Science
The Next Threat to L.A.? Rainfall That Could Cause Landslides
While winds and flames continued to ravage Los Angeles, small teams began creeping onto the charred soils left in their wake.
Roughly a dozen members of the California Watershed Emergency Response Teams and the United States Forest Service are studying the edges of the Eaton and Palisades fires to determine what patches of land burned most severely. Soon, they’ll issue hazard maps to help people prepare for what comes next: the near-certain threat of floods and landslides that will loom for days, months and even years while the city recovers.
“After a wildfire, the hazard to the public is not over,” said Jeremy Lancaster, California’s state geologist. He and his team spent Wednesday hiking in the steep canyons that flank the San Gabriel and Santa Monica Mountains. When it rains hard enough, the sediment on slopes like these can swiftly tumble downhill onto houses that increasingly push up against the fire-prone foothills.
The two major hazards after a wildfire are flash flooding and post-fire debris flows. While spongy soils typically absorb water, burned soils can become hard packed like concrete, repelling water as a raincoat would. Water then funnels downslope without much, or any, vegetation left after a fire to keep it in check.
Hazard maps use a combination of satellite images and field testing of soils to show where patches of moderately to severely burned soils could make these post-fire risks more likely. Recommendations for emergency services to engineer barricades against the danger accompany the maps.
The Palisades burn map was released on Thursday, and the Eaton map is still under review. Experts said the Palisades fire had mostly low to moderate burn severity, while the Eaton fire was likely to have more moderate to high burn severity.
Debris flows require three ingredients — steep slopes, burned soils and rain — and they’re often more dangerous than floods because the sediment they draw in claws at the landscape, creating a snowball effect that pulls a tumult of trees, vegetation, soil, rocks and anything else in its way.
“A debris flow is like a flood on steroids,” said Jason Kean, a research hydrologist with the United States Geological Survey. “It’s all bulked up with rocks and mud and trees.” While floods often have a longer reach, water churns faster in debris flows, which are less common but more destructive.
After the Thomas fire in 2017, a debris flow in Montecito, Calif., killed 23 people and damaged or destroyed more than 400 homes.
Neither homeowners’ insurance nor federal flood insurance covers the impact to properties of debris flows, which are defined by the Geologic Survey as landslides.
The fire-flood cycle is a long-studied relationship, but scientists say a warming planet has made the post-fire threat more likely. Fires burn bigger and more severely. Rains hit harder and more often. Those changes expand the target area for post-fire hazards, which could increase the size and the frequency of floods and flows.
“The fire scientists are telling us that wildfires are increasing in size and severity,” Dr. Kean said. “From that fact alone, you’re exposing more terrain and making more terrain vulnerable to post-fire problems.”
With a dangerous combination of very steep terrain, lots of sediment, high-fire activity and a lot of people pushed up against the mountains, Los Angeles faces an extreme risk.
“The Los Angeles area and Southern California are the world capital for post-fire debris flows,” Dr. Kean said.
Debris flows are so common in Los Angeles that at the edge of the San Gabriel Mountains, where the Eaton fire burned, the state has carved debris basins to collect waste from major flow events. In Southern California, more than two million people live on alluvial fans, landforms that are conducive to flash floods and debris flows, according to Dr. Lancaster.
The National Weather Service in Los Angeles collaborated with the Geological Survey to start the nation’s first early warning systems for these post-fire hazards in 2005. The geological survey sets a rainfall threshold that could set off landslides, and the Weather Service issues a warning if the rainfall they expect nears or surpasses it.
Jayme Laber, a senior hydrologist with the Los Angeles forecast office, has spent the last two decades issuing such warnings. While there’s still no sign of rain in the seven to 10 day forecast he issued on Wednesday, a garden-variety rainstorm that Angelenos see at least once or twice a year could be enough to kick off the next wave of hazards, which can develop within minutes.
“In a burned area, the kind of rain that would not cause problems would be a really light drizzly rain that just goes on and on,” Mr. Laber said. But at some point this winter, he added, “we’re going to get rainfall that has the potential to cause flash flooding and debris flows in these newly burned areas.”
This video of a small debris flow from 2016, which is close to the site of the current Eaton fire, shows how quickly large objects, like a six-foot boulder, can be sent rushing downhill.
Mr. Laber advised residents to prepare in case of a future evacuation, monitor the forecast and pay attention to local emergency officials if a warning is issued.
Science
Hantavirus fears heighten with 4 Californians exposed to the disease. Is the alarm warranted?
In the early weeks of the COVID-19 pandemic, health officials struggled to impress upon the public the grave risks associated with the disease, as well as how easily it could spread.
Now, six years later, public fears have surrounded another type of virus that has killed and sickened passengers on a Dutch-flagged cruise ship; four Californians who were exposed to the virus on the ship recently returned to the United States. This time, however, officials are taking a very different approach to messaging surrounding the deadly Andes virus — a type of hantavirus.
While officials and infectious disease experts have been quick to note the seriousness of the rodent-borne disease, they have also stressed key differences between hantavirus and COVID-19. Namely, that this virus is far less transmissible.
Public alarm over the illness began to grow following reports that three passengers died aboard the stricken vessel, MV Hondius. Worries grew further over the weekend when officials announced that 18 U.S. cruise passengers had disembarked and were returning home.
During a media briefing Monday, the California Department of Public Health said that four state residents had been exposed to the virus, but none had contracted it. Three of them were cruise ship passengers, while the fourth was a Sacramento resident who was on a plane with an infected person in South Africa.
As of now, all four individuals lack symptoms and appear healthy, according to Dr. Erica Pan, director of the department.
One passenger, a Santa Clara resident, disembarked the cruise before the outbreak was recognized and returned to California, she said.
“This person was reported to our department last week and is being monitored by the local public health department where they live,” Pan said.
She added that “the other two passengers disembarked over the weekend in the Canary Islands and have been flown” to a biocontainment facility at the University of Nebraska Medical Center — which is home to the National Quarantine Unit, the only federally funded facility of its kind in the nation.
The quarantine unit is designed to safely house and monitor people who may have been exposed to “high consequence” infectious diseases. A separate biocontainment unit was also created to care for such patients.
The individuals in Nebraska are undergoing a health assessment, and federal authorities will determine when they can return to California. After their return, local health officials will monitor them as necessary.
California’s current public health monitoring protocol includes daily temperature checks, assessment for any symptoms consistent with hantavirus, and direction to modify activities as necessary.
The MV Hondius had 147 people aboard: 86 passengers and 61 crew members. Sixteen passengers from the U.S. boarded a medical repatriation flight arranged by the federal government to Nebraska and remained there as of Monday, including one person who tested “mildly” positive for hantavirus. That person is staying in biocontainment at the University of Nebraska Medical Center.
Two other passengers, one of whom is showing symptoms of the virus, traveled to Atlanta and are staying in a biocontainment facility at Emory University.
That brings the total number of cases of hantavirus to nine, seven laboratory confirmed and two probable cause, including three deaths.
It’s reasonable for people to be concerned about this latest outbreak, said Dr. Nicole Iovine, chief medical epidemiologist and an infectious disease expert at the University of Florida Shands Hospital. Photographs of healthcare personnel in full personal protective equipment assisting cruise passengers are likely to spark recollections of the pandemic.
Even though this is not an easily transmitted disease, it is transmissible and has a high mortality rate, Iovine said. U.S. Centers for Disease Control and Prevention officials said 38% of people who develop respiratory symptoms from hantavirus may die from the disease.
“So it’s reasonable for the medical personnel to take maximal precautions so that they don’t contract it,” Iovine said. “It’s not a reflection of [the virus] being extremely contagious.”
In the U.S., hantavirus cases occur year-round and are transmitted via the urine, feces and saliva of wild rodents.
There were a total of 890 laboratory-confirmed cases of hantavirus in the U.S. between 1993 and 2023, according to the CDC.
The Andes virus, a strain of the disease that’s endemic to Argentina, similarly passes from the exposure of wild rodent particles. Infected humans can then transmit the virus to other people.
Unlike other infectious respiratory illnesses, hantavirus “infects cells very deep in the lungs, so it’s not as easily transmitted then when someone is speaking or coughing,” Iovine said.
COVID-19 transmission occurred when an infected person breathed out droplets and very small particles that contained the virus. Other humans could then inhale the particles or come into contact with them on the surface of objects.
“That’s one of the reasons that makes it much more difficult to transmit person-to-person, and is the reason why this is just not going to turn into a pandemic,” she said.
Experts say person-to-person transmission of the virus occurs only with close and prolonged contact. The hantavirus outbreak is rare but it’s not unusual for a viral outbreak to occur in a cruise ship, where people are packed in and close to each other, said Dr. Afif El-Hasan, member of the American Lung Assn.’s national board of directors.
“From an infectious disease standpoint, that is one of the most difficult and challenging situations and one where it’s more easy to catch something versus other situations,” El-Hasan said.
Experts including Scott Pegan, professor of biomedical sciences at UC Riverside, say the average American’s risk of contracting the disease — if they aren’t within close proximity of an infected individual for a prolonged period — is really low.
Pegan acknowledged it’s confusing to the public when a health incident like this occurs because “they hear ‘this is a really bad disease.’”
“At certain levels, we should worry about it because we don’t want to be interfacing with this virus,” he said.
When danger calls, some animals bare their teeth. Others take to the sky, or curl into protective balls. But the remora — a fish that often hitches a ride on larger marine animals like sea turtles, whales and sharks — sometimes follows a less dignified strategy: It disappears inside a manta ray’s rear end.
In a study published on Monday in the journal Ecology and Evolution, a team of researchers referred to this newly observed behavior as “cloacal diving.” While many questions about this fishy practice remain, there is one thing the team feels sure about.
“It does not look like the manta ray likes it,” said Catherine Macdonald, director of the shark research and conservation program at the University of Miami and senior author of the new study.
While remoras, also known as suckerfish, have been observed diving into the safety of whale-shark cloacae in the past, this is the first time anyone has documented the behavior in manta rays.
The paper uses seven instances of cloacal diving that took place between 2010 and 2025 across all three known species of manta ray. What’s more, the observations, which were gathered by the Marine Megafauna Foundation, occurred in three separate ocean basins, suggesting that this previously unobserved behavior could be common among rays and the remora species that associate with them.
In some cases, the remora forces itself so far inside the ray’s cloaca that only the very tip of its tail can be seen protruding from the exterior. In others, the ray is not large enough to accommodate the remora’s entire body, and half of the suckerfish hangs out of the ray, like a toddler playing peekaboo beneath a blanket.
“The remoras are pretty much as wide as the cloaca is,” said Emily Yeager, a Ph.D. student at the University of Miami and the lead author of the study. “So it’s fully filling that opening.”
To the researchers’ knowledge, no one has studied how sensitive manta ray cloacae are specifically, though Dr. Macdonald said that her lab would often swab the cloacae of sharks for fecal DNA to better understand what they’d been eating.
“They don’t especially like us sticking a swab up there,” she said. “And that swab is a big Q-Tip compared to a remora.”
While all of this may seem as if it’s a lark — News flash: Fish hides inside another fish’s backside — the findings contribute new information to a topic already hotly debated by scientists: the type of impact remoras have on their hosts.
Traditionally, experts have seen the interaction between remoras and manta rays as either commensal or mutualistic. In a commensal relationship, one animal benefits while the other is neither benefited nor harmed. In a mutualistic relationship, both creatures benefit: The remora gets a free ride and food, while the manta has its skin cleaned of parasites.
But cloacal diving almost certainly changes the equation, said Eleanor Caves, a sensory biologist at Brown University who was not affiliated with the new study. While the remora’s presence inside the ray is most likely brief, it could interfere with waste discharge or reproduction, or even damage the cloaca’s lining. This may mean the relationship between remoras and manta rays sometimes tilts into a parasitic interaction, in which one species benefits and the other is harmed.
While the researchers provide just seven instances of remoras using manta-ray cloacae as their own personal panic rooms, the fact that the animals are so difficult to see once inside suggests that the behavior is under-documented, at the very least.
“It’s really challenging to study these highly mobile relationships in marine systems,” Ms. Yeager said. “Oftentimes when researchers interact with these organisms, it’s just for a second in time, when we’re scuba diving in one location and one passes over us, or we’re fishing in a site and we bring one to our boat.”
“But these relationships persist 24/7, all of the time,” she added. “And we’re seeing just a snapshot.”
A Bay Area resident who was stuck on a cruise ship during a deadly hantavirus outbreak has returned to Santa Clara County and is being monitored by health officials.
The Santa Clara County Public Health Department confirmed Sunday that a county resident has returned to California after being exposed to the Andes hantavirus while on the MV Hondius. Three people on board the luxury cruise ship have died, and at least nine others have suspected cases.
The California resident is being monitored in coordination with the California Department of Public Health and the Centers for Disease Control and Prevention, the agency said.
CDPH acknowledged in a statement Friday that one California resident had already returned home, but didn’t disclose where they lived. The agency said another Californian remained on the ship as of Friday.
“At this time, there is no known risk to the public in Santa Clara County,” said Sarah Rudman, director of the Santa Clara County Public Health Department.
The CDC has emphasized that the risk to the American public “is extremely low” as American passengers stuck on the ship begin to return home.
Hantavirus is a rare disease typically transmitted to humans through inhalation of particles contaminated with the urine, droppings or saliva from a rodent.
Passengers began disembarking the ship Sunday in the Canary Islands. The CDC says it has sent a team to conduct a risk assessment for each American passenger.
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