The call came into the behavior specialists in San Antonio from a doctor in Afghanistan.

His patient had just been through a firefight and now was cowering under a cot, refusing to come out.

Apparently even the chew toys hadn't worked.

Post-traumatic stress disorder, thought Dr Walter F. Burghardt Jr, chief of behavioral medicine at the Daniel E. Holland Military Working Dog Hospital at Lackland Air Force Base. Specifically, canine PTSD.

If anyone needed evidence of the frontline role played by dogs in war these days, here is the latest: The four-legged, wet-nosed troops used to sniff out mines, track down enemy fighters and clear buildings are struggling with the mental strains of combat nearly as much as their human counterparts.

By some estimates, more than 5 percent of the approximately 650 military dogs deployed by US combat forces are developing canine PTSD. Of those, about half are likely to be retired from service, Burghardt says.

Although veterinarians have long diagnosed behavioral problems in animals, the concept of canine PTSD is only about 18 months old and still being debated.

But it has gained vogue among military veterinarians, who have been seeing patterns of troubling behavior among dogs exposed to explosions, gunfire and other combat-related violence in Iraq and Afghanistan.

Like humans with the analogous disorder, different dogs show different symptoms.

Some become hyper-vigilant. Others avoid buildings or work areas that they had previously been comfortable in. Some undergo sharp changes in temperament, becoming unusually aggressive with their handlers, or clingy and timid.

Most crucially, many stop doing the tasks they were trained to perform.

"If the dog is trained to find improvised explosives and it looks like it's working, but isn't, it's not just the dog that's at risk," Burghardt says. "This is a human health issue as well."

That the military is taking a serious interest in canine PTSD underscores the importance of working dogs in the current wars. Once used primarily as furry sentries, military dogs - most are German shepherds, followed by Belgian Malinois and Labrador retrievers - have branched out into an array of specialized tasks.

They are widely considered the most effective tools for detecting the improvised explosive devices, or IEDs, frequently used in Afghanistan. Typically made from fertilizer and chemicals, and containing little or no metal, those buried bombs can be nearly impossible to find with standard minesweeping instruments. In the past three years, IEDs have become the major cause of casualties in Afghanistan.

Across all the forces, more than 50 military dogs have been killed since 2005.

The number of working dogs on active duty has risen to 2,700, from 1,800 in 2001, and the training school headquartered at Lackland has gotten busy, preparing about 500 dogs a year. So has the Holland hospital, the Pentagon's canine version of Walter Reed Army Medical Center.

Burghardt, a lanky 59-year-old who retired last year from the Air Force as a colonel, rarely sees his PTSD patients in the flesh. Consultations with veterinarians in the field are generally done by phone, e-mail or Skype, and often involve video documentation.

In each case, Burghardt theorizes, the dogs were using an object, vehicle or person as a "cue" for some violence they had witnessed.

"If you want to put doggy thoughts into their heads," he says, "the dog is thinking: When I see this kind of individual, things go boom, and I'm distressed."

Treatment can be tricky.

Since the patient cannot explain what is wrong, veterinarians and handlers must make educated guesses about the traumatizing events. Care can be as simple as taking a dog off patrol and giving it lots of exercise, playtime and gentle obedience training.

More serious cases will receive what Burghardt calls "desensitization counter-conditioning", which entails exposing the dog at a safe distance to a sight or sound that might set off a reaction - a gunshot, a loud bang or a vehicle, for instance. If the dog does not react, it is rewarded, and the trigger - "the spider in a glass box", Burghardt calls it - is moved progressively closer.

Dogs that do not recover quickly are returned to their home bases for longer-term treatment. But if they continue to show symptoms after three months, they are usually retired or transferred to different duties, Burghardt says.