Migrating shorebirds stop each spring in New Jersey to feast on horseshoe-crab eggs. The birds have lots of flu strains in their guts.

Researchers swab the birds' butts (the technical term is "cloacas") and pick up their droppings in search of clues about potential future pandemics.

A pandemic flu that hops from birds to humans could quickly circulate around the world, killing hundreds of millions of people before scientists develop a vaccine.

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CAPE MAY, NEW JERSEY — On a May morning, birders and scientists descended on Reed's Beach with gunpowder and a cannon. They hid in the grass and mud, waiting for the right moment to shoot.

Hours passed as the sun drifted into the middle of the sky, then finally the rifle powder ignited, and the thud sent more than 100 stunned shore birds into a frenzy. They stretched their wings to soar into the air, then quickly tumbled to the ground, trapped in wide nets that had been shot from the cannon.

In an instant, dozens of people emerged from hiding spots in the beach dunes, rushing through the sand to pick the birds up. A few flu researchers went straight for the creatures' back ends.

Ths wasn't part of a war reenactment or violent uprising. It was a meticulously crafted plan to help scientists fight a dangerous yet almost invisible enemy: a potential flu pandemic that could originate from the guts of migrating birds.

Traveling birds are the primordial hosts of influenza viruses, which means they can harbor deadly flu strains that could get transmitted to chickens, pigs, and people. Every pandemic flu the world has ever seen has come from birds like these.

On the beach, the goal is to take a close look at these birds' butts and poop in the hopes of gathering information that could help scientists prevent the next deadly flu pandemic. This particular stretch of beach is especially good for bird research, since of thousands of tiny, well-traveled shorebirds — each small enough to fit into the palm of your hand — flock there during migration to gorge themselves on the protein rich-eggs of horseshoe crabs.

Migratory birds stop in Delaware Bay each spring to feast on horseshoe crab eggs before they fly further north. Horseshoe crab populations aren't what they used to be, but there are still crowds of them along the shore. Hilary Brueck/Business Insider

It's a ritual scientists have participated in every May for over three decades, when migratory birds stop along the Jersey Shore as they journey north from South America. For about three to four days, as the birds feast, researchers swab their throats and butts (technically they're called "cloacas") and collect hundreds of poop samples.

Their aim is to uncover clues about what the next dangerous bird flu might look like — while there's still time to prepare.

We are woefully unprepared for the next pandemic

The field work on Reed's Beach helped researchers discover that birds are the primary, natural hosts for pandemic flu.

The last big pandemic flu — the H1N1 virus nicknamed the Spanish flu — killed 50 million people in 1918. American soldiers brought the Spanish flu to the battlefields of WWI before the illness eventually spread as far as New Zealand, but it originally came from birds.

Most scientists agree that the next pandemic is a matter of when, not if. Experts think that if a pandemic flu similar to the 1918 one were to arise today, it could infect one in every three people alive. Without a good vaccine to combat it, the virus could kill more than one out of every 10 humans on the planet — upwards of 900 million people.

More recent pandemics include the 1957-58 "Asian flu;" the 1968 Hong Kong pandemic, which killed at least 1 million people; and the H1N1 pandemic of 2009, from which an estimated 284,000 died. Researchers have compelling evidence that each of those strains also came from bird guts.

Once the cannon goes off, birds are trapped inside a net, and then tagged and measured before they're released again. Hilary Brueck/Business Insider

The interconnected nature of our modern lives due to air travel and other daily migrations of people and food means the chance that a deadly virus could hit hard is only increasing.

"The world needs to prepare for pandemics in the same serious way it prepares for war," Bill Gates said last year, on the 100-year anniversary of the Spanish flu. "If history has taught us anything, it's that there will be another deadly global pandemic."

One pandemic simulation developed at the Johns Hopkins Center for Health Security estimated that if a "moderately contagious" and "moderately lethal" virus were unleashed today, it would kill as many as 150 million people in 20 months. That's almost half of the population of the US.

The beach is closed to people to let the birds feast in peace. Hilary Brueck/Business Insider

For birds, on the other hand, flu viruses are relatively benign, manifesting as gastrointestinal issues that are of little consequence to their health. The reason some flus become human-transmittable — then proliferate around the globe, with devastating consequences — while others don't remains a mystery.

"At the moment, we don't understand which ones are threats, and which ones aren't," Richard Webby, an infectious-disease expert at St Jude Children's Research Hospital who helps develop the annual flu vaccine, told Business Insider.

By studying the flu virus in birds, scientists hope to discover new strains that haven't hit humans yet and work on ways to fight those illnesses, perhaps by developing vaccines or creating antiviral drugs (like Tamiflu) that can make a case of the flu less severe.

'About 20% of them are pooping out influenza'

The more bird species interact with each other (in places where they congregate like markets, barns, or beaches), the more likely it is that their different flu genes will mix. More flu-virus interaction creates more opportunity for a virus to morph into a new strain capable of hopping into humans.

That's why Reed's Beach is one-of-a-kind: Up to 25 types of sea birds come together there each spring. There's the red knot, which travels more than 9,000 miles from the southern tip of Argentina to the Canadian Arctic, as well as ruddy turnstones on their way up from Brazil, plus sanderlings, semipalmated sandpipers, and several species of gulls.

It's a mixing ground for a hemisphere's worth of bird poop. (Similar research is done in poultry markets in Bangladesh, chicken and duck farms on the Nile river delta in Egypt, and on wild ducks in Alberta, Canada.)

"Those beautiful, healthy birds — about 20% of them are pooping out influenza," Robert Webster, a flu expert who pinpointed the Jersey shore bird hotspot in 1985, told Business Insider. "Where do so many infected birds come from? Where did they get their viruses from? We don't really know."

Trapping shore birds involves a cannon and nets. Hilary Brueck/Business Insider

Flu viruses can morph in two ways. The first is via a routine process called genetic drift, in which a virus multiplies and undergoes small genetic changes over time as it replicates. The second happens when two viruses mix together, which can create new (and dangerous) bugs capable of infecting chickens, pigs, or people.

At least 16 different "A" influenza subtypes — the kind responsible for pandemics in humans — are found in aquatic birds (and two additional influenza A subtypes were recently discovered in bats). Researchers looking at flu strains on Reed's Beach have found close relatives of the 1918 Spanish Flu (in the H1N1 group), as well as other strains that look like the Hong Kong pandemic bug.

Shore birds migrating from South America use New Jersey beaches as a rest stop on their way towards Arctic breeding grounds in May and early June. Hilary Brueck/Business Insider

"You're never going to know somebody had some bird flu unless they get really sick," Pamela McKenzie, a flu researcher from St. Jude Children's Research Hospital who participates in the annual bird-research frenzy on the beach, told Business Insider as she gathered poop samples.

The birds also harbor some strains that we haven't seen in humans in over 50 years, Webster said. That means if they were to hop into people now, our immune systems would be shocked.

"We know that the H2 viruses are still there in the wild birds and they're a potential threat to humans," he said. "We are concerned that there is no immunity in the human population, or only in the elderly, and so that is why we worry about them."

Tons of different flu viruses circulate among shore birds in New Jersey. Hilary Brueck/Business Insider

If a bird flu were to break out, public-health officials have a few anti-flu weapons to deploy. Stockpiled antiviral drugs called neuraminidase inhibitors can help, as long as they're dispensed within a couple days after a person gets sick. Otherwise, scientists would need to develop a new vaccine, but that could take months and ultimately prove ineffective if the virus morphed again before the vaccine was finished.

"Even with the very best technology that we have, it takes six months to make a new vaccine," Webster said. "And in the meantime, the virus is spread all over the world and if it's a deadly virus, it's going to kill many, many, many people."

This is another reason scientists are scrambling to develop a universal flu vaccine that would offer long-term protection against all kinds of flu, unlike the annual shot.

Trapping birds using cannons, nets, and tall grass

Susan Taylor says she comes to the New Jersey shore "every year" from Victoria, Australia, to watch birds migrating from South America. Hilary Brueck/Business Insider

Trapping the birds on Reed's Beach is a multi-hour operation that can be easily foiled if the birds get startled.

Before the big cannon-shooting moment, researchers used walkie-talkies to radio to one another in different spots around the bay. Slowly, they worked together to corner the birds into a small inlet. Then after the projectile net sent the birds gently toppling to the ground, the flu researchers unpacked their vials and swabs.

In addition to the live swabs, researchers also gathered about 600 poop samples from the beach using long-stemmed Q-tips.

None of these scientists seemed too concerned about coming into contact with a potential killer flu on the beach — they don't even wear gloves while picking up poop samples (though they do when swabbing the birds' cloacas). Instead, they simply maintain good hygiene on the sand and off, knowing they're more likely to get the flu from touching a door handle or sitting next to a sick person.

This tiny swab is specially designed to take saliva or poop samples from birds. Hilary Brueck/Business Insider

Once all the samples were packed in coolers and ready to go to the lab, the research crew left the birds alone on the beach. By the end of May, most of the birds had flown north; their dedicated beach area re-opened to the public in June.

In labs, researchers are now conducting DNA analyses to find out which influenza strains were circulating on the beach this year, how that compares to years past, and whether there was any new flu. That work will take about six months.

So far, after years of field research, scientists have gained some clues about which genes make an influenza strain more likely to be harmful and better able to jump from birds to poultry, pigs, and people. The new genetic information from this year will become part of that catalogue of three decades' worth of flu-gene segments, enlarging the database that helps researchers better track — and potentially fight — pandemic flu.

"You're looking for key components of the genetic sequence that make a gene more dangerous," McKenzie said.

But the scientists said their research is far from done.

"We would like to know if we can predict which are the viruses in the wild birds that are a real threat to either pigs, poultry or humans," Webster said. "We don't know the answers, but as we look at the genome of the viruses that are in the wild birds and in humans and so on, the goal is that in the future, we can make predictions of which viruses are really dangerous."

Pamela McKenzie, who works on the infectious disease team at St. Jude Children's Research Hospital, picks up bird poop samples along the beach in Delaware Bay. Hilary Brueck/Business Insider

That research cycle will start again next May, when both birds and bird-catchers return to the Jersey shore.

"Ultimately we dream about having a universal vaccine," Webster said. "In a perfect situation, we would have four or five anti-viral drugs and a universal vaccine. Then I could die happy."