How wet spring, El Niño can raise rare hantavirus risk

Weather can influence rodent habitat, food supply and activity, which can affect potential exposure risk.

Periods of heavy rain can help fuel rodent populations, which may increase the chance of human exposure to hantavirus in some areas, but health officials stress that the risk to the general public remains very low.

Hantaviruses are carried by rodents and can spread to people through contact with infected rodent urine, droppings or saliva. People can also become infected after breathing in contaminated particles stirred up from nesting materials or droppings, according to the Centers for Disease Control and Prevention (CDC).

“There are several different species of hantaviruses that cause infection. They're broken into two groups,” Dr. Steven Bradfute, associate professor and associate director of the Center for Global Health in the Department of Internal Medicine at the University of New Mexico Health Sciences Center, told AccuWeather. “One group is primarily in Europe and Asia, and it causes a kidney syndrome. Another group is found in North, South and Central America, and they cause what's called Hantavirus cardiopulmonary syndrome.”

Bradfute said Andes virus, the virus linked to the cruise ship outbreak, is part of the group found in the Americas. It can cause severe illness by affecting blood vessels in the lungs.

“It infects blood vessels in the lung, and it causes them to become leaky, so that fluid from the blood leaks into the lung,” Bradfute said. “So you can't oxygenate your blood well, because your lung has fluid in it, your blood pressure drops and your heart senses all this, goes into overdrive and that can cause the syndrome.”

Unlike most hantaviruses, Andes virus can spread from person to person in rare circumstances. Bradfute said it is the only known hantavirus that can spread that way.

“Of all the hantaviruses that we know, Andes virus, which is found primarily in Argentina and Chile, is the only Hantavirus that we know that can spread person to person,” Bradfute said.

Andes virus, like other hantaviruses, is originally spread to people through exposure to droppings from infected rodents. A person infected by a rodent can then spread Andes virus to another person through close contact, Bradfute said.

“So the virus can spread person to person, but it doesn't spread terribly well. Doesn't spread like covid or flu,” Bradfute said.

In the United States, Bradfute said Andes virus is not typically present. Instead, a related virus, Sin Nombre, is found in parts of the country. Sin Nombre virus was discovered on the Navajo Nation in 1993.

“New Mexico has more cases than any other state, and so that's one of the reasons that we study this virus, is because we're in New Mexico,” Bradfute said.

Weather’s role in hantavirus risk

Spring and early summer can be a good time to be aware of risk, especially in rural areas, mountain communities, campgrounds and seasonal homes. Above-normal precipitation can boost vegetation and food sources for deer mice, and people may encounter signs of rodent activity when reopening cabins, sheds, garages, campsites or vacation homes after winter.

Bradfute said research has shown a connection between weather patterns and hantavirus infections, although the relationship is not the same every year.

“There has been a lot of research on multiple hantaviruses where there has been seen to be an effect on weather patterns and infections,” Bradfute said. “So there's some nice evidence that if you have warmer, wetter periods leading up to spring and summertime, that can lead to more vegetation, which gives more food for rodents, and so you get more rodents, and these rodents, you know, more rodents you have, the more likelihood there is of transmitting infection to humans.”

Rainfall may also change where rodents nest and how close they come to people.

“There's also some potential evidence that more rainfall can flood burrows, and that leads the animals closer out of their burrows and to nesting more, you know, more closely to humans,” Bradfute said.

The link is not absolute, he added.

“I will say it's not 100%,” Bradfute said. “So we've had some years where we, you know, following an El Niño, where we expected to see more Hantavirus infections, and sometimes we have. And then there's been years where it hasn't been as much as expected.”

The original Four Corners outbreak, the first known hantavirus outbreak in the Western Hemisphere, was also linked to an El Niño event, Bradfute said.

“The previous weather leading up to those months was thought to lead to increased amount of vegetation, which led to more food for the rodents, which led to more rodents, which led to more interactions with humans and higher numbers of Hantavirus infections,” Bradfute said.

Cruise ship outbreak linked to Andes virus

The renewed attention comes as health officials monitor a hantavirus cluster linked to the MV Hondius cruise ship. The World Health Organization (WHO) said seven cases, including two laboratory-confirmed cases and three deaths, had been identified as of May 4. Recent reports said additional passengers evacuated from the ship, including an American and a French woman, tested positive for hantavirus.

Bradfute said there have been only a few known outbreaks involving person-to-person spread of Andes virus, and researchers are still learning about how it moves between people.

“It's only been a few outbreaks that we know of, and so we don't have all the information, but it appears to primarily be spread through what they call close contact,” Bradfute said. “So that's being in contact with someone who is sick, you know, pretty extensive contact.”

Bradfute said the cruise ship outbreak made headlines in part because it occurred on a ship, but the current theory is that the first infected person likely picked up the virus before boarding.

“It appears that it was probably the first individual to have the virus likely picked it up in Argentina before boarding the cruise ship, and then it spread from person to person,” Bradfute said. “That's the current theory.”

The possibility of person-to-person spread can raise concern, but Bradfute said questions should be addressed responsibly and in context.

“This virus is dangerous to people that get infected. About a quarter, about a third of people that get sick, perish from infection,” Bradfute said. “It is rare. It can transmit person to person. It doesn't transmit terribly well, so it doesn't transmit as well as the flu or covid or anywhere in that ballpark.”

Bradfute said past outbreaks have been controlled through isolation and contact monitoring.

“Once you identify the outbreak, you isolate people that are sick, and you see who they've been around, and you check them for symptoms,” Bradfute said. “That has been very effective in the past at controlling these viruses.”

There are no vaccines or specific treatments for Andes virus or other hantaviruses found in the Americas, Bradfute said, although research is ongoing.

How to safely clean areas with signs of rodents

Health officials recommend taking precautions before cleaning spaces with signs of rodents. Open doors and windows to ventilate the area, wear rubber or plastic gloves and avoid sweeping or vacuuming droppings or nesting materials, which can send contaminated particles into the air.

“What I like to tell people is, if you see any area of rodent activity, whether or not you think it's a rodent that carries a Hantavirus or not, it's because we don't always know which rodents carry which hantaviruses,” Bradfute said.

The CDC advises spraying droppings and urine with a bleach solution or EPA-registered disinfectant until very wet, letting it soak, then wiping up the material with paper towels.

“Basically, what you want to do is wear a well-fitted N95 mask and gloves and disinfect the area with either 10% bleach solution or household disinfectant,” Bradfute said. “You don't want to sweep, you don't want to use a shop vac, because that will aerosolize the particles.”

Bradfute said barns, sheds, cabins and other closed-up spaces can pose a risk when there has been rodent activity.

“There's no need for panic. Infections are rare, but it makes sense, if you see an area of rodent infestation, to treat it appropriately,” Bradfute said.

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