Spillover

Epidemiologist Jean Baptist du Prel on a potential threat scenario in which humans play a major role

Ebola, Marburg, Hanta, COVID-19 and bird flu are all deadly viruses that have jumped from animals to humans. This process is called spillover. Many pathogens have long coexisted with humans, but are now becoming increasingly dangerous to us. Epidemiologist PD Dr Jean Baptist du Prel works in the Department of Ergonomics at the University of Wuppertal and says: “You have to see the whole thing as a complex process. Environmental conditions change and human behaviour changes nature. Viruses replicate every day as normal and copying errors can occur.” Sometimes viruses then spread to other species as a result. “It’s always about probabilities. So what is the probability that an animal will encounter another species and that viruses that have changed will be transmitted to the other species and infect it? It’s a random game of mutation rate based on copying errors that makes it possible to jump to other species.” The contact rate then also plays a decisive role, and then humans come into play.

At least another 10,000 unknown viruses could infect humans

The diversity of pathogens in animals is astonishing. Biologists estimate that there are at least 10,000 unknown viruses on our planet that could infect humans. So the danger of further spillover has not yet been banished. “That’s right,” states du Prel, “the changed conditions mean that a large number of animals that harbour viruses are more likely to encounter humans. The moment animal populations shift, for example due to climate change or human behaviour such as slash-and-burn, they leave their old habitat and are therefore more likely to encounter humans. The other probability is that copying errors can lead to viruses changing, spreading to humans and becoming pathogenic (disease-causing, editor’s note). In this case, two probabilities practically come together. We make a decisive contribution to this through our behaviour by changing nature and the climate.” But what causes a virus to change host? In this context, the epidemiologist speaks of the “survival of the fittest”, i.e. the survival of the individuals best adapted to their environment, and makes it clear that this process is not a conscious, controlled process, but happens purely by chance. “If the virus carries certain structures that suit the host, then an infection begins in another species. It doesn’t necessarily have to be humans, there are also intermediate hosts. Frequent mutations are often to the disadvantage of viruses, only in rare cases are they to their advantage because they are then better adapted to a potential host.” A current example is the situation recently reported in the media on the cruise ship Hondius, where the hantavirus, a zoonotic pathogen that primarily “jumps” from infected rodents to humans by inhaling virus-containing aerosols from urine, faeces or saliva, was probably also transmitted at least partially from person to person through close contact. The so-called Andean virus is a specific variant of the hantavirus found in South America and the only one for which human-to-human transmission has been proven to date.

Predicting spillover

In order to prevent spillover from animals to humans, the development of spillover must first be understood. du Prel explains: “It’s a complex process. I first have to look at how viruses change and how pathogen reservoirs change, for example through movement. Then I also have to look at the virus’s ability to bind to humans and examine the extent to which humans themselves spread the virus. It’s always a Sherlock Holmes game. I have to see how these movements fit together. You can predict these spillovers to a certain extent in terms of probability theory by combining different mechanisms. There are viruses that have a relatively high mutation rate compared to other organisms, such as SARS-CoV-2, the pathogen that causes COVID-19. A high mutation rate, which is often the case with RNA viruses, increases the likelihood of a copying error occurring that enables the new virus variant to change host. In pandemics, humans themselves then play an important role again.”

Humans amplify pandemics through their behaviour

The Hendra virus (1994), the Nipah virus (1999) and presumably also the Ebola virus (1976) were transmitted by bats. There are 1400 species of bats worldwide. That is 25% of all mammals. It is now known that these animals had to leave their original homes due to slash-and-burn agriculture or climate change, for example, and settled in other places – closer to humans. The Marburg virus (1967), on the other hand, came from monkeys that had themselves become infected during a transport diversions at Gatwick Airport in London. Many different animals from all over the world were loaded there, allowing the virus to spread. “Humans are an amplifier of a process that is actually natural,” explains the expert. “These spillovers would also take place without humans, but humans as amplifiers make the occurrence of a spillover as well as the emergence and extent of a pandemic much more likely through their behaviour, for example by shifting habitats and contributing to the spread of the pathogen. Today, we have much more movement in the human population through flights, trade or flight. If there is then a mutant or a modified virus and we continue to move transcontinentally, the probability of a pathogen spreading worldwide is naturally also higher.” The proximity of humans to their livestock can also play a role in this context. Reports of farms on which thousands of animals are culled for safety reasons in suspected cases are no longer a rarity.

Viruses mutate every day

In 2003, a previously unknown coronavirus (SARS-CoV-1) killed people in Hong Kong for the first time. Until then, coronaviruses were considered harmless and perhaps caused the common cold. Today we know that coronaviruses can exchange genetic material with each other and create a new virus from it. Even back then, it was therefore only a matter of time before a pandemic would occur. Researchers predicted this as early as 2003, and viruses mutate every day. “Yes, that happens all the time and is completely normal,” says du Prel. “We have an incredible number of virus species and copying errors are constantly occurring. For the most part, the mutations that occur are not bad at all, because they also lead to the demise of a virus or have no effect at all. But some mutations can lead to them becoming dangerous for us.”
In 1980, the WHO (World Health Organisation) declared the eradication of monkeypox (MPOX). “But I think monkeypox never went away,” says du Prel firmly, “the only question is, where did it stay?” Although human infections were rarer, they were still present in monkeys and rodents. After the declared end of smallpox, people were no longer vaccinated against it. “Vaccinations became increasingly rare, but the smallpox vaccination also provided a certain degree of protection against monkeypox. In this respect, it is also inherent to the host (due to factors in the host itself, editor’s note) that we are more susceptible to monkeypox again.”

Global warming promotes potential pandemics

Scientists say that global warming is setting everything on earth in motion. Just 1 degree of warming causes animals to migrate hundreds of kilometres further, where they meet other animals they would otherwise never have encountered. These encounters promote the possibility of spillover. Seen in this light, climate change could also promote the accumulation of pandemics. “At the very least, it is an important amplifier,” concludes du Prel. “We will never eradicate mutations, they simply exist. It remains a game of different probabilities, both in the mutation and in the encounter of different species. These can be different animal species meeting each other, or humans meeting animals.”

Uwe Blass

Jean-Baptist du Prel studied human medicine at the University of Würzburg, where he also gained his doctorate in 2000, and public health at the University of Düsseldorf. He was a research associate in epidemiology at the German Diabetes Centre and taught epidemiology and medical biometry at the Universities of Mainz and Ulm. Since 2015, he has been a member of the scientific management team at the Chair of Occupational Science at the University of Wuppertal, where he also habilitated in 2022 and teaches subjects including preventive medicine and biological risks.

KNBA | By Alena Naiden

Published June 16, 2026 at 11:19 AM AKDT

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• Anaktuvuk Pass residents get long-sought permission to use ATVs to hunt in the Gates of the Arctic

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Marc John Morry has been hunting caribou around Anaktuvuk Pass since he was a child. But in the summer and fall, most of the land around the village has been off-limits to hunters like him.

That’s because residents were not allowed to use four-wheelers in the majority of the roadless Gates of the Arctic National Park and Preserve, which surrounds the village. But last month, the U.S. Department of the Interior announced it would restore off-road-vehicle access to the park for subsistence.

Residents have been advocating for that change for decades..

“This is life changing,” Morry said. “Now that we’re able to access the lands, we can learn ourselves and relearn what our ancestors taught us about certain areas that always have caribou.”

The Trump administration has been working to give hunters using off-road vehicles more access to protected federal lands across the country. However, the National Park Service officials said this action is specific to subsistence hunting in Gates of the Arctic and does not apply to sport hunters.

And it comes after decades of back and forth on the issue.

Before Anaktuvuk Pass became a permanent settlement about 80 years ago, the Nunamiut people were semi-nomadic and moved throughout the Brooks Range in search of caribou, their main food source.

“We heard many stories from our elders about hunting grounds that we weren’t able to access, which they remember before we even formed a community,” Morry said.

The federal government established the Gates of the Arctic around Anaktuvuk Pass when it passed the Alaska National Interest Lands Conservation Act. The law allowed subsistence hunting by residents using snowmachines, motorboats and other traditional transportation methods, but it didn’t mention ATVs.

Then, in 1986, the park interpreted the law to ban hunts on ATVs because they were not used traditionally. Lillian Stone, the city mayor of Anaktuvuk Pass, said the ban created invisible boundaries for residents relying on hunting for survival.

“It was like we were prisoners in our own land for 40 years, where before that it was — we could hunt anywhere, we could travel anywhere,” Stone said.

Susan Mekiana-Morry, the city vice mayor, said the ban didn’t just affect food gathering. Residents also couldn’t use the land to gather animal skins for making traditional clothing, tools and masks.

“We were deprived of our way of life, our culture, our heritage,” she said.

In the decades since, residents and local leaders have been advocating for ATV access to the park for subsistence — but without progress.

“We felt like it wasn’t getting anywhere, and we weren’t getting the answers that were needed,” Stone said.

Local Native corporations exchanged lands with the Park Service in the late 90s, making about 125,000 acres within the park available for subsistence ATV hunts, National Park Service spokesman for the Alaska region Scott Claggett said. Still, access remained limited.

“The people of Anaktuvuk Pass were limited to just 1% of the park’s 8.45 million acres for subsistence purposes,” Claggett wrote in an email. “Secretary Burgum’s decision last month vastly expands subsistence hunting access to indigenous peoples.”

A year after local leadership traveled to Washington D.C. to meet with the Interior Department, Interior Sec. Doug Burgum visited the village in May to announce the decision to restore ATV access to the park.

“No one knows or cares for this land more than the people who live here,” Burgum wrote in a social media post.

Claggett said that in the next six months, the Park Service will consult local communities to establish the new rule and consider any necessary protections for vegetation, cultural sites and wildlife.He said local subsistence hunters should contact the Gates of the Arctic for current information on using ATVs while the regulatory process is underway.

For Kristen Morry, an Anaktuvuk Pass hunter and a mother of two, the announcement means that she will be able to teach her children how to hunt and process the meat to continue the Nunamiut traditions.

“I have no words for what just happened, because it just makes me really emotional,” she said. “I’m excited to be out there and to no longer have to worry about when we have to stop, because I’m out there year-round.”

What is the virus? Where did it come from? Are humans at risk? What does it mean for the agricultural sector and wildlife? Experts answer your questions

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• ‘A genuine wildlife emergency’: everything you need to know about the arrival of H5 bird flu in Australia | Environment | The Guardian

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Petra StockSun 21 Jun 2026 02.11 EDTShare

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A deadly strain of bird flu, known as H5 bird flu, has arrived on the Australian mainland.

The federal agriculture minister, Julie Collins, this weekend confirmed that a brown skua – that had been found sick in Western Australia – had died from the H5N1 virus. The skua is a wild migratory bird.

A second bird, a sick giant petrel found nearby, was also likely infected.

Australia’s run as the only continent free of the virus has now ended. H5 bird flu has killed millions of birds and thousands of marine mammals since it began spreading across the globe in 2021.

A Pitt-led study reveals the biology of why the disease looked so different when it made the leap to cows and offers a framework for spotting its next new guise more quickly.

Home / News / How Bird Flu Hid in Dairy Cattle

Photography by Rayni Shiring, University of Pittsburgh

When H5N1 bird flu first began infecting U.S. cattle in early 2024, diagnosis was elusive, because in cows, the disease looked completely different. Instead of affecting the lungs, as H5N1 does in other mammalian species, it caused severe infection in the cows’ udders, largely sparing the lungs.  

A study by University of Pittsburgh School of Public Health researchers published June 19 in Science Advances provides the first mechanistic explanation for this peculiar new guise for H5N1, which now affects more than 100 bird and mammal species globally. The study also establishes a new way to help scientists spot bird flu’s next surprise move more quickly, saving precious time in mounting public-health measures to stem the spread. 

The disease first appeared in dairy cattle along the Texas Panhandle as stubborn cases of severe, necrotizing mastitis, a painful inflammatory condition that damages tissues in the mammary glands.  

“Mastitis is a classic disease in milk-production animals, and veterinarians were dutifully looking to all the usual suspects for the source, like bacterial pathogens,” said senior author Suresh Kuchipudi, professor and chair, Department of Infectious Diseases and Microbiology, School of Public Health, at Pitt. “When the real culprit turned out to be bird flu, everyone in the field was caught completely by surprise. We hadn’t even remotely considered that cattle could be a host for H5N1.”  

In the weeks before the virus was identified, it moved from herd to herd, sickening the cattle—and contaminating their environments.  

“If a cow is infected, it sheds a lot of virus into the milk,” said Kuchipudi. “This raised concerns about occupational risk for farm workers. Also, there is a habit of feeding raw milk to domestic pets, like cats, and there have been instances of cats dying, which we studied previously.” He stressed that fortunately, pasteurization is effective at killing the virus, underlining the importance of avoiding raw milk. 

Kuchipudi has been studying influenza viruses for his entire career, with a particular focus on how receptor biology determines which species—and which tissues—can be infected. Typically, such studies involve staining cells for the presence of receptors that are known to work in a lock-and-key relationship with influenza, a subset of sugar-based molecules known as glycans.  

In initial studies by other groups, such experiments suggested that flu‑related glycan receptors were present in the noses, tracheas and lungs of cows. The fact that the animals were nonetheless not developing respiratory infections told the team there was more to the story.  

“Glycan biology is very complex,” said Kuchipudi. “We realized that, to understand what was really going on, we would need to use more innovative technologies and map out the finely detailed architecture that enables the virus to bind to cells.” Kuchipudi collaborated on the study with Harvard Medical School’s Lauren E. Pepi, an expert in the methodology for comprehensively cataloging the entirety of glycan structures, dubbed glycomics.   

Using a multimodal approach that combined binding experiments, staining methods and ultrahigh‑resolution imaging, the team revealed that not all glycan receptors were functioning the same in animals infected with bird flu. Only a particular subtype, known as N‑linked sialic acid receptors, could bind to H5N1. These receptors were virtually absent in cow airway tissue, but pervasive in udders, making them a “perfect breeding ground for the virus,” Kuchipudi said. 

The research provides a framework other scientists can use to potentially predict not just whether H5N1 can jump to new hosts, but also how.  

“We can preemptively screen different species and different tissues within them for susceptibility,” said Kuchipudi. “For example, would they exhibit respiratory symptoms? Would they show only mastitis, as in cows? Or would they show neurological disease, as our team has shown in cats? The lessons learned could potentially help prevent us from being caught by surprise again.”  

Other authors on the study were Surabhi Srinivas, Shubhada K. Chothe, Santhamani Ramasamy, Sougat Misra, Noel Chandan Nallipogu and Lindsey LaBella, all of Pitt; Yin-Ting Yeh, of The Pennsylvania State University; May Wang, of Harvard University; and Heidi L. Pecoraro and Brett T. Webb, of North Dakota State University. 

This research was supported by Pitt Public Health, and the U.S. Department of Agriculture’s National Institute of Food and Agriculture (FP00039373/AWD00010780). 

How Bird Flu Hid in Dairy Cattle – Health Sciences | University of Pittsburgh