El Niño, Andes hantavirus and human health: when climate sets the stage for the next emergence

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Introduction

When we talk about emerging diseases, attention often focuses on the viruses themselves. Yet in many cases the real triggers lie much further upstream, within ecosystems.

The Andes virus (ANDV), responsible for hantavirus cardiopulmonary syndrome in South America, perfectly illustrates this reality. Unlike many infectious diseases whose emergence is directly linked to human activity, the risk associated with hantavirus depends heavily on environmental, climatic and ecological conditions.

In this context, the onset of an El Niño episode deserves particular attention. This climatic phenomenon could indirectly promote an increase in reservoir rodent populations and, consequently, raise the risk of human infections.

The Andes virus: a particularly concerning hantavirus

The Andes virus circulates mainly in Chile and Argentina. Its natural reservoir is a small wild rodent called the long-tailed pygmy rice rat (Oligoryzomys longicaudatus).

In this rodent, the infection is generally asymptomatic. The animal nevertheless excretes the virus in its urine, faeces and saliva, progressively contaminating its environment.

In humans, the situation is quite different. The infection can cause a hantavirus cardiopulmonary syndrome characterised by:

• an initial flu-like phase;
• acute respiratory distress;
• massive capillary leak;
• potential cardiogenic shock.

Case fatality varies between series but frequently reaches 30 to 40%, making it one of the most severe viral zoonoses currently known.

Another feature distinguishes the Andes virus from most other hantaviruses: its documented capacity to be transmitted from one human to another during close and prolonged contact.

El Niño: a global climatic phenomenon

El Niño corresponds to an abnormal warming of the surface waters of the equatorial Pacific. This anomaly profoundly alters global weather patterns:

• increased rainfall in some regions;
• droughts in others;
• changes in temperature;
• disruption of the biological cycles of plants and animals.

South America is among the regions most sensitive to these changes. For epidemiologists, El Niño is not only a climatic phenomenon. It is also an event capable of altering the dynamics of many infectious diseases.

From rain to rodents: an ecological cascade

El Niño’s effect on hantavirus does not pass directly through the virus. It acts first on the environment. Increased rainfall promotes:

• plant growth;
• seed production;
• the abundance of food resources available to rodents.

The biological consequences are rapid:

• better adult survival;
• lower juvenile mortality;
• increased reproductive success;
• more litters.

A few months later, rodent populations can experience a genuine demographic explosion. In some regions of Chile and Argentina, these outbreaks are known as « ratadas ».

More rodents, more virus

When rodent population density increases, contacts between individuals become more frequent. The animals compete more for:

• territories;
• food resources;
• reproductive partners.

These interactions favour bites and exchanges of biological fluids, accelerating transmission of the virus within animal populations. The result is twofold:

• more rodent carriers;
• more viral excretion into the environment.

Rural dwellings, farm sheds, barns and forest cabins then become potential sites of exposure.

The jump to humans

The majority of human infections occur through inhalation of contaminated particles. The at-risk situations are well known:

• cleaning an enclosed space;
• handling stored wood;
• sweeping dust;
• agricultural work;
• forestry activities;
• camping or hiking in an endemic area.

When rodent populations increase, opportunities for contact between humans and the animal reservoir mechanically become more numerous. It is often several months after the climatic anomalies that the first human cases appear.

Why is this a major weak signal?

In public health, a weak signal corresponds to a discreet event that may nonetheless herald a more significant phenomenon. The Andes virus fits this definition perfectly. The early indicators may be:

Ecological signals

• abnormally high rainfall;
• exceptionally abundant vegetation;
• mass flowering of certain bamboo species;
• increased rodent captures.

Veterinary signals

• rising viral prevalence in rodents;
• geographic expansion of the reservoir.

Human signals

• unusual increase in hantavirus pulmonary syndromes;
• cases clustered in the same region;
• appearance of secondary human-to-human transmission.

Early detection of these signals theoretically makes it possible to intervene before an epidemic wave becomes established.

A pandemic risk?

To date, nothing suggests that an increase in Andes virus cases could lead to a global pandemic comparable to COVID-19. Several factors limit its spread:

• mainly zoonotic transmission;
• the need for close contact for human-to-human transmission;
• a relatively long incubation period;
• the rapid onset of severe symptoms.

Nevertheless, its high case-fatality rate and its demonstrated capacity for human-to-human transmission justify close surveillance. For international health agencies, the Andes virus remains an emerging agent with high potential impact despite its low number of annual cases.

Conclusion

The link between El Niño and the Andes virus perfectly illustrates the interconnection between climate, biodiversity and human health. Infectious emergence does not begin in hospitals. It often begins much earlier: in forests, grasslands, animal populations and the ecological balances that climate change disrupts.

Thus, when an El Niño episode sets in, epidemiologists do not only monitor the weather. They also watch the plants, the rodents and the weak signals that may herald the next infectious threat. Because sometimes, the next epidemic begins simply with a few extra weeks of rain.