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New 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.
South Delhi Municipal Corporation, the nodal organization that maintains data on the vector-borne disease for all three municipalities, has collected data showing that Delhi reported 33 cases of dengue between January 1st and February 5.. Last year, the Capital saw 1,337 cases in December, 6,739 in November, and 1,196 in October. Delhi reported 122 cases last December, 338 in November, and 346 in October 2020.
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 have noticed that the mosquito breeding seasons is gradually expanding 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.
It is a recognized disease by the Delhi government, and hospitals must 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. According to reports, January 2022 was the coldest January in Capital history in 19 years.
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. She also said that this was causing a longer transmission season in endemic regions.
“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
Experts in climate science also stated that prolonged rains and high humidity levels are creating a breeding ground for mosquitoes, which in turn is extending the dengue cycle.
“This year we saw that November was completely dry; usually there are three-four western disturbances in this month. However, January saw the breaking of all rainfall records. The rainfall patterns in Delhi have become more irregular. Waterlogging is a result of shorter and more intense rainfall. 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 88.2mm rains, breaking the record of January’s highest rainfall for 122 years.
Experts believe that this year’s dengue cases may be an exception due to 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 experienced unusually heavy rainfalls from November to December. With such unseasonal rainfalls and stagnant water increases, the mosquito population would explode. 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. Also, a dry December following heavy rains would help mosquito breeding. Other human factors like how much time people spend outside will contribute as well.”
