The strain of hantavirus that has killed three people and sickened five others on a cruise ship is the Andes strain, which is typically found in South America including parts of Argentina, where the luxury cruise ship MV Hondius departed.

This strain is the only type that’s known to spread from person to person. As of Wednesday, there were eight cases; three have been confirmed as hantavirus by laboratory testing. Three people, a married couple who were Dutch nationals and another woman, have died. Another person is in intensive care in South Africa.

Dr. Abraar Karan, an infectious disease physician at Stanford Medicine, said the death of the woman, 69, who was on a plane just 24 hours prior to her death on April 26, “tells you how rapidly this disease can progress.”

The investigation is continuing, but experts insist that the cruise ship outbreak is not similar to Covid and any risk to the public is very low.

How does the Andes strain spread?

Dr. Lucille Blumberg, an infectious disease specialist at the National Institute for Communicable Diseases in South Africa, said the Andes hantavirus doesn’t spread easily through casual contact.

There are nearly 40 strains of hantavirus found all over the world, and different strains cause different illnesses, according to the U.S. Centers for Disease Control and Prevention.

The virus typically spreads through contact with rodents like rats and mice. The Andes strain is primarily carried by the pygmy rice rat. Infection is usually caused by inhaling germs from the rodents’ urine, droppings or saliva that have become airborne.

Dr. Emily Abdoler, an infectious disease physician at University of Michigan Health, said the Andes strain is the only one capable of human-to-human transmission and, unlike other types, can spread through respiratory droplets, which are created when someone coughs, sneezes or talks.

The virus can also spread through bodily fluids, including the exchange of saliva and other intimate contact.

Dr. Pablo Vial, an infectious disease doctor who studies hantavirus at the Institute of Sciences and Innovation in Medicine in Santiago, Chile, has treated many patients with the Andes strain. Person-to-person transmission causes only 2% to 5% of all Andes virus cases, he said.

“In previous outbreaks, there have been instances of human-to-human transmission, mainly among close contacts either providing clinical care or people who have had close physical contact,” Dr. Maria Van Kerkhove, an infectious disease epidemiologist with the World Health Organization, said in a call Wednesday.

“This is not Covid. This is not influenza,” she said. “This spreads very, very differently.”

But there has been at least one documented “superspreader” event, in Epuyén, Argentina, in 2018, according to a report in The New England Journal of Medicine. The outbreak was triggered by three people with symptoms who attended crowded social events, including a birthday party. Ultimately, 34 cases were confirmed, with 11 deaths.

“Some people are going to be superspreaders biologically,” said Karan. “They actually shed more virus than others do.”

Gustavo Palacios, the senior author of the Argentina superspreader report, said the vast majority of known cases of human-to-human spread of the Andes virus have been among people who were close contacts — for example, living together, in relationships, ora healthcare workers treating sick patients. Palacios is a microbiology professor at the Icahn School of Medicine at Mount Sinai in New York.

Fewer than 50 cases, Palacios said, have involved more casual contact, including one instance described in the report in which a person got sick after a brief conversation with someone who was infected.

Still, he emphasized that with such little available information about human-to-human transmission, it’s difficult to draw conclusions about exactly how the virus spreads. “We need to work with what we know, and I would not be sure to say that yes, I know you need very close contact,” he said.

Palacios also noted that a cruise ship is a completely different environment from the previous outbreaks — “a lot less ventilated, a lot more dense” — meaning what’s known about transmission in other scenarios may not apply in this case.

Vial said the virus is most transmissible to other people in the early stages of an infection, before the body’s immune system has started responding.

The incubation period between exposure to the virus and developing symptoms can range from nine to 40 days. If there are more cases from the cruise ship, they will likely show up in the next week, Karan said.

Most of the documented cases of human-to-human transmission involve people with intimate contact or who were in healthcare settings with limited protection, Blumberg said.

The risk of being a secondary case — that is, catching it from someone else — is more than 10 times higher in sexual partners than in other family members, Vial said.

“The fact that additional cases occur in sexual partners does not necessarily mean it is a sexually transmitted disease,” he said. “Rather, it indicates more frequent intimate contact and that the partner is generally the one who cares for the infected person.”

Is there a test for hantavirus?

There are a few ways to confirm hantavirus infection.

A KLM Royal Dutch Airlines flight attendant has been hospitalized and is being tested for hantavirus after being exposed to the Dutch woman who was on the plane briefly before her death. It’s not yet clear if the flight attendant was on the same flight.

A blood test can measure levels of the antibodies IgM and IgG, which tend to rise early and late in an infection. Another is polymerase chain reaction, or PCR, which looks for fragments of the virus’ genetic material, typically in a blood sample. PCR tests were also used early in the coronavirus pandemic and are often used to diagnose other viral illnesses. Some of the cases so far have been confirmed through PCR testing, according to the World Health Organization.

For high-priority situations, a PCR test for viral illnesses can usually be done in several hours.

There are still nearly 150 people aboard the MV Hondius. However, because the incubation period for hantavirus can be as long as eight weeks, it’s still unclear how long passengers will need to remain on the ship.

It’s a “tricky situation,” said Karan, because many patients may be incubating the virus and not be testing positive.

“Let’s say you were on the ship and you were a passenger and you may have been exposed, but you’re incubating. A lot of tests won’t pick you up,” said Karan. “You may not even be shedding enough virus yet for a swab to detect it.”

What makes the Andes virus so deadly?

While hantavirus infections are rare, the fatality rate can be as high as 50% because the virus tends to affect organs like the lungs and kidneys that are critical for survival.

Vial said he’s observed in his practice that the Andes strain often affects young, previously healthy men in their 30s.

Not everyone will get severely ill. About 40% develop a mild form of the infection, and 60% develop the severe form known as cardiopulmonary syndrome, which can lead to respiratory failure and cardiovascular shock. Overall, 30% to 40% of patients die, Vial said.

What are the symptoms of an Andes virus infection?

Cases of the Andes strain may initially go undetected, say experts, as the virus tends to first present with vague symptoms.

“It often starts with nonspecific symptoms, meaning they’re symptoms that people can have with a lot of different types of infections and illnesses, such as headaches, muscle aches, abdominal pain,” Abdoler said.

These tend to occur within one to three days of symptom onset, said Karan.

“Then it can progress to this more critical stage where you start to get respiratory symptoms and respiratory failure and cardiac collapse,” said Abdoler. “It causes a collapse of body systems critical for survival on a second-by-second basis.”

Vial said some patients with the Andes strain can deteriorate within hours.

Depending on the region, the virus can present with different symptoms and have a different disease course. In the Western Hemisphere, the virus tends to cause hantavirus pulmonary syndrome, or HPS, which can cause shortness of breath and cough and could ultimately lead to respiratory failure. The Andes strain tends to cause HPS.

“A lot of these viruses end up killing people not just from the infection itself but from the body’s own immune response,” Karan said.

In Europe and Asia, hantavirus can cause hemorrhagic fever with renal syndrome, leading to internal bleeding and ultimately kidney failure and death.

There’s no specific treatment for the virus, said Abdoler, and patients are typically given supportive care directed at fighting the symptoms. This could include things like medications to fight a fever or IV fluids to help dehydration.

You can use a ventilator to support breathing, or you can even use extracorporeal membrane oxygenation (ECMO), a machine that can take over when the heart and lungs are failing, Abdoler said.

Medications called vasopressors can also be given to boost blood pressure and make sure organs such as the brain and kidneys are getting nutrients such as oxygen.

“That’s really what it means by supportive care, is supporting oxygenation and supporting circulation,” Abdoler said.