An outbreak of the rare Andes strain of hantavirus occurred on a ship, sparking fear due to its person-to-person transmission. However, experts state its high lethality and rapid onset of severe illness prevent it from becoming a global pandemic.
. Illness struck rapidly. Fever, gastrointestinal symptoms, pneumonia, respiratory distress, shock. One patient in critical condition.
Another evacuated from the ship in an ambulance boat. The rare Andes strain of hantavirus, confirmed May 6, is known for something that should terrify public health officials: It can spread from person to person. Yet epidemiologists are not panicking. There will be no lockdowns, no travel bans, no breathless warnings of a pandemic-in-waiting.
The disease that killed between 30 and 60 percent of those infected in previous outbreaks is, paradoxically, the very reason it cannot cause a global crisis. Hantavirus's extreme lethality is its limitation.
"Pandemic potential is mostly about transmission architecture, not lethality," Amesh Adalja, senior scholar at Johns Hopkins Center for Health Security, toldThomas G. Ksiazek, a virologist with extensive experience tracking emerging infectious diseases, echoed that assessment. "This virus is not new to the world," Ksiazek toldThe Transmission Problem, Not the Deadlinesspublic anxiety spiked. People online speculated about pandemic risk. Social media filled with fears.
But the mechanism experts worry about when assessing pandemic threat has almost nothing to do with how deadly a virus is. It has everything to do with whether people can spread it before they get so sick, they stop moving around.
"It's not the case fatality rate that matters for pandemic potential, it's the ability to transmit between humans," Bill Hanage, epidemiologist at Harvard School of Public Health, told. "If transmission happens efficiently enough before people become seriously ill, then there are very few constraints on virulence. " Hantavirus fails this test catastrophically.
Andes virus, the only human-transmissible strain among roughly 50 species of hantavirus, does so poorly that even on a ship with hundreds of people in close quarters—a worst-case scenario for disease spread—transmission remained rare and limited to people with the closest contact.
"The key to transmission is shedding virus in the presymptomatic and asymptomatic phase," Vincent Racaniello, Higgins Professor of Microbiology and Immunology at Columbia University, told. "Influenza and SARS-CoV-2 are very good at it. For hantavirus, the barrier is efficient human to human transmission," he added.. For most infected, the virus incubates for days or weeks—up to eight weeks in some cases—before symptoms begin.
Once symptoms start, people sicken quickly.
"Pathogens with the greatest pandemic potential often allow people to remain mobile and socially connected during the infectious period," Zoe Weiss, director of microbiology at Tufts Medical Center, told. "COVID-19, for example, can spread before symptom onset and from people with mild or minimally symptomatic infection, which makes containment very difficult. "Marc Lipsitch, a professor of medicine and biology at Stanford and infectious disease epidemiologist, toldthat while the cruise ship outbreak warrants attention, the virus has fundamental limitations.
"It's very hard to transmit it from one person to another," Lipsitch said. This is where lethality becomes an unexpected advantage for containment. But the relationship is not straightforward. Killing a host quickly does not automatically prevent spread—Ebola kills 50 to 70 percent of those infected and spreads through blood and body fluids, reaching people who tend to the dying.
The real constraint comes from when the virus is most transmissible and how symptoms affect human behavior..
"A highly lethal virus can still cause serious outbreaks if it transmits efficiently enough. High lethality can reduce pandemic potential when severe illness limits the time people remain mobile and socially connected while infectious.
" For hantavirus, severe illness arrives fast. A patient incubates for weeks with minimal symptoms, then collapses into respiratory failure over days. The timeline is brutal but brief.
"For many of these very severe and deadly infections, patients are most contagious when they are already very ill," Weiss told. "At that point, they are less likely to be traveling, working, attending school, or interacting with people in public. Hospitalization, breathing problems, fatigue—all reduce movement and interrupt what we call transmission chains.
"Compare this to influenza, which kills far less people but spreads readily. "The flu makes you feel really crappy but is rarely deadly," Angie Luis, associate professor in the Wildlife Biology Program at the University of Montana, told. "It is virulent enough to make you cough and sneeze, and that helps the virus get transmitted. But it is usually not virulent enough to kill you because that would decrease the virus's fitness—its ability to transmit.
" Hantavirus strikes a different balance. It is lethal enough to incapacitate people before they can spread it widely. It is not transmissible enough to overcome that incapacitation through presymptomatic spread.of hantavirus.
"Right now, hantaviruses are not adapted to transmit between people. They are adapted to get transmitted between their rodent hosts.
" The cruise ship outbreak illustrates this principle. Nearly 150 people lived in extreme proximity for weeks. Closed quarters, recycled air, shared surfaces, intimate spaces. If hantavirus were a respiratory virus that spread before people showed symptoms—if it were, say, measles—it would have gutted that ship.
Instead, seven cases emerged among 150 people, and transmission required"very close and sustained contact," according to the World Health Organization .
"Even the Andes version of hantavirus is very inefficient in its spread requiring close and prolonged contact," Adalja toldWhat Would Have to Change The question that keeps epidemiologists vigilant—though not alarmed—is what would need to happen for hantavirus to evolve into a genuine pandemic threat. The answer unsettles them more than the virus itself: It would need to become more efficiently transmissible. And that would likely require multiple coordinated evolutionary changes..
"That's what we are scared about for influenza H5N1. Nobody can tell you if this will happen or not. It could be that the limited human transmission we see on the cruise ship might be enough to select for efficient human transmission.
"The risk is real in theory. Evolution selects for traits that help a virus spread. If human transmission suddenly became advantageous to the virus—say, through a chance mutation that increased respiratory shedding—natural selection could favor variants with that trait.
"Hantavirus would need to make a major evolutionary leap to become a pandemic threat," Weiss told. "It would have to evolve to spread more efficiently from person to person, most likely through respiratory transmission. That would likely require multiple coordinated changes affecting how the virus enters human cells, replicates in human tissue, and evades early immune defenses. Hantavirus does not currently spread well enough between people for that to be an easy or plausible evolutionary path.
This is not something experts think is close to happening.
", not human airways. Adapting to human transmission efficiently would require not just one mutation but a cascade of cellular changes—the kind of major evolutionary leap that takes time and requires many opportunities for natural selection to act..
"You need many many human infections and opportunities for mutation and selection. What would be more anxiety inducing is if transmission in the natural host happens to be favored by viral properties that also favor transmission among humans. That would increase the number of opportunities to adapt to humans. But that's speculative. "
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