El Nino and its impact on dengue outbreaks

Danger: A study in Brazil found that during El Nino years, the number of Aedes mosquito larvae jumped by 30% compared with normal years.

SOME of the most important warnings in public health don’t come from a doctor or a lab report. They come from the ocean. 

Right now, the Pacific Ocean is sending us one. The tropical Pacific has already entered conditions that usually give rise to the weather cycle known as El Nino, and the phenomenon is continuing to strengthen. Current forecasts indicate a greater than 90% probability that El Nino will persist until early 2027, with a substantial likelihood of developing into a very strong event.

For those of us who track dengue fever, this isn't just a weather update. El Nino remains one of the most valuable multi-month climate signals available to public health, providing an early warning of a heightened risk of dengue outbreaks across much of the tropical world. The signal is clear, and we have time to act.

The El Nino-mosquito connection

Think of El Nino as the ocean breathing a warm breath. Normally, trade winds push warm surface water towards Asia and allow cold, nutrient-rich water to rise off the coast of South America. During an El Nino cycle, those winds weaken. The warm water slides back eastward across the Pacific, heating the ocean surface and, in turn, the atmosphere.

This shift has ripple effects on weather patterns thousands of kilometres away. It often brings hotter, drier conditions to South-East Asia and Australia, and wetter conditions to parts of South America and East Africa. These are the same regions that carry the heaviest burden of dengue.

How does a warmer ocean lead to more dengue? The link is one of the most well-studied connections between climate and infectious disease, and it works through two main pathways.

First, there is the biological effect. The Aedes mosquito, which carries the dengue virus, is extremely sensitive to temperature. Warmer conditions are a boon for it. They help it live longer, bite more frequently, and, crucially, allow the dengue virus inside the mosquito to replicate faster. This means a mosquito can become infectious and start spreading the virus sooner.

A study in Brazil, for example, found that during El Nino years, the number of Aedes mosquito larvae jumped by 30% compared with normal years.

Second, there is the hydrological and behavioural effect. El Nino doesn't just bring heat; it redistributes rainfall. Where it causes drought, families store more water in containers around the home – perfect breeding grounds for mosquitoes. Where it causes flooding, it creates countless new pools of stagnant water. Either way, the mosquito wins.

This is not a new phenomenon. Studies from Venezuela and Brazil have shown that dengue outbreaks tend to follow a cyclical pattern, often every three to four years, that aligns with El Nino cycles. What is troubling is that these cycles appear to be getting shorter, compressing to every two to three years. This suggests that more frequent El Ninos and rising global temperatures are disrupting the virus's natural rhythm and making outbreaks more common.

The data is undeniable

If the mechanisms are clear, so is the data. A major study published last year in Nature Communications, which analysed 57 countries over four decades, provides a staggering quantification of the risk.

It found that El Nino-related temperature changes could explain about 63% of the year-to-year variation in dengue cases. For every 1°C rise in a key Pacific Ocean temperature index, dengue cases increased by 48%.

The study also showed that the scale of these outbreaks is growing exponentially. The four largest El Nino events on record produced an estimated excess of 0.2 million, 1.4 million, 4.1 million, and a staggering 9.6 million dengue cases, respectively. Each event has been bigger than the last.

Country-level data confirms this. A 2026 study in Indonesia found that during strong El Nino years, dengue incidence was 96% higher than in neutral years. The country's massive 2024 outbreak of roughly 250,000 cases fits squarely within this pattern.

The 2014-2016 El Nino was one of the strongest events on record. It aligns very well with the surge of dengue cases and deaths in the 2014-2016 outbreaks in Malaysia.

Perhaps most importantly, a recent analysis in The Lancet Planetary Health delivered a sobering conclusion: about half of the increased dengue risk we see during El Nino is no longer just natural weather variability. It is being supercharged by human-caused climate change.

Why 2026 is different

The El Nino forecast for 2026 is a warning we cannot afford to ignore, for three reasons.

1. The baseline is hotter. The world is warmer now than during previous El Nino events. The tropics, once too cool in some areas for the Aedes mosquito, are now becoming more hospitable to it.

2. The events are stronger. Climate models project that El Nino events themselves will become more intense, with one study estimating a 39%-82% increase in the dengue burden they drive by the end of the century.

3. The danger lasts. The impact of an El Nino event is not a single season. Its climate signature can persist for up to two years, meaning a 2026 event is also a 2027 problem.

From warning to action

This forecast gives us something incredibly rare in public health: a head start. We know what’s coming, roughly when, and where.

This means we can no longer afford a fragmented, reactive approach. The climate signal is already visible; we must act on it. This is the time to:

> Strengthen surveillance: Health ministries must use this climate data to prepare. Early-warning systems based on El Nino forecasts are not a luxury – they are a necessity. We need to ensure our systems for tracking cases are robust and ready.

> Pre-position resources: Vector control operations, hospital surge capacity, and essential supplies must be planned and ready to go before cases begin to rise, not after hospitals are overwhelmed.

> Empower communities: For clinicians and communicators, the message is one of foreseeable risk. We must engage communities now on how to eliminate mosquito breeding sites, protect themselves from bites, and consider vaccination where available.

> Integrate our response: We must see vector control, community engagement, climate-informed early warning, and vaccination not as separate tools, but as a single, integrated portfolio of defence. No single measure is enough.

The science is converging across dozens of studies in very different settings. The conclusion is unanimous: El Nino is a probabilistic, multi-month early warning for dengue. And the signal is strengthening.

The 2026 El Niño has arrived and the climatic signals are intensifying. Yet the value of El Nino lies precisely in the advance warning it provides. The forecast is here. The evidence is here. The lead time is here. The ocean has breathed its warm breath. The question is whether we will listen.

What we do in the coming months and year ahead will determine whether the next outbreak arrives as a surprise or as a signal we had the opportunity to anticipate and prepare for.

DR MUSA MOHD NORDIN

Paediatrician

Petaling Jaya

DR HUSNA MUSA

Paediatric neurologist

Universiti Putra Malaysia