cities and towns, has intensified. As human populations expand, so do the habitats for mosquitoes that thrive around people.
This close association has led to a significant increase in dengue cases, as mosquitoes track and exploit the growing urban environments,” says Cameron Simmons, executive director delivery at the World Mosquito Program (WMP), a not-for-profit group that works to protect the global community from mosquito-borne diseases such as dengue, Zika, yellow fever and chikungunya.
As a result, dengue, once limited to tropical and subtropical regions, has now become endemic in over 100 countries. Naturally then, the global incidence of dengue has skyrocketed over the past two decades. From 2000 to 2019, the World Health Organization (WHO) recorded a staggering 10-fold increase in reported cases, soaring from 5,00,000 to 5.2 million.
The year 2019 was particularly alarming, with dengue cases reaching a record high across 129 countries. While the Covid-19 pandemic led to a temporary dip in reported cases between 2020 and 2022, dengue made a dramatic resurgence in 2023. The frequency and scale of outbreaks surged, reaching regions previously untouched.
CRITTERS SPREAD THEIR WINGS
This year, by May 2024, the European Centre for Disease Prevention and Control has reported another chilling statistic: over 10 million dengue cases and more than 5,000 deaths across 80 countries and territories. In Brazil alone, the virus spread like wildfire, with over a million cases reported in just the first two months of the year—a staggering record for this period (cases normally peak from March to May).
Peru’s situation was equally dire; with more than 34,000 cases reported early in 2024, the country was forced to declare a health emergency. The infection’s relentless march hasn’t spared even countries previously considered low-risk. Croatia, France, Italy, Portugal (Madeira) and Spain are now grappling with outbreaks, marking a disturbing shift in the disease’s geographical spread.
In Africa and Asia, where data collection is often sparse, the true burden remains unknown. “Cases are also being reported from parts of Latin America, southeast Asia, Argentina, southern China and southern Europe,” says Simmons. India’s dengue crisis mirrors global trends. The Indian Council of Medical Research (ICMR) in 2023 reported an 11-fold increase in dengue cases over the past two decades, with recurrent outbreaks now a troubling norm.
What was once a disease confined to just eight states in 2001 has now spread its tentacles to all states and Union Territories. Remarkably, it has even reached Ladakh, with two cases reported in 2022—a stark indication of how far the virus has travelled. The reasons behind the spread is the usual suspect: man’s disregard for Nature’s balance.
Climate change: According to the Intergovernmental Panel on Climate Change (IPCC), dengue’s escalation is being fuelled by rising global temperatures and increased rainfall, which create ideal conditions for dengue-carrying mosquitoes to thrive. “Climate change is lengthening mosquito breeding seasons, expanding their range, and enhancing their survival rates, all of which contribute to the spread of dengue,” explains Dr Naik. Research published in Nature Microbiology paints an even more alarming picture.
By 2050, nearly half of the world’s population could be at risk of dengue. The report warns of significant increases in regions previously untouched by the disease, particularly in Europe and North America. This anticipated surge is directly linked to a projected 2°C rise in global temperatures. It could dramatically enhance the transmissibility of the virus and the ability of mosquitoes to spread it.
Moreover, extreme weather events like hurricanes and floods—already becoming more frequent due to climate change—are exacerbating the problem. “While climate change is and will continue to significantly impact mosquito populations, the way humans respond to it will also have an impact. For instance, droughts often lead people to store water. It creates perfect breeding grounds for mosquitoes.
More frequent droughts mean more mosquitoes,” says Simmons. Recent research also sheds light on the evolving breeding preferences of Aedes aegypti mosquitoes. A study conducted during 2018-2019 in 40 localities within Delhi revealed a significant preference for breeding in human-made containers. Clay containers were found to be the most favoured for egg-laying, with a preference rate of 81.8 per cent over other types like plastic, paper, metal and glass, irrespective of their location.
Interestingly, despite providing suitable conditions for breeding, cement tanks and tyres were not identified as major contributors to Aedes breeding in this study, published by the NIH in 2023. These unwelcome guests are manifesting surprises. The Aedes vittatus—“a proven vector of chikungunya, Zika, dengue, yellow fever viruses and many other diseases”, according to the team of scientists who discovered it—considered endemic to the Indian subcontinent has invaded the western hemisphere.
Urbanisation and land use: With over 4.4 billion people—more than 50 per cent of the global population—living in cities, and this number expected to rise to 68 per cent by 2050, urbanisation is exacerbating the spread of dengue and complicating control efforts. Cities, with their abundance of stagnant water, neglected green spaces and specific agricultural practices, create ideal breeding grounds for Aedes mosquitoes. In India, for instance, dengue is an urban epidemic.
The Urban Heat Island effect, which causes higher temperatures in densely populated areas, accelerates the problem. The heat, combined with dense populations, rapid construction, poor waste management and inadequate water storage practices, creates a perfect storm for the spread of dengue. Not surprisingly, urban areas account for a significant share of dengue cases in recent years.
According to the Ministry of Health and Family Welfare, urban regions were responsible for 55-58 per cent of dengue cases, a figure that surged to around 68 per cent in 2023. “Cities in Maharashtra, West Bengal, and Delhi are heavily impacted due to high population density and water stagnation,” explains Dr Naik, adding “Even areas previously less affected, such as Rajasthan and Gujarat, are now at risk due to rising temperatures.”
Social demography: A 2020 paper published in the International Journal of Health Geographic links socio-economic and demographic factors to dengue risk. It shows that higher incomes, better healthcare, and higher education levels are associated with a lower risk of infection.
Travel: The interconnectedness of our global society means that diseases like dengue, once confined to specific areas, can now easily cross borders. In 2023, for instance, England, Wales and Northern Ireland reported 634 dengue cases among travellers returning from affected areas.
Dengue, chikungunya and Zika co-circulation: Dengue, chikungunya and Zika viruses often co-circulate, all transmitted by Aedes mosquitoes, creating a complex web of interactions that complicates public health responses. These viruses share common vectors and can influence each other’s immune responses, but the full extent of their interactions remains poorly understood.
Natural evolution: Both the dengue virus and the Aedes mosquito are also evolving, creating new challenges in the fight against these diseases. “For example, the eggs of Aedes mosquitoes can now withstand prolonged periods of drying. When submerged in water, these resilient eggs can hatch almost immediately, ensuring that the mosquito population can quickly rebound even after dry spells. Similarly, the dengue virus continuously adapts to selection pressures, which can lead to the emergence of new strains,” says Dr Rakesh Mishra, director, Tata Institute for Genetics and Society, Bengaluru.
The recurring nature of dengue outbreaks places a tremendous burden on hospitals and healthcare providers. “Dengue transmission has traditionally peaked during the monsoon season, but recent trends reveal an increase in cases even during the summer, suggesting it has become a year-round issue. Healthcare providers are also encountering more fever cases overall, signalling a broader public health concern,” explains Dr Hemalata Arora, senior consultant in internal medicine at Nanavati Max Super Speciality Hospital in Mumbai.
HUMAN-MOSQUITO WAR ESCALATES
The scientific and medical world is exploring various ways to prevent or contain dengue outbreaks. But they come with their own challenges. Biological control measures, such as introducing natural predators to manage mosquito populations, can backfire. For instance, local administrations worldwide use mosquitofish, dragonflies, and aquatic turtles to control mosquitoes and prevent diseases like dengue and malaria. To this end, several districts in India have released Gambusia fish into local water bodies to tackle outbreaks.
These fish feed on mosquito larvae. However, the use of Gambusia affinis, commonly known as mosquitofish, is not without controversy. The International Union for Conservation of Nature (IUCN) lists Gambusia affinis among the 100 worst invasive alien species globally. Experts like Nobin Raja, who conducts research in genetics, evolutionary biology, and ecology at the Ashoka Trust for Research in Ecology and the Environment, Bengaluru, raise significant concerns.
“Gambusia’s effectiveness is limited. While it might reduce mosquitoes in small ponds, it often prefers other prey and can cause ecological harm by displacing native species and preying on other fauna in freshwater ecosystems. This can lead to a decline in native biodiversity,” he says. Broad-spectrum insecticide sprays can have serious environmental consequences. These not only target mosquitoes but also harm beneficial insects such as bees and butterflies, as well as songbirds that rely on insects for food. Moreover, they are less effective in reducing mosquito populations since they mainly target adult mosquitoes.
The race between humans and mosquitoes to survive is also getting more complicated. Moreover, the insects are developing insecticide resistance because of persistent use of chemical insecticides, making mosquito breeding difficult to control. For example, DDT-resistant mosquitoes were first identified in India in 1959. Since then, resistance has grown so rapidly that local mosquito populations often become resistant within months of the initiation of a spray programme, rather than years. Community participation in preventive measures is crucial but often inconsistent. “But preventive measures are often applied too late and not on a sufficient scale,” says Simmons.
THE ERADICATION QUANDARY
As Bill Gates famously noted, mosquitoes are responsible for more human deaths than any other animal, including humans. A 2018 study published by the National Institutes of Health (NIH), the primary agency of the US government responsible for biomedical and public health research, only three species—Aedes, Anopheles and Culex—bear primary responsibility for the spread of human disease. Because of their small size and strong adaptability, these mosquitoes can carry and transmit bacteria, viruses or parasites easily.
But not all agree. While it is often said that mosquitoes serve no purpose other than to annoy humans, this perception oversimplifies a complex ecological reality. These ancient insects, dating back to the dinosaurs, likely originated in what is now South America when it was part of the supercontinent Gondwana. Continental drift and other major events shaped their evolution. It is estimated that there are over 3,500 mosquito species globally, and over 390 are found in India alone.
Many of these live far from human habitats. Some species look somewhat attractive, like the Toxorhynchites rutilus, found in the US, and adorned with shiny metallic dark-blue scales and patches of iridescent silvery-white, purple, and green. Another example is Uranotaenia sapphirina, which is known for its iridescent blue scales. Neither species feed on humans. Even among the ones that do, male mosquitoes and non-fertilised females primarily thrive on nectar, contributing to plant pollination.
Mosquitoes are also crucial to the food web: the complex chain of interconnected relationships between feeding patterns in a community. Adult mosquitoes and aquatic larvae serve as prey for various wildlife such as dragonflies, turtles, bats and birds. Hummingbirds, for instance, rely on small flying insects and spiders as a primary food source. Only female mosquitoes preparing to produce eggs bite humans, since they need the extra nutrients from blood meals.
To be fair, they are not driven by bloodlust; the insects are simply doing what it takes to be good mothers. Which means that while the idea of swatting these pests into oblivion is appealing, it’s not advisable. “As a scientist, the idea of eradicating an entire species, even those as harmful as certain mosquitoes, makes me uneasy. The consequences on other organisms and the ecosystem could be profound and unpredictable,” says Dr Mishra.
MOSQUITO CHALLENGED INDIA
India simply doesn’t react fast enough in the face of any medical challenge, as the response to the pandemic exemplified. Currently, while a complete prevention of dengue remains elusive, two vaccines—Dengvaxia and QDenga—are available. According to the Centers for Disease Control and Prevention (CDC), Dengvaxia, with about 80 per cent efficacy, is recommended for children aged nine to 16.
QDenga, approved by the European Medicines Agency (EMA), can be given to adults, adolescents and children from four years of age. However, it is not recommended for senior citizens, pregnant women, or those with compromised immune systems. “But these are not yet available in India, since the regulators either require additional safety data, or the clinical trials in local conditions are yet to be completed.
Developing a vaccine tailored to India’s needs is essential,” says Dr Mishra. Which means, at the moment, these vaccines alone do not address India’s needs. “Diagnosing dengue remains challenging due to its symptom overlap with other viral fevers. Early and accurate diagnosis is critical for effective treatment,” says Dr Arora.
Innovative approaches that focus on prevention offer the best hope. Among these are early warning systems, insecticide-infused house paint, and a preventative pill currently in trials by Johnson & Johnson. However, one of the most promising methods is the Wolbachia approach. Wolbachia are naturally occurring bacteria found in about 50 per cent of insect species, including some mosquitoes.
These bacteria are harmless to humans and the environment, yet they have a remarkable ability to inhibit the growth of viruses like dengue, chikungunya and Zika within Aedes aegypti mosquitoes. By infecting mosquito populations with Wolbachia, it will reduce the mosquitoes’ capacity to transmit these viruses.
In Australia, this method has shown success. Cities like Cairns and Townsville in Queensland, key gateways to the famous Great Barrier Reef, historically experienced annual dengue outbreaks often triggered by travellers bringing in the virus. The release of Wolbachia-infected mosquitoes by WMP has led to a notable decline in dengue cases, with no local transmission reported in Queensland.
“Far North Queensland is now essentially a dengue-free area for the first time in well over 100 years,” says Dr Richard Gair, director and public health physician at Tropical Public Health Services in Cairns. This method has also seen positive outcomes in parts of Colombia and Indonesia. It is currently being developed in countries like Laos, Sri Lanka, Vietnam, and various regions of Latin and South America.
In India, the Tata Institute for Genetics and Society is pioneering a similar approach. “We’re infecting mosquitoes with a Wolbachia strain that we’ve identified, which is harmless to humans and well-suited to our environmental conditions,” explains Dr Mishra. Despite the promising potential of this method, he notes that deployment will take time.
“Programmes like ours, along with vaccines, might be useful in certain settings, but dengue will remain a significant issue for years to come,” regrets Simmons, adding, “Large-scale solutions are slow to implement, and dengue often flies under the radar, despite its profound impact on communities. I’m not optimistic about a quick fix, but we must continue our efforts.”
The clock is ticking. For now, the cooties are far ahead.
Bite me not
There are around 3,500 species of mosquitoes worldwide, but only a few transmit diseases to humans. While most mosquito bites are merely irritating, certain species pose significant health risks:
● Aedes aegypti: Transmits yellow fever, dengue, chikungunya, and Zika viruses
● Aedes albopictus: Known for spreading chikungunya, dengue, and dirofilariasis
● Aedes atropalpus: Can carry West Nile, La Crosse, Japanese encephalitis, Saint Louis encephalitis, and Eastern equine encephalitis viruses
● Aedes koreicus: Native to Japan, northeastern China, South Korea, and parts of Russia; a vector for Japanese encephalitis in some regions
● Aedes triseriatus: Primary vector of La Crosse virus in North America
● Anopheles atroparvus: Widespread in Europe, particularly coastal areas; historically a key malaria vector
● Anopheles labranchiae: An aggressive species known for attacking humans
● Anopheles plumbeus: A persistent human biter, suspected of contributing to past malaria outbreaks
● Anopheles sacharovi: A major malaria vector in its distribution areas
● Culex pipiens: A common urban pest in Europe
Source: European Centre for Disease Prevention and Control