When faced with a minor cut or bruise, most people will instinctively lick the site of injury. It's almost second nature as if it's inscribed in our DNA. Everybody does it -- humans, dogs, virtually anyone with a tongue and saliva. There's even a widely used idiom, 'lick your wounds', which means to spend time getting back your strength or happiness after a defeat or bad experience.
You've likely done it countless times so far and you're still alive. So, without even doing proper science we at least know that licking a wound isn't fundamentally bad. But does it accelerate healing in any way or does it just act like a comforting placebo?
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Wound licking: good or bad?
A 2008 study published by Dutch researchers suggests putting saliva in contact with an open wound comes with many benefits. It seems a certain compound of saliva called histatin not only kills bacteria, preventing infections, but also accelerates healing.
The researchers first collected epithelial cells from the inner cheek then cultured them in multiple petri dishes until the surface was completely covered in cells. An incision was then made in the cell layer by scratching away a small area of the cells.
One dish was bathed in isotonic fluid containing the same number of dissolved particles as blood. But other dishes were bathed in glorious human saliva. Sixteen hours later, the scientists reported the saliva-treated artificial wound was almost completely closed while untreated dishes had a substantial part of the 'wound' still open. Then, it was only a matter of singling each saliva component to find out which one was responsible for the accelerated healing property.
"This study not only answers the biological question of why animals lick their wounds," said Gerald Weissmann, MD, Editor-in-Chief of The FASEB Journal, "it also explains why wounds in the mouth, like those of a tooth extraction, heal much faster than comparable wounds of the skin and bone. It also directs us to begin looking at saliva as a source for new drugs.
Another added benefit of wound-licking is that small wounds and injuries can be cleansed of debris like dust, infected tissue, and other contaminants.
There are some risks, though
Though there are proteins and enzymes in saliva that promote wound healing, it's worth remembering that our mouths are also host to scores of bacteria. It's estimated that there are over 100 million microbes composing more than 600 different species in each milliliter of saliva. These bacteria are completely harmless as long as they stay in the mouth and there are no open wounds inside it. In fact, mouth bacteria are responsible for some of the most common diseases in humans, particularly gum disease and tooth decay (cavities).
As such, in some cases, licking your wounds may be a bad idea, especially if you have a history of decreased immunity. For instance, there's one odd case reported in a 2002 paper published in the New England Journal of Medicine. The paper describes how German doctors were forced to amputate the thumb of a diabetic man who licked a small wound inflicted from falling off his bike. The diabetic patient had fallen victim to necrotizing fasciitis, which can destroy tissue in as little as 12-24 hours and absent urgent medical care can be fatal. Subsequent examination revealed two types of bacteria: Eikenella corrodens, commonly found inside the mouth, and Streptococcus anginosus, often found on the skin and in the throat, were responsible for the infection. It should be noted that this sort of infection is rare and only occurs if the victim is vulnerable somehow; in this case suffering from diabetes.
And don't let pets lick your wounds, seriously
There are as many bacteria in our bodies as are human cells. This flora, as scientists call it, includes skin bacteria, mouth bacteria or gut bacteria, as well as yeast and other eukaryotes. Our immune system has adapted to this flora and learned to live in harmony with it. Each person's flora is unique and absent many types of bacteria, we wouldn't be able to survive. However, getting exposed to foreign bacteria can be very dangerous.
Your dog will have its own colonized set of bacteria and yeast flora. If you raised the dog since he was a pup, then it's likely his flora has come in contact with yours so you've become immune. That being said, don't trust someone else's dog to lick your cut finger or bruise. You shouldn't let your own dog do it, for that matter.
Animals lick their wounds because they have no other recourse. We humans, however, are blessed with knowing how to use soap and water, as well as disinfectants and, if required, antibiotics. Licking your own wound or letting your pet do it for you shouldn't cause disease, but it does come with risks. In most cases, the safest thing to do with your mouth is to ask for help. Otherwise, just use a band-aid.