Flickr user Mark Watson

From drug-sniffing beagles at airports to bloodhounds on the trail of an escaped convict, dogs are famous for their acute sense of smell.

In recent years, studies have suggested that their olfactory abilities go beyond what we had imagined, allowing them to sniff out cancer in human beings, as well as alert us to dangerous drops in our blood sugar.

We've all heard of CAT scans—but are we ready for DOG scans? Can dogs really smell disease?

In fact they can, thanks to their cold, wet, amazing noses that can pick up odors in the low parts-per-billion range, says Nancy Dreschel, a veterinarian in Dairy and Animal Science at Penn State University. She describes a stunning example presented at a conference on working dogs she attended last May. "A scientist was training dogs to detect bladder cancer in humans by smelling their urine. She said a dog alerted them to a sample from a supposedly healthy person who was being used as a control. On reexamination that person was found to have bladder cancer, so the dog caught it before anyone else did."

It has been known for decades that malignant tumors exude tiny amounts of chemical compounds called alkanes and benzene derivitatives that are not found in healthy tissue.

One 2006 study showed that, after a training period, five test dogs could identify breast cancer patients 88 percent of the time with no false positives, and lung cancer patients with 99 percent accuracy—based solely on sniffing people's breath.

Dogs seem able to sniff out blood sugar changes as well. Research from the United Kingdom found that 65 percent of dog-owners with Type 1 diabetes said that their pets warned them about impending hypoglycemic attacks by barking and whining.

Even with promising stories like that, "biodetection dogs" aren't showing up in hospitals and clinics just yet, says Dreschel, pointing to two challenges.: First, we need to figure out what chemicals the dogs are associating with a disease. , so they can be trained more efficiently and consistently. "For bladder cancer, they train using urine samples from known cancer patients," Dreschel explains, "but they don't know exactly what the dogs are detecting in those samples." Knowing more precisely what the dogs are noticing would allow their training to be standardized.

The other challenge is overcoming skepticism from the medical community. "Understandably, a lot of physicians don't necessarily want a dog making the diagnosis," says Dreschel. But further studies may help convince reluctant doctors to use them as part of the preliminary screening process.

"It's biologically plausible," an American Cancer Society spokesperson has noted, "but there has to be a lot more study and confirmation of effectiveness."

Says Dreschel, the genetics and the physiology of dogs are perfectly suited for sniffing. "Dogs have so many more genes that code for olfactory ability, and many more olfactory nerve cells than humans," says Dreschel. "They have an exquisite sense of smell which humans have been taking advantage of for centuries, in hunting and tracking, search and rescue, and detecting drugs and explosives."

We're already used to dogs taking a whiff of our suitcases at the airport. Don't be surprised if one day, you are greeted at the doctor's office by a nurse with a spaniel. Just another case of a dog doing its job.

Nancy Dreschel, DVM, PhD, is an instructor in Dairy and Animal Science at Penn State, University Park; (814) 863-4197 nad5@psu.edu.