Click image for more photos of bomb dogs. Above, TSA explosives detection handler Diego Villanueva and his dog Joe train at the Atlanta airport in December. (AP/John Bazemore) More

The now ubiquitous surveillance image of the Brussels suicide bombers strolling through the airport corridor is enough to instill fear in anyone with plans to fly.

In the U.S., security officials are partly depending on bomb-sniffing dogs to thwart similar terror plots.

Since the 9/11 attacks, the number of canines deployed to protect the nation’s busiest airports, train stations and other transit centers has surged 400 percent.

A similar strategy is employed across the world. But the global war on terror’s ever-increasing reliance on man’s best friend is presenting a new problem — a deficit of high-quality bomb dogs.

“More developing countries are incorporating detection dog teams into their national security plan,” Cynthia Otto, executive director of the Penn Vet Working Dog Center at the University of Pennsylvania, told Congress earlier this month at a hearing on canines used for homeland security. “The demand for detection dogs has increased to the point that the quality of dogs has suffered and the price has increased dramatically.”

SLIDESHOW – Bomb-sniffing dogs at work>>>

No agency outside of the U.S. military employs more bomb-sniffing canines than the Transportation Security Administration.

“It’s our best asset out there to find explosives,” Henry Sergent, chief of the TSA’s National Explosives Detection Canine Team Program, told Yahoo News.

This year, more than $120 million is budgeted for the TSA to position nearly a thousand bomb-sniffing dogs and their handlers at transit hubs.

But according to a recent federal audit, the effort is currently short 210 detection teams — in part because they don’t have enough dogs.

Source: Transportation Security Administration

A TSA spokesman said the exact deficiency wasn’t known, but for the first time since 9/11 the agency is seeking to purchase privately trained dogs for possible use.

“We’re just thinking ahead and making sure that we cover our bases so that we have dogs available for any sort of purpose or need that comes up,” Sergent said. “We don’t want to be caught in any sort of position of where … we don’t have an adequate supply.”

But that problem already exists, according to some veteran canine trainers, academics and law enforcement officials. The market for labradors, Belgian malinois, vizslas and other breeds that make good explosives detectors is so competitive that some trainers wouldn’t reveal their suppliers to Yahoo News.

“There is such a demand for high-quality labradors in the United States that we have to look outside of the United States for labs as well,” said Sue Kjellsen with K2 Solutions, a North Carolina private contractor that has made tens of millions in recent years supplying and training dogs to spot improvised explosive devices (IEDs) for the U.S. military.

Three weeks ago, the Senate Homeland Security and Governmental Affairs Committee summoned Otto, the Penn veterinarian and canine researcher, along with representatives from the TSA, Border Patrol and the Government Accountability Office to discuss how federal dog programs contribute to keeping Americans safe.

“At the same time, these valuable tools are not free,” Sen. Tom Carper, D-Del., said in his opening statement. “Dogs with the proper abilities and temperament to conduct searches are expensive to buy and even more expensive to train and deploy effectively. As with all of our security investments, we must make sure we are deploying these canine teams in a cost-effective way.”