Dogs have an extraordinary sense of smell—at least 10,000 times as acute as ours. While humans boast 6 million olfactory receptors, dogs are blessed with up to 300 million. It's no wonder they're being trained to detect a wide range of objects by smell, from bombs to viruses to fertile cows. Dogs are proving themselves more than our best friends—they are also astoundingly precise odor hunters. Here are seven unusual items they've been taught to sniff out.

1. The Novel Coronavirus

Viruses have specific odors, and scientists have discovered that dogs can detect them. Dogs have been able to distinguish between three types of bovine viruses, for example—those causing diarrhea, herpes, and influenza. Now, dogs are being taught to smell viruses that affect humans. Cindy Otto, director of the University of Pennsylvania's Working Dog Center, is training eight Labrador retrievers to sniff out SARS-CoV-2, the coronavirus responsible for COVID-19. If the pilot study works, canine surveillance teams may one day help screen travelers at airports, patients at hospitals, or attendees at conferences.

Dogs that have already been trained to detect malaria are being put to a similar test at the London School of Hygiene & Tropical Medicine. Scientists have collected viral samples to train the dogs, and if all goes well, the UK's government hopes to deploy six dogs to screen travelers from abroad at airports.

2. Bronze-Age Human Remains

Cadaver dogs, or human-detection dogs, can sniff out the scent of decomposing remains in decades-old cold cases. But could dogs’ noses track odors that go back thousands of years? In 2015, Croatian archaeologist Vedrana Glavaš and dog trainer Andrea Pintar decided to test dogs on Croatia’s Velebit mountain range, where Glavaš had uncovered portions of a 3000-year-old hill fort and necropolis. According to their 2018 study in the Journal of Archaeological Method and Theory, the cadaver dogs found graves with burial chests and human finger and toe bones. The dogs, the paper contends, are searching for the molecules of human decomposition. Glavaš believes the rocky, porous karst landscape of the area preserves those molecules well, perhaps better than plain soil.

3. Fertile Cows

Knowing when a cow is fertile—or in estrus—is important to dairy farmers so they can choose the best time for artificial insemination. In the past, farmers tried to determine estrus by observing changes in cows’ behavior, such as mounting or standing to be mounted. Dogs that have been trained to identify odors specific to estrus might be a more accurate way. Dogs can pick up such scents in vaginal fluid, milk, urine, or blood samples with an accuracy of more than 80 percent overall, and in the case of milk, 99 percent. In one 2013 study, 13 dogs, eight of which had previously been trained to detect narcotics or cancer, were taught to detect estrus-specific scents in cow saliva. They were able to pinpoint estrus in nearly 60 percent of the samples.

4. People Carrying Explosives

Though we can’t see them, we all trail thermal “plumes” in our wake as we move through air—and dogs have been trained to detect them for traces of explosives. In Auburn University's Canine Performance Sciences program, researchers have taught Labrador retrievers to detect body-worn explosives in crowds, including at large concerts and airports, by following scent particles in human heat plumes. This patented method, dubbed Vapor Wake, has been used to train dogs for law enforcement agencies and for mass transit organizations.

5. Invasive Pythons

A dog detects invasive snakes as part of the USDA's Animal Plant Health and Inspection Service.U.S. Department of Agriculture, Wikimedia Commons // CC BY 2.0

Auburn University’s detection dogs have also been trained to smell Burmese pythons [PDF] slithering through Florida's Everglades National Park. Thousands of the pythons, an invasive species, hide in the Everglades’ tall grasses and prey on native mammals and birds. In 2010, two dogs named Ivy and Jake found 19 snakes in the park, and in 2017, dogs named Floyd and Vito sniffed out five pythons on North Key Largo in the Florida Keys. The python-sniffing pups were trained as part of Auburn's Eco Dogs program.

6. Bed Bugs

When bed bugs invade homes, they hide in crevices and cracks and can be very hard to get rid of. One reason is because they're difficult to see, but dogs—who hunt by smell—have proven to be good detectives. In 2008, researchers at the University of Florida trained dogs to discern the odor of bed bugs and their eggs from those of other insects with 97.5 percent accuracy. The dogs were able to smell the difference between live bed bugs or eggs and dead bed bugs, their skins, and their feces. But in the real world, bed bug-detecting dogs sometimes they detect bugs where there are none, wasting consumers' time and money.

7. Cancer

In 2013, a 75-year-old man visited his doctor after his dog kept licking at a small lesion behind the man’s ear. The lesion turned out to be a malignant melanoma—and it wasn’t the first case in which a dog had sniffed out cancer in humans. Different cancer types have specific odor signatures. Some researchers suggest that detecting cancer by that signature, potentially before a visible tumor forms, may catch cancer at earlier stages and prolong survival. The Penn Working Dog Center's cancer detection program is training dogs to imprint on tissue and blood samples donated from ovarian cancer patients. The researchers hope to isolate the odor the dogs identify and develop an electronic sensor that will screen cancers earlier.