Year After Year, Why Does India Fail to Control Dengue Outbreaks?

Despite knowing how it spreads and when cases tend to spike, why is it so hard to contain dengue outbreaks in India?

Kanika sits nervously in the waiting room of a children's hospital in Bengaluru where her 9-year-old son is being treated for dengue. "This is the second time he's had dengue, and this time it's more severe. This time his platelet count fell so low that we have had to admit him," she tells FIT.

Kanika's son is one in several dengue-positive children admitted in the hospital at this time.

Karnataka is currently seeing a surge in dengue cases. So far, the state has reported 4,827 cases of dengue since the beginning of 2024, with over 2,000 of them recorded in Bengaluru alone.

Moreover. the first dengue-related death in the city this year was recorded on Saturday, 29 June.

As the blistering heat makes way for monsoons across the country, like clockwork, the season for vector-borne diseases, particularly dengue, also arrives in the country, year after year.

We know how it spreads and when it spreads. Why, then, is it so hard to control the spread of dengue in India?

What Data Says: Dengue Cases in India Are on the Rise

First let us take a quick look at the trajectory of dengue cases in India in the last few years.

"Dengue is endemic in India," Says Dr PS Indu, Professor and Head, Department of Community Medicine, Government Medical College, Kollam, Kerala. This means that the disease is regularly found in this area, and its cases have plateaued.

With the exception of 2020 when the COVID-19 pandemic was at its peak, dengue cases have spiked in parts of the country consistently every year. The spikes generally occur between the months of July and September when most parts of the country experience heavy rains.

Latest data from the National Centre for Vector Borne Diseases Control suggests that the total annual cases in India have been significantly higher since the pandemic compared to the years immediately before COVID-19.

Data source: National Centre for Vector Borne Diseases Control and the Ministry of Health and Family Welfare.

This year, at least 19,447 cases have already been recorded in India.

Moreover, according to government data, some states like Karnataka, Delhi, Kerala, Sikkim, and Haryana tend to see the highest cases every year.

Speaking to FIT, Dr Suneet Verma, an internal medical specialist in Panchkula, says that dengue is grossly underreported and that the actual number of cases is likely far higher.

"This is not only the case in India, but in nearby countries like Pakistan, Sri Lanka, Thailand, and Bangladesh as well. We are seeing a tremendous increase in the cases of dengue everywhere," he explains.

'Dengue Is a Tricky Virus to Conquer'

Although the dengue virus is well studied, the nature of the virus and how it spreads makes it difficult to contain, say experts. There are a few major factors that contribute to this.

• Multiple serotypes

For one, there are four known serotypes of the dengue virus (DEN-1, DEN-2, DEN-3, and DEN-4).

Dr Indu, who led a joint study by the Government of Kerala and the World Health Organization (WHO) on dengue in India earlier this year, explains that people who are infected with one serotype develop an antibody response to that serotype.

However, she says, "People can get infected again with another serotype, it triggers an abnormal immune response and this leads to severe dengue."

"Areas that have a high rate of severe infections are places where you have all four serotypes. We call this a hyper-endemic situation. This is why places that are considered dengue hotspots experience repeated outbreaks."

Dr PS Indu

Adding to this, Dr Verma says, "Researchers have even found evidence of another fifth serotype, which is not well documented, and there is no test available for that."

• The virus can survive in the eggs of the moqsuitoes

According to Dr Srinivasa Rao Mutheneni, a scientist in the Biology division of CSIR-Indian Institute of Chemical Technology, Hyderabad, "Aedes aegypti and Aedes albopictus mosquitoes are highly adaptive and can breed in a variety of environments."

He explains,

"The Aedes mosquitoes can breed indoors and even in small amounts of stagnant water making it difficult to eliminate all potential breeding habitats in urban and rural areas."

They can also be very stubborn, adds Dr Indu. "It isn't enough to just empty out stagnant water. The eggs of the mosquitoes can cling to the surfaces of dry buckets and water storage units even after they've been drained. They need to be scrubbed and cleaned to prevent breeding which people rarely do."

• Asymptomatic cases

Another peculiar characteristic of the dengue virus is that most people who are infected by it do not present any symptoms.

"Out of 100 cases of dengue, 80 are asymptomatic. In fact, during one of our studies in children, we found that 89 percent of kids infected with dengue were unaware that they have it."

Dr PS Indu

It's harder to contain cases when people don't even know they are dengue-positive and may be spreading the virus.

According to a recent modelling study conducted in India, around 84 percent of dengue transmission in the community happens via asymptomatic cases.

• Rapid urbanisation and poor urban planning

Rapid unplanned urbanisation is also responsible for creating numerous potential breeding habitats for mosquitoes, say experts.

Crowded cities with inadequate infrastructure and sanitation often lead to poorly managed waste water, waterlogging, and stagnant water (especially in construction sites), says Dr Mutheneni.

Moreover, due to water shortage in urban cities, particularly during the monsoons, people tend to store large quantities of water in and around their homes in containers that are often left open or unattended, further increasing the risk of mosquito breeding.

"Every household in areas prone to water shortage has extra drums which are sitting uncovered and having water in it, around the clock. This is a huge problem."

Dr Suneet Verma

• Climate change

"The growth and life cycle of Aedes mosquitoes is affected by various climatic factors like temperature, rainfall, humidity, and wind speed etc,"explains Dr Mutheneni. "Increased temperatures accelerate the development of Aedes mosquitoes from eggs to adults, leading to more rapid breeding cycles."

"Higher temperatures, heavier rainfall, and frequent flooding are conducive to mosquito breeding and extend the duration of transmission periods."

Dr Srinivasa Rao Mutheneni

He adds, "Climate change can lead to increased migration and displacement of populations due to extreme weather events which introduce the dengue virus to new areas and facilitate its spread."

What About the Vaccine?

CYD-TDV (Dengvaxia) is the first licenced dengue vaccine which has been approved for use by the European Medical Association, the United States Food and Drug Administration, and some Asian countries, including India. It is also in the WHO's Essential Medicines List.

However, experts say, it has its limitations.

"This vaccine (Dengvaxia) was effective in those who have had dengue in the past. In those who had not been previously infected, the vaccine could potentially increase the risk of severe dengue if they contracted the disease after vaccination...this limited the use of CYD-TDV in national immunization programmes."

Dr PS Indu

The reason this happens is because of the complex immune response that the dengue virus triggers. This is also why a person develops more severe illness if they get infected with dengue naturally, more than once.

This makes immunisation programmes more complicated and expensive because it necessitates a trustworthy pre-screening system to verify prior infection.

According to Dr Verma, because the immune mechanism for dengue is so complex, creating a stable vaccine for it has been challenging. He explains, "This is the reason why we haven't been able to develop a vaccine for mass use."

There are currently other dengue vaccines that have been approved or are in late stages of development, and the goal is to develop a safe and effective dengue vaccine that can be included in routine immunisation programmes globally, especially in dengue-endemic regions, say experts.

Can’t Tackle Dengue Without Better Sanitation & Public Health

According to Dr Verma, "Once an outbreak starts, you can't reverse it. There is no treatment for it. We can only give symptomatic support. Every year we see young people dying in hospitals because of this."

Which is why, according to Dr Verma, dengue alone cannot be curbed in a targeted manner if larger public health issues like poor sanitation, water shortage, and poor wastewater management are not addressed.

"Many dengue-prone areas are in low- and middle-income countries with limited resources for vector control and poor healthcare infrastructure to manage disease outbreaks effectively," says Dr Mutheneni.

Moreover, he adds that poor surveillance systems are responsible for the delayed detection of outbreaks and the implementation of control measures.

"Containment of dengue requires a multi-faceted approach involving vector control, public education, improved surveillance, and effective healthcare response system."

Dr Srinivasa Rao Mutheneni

Dr Indu, adds, "right now we need to have an integrated approach focusing on vector control through community engagement and meticulous clinical management to reduce mortality."

However, according to Dr Verma, "So far, government machines and healthcare department machinery have not been proactive in preventing the illness at a community level."