Leeches are segmented worms, closely related to earthworms, and are often found in freshwater environments, though some species inhabit oceans or moist terrestrial areas like rainforests. While there are over 600 distinct species of leeches, only a small fraction are sanguivorous, meaning they feed on blood and interact with humans. This interaction, though often feared, is a biological process that has also been harnessed for medical purposes.
The Leech Bite Process
Leeches are adept at locating potential hosts by detecting cues such as body heat, movement, and the carbon dioxide released from breathing. Once a suitable host is found, the leech uses its anterior, or oral, sucker to attach to the skin. This sucker contains its jaw and is used for feeding, while a posterior sucker is used for leverage and movement. The bite itself is painless because the leech’s saliva contains a natural anesthetic that numbs the area.
Most blood-sucking leeches have three separate jaws arranged in a triangular formation, each lined with up to 100 tiny, sharp teeth. These jaws move in a sawing motion to create a distinctive Y-shaped incision in the skin. As the leech makes this cut, its salivary glands secrete bioactive compounds directly into the wound. This saliva is rich in hirudin, a potent anticoagulant that prevents blood from clotting.
This continuous, non-clotting flow of blood is further facilitated by other substances in the saliva, including a vasodilator that widens blood vessels. A leech can consume up to ten times its weight in a single feeding session that can last from 20 to 40 minutes. After feeding to its capacity, the engorged leech will naturally detach and fall off the host.
Removal and Post-Bite Care
If a leech is discovered before it has finished feeding, it should be removed correctly to minimize complications. The recommended method involves using a flat, thin edge, such as a fingernail or a credit card, to slide underneath the oral sucker at the site of the bite. This action breaks the seal of the suction, causing the leech to detach. Once the anterior sucker is free, the leech can be removed completely.
Certain removal methods should be avoided because they can cause the leech to regurgitate its stomach contents back into the open wound. Applying salt, vinegar, or heat can induce this regurgitation, which introduces bacteria from the leech’s digestive system into the bloodstream, increasing the risk of infection. Pulling the leech off forcefully is also discouraged, as its jaws may remain embedded in the skin, which can also lead to infection.
After the leech is removed or falls off, prolonged bleeding from the wound is normal. This is due to the anticoagulants in the saliva, which can take several hours to wear off. The bite area should be cleaned with soap and water and then covered with a sterile bandage. Applying firm, steady pressure to the site can help control the bleeding.
Monitor the wound for signs of infection, such as increasing redness, swelling, heat, or the formation of pus.
Medicinal Leech Applications
Beyond encounters in the wild, leeches have a long history of use in medicine, a practice known as hirudotherapy. Today, this application is highly specialized and used in the fields of plastic and reconstructive surgery. Following procedures to reattach severed body parts or to secure skin grafts, venous congestion (the pooling of blood) can threaten the reattached tissue. Medical leeches are applied to these areas to alleviate this pressure.
The species used for these procedures, Hirudo medicinalis, is raised in sterile environments to prevent disease transmission. When a medicinal leech is applied to a congested tissue flap or reattached digit, its saliva delivers anticoagulants. These prevent the formation of blood clots in the repaired veins, allowing blood to flow out of the area. This promotes circulation and helps the tissue survive until the body can re-establish its own circulatory pathways.
This controlled application helps drain excess blood, which increases the chances of a successful surgical outcome. The bleeding that continues for hours after the leech detaches is considered a beneficial part of the therapy, as it further prevents venous congestion.