The risk to humans from zoonotic diseases like bird flu is low, scientists tell Global News, but they’re still concerned.

The H5N1 avian flu is spreading in the United States. So far there have only been a handful of detections in humans.

The worry is that it could spread further.

It’s a zoonotic disease, meaning it jumped from animals to humans in what’s called a spillover event.

They can, on occasion, pose major risks to humans. Three out of every four new or emerging infectious diseases in people come from animals, according to the U.S. Centers for Disease Control and Prevention.

“SARS-1 was from a civet (an African cat-like creature), MERS was from a camel, H1N1 was likely from a pig farm,” infectious disease specialist Dr. Isaac Bogoch told Global News, referring to severe acute respiratory syndrome first detected in China in 2003 and Middle East respiratory syndrome coronavirus, which was first detected in Saudi Arabia in 2012.

While some infections have jumped to humans and resulted in pandemics and epidemics, Bogoch was careful to point out that there are also “isolated spillover events with one or two other transmission events in humans.”

He also warned that factors like climate change are pushing non-human animals into close contact with people, “where we’re going to see those spillover events becoming increasingly common. And you can’t be surprised if you start to see more zoonotic infections.”

Different animals will harbour very different infections, Bogoch said, speaking from Toronto. They might not cause illness in the non-human host but they could, eventually, cause a human to become sick.

“The more a virus is transmitted and infects new hosts, it does accumulate mistakes in its genome and its genetic code, as it is replicating and making more of itself,” virologist and vaccinologist Dr. Alyson Kelvin said.

One of those genetic mistakes can give the virus an advantage that helps it infect other animals of the same species, she told Global News.

“Once it jumps a species barrier, say, from birds to mammals such as otters or raccoons or skunks, as we’ve been seeing (with H5N1), it can accumulate more mutations that make it more fit for different, say, mammalian hosts,” she said.

“And that is a concern, as we’re seeing right now as H5N1 is transmitting and infecting dairy cattle.”

Bogoch said H5N1 “is not the most transmissible virus from mammal to mammal, especially through the respiratory pathway.”

He said the key now is to quell the epidemic.

Chronic wasting disease (CWD) is a highly contagious and fatal brain infection in cervids like deer and moose.

It’s spread across a swath of the U.S. and Canada. It was detected for the first time in British Columbia last February.

It’s a disease the B.C. Centre for Disease Control (BCCDC) is “watching very closely,” according to that organization’s medical director of public health response, Dr. Aamir Bharmal.

He said the BCCDC monitors sick and dead animals and human hospital admissions to try to spot diseases that are circulating.

“If we’re starting to see more of a specific virus that might be circulating, then we want to understand a little bit more about what might be some of the causes for that,” Bharmal said from Vancouver.

“If we don’t really know what’s happening on that fringe of animals, then we don’t really have a picture of what’s going on.”

Monitoring is crucial, Bogoch agreed.

So is recognizing humans’ ability to quickly traverse the planet — and therefore spread pathogens.

“What’s happening in someone’s backyard can be our issue in as little as 24 hours,” Bogoch said.

MERS was first spotted in Saudi Arabia in 2012. Within a few years, officials in 27 other countries detected it. In 2015 the disease killed 38 people in South Korea.

“Is a South Korean doctor going to think ‘MERS’ as the first thing on their differential diagnosis when they’re seeing someone presenting to their hospital with a severe respiratory illness?” Bogoch asked.

“No, of course not.”

Bogoch called for health authorities around the world to implement a “one health” approach.

The U.S. Centers for Disease Control and Prevention (CDC) describes it as an approach that recognizes the connection between “people, animals, plants and their shared environment.”

Bogoch said it includes steps like careful urban planning to create fewer spaces to have non-human and human interaction that could potentially lead to spillover events, farmers using personal protective equipment when needed and creating infrastructure for the rapid detection of a pathogen.

“The key here is it’s got to be global, right?” he said.

“It’s not sufficient for this to be a Canadian strategy or a European strategy.”

Bharmal said the BCCDC has adopted parts of the one health approach “but there’s a need to just embrace it even more broadly.”

Quelling a zoonotic outbreak doesn’t mean culling animals, Bogoch said. When humans detect infections in animals that could jump or have in the past, he said it’s important to track how the illness is spreading and prevent it.

That can include making sure sick animals don’t interact with healthy ones, monitoring wastewater and screening cows’ milk, he explained.

Spillover events “probably happened all the time,” he said. But examples like the Ebola virus and SARS-CoV-2, which U.S. and Canadian health authorities say likely came from a bat and which caused COVID-19, prove that sometimes zoonotic diseases require vaccines.

Whether a vaccine is needed is “a bit of an economic question, a public health question and a scientific question,” Kelvin said.

Kelvin works at the University of Saskatchewan’s Vaccine and Infectious Disease Organization (VIDO), where she researches coronaviruses and influenza viruses.

“Obviously a lot of vaccine manufacturers look at their profits, but also they need to cover the basic manufacturing process and process development of making a vaccine,” she said, noting that non-profit and public health bodies like the World Health Organization (WHO) work to support vaccine equity.

An illness also needs to be severe enough and be poised to infect enough people to warrant a vaccine, she said.

“Not all pathogens cause severe disease requiring hospitalization or potentially cause deaths. So we usually target those ones first instead of more mild diseases like the common cold,” she said, speaking from Saskatoon.

Finally, it depends on whether the scientific community can create a cure.

“The actual science of developing an immune response that will be protective against the bacteria Borrelia burgdorferi, which causes Lyme, is more difficult than, say, what we see with influenza or SARS-CoV-2,” she said.

If untreated, Lyme disease can affect the nervous system, joints and heart.

There is no vaccine for Lyme disease, though there once was.

When a spillover event happens and a vaccine is needed, Kelvin said scientists can tailor previously created vaccines to the new illness – if they’re similar enough.

She pointed to the COVID-19 vaccines and influenza vaccines, or flu shots.

The WHO’s global pathogen surveillance system collects and samples sequences of circulating viruses. Scientists can then use that data to update vaccines, Kelvin said.

To that end, Kelvin and Bogoch said the U.S. already has a stockpile of updated H5N1 vaccine.

“Even though H5n1 wasn’t causing as much global concern as it is in the last two years previously, we still identified a good vaccine strain and made a potential vaccine lot that could be used,” she told Global News.

If scientists don’t have a pre-existing shot, they begin preclinical studies, like with the COVID-19 vaccines.

Bharmal said national bodies, like the U.S. CDC or Canada’s National Advisory Committee on Immunization (NACI), decide when diseases have hit a critical mass and vaccines should be distributed.

— with files from Katie Dangerfield and The Associated Press