Wound licking

Wound licking is an instinctive response in humans and many other animals to lick an injury. Dogs, cats, small rodents and primates all lick wounds. Saliva contains tissue factor which promotes the blood clotting mechanism. The enzyme lysozyme is found in many tissues and is known to attack the cell walls of many gram-positive bacteria, aiding in defense against infection. Tears are also beneficial to wounds due to the lysozyme enzyme. However, there are also infection risks due to bacteria in the human mouth.

Oral mucosa heals faster than skin, suggesting that saliva may have properties that aid wound healing. Saliva contains cell-derived tissue factor, and many compounds that are antibacterial or promote healing. Salivary tissue factor, associated with microvesicles shed from cells in the mouth, promotes wound healing through the extrinsic blood coagulation cascade. The enzymes lysozyme and peroxidase,defensins,cystatins and an antibody, IgA, are all antibacterial. Thrombospondin and some other components are antiviral. A protease inhibitor, secretory leukocyte protease inhibitor, is present in saliva and is both antibacterial and antiviral, and a promoter of wound healing.Nitrates that are naturally found in saliva break down into nitric oxide on contact with skin, which will inhibit bacterial growth. Saliva contains growth factors such as epidermal growth factor,VEGF,TGF-β1,leptin,IGF-I,lysophosphatidic acid,hyaluronan and NGF, which all promote healing, although levels of EGF and NGF in humans are much lower than those in rats. In humans, histatins may play a larger role. As well as being growth factors, IGF-I and TGF-α induce antimicrobial peptides. Saliva also contains an analgesic, opiorphin. Licking will also tend to debride the wound and remove gross contamination from the affected area.

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