Of dengue, climate change and El Nino

On 22 August, hospitalisations from dengue had already crossed the 1 lakh mark in Bangladesh. While the death toll is nearing the 500 mark this year –  the highest ever in the country's history. It's only August. 

Dengue statistics (both in terms of death toll and cases) over the last five years do not inspire hope – indicating an exponential rise in the numbers since 2021. For instance, dengue deaths were 105 in 2021, 281 in 2022 and 493 (till 22 August 2023), according to the Directorate General of Health Services (DGHS). 

Every year, dengue outbreaks bring the lack of preparedness among the authorities to counter it front and centre in national discourse – but not to any avail, track record shows. 

Top entomologists in the country blamed the local government and city corporations for the current dengue crisis. The experts also said the health ministry data on the virus was fragmented, failing to paint a true picture of the scenario.

This perhaps begs the question: how can a flawed healthcare management system fail to address an annual outbreak that has been around for more than two decades in Bangladesh? Did the Covid-19 pandemic not teach us anything in terms of preparedness? In fact, last year, The Business Standard published a story addressing these very questions. 

In the story, public health expert Professor Dr Be-Nazir Ahmed said, "Dengue has a connection with the environment - a specific amount of rainfall and humidity. This year [2022] the monsoon has lasted for a long time." He also said a clean environment is crucial, adding "Now it depends on the government whether they would focus on strategic planning or not."

Chain reactions?

For the uninitiated, dengue is a viral disease transmitted primarily by the Aedes aegypti mosquito. While the mosquito lays eggs in water containers, it also thrives in warm and humid environments and this weather condition generally follows after heavy rainfall. 

Having said that, increased rainfall leads to more breeding sites such as discarded tires, flower pots and standing water (puddles, waterlogged areas, etc). 

Additionally, the densely populated urban areas and inadequate drainage systems only exacerbate the impact of increased rainfall as water can pool in areas where people live and work, creating more breeding sites for mosquitoes. 

All of this facilitates the disease transmission at warp speed.  

It's important to note that while increased rainfall can contribute to the conditions conducive to dengue outbreaks, other factors also play a role — including but not limited to the availability of healthcare, public health efforts to control mosquito populations, community awareness and practices and the immunity levels of the population to the circulating strains of the virus.

And why is Bangladesh experiencing increased rainfall? It has been studied and proven that climate change is directly responsible for changes in weather patterns in Bangladesh, including increased rainfall during the monsoon season. 

Taking all this into account, and the intensifying adverse effects of climate change — it is not exactly far-fetched to deduce that, ceteris paribus, Bangladesh will not only continue to have dengue outbreaks in the near future but this public health crisis is poised to only get worse in the coming years. 

Of climate change and disease burden 

Not just dengue and not just in Bangladesh, but climate change will increase disease outbreaks around the world in several ways.  

Changes in temperature, precipitation patterns and ecological systems can alter the distribution of disease vectors (organisms that transmit diseases), impact the life cycles of pathogens (disease-causing microorganisms) and affect human behaviour and vulnerability. 

Climate change can affect the distribution and behaviour of disease-carrying vectors like mosquitoes, ticks and fleas, according to the Centers for Disease Control and Prevention. 

Malaria (warmer temperatures can expand the geographic range of malaria-transmitting mosquitoes to higher altitudes and more temperate regions), dengue fever and Lyme disease (warmer temperatures can expand the range of ticks that carry the bacteria causing the disease, leading to its spread to new areas) are a few examples. 

"Vectorial capacity (a summary measure of the capacity of a vector to transmit disease that integrates information on vector abundance, survival, competence and feeding rate, and the length of the extrinsic incubation period), which is dependent on temperature conditions, helped explain the 2017 chikungunya outbreaks transmitted by A. albopictus in Europe," wrote  Joacim Rocklöv and Robert Dubrow in a study published in Nature about climate change and vector-borne diseases.

Additionally, climate change will also give rise to water-borne diseases (for example cholera) and air-borne diseases (for example influenza).  

To recap, the disease burden will reach far and wide. Some regions will see new outbreaks, while some regions will experience deadlier outbreaks. A harrowing climate change memo that many climate scientists have been signing to get our attention. 

While  Joacim Rocklöv and Robert Dubrow said "Predicting how future climate change will affect vector-borne diseases is not possible because there are too many uncertainties," they also wrote, "Climate change will only get worse in the foreseeable future, with models projecting a substantial expansion of regions with a suitable climate for a number of vector-borne diseases." 

But there's more. "For decades, climate change has proceeded at roughly the expected pace, says David Armstrong McKay, a climate scientist at the University of Exeter, in England. Its impacts, however, are accelerating—sometimes far faster than expected," wrote Lois Parshley in July this year in The Atlantic explaining how climate collapse could happen fast. 

To contextualise all this information for Bangladesh, unless immediate and effective interventions are taken in terms of preparedness at the national level, Bangladesh is highly likely to experience exponentially worse cases of dengue outbreaks in the coming years. 

El Nino and Bangladesh

The US National Oceanic and Atmospheric Administration is now forecasting a moderate-to-strong El Niño season to continue through 2024. 

How is climate change affecting El Nino? "The El Nino Southern Oscillation (ENSO) is a cyclic pattern of warming and cooling in the waters of the east-central Pacific Ocean. ENSO oscillates between cool (La Niña) and warm (El Nino) phases at intervals averaging five to seven years. 

Climate change is giving ENSO a "push," making the swings back and forth deeper and potentially longer lasting. 

The currently forming El Nino follows a rare "triple dip" La Nina that ended in June and lasted nearly three years—the longest in over 50 years," wrote Cullen Hendrix, a senior fellow at the Peterson Institute for International Economics, in Foreign Policy. 

The world has started to experience more intense El Nino. 

Developing countries and commodities markets are expected to be hit hard by the effects of more intense El Nino. 

Broadly speaking, the Amazon Basin, Australia, the Indian subcontinent, the Sahel, Southeast Asia and southern Africa often suffer drier conditions; Central and East Asia, the Horn of Africa, the southern cone of South America and the southern United States tend to get wetter.

Sometimes, the consequences can be devastating. 

A relatively mild El Nino in 2018-19 helped to fuel some of the worst wildfires in Australia's history. The exceptionally strong El Nino of 2014-16 brought droughts and floods that left some 60m people around the world short of food, drove huge outbreaks of Zika virus across South America and bleached 29% of Australia's Great Barrier Reef."  

The correlation between El Nino and the worsening effects of climate change spells bad news for disease burdens. Because, "analysing the strong El Nino of 2014 to 2015, researchers found increases in cases of plague in Colorado and New Mexico, cholera in Tanzania, and dengue fever in Brazil and Southeast Asia," wrote Hendrix.  

How does El Nino affect Bangladesh? The weakening of the monsoon circulation during El Nino events can lead to drier conditions and reduced rainfall. This can result in water shortages, droughts and impacts on agriculture.

If El Nino means drier weather for Bangladesh and less rainfall, can we assume that it will have a positive effect on the dengue outbreak by shrinking it in scale and size? Not quite so, it's not that simple.  

"This is quite hard to say or predict," said Dr Be-Nazir, because there will always be rainfall in Bangladesh. Lesser rainfall might not mean relief from the dengue outbreak, because there are other factors at play here.  

"I look at it like this, Aedes mosquitoes thrive at a junction between temperature, humidity, and rainfall --- but no one variable can be used to forecast a dengue outbreak," he said. So lesser rainfall might not give us much relief, while the other two factors (temperature and humidity), coupled with a failing health management system persist. 

Dr Be-Nazir added that this year there had been some curious observations across the world in terms of dengue outbreaks.  

"We are not seeing any outbreaks in African regions because of increased temperature," he explained. But at the same time, "recently, there have been dengue [fever] cases in South Europe. In places like Spain, Greece -- which also has seen an increase in temperature," said Dr Be-Nazir. 

Also this year, countries in South America such as Argentina, Paraguay and others saw an increase in dengue cases. The professor explained that all these observations make it very difficult to assume or forecast a dengue outbreak based on one variable such as temperature in this case.

Moreover, crop failure and food security, water scarcity, impact on fisheries and aquaculture and more are ways in which El Nino can spell more trouble for Bangladesh.

All signs are telling of a more sinister climate and more grievous consequences to befall Bangladesh. At this point, if we continue to fail even in basic preparedness at the national level, to counter annual outbreaks such as dengue – what does that tell about our chances against climate change?  

While the Reuters report on a dengue vaccine (vaccine manufacturer Indian Immunologicals Limited (IIL) expects to commercially launch its dengue fever vaccine by early 2026), inspires some hope, it is not much for Bangladesh to inspire optimism, given the circumstances.