Wading through shin-deep water, your face twisting in a grimace every time some unseen thing grazed past your ankle, you cursed the authorities for the inundated streets, the overflowing drains, and the putrid water creeping up your legs. A couple of days later, when the flood has receded into potholes dotting the lanes, into open drains, and garbage heaps at streetcorners where sunlight never ventures, you breathed a sigh of relief believing it was the end of your woes, at least until rains came lashing again.
But beware! For in the dark, damp recesses of the city, festering floodwater pools become the breeding ground of disease-bearers. Nurtured in muck, they creep into your homes, schools, and workplaces like unwelcome guests – dengue-infested mosquito hoards. Dengue, in the national capital, has become a recurring monsoon bane when stagnant water provides ideal breeding conditions for the Aedes aegypti, the carrier of the dengue virus.
Despite advances in healthcare and sanitation, the national capital remains a hotbed for vector-borne illness. In the past eight years, the city has witnessed three of its worst dengue outbreaks, casting a shadow of doubt over claims by civic agencies and the government about controlling the spread.
The worst dengue outbreak was reported in 2015 when the city saw 15,867 cases of the vector-borne viral infection and 60 deaths due to it — a significant surge attributed to a particularly virulent strain.
In 2021, the capital faced its second-worst dengue situation, logging approximately 9,613 cases and 23 deaths. Many believed that the severity of the outbreak was underreported, prompting the city government to classify dengue as a notifiable disease through a gazette notification, making it mandatory for healthcare facilities to report every case.
Then in 2023, while G-20 fever swept the city, so did the authorities; extensive sanitation and beautification efforts were undertaken by governments and civic agencies alike, to make the capital VIP-ready. Yet, another major dengue outbreak marred the festivities. National Centre for Disease Control data showed that the capital recorded 9,266 cases of dengue and 19 deaths in 2023.
This year, Delhi has seen over 256 dengue cases till July 6, nearly double the 136 cases recorded in the corresponding period in 2023; and the highest since 2020. In areas under the jurisdiction of other agencies like New Delhi Municipal Corporation (NDMC), and Delhi Cantonment and Railways, around 10 dengue cases have been reported until July 6.
Civic body officials attributed the significant rise in dengue cases in the national capital this year to an increased number of testing centres, which has grown from 36 last year, to 900. “The rise in the number of cases this year is because more testing centres have started collecting samples and reporting dengue cases to the civic authorities. Until last year, there were about 36 testing centres. Now, this number has increased to 900; because of this the number of cases appears to be inflated,” a senior civic body official said.
Infestation
But what makes this bustling metropolis so vulnerable to dengue? The answer probably lies in a complex interplay of environmental, infrastructural, and social factors. According to experts, a host of factors contribute to the alarming transmission of dengue in Delhi; first and foremost, the city’s tropical climate, provides a conducive environment for the Aedes mosquito to thrive. Dengue is primarily transmitted by the Aedes aegypti, which thrives in warm, humid climates. Monsoons in Delhi, characterised by heavy rains and high humidity, provide the perfect breeding ground for these mosquitoes.
Ideal climate
Monsoons in Delhi, typically from June to September, bring heavy rains, often leading to waterlogging in several areas due to its loosened civic infrastructure. This creates ideal breeding grounds for the Aedes mosquito, the primary vector of dengue. These mosquitoes lay eggs in stagnant water. The optimal temperature range for Aedes activity and dengue virus replication (growth) is between 25-30°C. Delhi’s climate often falls within this range, facilitating dengue spread. High humidity levels during and after monsoons are also favourable for mosquito survival and reproduction. High humidity increases the mosquito’s longevity, enhancing the likelihood of disease transmission.
Dense population
The unchecked urbanisation in motion in the national capital further compounds the problem; the high population of human hosts to transmit the virus increases the chances of mosquito-human contact, facilitating the spread of dengue. Besides, the high population density exponentially multiplies ‘man-made breeding grounds’ – stagnant water collecting in and around homes, in clogged drains, open containers, pots, discarded tyres, and construction sites – serving as ideal breeding sites. “Densely populated areas could play a role in dengue transmission due to increased chances of human-mosquito contact, the presence of more diverse and suitable vector habitats and breeding sites, and changes in land use,” said Dr Manali Agrawal, former microbiologist, at AIIMS Delhi.
Civic inadequacy
Lack of awareness among the public also plays a crucial role in the unchecked proliferation of dengue cases in the national capital. Many residents remain unaware of effective preventive measures, such as maintaining clean surroundings, using mosquito repellents, and wearing protective clothing. Education campaigns must be intensified to empower individuals with knowledge to protect themselves and their communities.
According to the National Institute of Malaria Research, inadequate sanitation and waste management contribute significantly to the problem. “The accumulation of improperly disposed waste can become breeding sites for mosquitoes. Therefore, local authorities must prioritize proper waste disposal and drainage system maintenance to break the dengue transmission cycle,” the institute said in a research published in the International Journal of Infectious Diseases.
“Moreover, population mobility and urban migration further complicate disease control efforts. People moving in and out of the city can inadvertently carry the virus to new areas, causing outbreaks in previously unaffected regions. This highlights the need for a coordinated, region-wide strategy to combat dengue,” it added.
Clinical epidemiology (study of the epidemic) of multiple strains of Dengue Virus (DENV) also suggests that co-infection among the population is an epidemic driver of vector-borne disease.
Epidemic
Co-circulation of all four serotypes (strains) with predominance of DENV-2 was observed during the 2015 dengue outbreak, i.e. concurrent infection with multiple serotypes. This was the first outbreak in the capital where concurrent infections from all four strains were observed in a patient, thus necessitating a clinical recourse.
Dengue virus has four serotypes, namely DENV1, DENV2, DENV3, DENV4. Severe clinical manifestations of the virus is commonly seen with DENV -2, hemorrhagic manifestations with DENV-4, liver complications with DENV- 3; while DENV-1 usually has mild clinical symptoms.
“Dengue patients infected with multiple serotypes of DENV are more prone to have severe manifestations than with mono-infection. One particular serotype increases the risk of getting severe infection if infected again with a different serotype. This is known as antibody dependent enhancement,” Dr Aggarwal explained.
“After you’ve recovered from dengue fever, you have long-term immunity to the virus strain that infected you, but not to the other three types. The fatality rate increases severely when a secondary infection occurs by a virus serotype different from that of the initial infection,” she added.
Experts suggest, identifying which serotype is causing an outbreak helps in understanding the epidemiology and potential severity of the disease, as subsequent infections with different serotypes increase the risk of severe dengue fever.
The genome sequencing data, combined with serotype information, helps in predicting potential outbreak patterns, identifying high-risk areas, and understanding the transmission. This predictive capability allows for proactive measures to prevent large-scale outbreaks. With the frequency of outbreaks in Delhi, there is an urgent need to conduct epidemiological and entomological surveillance which will asssist in early detection of the outbreak, thus aiding mitigation.
Serotype identification is as important as virus detection, Dr Aggarwal asserted. “To know how the virus behaves, it’s important to do virus sequencing. Genome sequencing and serotyping will be very helpful in medical management of dengue and also in implementing prevention measures by the authorities,” she said, adding, “Apart from providing information on which serotype is dominant in a particular area, contact tracing and surveillance, the sequencing would also let researchers know if any mutation has occured in the genetic makeup of the virus, which would eventually help in drug and vaccine development,” she added.
AWARENESS
Meanwhile authorities, apart from taking steps to control infection spread, have also tuned to widespread genome sequencing since 2023. Health Minister Saurabh Bharadwaj has issued strict instructions for genome sequencing of the dengue infections. Last year, 19 out of 20 samples sent for the genome sequencing returned with prevalence of DENV-2 which is considered the most potent serotype of the dengue virus.
Both Centre and states have issued multiple directions to hospitals under their jurisdictions to prioritise dengue patients for immediate treatment. Union health and family welfare minister JP Nadda has directed officials to create a 24/7 central helpline number for dengue prevention, awareness, and support for queries on the symptoms, treatment protocols and during emergency situations. Nadda has directed all central government hospitals to have dedicated dengue wards fully equipped with manpower and resources.
Delhi health minister Bharadwaj also issued directions to take preventive and infection control measures for dengue in city hospitals. The availability of essential resources such as mosquito nets in dengue wards has been highlighted as a priority to prevent patients from being bitten by mosquitoes while admitted.
Many hospitals said they have already reserved 5% of their beds for dengue patients, designating dedicated dengue wards. This also ensures that the diseases do not spread to other patients. Besides, cleanliness in hospital premises is also a high priority. Bharadwaj has ordered hospitals to ensure medical waste is disposed of properly.