How climate crisis is accelerating the global spread of deadly dengue fever

(CNN)Explosive outbreaks of dengue fever have rapidly spread in countries across Asia, killing more than 1,000 people, infecting hundreds of thousands and straining hospitals packed with sick families.

Images from Bangladesh show patients in teeming hospital wards, lying beneath mosquito nets under lurid electric strip lights.
Mothers cool their children from the sticky summer heat with hand-held fans while others rest on hospital floors, holding drips, waiting for a free bed.
This is the worst outbreak Bangladesh has ever seen. Five times as many people were infected in August than the whole of 2018, and so far this year 57 people have died.
    Bangladeshi patients suffering from dengue fever receive treatment at the Shaheed Suhrawardy Medical College and Hospital in Dhaka on September 3, 2019.
    It's a similar story across the region. Cambodia, Laos, Malaysia, Singapore, Sri Lanka, Thailand, and Vietnam have all reported higher than normal cases of the disease and deaths compared to last year. And the Philippines has declared a national dengue epidemic -- 1,107 people have died there since the start of the year and more than 250,000 have been infected.
    Dengue is a seasonal, mosquito-borne disease commonly found in hot, wet regions of the tropics and subtropics during the rainy months.
    Scientists say hotter, wetter weather brought on by climate change has created ideal conditions for female mosquitoes to lay their eggs. Not only are there more mosquitoes, but the rapid urbanization occurring in many Asian nations means that susceptible populations are living in closer contact with disease-carrying insects.
    Add to the mix inadequate or unprepared health services and the circulation of a virus, and you have the ingredients for an outbreak of epidemic proportions -- and one that is likely to spread.

    The 'break-bone' fever and climate crisis

    Dengue is a viral infection transmitted by the Aedes mosquito, the same insect responsible for spreading Zika, chikungunya and yellow fever. It causes flu-like symptoms, including piercing headaches, muscle and joint pains, fever and rashes, though only 25% of those infected show symptoms. Extreme cases can bring bleeding, shock, organ failure, and potentially death.
    It's the most rapidly spreading mosquito-borne viral disease in the world, increasing 30-fold in the past 50 years. While it was once found in only nine countries, today the disease is endemic in more than 100 -- putting more than half the world's population at risk, according to the World Health Organization (WHO).
    There have been numerous studies on the link between climate change and mosquito-borne diseases and experts say the climate crisis is playing a major role in dengue's spread.
    Scientists have repeatedly warned that rising temperatures are triggering more extreme weather events, and a warmer, wetter world could put us at greater risk of vector-borne diseases -- those transmitted by mosquitoes, ticks or other organisms.
    Bangladeshi patients suffering from dengue fever rest on the floor of a hospital ward in Dkaha.
    One recent study suggested that 1 billion more people might be exposed to mosquito-borne disease by 2080 as temperatures continue to rise with the climate crisis.
    Dengue-carrying mosquitoes thrive in urban areas and lay their eggs in shallow pools of water, in household containers, buckets, even upturned bottle caps. But environmental changes can affect how quickly viruses replicate in mosquitoes and how long they live, increasing the risk to humans.
    "As the average temperature increases the survival of mosquitoes and the actual replication of the pathogen with mosquitoes becomes more efficient," said Dr. Rabindra Abeyasinghe, Coordinator of Malaria, Other Vectorborne and Parasitic Diseases, at the WHO's Regional Office for the Western Pacific. "So the mosquitoes only need to survive for shorter periods of time."

    Erratic weather makes it harder to predict outbreaks

    Since the start of the year, more than 7 million people have been displaced worldwide as a result of disasters such as flooding, cyclones, and droughts -- a phenomenon described by the researchers at Internal Displacement as becoming the norm.
    "Climate change is altering weather patterns across the globe. Seasonality is changing, we're having less predictable extreme events," said Dr. Rachel Lowe of the London School of Hygiene and Tropical Medicine.
    Lowe said that these changing patterns can cause the dengue season to shift "making it harder to know when and where an epide