90-Second Read: Hantavirus: A cruise ship, a deer mouse, and the fictional line between human and animal health

The Hantavirus outbreak that began on a cruise ship is a reminder of something we keep having to relearn: When humans push into ecosystems they don't normally inhabit, they are exposed to viruses. This Hantavirus outbreak is a smaller, slower-moving version of the same lesson: the line between human health, animal health and the places we travel for pleasure is much smaller than we like to think. But what connects these stories is a world where the boundaries between human health, animal health, climate and travel are largely fictional, and shrinking further every year. Fourteen months later, 11 people on the Dutch cruise ship Hondius have been infected with a different Hantavirus strain.

A New England Journal of Medicine study of a 2018 Andes outbreak reconstructed how a single zoonotic spillover from a rodent reservoir in Argentina produced 34 human cases and 11 deaths over three months, driven by three symptomatic super-spreaders at crowded social events. Though much of the focus has been to reassure people that this is not the next COVID-19, what this outbreak points to is a real-time One Health story, a framework that recognizes human, animal and environmental health as a single, interconnected system. Deer mouse populations in North America boomed roughly tenfold following the wet, warm El Niño winter of 1991, 1992, triggering the 1993 Hantavirus outbreak. Hantaviruses are not one virus but a family of related viruses, carried by different rodent species in different parts of the world.

The pathogen that killed her was Hantavirus, almost certainly picked up from deer mouse droppings on the property. Andes virus, found in South America, is the only Hantavirus known to spread between people, and through close, interpersonal contact. Last-chance tourism into fragile and isolated ecosystems, wildlife photography in remote habitats, cruises that promise experiences into uninhabited shores: this is a growing category of travel despite being potential One Health exposures. The World Health Organization's (WHO) own 2016 handbook for managing public health events on board ships calls for an "all-hazards" precautionary approach when a cause cannot be identified.

The same dynamic plays out elsewhere: in Southeast Asia for example, rodent trade networks, deforestation and intensifying agriculture continually create new interfaces between people and pathogens. Hantavirus was not identified until May 2, three weeks during which the ship continued its route, calling at multiple ports. Old pathogens now have more opportunities to expand, and interact with humankind.