Editor’s note: As COVID-19 continues to spread worldwide, scientists areanalyzing new ways to track it. One promising approach istraining dogs todetect people who are infected by smelling samples of human urine or sweat.Research scientist Glen Golden, who has trained dogs and ferrets to detectavian flu in birds, explains why certain animals are well suited to sniff outsickness.
1. Which species have a nose for disease?
Some animals have highly developed senses of smell. They include rodents; dogsand their wild relatives, like wolves and coyotes; and mustelids – carnivorousmammals such as weasels, otters and ferrets. These species’ brains have threeor more times more functional olfactory receptor neurons – nerve cells thatrespond to odors – than species with less keen smelling abilities, includinghumans and other primates.
These neurons are responsible for detecting and identifying volatile olfactorycompounds that send meaningful signals, like smoke from a fire or the aroma offresh meat. A substance is volatile if it changes readily from liquid to gasat low temperatures, like the acetone that gives nail polish remover itsfruity smell. Once it vaporizes, it can spread rapidly through the air.
When one of these animals detects a meaningful odor, the chemical signal istranslated into messages and transported throughout its brain. The messages gosimultaneously to the olfactory cortex, which is responsible for identifying,localizing and remembering odor, and to other brain regions responsible fordecision-making and emotion. So these animals can detect many chemical signalsover great distances and can make rapid and accurate mental associations aboutthem.
2. How do researchers choose a target scent?
In most studies that have used dogs to detect cancer, the dogs have identifiedphysical samples, such as skin, urine or breath, from patients who either havebeen diagnosed with cancer or have undiagnosed cancer at an early stage.Scientists don’t know what odor cue the dogs use or whether it varies by typeof cancer.
The U.S. Department of Agriculture’s National Wildlife Research Center inColorado and the Monell Chemical Senses Center in Pennsylvania have trainedmice to detect avian influenza in fecal samples from infected ducks. Bird fluis hard to detect in wild flocks, and it can spread to humans, so this work isdesigned to help wildlife biologists monitor for outbreaks.
The Kimball lab at Monell taught the mice to get a reward when they smelled aconfirmed positive sample from an infected animal. For example, mice would geta drink of water when they traveled down the arm of a Y-shaped maze thatcontained feces from a duck infected with avian influenza virus.
By chemically analyzing the fecal samples, researchers found that theconcentration of volatile chemical compounds in them changed when a duckbecame infected with bird flu. So they inferred that this altered smellprofile was what the mice recognized.
Building on that work, we’ve trained ferrets and dogs to detect avianinfluenza in fowl, such as wild ducks and domestic chickens, in acollaborative study between Colorado State University and the NationalWildlife Research Center that is currently under review for publication.
With ferrets, we started by training them to alert, or signal that they haddetected the target odor, by scratching on a box that contained high ratios ofthose volatile compounds and to ignore boxes that contained low ratios. Nextwe showed the ferrets fecal samples from both infected and noninfected ducks,and the ferrets immediately began alerting to the box containing the fecalsample from an infected duck.
This approach is similar to the way that dogs are trained to detect knownvolatile odors in explosives or illegal drugs. Sometimes, though, we have tolet the detector animal determine the odor profile that it will respond to.
3. Can animals be trained to detect more than one target?
Yes. To avoid confusion about what a trained animal is detecting, we can teachit a different behavioral response for each target odor.
For example, the dogs in the U.S. Department of Agriculture’s WildlifeServices Canine Disease Detection Program respond with an aggressive alert,such as scratching, when they detect a sample from a duck infected with birdflu. When they detect a sample from a white-tailed deer infected by the prionthat causes chronic wasting disease, they respond with a passive alert such assitting down.
Research at the University of Auburn has shown that dogs can remember andrespond to 72 odors during an odor memory task. The only limitation is howmany ways a dog can communicate about different odor cues.
4. What kinds of factors can complicate this process?
First, any organization that trains animals to detect disease needs the righttype of laboratory and equipment. Depending on the disease, that could includepersonal protection equipment and air filtering.
Another concern is whether the pathogen might infect the detection animals. Ifthat’s a risk, researchers may need to inactivate the samples before theyexpose the animals. Then they need to see whether that process has altered thevolatiles that they are teaching the animals to associate with infection.
Finally, handlers have to think about how to reinforce the desired responsefrom detection animals in the field. If they are working in a population ofmostly noninfected people – for example, in an airport – and an animal doesn’tget a chance to earn a reward, it may lose interest and stop working. We lookfor animals that have a strong drive to work without stopping, but working fora long time without reward can be challenging for even the most motivatedanimal.
5. Why not build a machine that can do this?
Right now we don’t have devices that are as sensitive as animals with well-developed senses of smell. For example, a dog’s sense of smell is at least1,000 times more sensitive than any mechanical device. This could explain whydogs have detected cancer in tissue samples that have been medically clearedas not cancerous
We also know that ferrets can detect avian flu infection in fecal samplesbefore and after laboratory analysis shows that the virus has stoppedshedding. This suggests that for some pathogens, there may be changes involatiles in individuals who are infected but are asymptomatic.
As scientists learn more about how mammals’ sense of smell works, they’ll havea better chance of creating devices that are as sensitive and reliable insniffing out disease.
This article is republished fromThe Conversation, a nonprofit news sitededicated to sharing ideas from academic experts. It was written by: Glen J.Golden, Colorado State University.
Source: Glen J. Golden, Research Scientist/Scholar I, Colorado StateUniversity
Image: Moose, a mixed-breed dog from the Nebraska Humane Society, trains inodor-detection work. Bill Cotton/CSU, CC BY-ND
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