Mosquitoes move up mountains as the earth warms

Global heating has fanned the spread of vector-borne diseases like dengue across the Himalaya

BAD BITER: The dengue-carrying female Aedes aegypti mosquito (above) can now survive in higher altitudes and latitudes where it could not before because of climate change.

With over 8,000 reported dengue cases and an estimated 80,000 infections in five months, Nepal’s viral fever epidemic this year has gone viral. So far six people have died, and dengue has spread to higher altitudes where mosquito-borne diseases were unheard of.

In Kathmandu Valley it has struck ministers, politicians, businessmen, even doctors and their families, but it has also drawn attention to tropical diseases migrating to the mountains. Agriculture Minister Chakrapani Khanal, renowned ophthalmologist Sanduk Ruit and Nepal Communist Party leader Bamdev Gautam are all down with fever, but the government’s response has been inadequate.

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“Vector insects are moving up the Himalaya as warming increases, and so are the diseases they carry. Climate change is leading to changes in the geographic pattern of diseases,” said Meghnath Dhimal of the Nepal Health Research Council. His research has shown that mosquitoes do not just travel to higher altitudes in vehicles, but increasingly complete their lifecycles in what used to be colder places.

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“Not just dengue: at any moment you can also have outbreaks of Chikungunya, Japanese encephalitis and Zika in the mountains of Nepal due to global warming,” Dhimal says.

This year’s unusually wet monsoon in Kathmandu Valley provided suitable breeding grounds for mosquitoes. The increased mobility of Nepalis has helped spread diseases like malaria, dengue and encephalitis to higher valleys. But it is climate change that has made disease-carrying mosquitoes themselves migrate to higher altitudes and latitudes.

While mosquitoes and the microorganisms they carry now thrive in places that used to be too cold for them, extreme heat in the plains is killing off the insects. Recent studies have shown that dengue, malaria and other vector-borne infections are now found in areas where they were not before, and have disappeared in places where they were common.

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In 2014, ecologists at the University of Michigan found the first hard evidence that malaria travels to higher altitudes during warmer years. Another study by the London School of Hygiene and Tropical Medicine confirmed that the movement of malaria is temperature dependent.

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Nepal has set a target to eliminate malaria by 2022, and the climate crisis adds to the challenge, while also stressing the need to expand existing health programs to districts previously unaffected by the disease.

“We need a climate-relevant and sensitive health system so that we are prepared for more frequent outbreaks,” added Dhimal. “This means training healthcare personnel and ensuring a robust monitoring, evaluation and surveillance mechanism.”

Ironically, the Kathmandu hospital that is supposed to control infectious diseases has become an epicentre for the spread of the virus because of overcrowding. When one 24-year-old banker based in Pokhara travelled to Teku to confirm her dengue status, she met many others like her from Dharan, Hetauda and Salyan.

“Relatives waiting on their infected family members were also starting to get sick because the mosquitoes there were biting everyone,” she said.

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Dengue is spread when a female Aedes aegypti mosquito bites an infected patient and then bites healthy people within the following week. The mosquito can bite up to 10 different people before she has had her fill, which is why multiple members of the same family are falling ill.

“We are receiving more than 500 cases per day and more than 70% of those I oversee have been tested positive,” said Sher Bahadur Pun, Chief of the Clinical Research Unit at Sukraraj Tropical and Infectious Disease Hospital in Teku. “Our response is not proportionate to the enormity of the crisis. We are risking our own health to treat infected patients.”

A female Aedes mosquito lays from 500-1,000 eggs in her lifetime, and if infected, all her eggs are also infected from birth and the epidemic multiplies. The best strategy to stop this cycle is destroying mosquito habitat, experts say. Personal protective measures are equally important.

“No one is safe since the mosquitoes are everywhere, from plains to the mountains,” said Prakash Prasad Shah of the Epidemiological Diseases Control Division. “Also, fogging is not effective — it only kills adult mosquitoes. We need search and destroy missions and this requires a collective effort.”

After Dharan started an anti-dengue campaign by targeting the mosquito’s breeding spots, there has been a drop in the four-month outbreak there. Similarly, dengue cases in New Delhi fell 80% after 2015 due to a massive public awareness campaign.

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Only 10% of those infected with the dengue virus show symptoms and need further treatment. Very few suffer from complications due to haemorrhage and internal bleeding. Of the four types of dengue viruses, most Nepalis seem to be afflicted with the milder forms, either type 1 or 2.

There is no vaccine or definite treatment for dengue fever yet. Patients are kept under observation and given supportive treatment for fever and with a blood transfusion if there is a drop in blood platelet count.

This story is part of Covering Climate Now, a global collaboration of more than 250 news outlets to strengthen coverage of the climate story.

Sonia Awale


Sonia Awale is Executive Editor of Nepali Times where she also serves as the health, science and environment correspondent. She has extensively covered the climate crisis, disaster preparedness, development and public health -- looking at their political and economic interlinkages. Sonia is a graduate of public health, and has a master’s degree in journalism from the University of Hong Kong.

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