Ocean, desert rainfall and a parasitic disease: researchers connect patterns

Climate does more than shape the weather. It can influence when and where infectious diseases emerge by changing how disease-carrying vectors and animal hosts survive and behave. Scientists use weather data to anticipate some disease risks days or weeks ahead. But forecasting how those risks will change across seasons and years remains difficult. We asked Adrià San José Plana, a researcher in infectious disease modelling, and Xavier Rodó, a research professor and founding director of the Catalan Institute of Climate Sciences in Barcelona, about their recent research.

What’s known about the relationship between climate and diseases?

Research has shown that vector-borne diseases – those transmitted by mosquitoes, ticks, sandflies and other arthropods – are particularly sensitive to climate.

These insects are ectotherms, meaning that they do not regulate their internal temperature in the same way humans do. They are therefore strongly affected by environmental temperature. Temperature influences their survival and longevity, how frequently they bite, and how quickly pathogens develop inside them before they can be transmitted.

Rainfall and humidity also dramatically affect their breeding and influence vector abundance.

Climate also affects the spread of zoonotic diseases, but often through a different mechanism. This can happen when zoonotic host animals are forced to look for scarce food and move closer to human settlements.

What was the focus of your study?

Our research focused on cutaneous leishmaniasis, a vector-borne disease transmitted by infected female sandflies.
There are around 1 million cases a year of cutaneous leishmaniasis in the world. It’s rarely life-threatening and often resolves on its own, but it leaves skin lesions and ulcers that can last months or years. In some cases it leaves permanent scars and lasting physical, psychological and social harm, including stigma.

In north Africa, cutaneous leishmaniasis is mainly caused by the zoonotic parasite Leishmania major, which is maintained in animal reservoirs, particularly rodents in arid environments close to villages.

In these settings, the disease is strongly associated with succulent, salt-tolerant desert shrubs. These plants provide rodents with an important source of food and vegetation cover. The burrows the rodents make beneath or near the vegetation provide shelter and suitable habitats for both rodents and sandflies.

Especially in Morocco, another form of the disease (Leishmania tropica) also circulates with humans as the main reservoir. It was included in our analysis too.

How does climate affect its spread?

Previous research has identified rainfall as an environmental factor influencing zoonotic cutaneous leishmaniasis transmission in the region. This happens through a cascade of ecological effects: increased rainfall; growth of desert shrubs; more rodents and sandflies; greater transmission.

In the tropics, phenomena like El Niño create climate signals that are predictable months to years in advance. Diseases sensitive to temperature or rainfall can inherit some of that predictability.

Outside the tropics, the atmosphere usually loses its memory much faster. Knowing what the weather is doing today tells us much less about what conditions will be like months from now. That makes it much harder to predict climate conditions months ahead.

But could north Africa be different? We wanted to find out whether this region might have enough long-term climate predictability to allow us to forecast cutaneous leishmaniasis months ahead of time.

How did the north African climate help your research?

Rainfall is normally hard to predict because it depends on many competing processes at once: land-sea interactions, mountains, shifting moisture in the atmosphere. Pin down a signal in one of those processes, and the others tend to drown it out.

Sandfly from Tunisia.

Adel Rhaim

North Africa’s arid zones are different. With few other rain-generating processes at play, rainfall there is governed almost entirely by one thing: large-scale westerly winds carrying Atlantic moisture inland. The desert acts as a filter, letting that single Atlantic influence show through clearly in local rainfall.

That makes the region promising to study: if the Atlantic circulation is predictable, and it strongly influences local rainfall, then rainfall – and potentially leishmaniasis transmission – may be predictable too.

What are your main findings?

We found that year-to-year changes in cutaneous leishmaniasis transmission in north Africa can be linked to climate variability originating in the Atlantic Ocean.

The chain is remarkably coherent:

• Slowly varying temperatures in the Atlantic influence large-scale atmospheric circulation and the strength of westerly winds.
• These winds transport moisture towards north Africa and influence rainfall in areas where leishmaniasis is transmitted.
• Rainfall affects vegetation in the region’s arid ecosystems.
• Changes in vegetation affect the rodent populations that act as reservoirs for leishmania major, as well as the conditions in which sandflies thrive.

These ecological changes are then reflected in changes in cutaneous leishmaniasis transmission.

In other words, a climate signal that begins in the Atlantic can be linked to disease transmission in north Africa.

That Atlantic signal can also improve our ability to anticipate changes in leishmaniasis transmission.

What’s significant about what you’ve found?

Scientifically, the findings matter because they show long-range climate predictability isn’t limited to the tropics, where most seasonal forecasting research has focused. A signal originating in the Atlantic can travel through the atmosphere and local ecosystem and surface, months later, in patterns of disease transmission. Deserts may be especially good places to detect this kind of signal, precisely because their aridity strips out the competing processes that would otherwise mask it. Whether the same approach transfers to other diseases and regions depends on how tightly their local ecology is tied to rainfall.

From a public-health standpoint, the findings lay groundwork for forecasting cutaneous leishmaniasis in Morocco and Tunisia. The forecasts could give health authorities advance warning to ramp up surveillance, plan vector control and prepare health services.

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