Wound licking

Wound licking is an instinctive response in humans and many other animals to lick an injury. Dogs, cats, rodents and primates all lick wounds. 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.

Mechanism


Oral mucosa heals faster than skin, suggesting that saliva may have properties that aid wound healing. Saliva contains many compounds that are antibacterial or promote healing. 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.

In non-human animals


It has been long observed that the licking of their wounds by dogs might be beneficial. Indeed, a dog's saliva is bactericidal against the bacteria Escherichia coli and Streptococcus canis, although not against coagulase positive Staphylococcus or Pseudomonas aeruginosa. Wound licking is also important in other animals. Removal of the salivary glands of mice and rats slows wound healing, and communal licking of wounds among rodents accelerates wound healing. Communal licking is common in several primate species. In macaques, hair surrounding a wound and any dirt is removed, and the wound is licked, healing without infection.

Risks
Too much licking of wounds can be harmful. An Elizabethan collar is sometimes worn by pet animals to prevent biting or excessive wound licking, which can cause a lick granuloma. These lesions are often infected by pathogenic bacteria such as Staphylococcus intermedius. Infection is another risk. Horses that lick wounds may become infected by a stomach parasite, Habronema, a type of nematode worm. The rabies virus may be transmitted between kudu antelopes by wound licking.

In humans
In an unusual case, an Oregon teacher was reprimanded after licking blood from wounds on a track team member's knee, a football player's arm, and a high school student's hand. An Oregon public health officer commented that "We do know that animals lick their own wounds, and it may be that saliva has some healing properties. But my very strong recommendation is that you confine yourself to licking your own wounds."

In legend
There are many legends involving healing wounds by licking them or applying saliva. The Saint Magdalena de Pazzi is said to have cured a nun of sores and scabs in 1589 by licking her limbs. The Roman Emperor Vespasian is said to have performed a healing of a blind man using his saliva. Pliny the Elder in his Natural History reported that a fasting woman's saliva is an effective cure for bloodshot eyes. A Filipino belief, usog, holds that a child afflicted by the evil eye by a stranger can be relieved of their distress by applying the stranger's saliva to their body.

Risks
There are potential health hazards in wound licking due to infection risk, especially in immunocompromised patients. Human saliva contains a wide variety of bacteria that are harmless in the mouth, but that may cause significant infection if introduced into a wound. A notable case was a diabetic man who licked his bleeding thumb following a minor bicycle accident, and subsequently had to have the thumb amputated after it became infected with Eikenella corrodens from his saliva. The practice of metzitzah during circumcision is controversial as it can transmit the herpes virus to the infant. Attempting to suck out venom following a snakebite may also introduce infection.

In history and legend
Dog saliva has been said by many cultures to have curative powers in people. "Langue de chien, langue de médecin" is a French saying meaning "A dog's tongue is a doctor's tongue", and a Latin quote that "Lingua canis dum lingit vulnus curat" or "A dog's tongue, licking a wound, heals it" appears in a thirteenth-century manuscript. In Ancient Greece, dogs at the shrine of Aesculapius were trained to lick patients, and snake saliva was also applied to wounds. Saint Roch in the Middle Ages was said to have been cured of a plague of sores by licking from his dog. The Assyrian Queen Semiramis is supposed to have attempted to resurrect the slain Armenian king Ara the Beautiful by having the dog god Aralez lick his wounds. In the Scottish Highlands in the nineteenth century, dog lick was believed to be effective for treating wounds and sores. In the Gospel of Luke (16:19-31), Lazarus the Beggar's sores are licked by dogs, though this does not seem to be curative.

Modern cases
There are contemporary reports of the healing properties of dog saliva. Fijian fishermen are reported to allow dogs to lick their wounds to promote healing, and a case of dog saliva promoting wound healing was reported in the Lancet medical journal.

Risks
As with the licking of wounds by people, wound licking by animals carries a risk of infection. Allowing pet cats to lick open wounds can cause cellulitis and septicemia  due to bacterial infections. Licking of open wounds by dogs could transmit rabies if the dog is infected with rabies, although this is said by the CDC to be rare. Dog saliva has been reported to complicate the healing of ulcers. Another issue is the possibility of an allergy to proteins in the saliva of pets, such as Fel d 1 in cat allergy and Can f 1 in dog allergy. Cases of serious infection following the licking of wounds by pets include:


 * Dog
 * A diabetic man who was infected by Pasteurella dagmatis due to the licking of his injured toe by his dog, causing a spinal infection.
 * A woman recovering from knee surgery suffered a persistent infection of the knee with Pasteurella after her dog licked a small wound on her toe.
 * A dog lick to an Australian woman's minor burn caused septicemia and necrosis due to Capnocytophaga canimorsus infection, resulting in the loss of all her toes, fingers and a leg.
 * C. canimorsus caused acute renal failure due to septicemia in a man whose open hand wound was licked by his dog.
 * A 68 year old man died from septicemia and necrotizing fasciitis after a wound was licked by his dog.
 * A patient with a perforated eardrum developed meningitis after his dog passed on a Pasteurella multocida infection by licking his ear.
 * Cat
 * A woman recovering from surgery for endometrial cancer suffered from Pasteurella multocida infection causing an abscess after her cat licked the incision.
 * A blood donor whose cat licked her chapped fingers passed on Pasteurella infection to a 74-year old transfusion recipient.
 * A seven-week old boy contracted meningitis due to Pasteurella from contact with pet saliva.

Idiomatic use
To "lick your wounds" means to "to withdraw temporarily while recovering from a defeat"

The phrase was spoken by Antony in John Dryden's seventeenth century play All for Love: "They look on us at distance, and, like curs Scaped from the lion's paws, they bay far off And lick their wounds, and faintly threaten war."