SAN DIEGO (Reuters) - One of America's most valuable defenders against fentanyl trafficking at the Mexico border uses his nose to root out illicit drugs, an old-school technique that authorities say is a key to reducing the flow of deadly synthetic opioids.

Goose, an enthusiastic Golden Retriever, weaves through a sea of idling cars on a warm afternoon at San Diego's massive legal border crossing, one of the most transited in the world with roughly 100,000 people entering the U.S. each day.

The border crossing is open around the clock and dogs contend with exhaust fumes, hot pavement and unpredictable workdays that can go from routine to tense in seconds.

Now Goose and his handler, customs officer Joseph Arcia, trek inside to demonstrate to Reuters how the six-year-old canine can sniff out his training chew toy among the throngs of pedestrians crossing into the U.S. on foot, replicating what he and other dogs do to detect fentanyl and other contraband daily.

Goose sits when he finds the toy planted on a random volunteer crossing the border. Mission accomplished.

The Golden Retriever is one of 536 U.S. Customs and Border Protection canines trained to sniff out drugs, guns, ammunition, money and hidden passengers at America's land border crossings, airports and seaports. The rise of illicit fentanyl and the epidemic of related overdoses prompted CBP to take the then-unprecedented step in 2017 of training drug-sniffing dogs to detect it, a program that has proved crucial to the agency's efforts.

Despite millions of dollars in technology that allows CBP to scan vehicles and data analytics that help target possible smugglers, a dog's sense of smell remains a vital tool for uncovering fentanyl and other narcotics.

Fentanyl is a powerful synthetic opioid that was approved by U.S. authorities for use as an anesthetic in 1968, but a spike in clandestine production and fatal overdoses in the past decade has made it a priority for law enforcement and health providers.

An estimated 75,000 people died from synthetic opioid overdoses in 2023, mostly involving fentanyl, according to the U.S. Centers for Disease Control and Prevention.

The vast majority of CBP fentanyl seizures occur at legal border crossings in Arizona and California, according to CBP statistics. Most convicted fentanyl traffickers in recent years have been U.S. citizens, U.S. Sentencing Commission figures show.

CBP Office of Field Operations canines have been involved with seizing more than 63,000 pounds [28,576 kg] of fentanyl since the program started, according to agency statistics.

President Joe Biden, a Democrat running for another term in Nov. 5 elections, has called on Republicans in Congress to increase funding for border security, including counter-fentanyl efforts at legal border crossings. CBP officials say the funding could help expand the use of canines, which includes a pilot program that has trained six dogs to smell for "precursor" chemicals used to make fentanyl.

Sidney Aki, the CBP field office director for the San Diego area, worked as a canine handler at the start of his career in the 1990s. Speaking to Reuters at the San Ysidro port of entry in late May, he said the dogs operated in conjunction with scanners and data analytics to root out fentanyl and other contraband.

"Of course, if we had more canines, if we had more personnel partnered up with canines, we would continue to do more and more," he said.

STRONG MOTIVATION

At CBP's canine academy in Front Royal, Virginia, customs officers from around the country are paired with their new four-legged partners, part of a four- to six-month process to teach the dogs to seek out contraband.

The drug-sniffing dogs are trained to detect six substances: marijuana, cocaine, heroin, methamphetamine, ecstasy and fentanyl, initially by filling chew toys with "pseudo-narcotics" that smell like the actual drug.

"That toy, that's all they're thinking about," said Donna Sifford, the academy's director, during an exclusive tour of the facility in mid-June. "When they smell those odors and sit, all they want to do is play with that toy."

The academy - located on a picturesque 300-acre property in the Shenandoah River Valley - has training areas that simulate what will become the dogs' real-world work environments, including an airport baggage screening room, mail room conveyor belt and an outdoor parking lot with dozens of dusty cars.

The dogs tend to be German Shepherds, Labradors, Dutch Shepherds and German Shorthaired Pointers, Sifford said. Goose is one of three Golden Retrievers in the program.

Most come from breeders in Europe, mainly Germany and the Czech Republic, while a smaller portion are American. They cost on average $11,000-$12,000 per dog and tend to retire when they reach age 8 or 9, she said.

While other hard drugs can have distinct scents - heroin sometimes smells like vinegar, for instance - fentanyl is usually odorless, at least to humans.

Dogs can learn to detect a new scent in three days on average, Sifford said, but before CBP could start training on fentanyl, the agency needed to develop safety protocols. Trainers always carry four doses of the opioid overdose drug Naloxone - which can also be administered to dogs - although they have not yet needed it, she said.

CBP has heard from authorities in other countries who want to learn about the fentanyl training techniques, including from recent outreach from Argentina and France, Sifford said. They have also had requests from U.S. states and localities.

Sifford acknowledges the work can be challenging for dogs at the border who have to deal with seasonally high temperatures, long work days and the stress of navigating traffic but said the work matches their breeding and temperament.

"When we're selecting the dogs to go down to the southwest border, we're looking for the higher-drive dogs that we know can actually work in that environment and maintain that pace," she said.

'BEST TECHNOLOGY'

Dogs have a sense of smell that is exponentially more powerful than humans with up to 200 times more olfactory receptors, according to a 2022 study published in the peer-reviewed Journal of Neuroscience.

Dogs can rapidly sweep through vehicle traffic, search suspicious cars and check lines of passengers. They are particularly useful for uncovering fentanyl, which can be moved in small quantities as pills or powder.

"They're like biosensors," said Michael Gould, a founding member of the New York City Police Department's canine unit who now works as an expert witness in legal cases involving police dogs. "It's really the best technology that's available."

The dogs do have limitations. They can typically only be out searching vehicles or people for about 20 minutes in warmer weather before they need a break, officials said. Drug-sniffing canines can also send false alarms, with studies showing a range of effectiveness.

And while CBP's fentanyl seizures increased in recent years, the agency only appears to intercept a small percentage coming into the U.S.

A 2022 report on synthetic opioids issued by a group of U.S. lawmakers, government officials and outside experts estimated that only five metric tons of pure fentanyl would be enough to supply all U.S. opioid users for a year - a fraction of comparable heroin consumption by weight.

"That's a whole year's worth of fentanyl use in the United States that you can provide with three pickup truck loads," said David Luckey, a senior researcher with the non-partisan, public policy-focused RAND Corporation, who worked on the report.

The street price of fentanyl has dipped to $1 per pill or lower in certain parts of the U.S., suggesting ample supply.

Pete Flores, CBP's acting deputy commissioner, told Reuters his agency does not estimate how much fentanyl or other drugs might cross into the U.S. undetected but said the efforts aim to disrupt transit routes and business models used by criminal organizations.

"Every single shipment of narcotics, and particularly fentanyl, that we stop, we're saving lives," Flores said.

(Reporting by Ted Hesson in San Diego and Front Royal, Virginia; Additional reporting by Kristina Cooke in San Francisco; Editing by Mary Milliken and Diane Craft)

By Ted Hesson