Experts examine climate crisis, dengue link in Delhi

With 33 dengue cases reported in the national capital so far this year (a highly unusual spike when compared to previous years), health and environmental experts suspect the climate crisis — an extended rain cycle and high humidity levels — may be leading to the higher incidence of the infection and the gradual extension of the cycle of the illness well into Delhi’s winter.

Data collated by the South Delhi Municipal Corporation (SDMC), the nodal agency that maintains data for the vector-borne disease for all three municipal agencies, shows that Delhi reported 33 dengue cases between January 1 and February 5. Last year, the Capital recorded 1,337 cases in December; 6,739 cases in November; and 1,196 cases in October. In 2020, Delhi reported 122 cases in December, 338 cases in November and 346 cases in October.

Municipal officials agree that the high dengue count is “unusual for this time of the year” but added that the trend of increasing dengue cases during peak winter months has come to the fore over the last few years.

“Data is proving this, but even if you ask people from older generations, they can tell you that till the 1980s and ’90s, the dengue season would start by July and extend only till around September, when the monsoon season would abate. Over the last decade, we are seeing that the mosquito breeding season is gradually extending well into the winter months… This could definitely be because of the change in precipitation patterns and higher humidity levels in Delhi, but another factor is that the aedes aegypti mosquito (which spreads dengue) is also evolving,” said a senior official of SDMC’s health departmentww.

To be sure, the Delhi government has declared it a notifiable disease in the Capital, making it mandatory for hospitals to report all cases to the state.

Noting that the adult aedes aegypti mosquito becomes less active in the open environments as the temperature drops below 20 degrees Celsius (°C) and gradually becomes inactive as the mercury level falls below the 10°C mark, health experts said that the larger role of climate crisis cannot be ignored in the extending dengue cycle in the national capital, adding that multiple studies across the world have proved this. January 2022 was the coldest ever in the Capital in 19 years, according to reports.

Dr Manisha Kulkarni, associate professor in the School of Epidemiology and Public Health at the University of Ottawa, who recently wrote a review paper summarising climate change effects on malaria and dengue, said that while there were no studies proving this link specifically in India, “given the observations from other countries in Asia”, climate change was leading to an expanded geographic range of the aedes aegypti mosquito as well as increasing transmission suitability for the infection in many areas. This was also leading to the lengthening of the transmission season in endemic areas, she said.

“It is important to note that other factors such as urban intensification and potential introduction of new virus strains can also result in surges in dengue transmission, but the warming temperatures and changing precipitation patterns due to climate change are certainly an important driving force,” Dr Kulkarni told HT.

Dr Kulkarni’s paper, which was published in January this year, said, “There is strong evidence for the impacts of climate crisis, including climate variability, on the transmission and future spread of malaria and dengue, two of the most globally important vector-borne diseases.”

“…Climatic factors, such as temperature and rainfall, are intricately linked to the biology and transmission of vector-borne diseases. According to the IPCC (Intergovernmental Panel on Climate Change), global climatic changes, which include but are not limited to changes in temperature and rainfall, have altered the distribution of disease vectors and the risk of vector-borne diseases, and will continue to do so in the future decades with the largest impacts experienced by populations living in resource-poor settings,” it added.

Extended rain cycle

Climate experts also said that extended rains and high humidity levels, which is being observed over the last decade in Delhi, is creating a conducive breeding ground for mosquitoes and thereby extending the dengue cycle.

“This year we saw that November was completely dry; usually there are three-four western disturbances in this month. But in January, all rainfall records were broken…The rainfall patterns in Delhi are becoming more erratic. Shorter and intense spells are resulting in waterlogging. Such stagnant water is the ideal breeding ground for dengue spreading mosquitoes,” said Mahesh Palawat, vice-president (meteorology and climate change), Skymet Weather Services.

Delhi received 88.2 mm of rain, breaching the record for the highest rainfall recorded in January in 122 years.

Some experts, however, said that the dengue cases this year may be an anomaly because of the above-average monsoon season.

“…The 2021 monsoon was very unusual with a deficit June-July but extremely heavy rains from late August into November. Delhi got very unusual heavy rains into November and even December. The mosquito population would explode with such unseasonal rains and increases in stagnant waters. Delhi received over 1,500 mm of rain, which was the highest in more than 80 years,” said Raghu Murtugudde, professor of atmospheric and oceanic science and earth system science at the University of Maryland, and visiting professor at IIT-Bombay.

He added, “This is probably the most important factor for record dengue outbreak now. The La Niña winter should have brought cold nights but night-time minimum temperatures didn’t break any records. So the mosquito population didn’t get squashed. A drier December following heavy rainfall earlier would also help mosquito breeding. Other human factors like how much time people spend outside will contribute as well.”