Transmission windows of dengue, a mosquito-borne viral illness, have been expanding due to changing climate, and the number of cases has been increasing, an IndiaSpend analysis of prevalence data in Uttar Pradesh, particularly its capital Lucknow, has shown.

Between October 31 and November 1, Lucknow reported 91 new cases of dengue, as per the data accessed from the chief medical officer. In the week to November 1, more than 400 cases were confirmed. There hasn’t been a single day in October this year without the detection of new cases. Overall, more than 2,100 dengue cases have been reported in the city so far this year.

Across India, cases have risen from 157,315 in 2019, before the Covid-19 pandemic, up to 289,235 last year. The disease, transmitted by the Aedes aegypti mosquito, causes a wide range of symptoms, from mild fever to severe, life-threatening illness.

There is no known cure for dengue, and cases can be treated at home with regular oral hydration and fever management, as IndiaSpend reported in September 2022. However there is danger of bleeding, in which case the patient would need to be hospitalised and need blood transfusions, a 2017 study found. During an outbreak this causes shortage of blood platelets, says the study.

Risk of dengue increases with increasing temperatures and increasing rain in India, found a study published in 2019.

Innocuous infections can quickly turn deadly

Girish Chandra Dubey, a 56-year-old resident of Aliganj in Lucknow, had a fever that persisted for several days. Soon after, he began experiencing severe pain in his limbs. His family rushed him to King George Medical University where tests confirmed he had contracted dengue. His platelet count had dropped to 20,000. After two days of treatment at KGMU, the hospital was unable to provide the necessary platelets, and Dubey was transferred to a private facility.

In a healthy individual, the normal platelet count ranges from 150,000 to 400,000 per microlitre of blood. However, when platelet levels fall below 20,000, the risk of developing dengue shock syndrome (DSS) becomes imminent, N.B. Singh, chief medical superintendent at Balrampur Hospital explained.

“Platelets are necessary for blood clotting. A deficiency can put the patient’s life at risk,” said Tulika Chandra, head of blood and transfusion medicine at KGMU.

DSS is a severe complication that occurs in the later stages of the disease. Symptoms include persistent fever, body aches, and the development of rashes and red spots on the skin. As the condition worsens, the patient may experience a slowing pulse and their lips may begin to turn blue – a sign of circulatory collapse. In some cases, internal bleeding can occur, and if left untreated, dengue can be fatal.

On October 19, an 84-year-old woman died of dengue at Balrampur Hospital, the only such fatality this year in Lucknow, according to data from the CMO’s office. Singh noted that “her immunity was already very weak”, which made her more susceptible to the complications of dengue.

Cases have shown an uptick, as we said. In September 2024, there were 388 confirmed dengue cases in Lucknow. By October, this number rose to over 2,100.

In the pandemic year of 2021, the city had reported 1,104 cases up until November 4, with 95% recorded between September and November. In 2022, Lucknow recorded 1,677 dengue cases and in 2023, there were 2,700 dengue cases in the city.

How the vector has adapted

Singh and other medical professionals agree that the Aedes aegypti mosquito thrives in areas with stagnant water. Changing weather, with heavy rainfall followed by heat, creates the perfect breeding ground for these mosquitoes.

Warmer temperatures accelerate mosquito breeding, enabling the mosquitoes to bite more frequently, further increasing the chances of spreading the virus, research shows. High humidity, which supports the survival of mosquitoes, and the accumulation of floodwater from heavy rainfall also contribute to the growth of mosquito populations, creating additional breeding sites. As a result, areas that were once considered low-risk are now seeing outbreaks of the disease.

Experts also point out that the change in human habitations has helped mosquito populations spread faster, as IndiaSpend reported in September 2022. India’s population density in 1961 was 155 people per sq km, which in 2021 increased to 469 people. The Aedes Aegypti mosquito breeds in natural containers such as tree holes and bromeliads, but nowadays it has adapted to urban habitats and breeds mostly in man-made containers, including buckets, mud pots, discarded containers, used tyres, storm water drains etc, making dengue an “insidious disease in densely populated urban centres”, according to this World Health Organisation article.

Dengue symptoms typically begin with fever, severe headaches, joint and muscle pain, nausea, vomiting, pain behind the eyes, and skin rashes. In the most severe cases, the virus causes internal bleeding and can lead to death if not treated promptly. While the majority of people experience only mild symptoms, the potential for severe complications makes dengue a serious public health threat.

“There are a lot of overlaps of symptoms between malaria and dengue, and other tropical diseases, and viral fever. In the early phases even swine flu and leptospirosis [have similar symptoms],” which could lead to incorrect diagnosis or reporting of malaria and dengue cases, said Bharat Agarwal, internal medicine specialist at Apollo Hospital in Navi Mumbai, had told us in September 2022.

Sanjay Kumar, the district medical chief of Bahraich, told IndiaSpend that the rise in dengue cases is indeed related to the shifting weather patterns. “This time the winter is late and the warm temperature combined with water logging has become a perfect ground for the breeding of mosquitoes. The preventive measures by the government have been scaled up but it needs more attention,” he said.

India’s malaria and dengue data are collected mainly by the Health Management Information System and the Vital Registration System and Medical Certification of Cause of Death. But not all malaria and dengue cases are diagnosed, leading to undercounting, as IndiaSpend reported in September 2022. And India doesn’t register about 3 million deaths and does not certify the cause of death of about 8 million people, IndiaSpend reported in August 2021. No certification could mean that some dengue deaths are also not certified, and thus undercounted.

Rising platelet  consumption

At Balrampur Hospital, 36 beds are specifically allocated for dengue patients. Till November 1, 2024, a total of 28 patients were receiving treatment for the viral infection. Ajay Tripathi from Lokbandhu Hospital shared similar figures: Of the 20 beds available, 13 were occupied by dengue patients.

Of the 150 fever cases presented at both hospitals daily, around 15-20 are diagnosed with dengue. With platelet counts in many patients plummeting rapidly, recovery times are lengthening, sometimes taking more than a week for patients to stabilise.

Major hospitals said they are maintaining adequate stock of platelets. However, a growing strain on the city’s blood banks is evident. Private and government hospitals have reported that their platelet consumption, which was about 200 units per day just three weeks ago, has now surged to over 400 units daily.

Despite these challenges, Manoj Agarwal, the CMO of Lucknow, said there is no shortage of beds or platelets in hospitals. “The increase in cases this month is less than last year. We have enough resources to handle the current situation,” he said, calling for hygiene and usage of mosquito nets as preventive measures.

Agarwal points out that the city’s increased efforts in testing and raising awareness have helped in detecting more cases early. “The number of cases in October increased by about five percent compared to last year, but the total number recorded this year remains significantly lower,” he explained. He attributed this to heightened surveillance and 135 testing centres across the city – up from 83 last year.

“The higher number of cases this month is primarily due to increased testing,” Agarwal added. “Overall, we are managing the situation better than in previous years.”

Rajesh Srivastava, CMO at Dr Shyama Prasad Mukherjee Civil Hospital in Lucknow, told IndiaSpend that the condition of dengue patients in the district is improving by the day. Special precautions are being taken for dengue patients at the Civil Hospital, such as the provision of mosquito nets and special attention to cleanliness.

However, the in-charge of a community health centre in rural Lucknow, on condition of anonymity, stated that if proper testing is done, the number of cases could rise even further. “When the report comes back positive at our centre, the patient either doesn’t come to the hospital for treatment at all or goes to a private hospital,” they said. “In such cases, they are not even counted. Patients go home and start treating themselves.”

IndiaSpend reached out to the principal secretary for health in Uttar Pradesh for comment. We will update this story when we receive a response.

Climate change and its impact

Gaurav Kumar, physician at a private hospital, explained how temperature plays a crucial role in mosquito breeding. “Studies show that in September, October, and November, temperatures between 20°C and 35°C are ideal for the rapid growth of Aedes aegypti mosquito larvae,” he said. “The recent climate changes, including rains in October, have created favourable conditions for the mosquitoes. As temperatures begin to drop, mosquito populations will likely decrease.”

October 2024 saw the highest average night temperatures since 1901 in all regions except east and northeast India. Meteorologists attribute this anomaly to delayed monsoon withdrawal and the formation of low-pressure areas. Atul Kumar Singh, a senior scientist at the India Meteorological Centre in Lucknow, explained, “The lack of neutral El Niño conditions and the presence of active western disturbances have caused the temperatures to remain higher than usual.”

Last month, Lucknow’s minimum temperature was 3°C higher than usual while the days were, on average, 1.2°C warmer, he said. This abnormal heat has contributed to mosquito breeding cycles and, by extension, the rise in dengue cases.

Vijay Nath, former medical superintendent at Sir SunderLal Hospital at Banaras Hindu University and professor at BHU, said climate change is a key factor contributing to the global rise in Aedes mosquitoes. “As temperatures rise and rainfall patterns shift, we could see a significant change in the geographical distribution of these mosquitoes,” he explained. “This could lead to their spread in regions like Europe and parts of Asia, along with Northern Hemisphere where they were once uncommon.”

About half the world’s population is now at risk of dengue, with an estimated 100-400 million infections every year, according to the WHO. Countries in Southeast Asia, including Indonesia, Bangladesh, Nepal, and Thailand, have seen an increase in cases and deaths this year, compared to the same period in 2023.

As temperatures rise, the range of mosquitoes expands, pushing them into previously cooler regions. For example, regions like northern Himachal Pradesh and Jammu and Kashmir, which traditionally had cooler climates, are now experiencing an uptick in dengue cases.

Increasing temperatures may further exacerbate this situation by enabling greater spread and transmission in low-risk or currently dengue-free parts of Asia, Europe, North America and Australia, according to research published in Nature Microbiology in June 2019, which derived estimates based on climate, population and socioeconomic projections.

With inputs from Azeem Mirza in Bahraich.

This report is republished with permission from IndiaSpend, a data-driven, public-interest journalism non-profit. It has been lightly edited for style and clarity.

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