Shifting weather patterns driving dengue cases in Manipur

Nongmaithem Debarani *

Shifting weather patterns driving dengue cases in Manipur :: Picture Courtesy - Chronicle News Service / Nongmaithem Debarani

Manipur, with its warm tropical climate and rich biodiversity, has a history of battling vector-borne diseases like Japanese Encephalitis (JE), dengue, and malaria. While JE outbreaks were recorded in 2017 and 2019, and dengue cases surged in 2024, malaria remains relatively low but consistent in pockets of Chandel and Churachandpur districts.

Although there were no major outbreaks of Zika virus or Chikungunya, the risk is imminent due to the favourable climatic conditions for the vectors. Rising temperatures and erratic rainfall also contribute to spread of these diseases, forcing authorities to implement preventive measures like vaccination drives, fogging, and community awareness initiatives.

Malaria is caused by the Plasmodium parasite and spread by Anopheles mosquitoes. In 2015, Manipur reported over 200 cases, but by 2021, the number declined to just 19 .

The National Vector Borne Disease Control Programme (NVBDCP) indicates that the highest number of malaria cases in recent years was in 2020, with 36 confirmed cases, which was preceded by 19 persons infected in 2018 and 2021, 16 in 2019, 33 in 2022, and 34 in 2023, and 41 cases recorded till November 2024.

The Anopheles mosquito thrives in specific environmental conditions, such as cooler and more stable environments, stagnant water bodies, and specific habitats.

Over the years, malaria cases have consistently decreased due to effective management and improved healthcare interventions by authorities concerned, such as the distribution of chemically-treated bed nets, better access to antimalarial treatments, and government-led awareness programmes.

While climate change has affected transmission by altering mosquito habitats and changing seasonal disease patterns, it remains a concern in regions experiencing fluctuating rainfall and rising temperatures.

On the other-hand, dengue, a viral disease transmitted by the Aedes mosquito, has traditionally been less common. However, in recent years, dengue fever has shown a remarkable surge in Manipur. Between 2015 and 2024, the state experienced serious fluctuations in dengue cases.

In 2019, there were 359 dengue cases, which declined to 37 in 2020 and rose to 203 in 2021 . In 2022, the state reported 503 cases and four deaths. The situation turned grave in 2023, with 2,548 cases, though no fatalities were reported.

By October 2024, Manipur logged 2,022 dengue cases and five deaths, with a significant spike observed that month. Imphal West and Imphal East districts were the most affected, accounting for the majority of cases. Some experts argue that this shift is closely linked to changes in climate patterns.

The Aedes mosquito is highly adaptable to urban and semi-urban environments and is more resilient to temperature fluctuations and humidity changes. It thrives in warmer climates, and with increased rainfall, an ideal environment is created for the Aedes mosquito.

The Aedes mosquito is a container breeder and can lay eggs near the surface of waterlogged areas, water bottle caps, used tyres, and planters or vases. It can even breed in a spoonful of water. Even if these containers are cleaned or the temperature plummets, the eggs can remain dormant until conditions become favourable for hatching.

According to entomologist Sujalata Ngangom of the State Malaria Office, the Aedes mosquito thrives in human environments. Unlike the Anopheles mosquito, which feeds on a single human, the Aedes mosquito feeds on 10-15 humans, which contributes to the higher rate of transmission.

She also mentioned that while people consider fogging the best way to prevent dengue, the chemicals remain effective for only 2-3 days and only eliminates adult mosquitoes, not the eggs. Thus, the best solution is larval source management, which includes removal of stagnant water in the vicinity, even if they are for gardening or landscaping purposes.

Rearing guppy fish can also help, as they feed on mosquito larvae. However, rearing guppy fish can be difficult as they cannot survive in the cold or if the water becomes contaminated, she conveyed.

Sujalata also stated as the Aedes is day-biter mosquito, people should use mosquito repellents in the day and wear full-length clothes. She further pointed out while there are vaccines available for malaria, there is no vaccine for dengue at the moment.

Meanwhile, ICAR entomologist Dr Arati Ningombam said that urbanisation has led to clogging of the drainage system and expanding breeding sites, compared to gradual decline in the populations of frogs and fish in the wild, which used to feed on mosquito eggs and larvae.

Rising average temperatures in Manipur over the past 50 years are a reality that cannot be ignored, she stated, adding that higher temperatures will always favour insect populations. She stressed the need for people to be more mindful of their surroundings, especially regarding stagnant drains and water, which can serve as egg-laying sites for Aedes mosquitoes.

When planning cities and local surroundings, ecological balance should be kept in mind as urbanisation should not only prioritise human convenience but also account for both the big and small organisms in the ecosystem, stressed the entomologist.

Dr Arati also noted that construction of concrete retaining walls to strengthen river banks and community ponds in urban areas obstructs mobility of water species ranging from tadpoles to frogs, which feed on insects, including mosquitoes. Consequently, they perish from starvation and exhaustion in trying to climb the step structures.

Naturally, the frog population declined each year, and the insects, including viral vectors, continue to increase. Therefore, city planning should not only be for engineers but also involve biologists and be carried out in consultation with them, ensuring maintenance of ecological balance.

While the decline in malaria cases is an encouraging sign of effective public health measures, the rise in dengue cases entails immediate action.

Taking all of the above into account, it is imperative to adopt climate-adapted vector control strategies, such as integrated vector management (monitoring and mapping, larviciding, and biological control), strengthen surveillance and data collection, engage the public in awareness campaigns, and build climate-resilient infrastructure.

Climate change mitigation and adaptation are the need of the hour. A multi-pronged approach that integrates these strategies into public health responses is essential for overall health of the people.

(The report is filed under the 5th Media Fellowship Programme on Climate Change Reporting under the sponsorship of Directorate of Environment and Climate Change, Government of Manipur) .