A leech is a segmented worm belonging to the phylum Annelida that primarily inhabits freshwater environments such as lakes and ponds. While the thought of a leech attaching to skin may cause alarm, the vast majority of species found in North American lakes are not dangerous to human health. The few blood-feeding species that exist are generally considered nuisance parasites but do not pose a serious threat. Understanding their nature helps demystify the fear surrounding them.
Identifying Common Freshwater Leeches
The nearly 700 species of leeches worldwide vary significantly in their feeding habits, with most North American lake species being predatory rather than parasitic. These common species primarily prey on small invertebrates like insect larvae, snails, or worms, and they are not interested in human blood. Leeches are characterized by a soft, muscular body that is flattened and can range from under an inch to several inches long.
They are typically dark in color and possess a sucker at each end of their body. The smaller, anterior sucker contains the mouth and is used for feeding, while the larger posterior sucker is used for movement and anchoring. The North American medicinal leech, Macrobdella decora, is one of the few that may feed on humans and is recognized by its olive-green body with a line of orange spots. Swimmers sometimes encounter these animals because they are attracted to water disturbances and chemical cues in shallow areas.
The Actual Health Risks of a Leech Bite
Contrary to popular belief, a bite from a common lake leech is generally painless and carries minimal health risk. When a blood-feeding leech attaches, it secretes an anesthetic compound in its saliva, preventing the host from feeling the initial incision. It then injects a powerful anticoagulant, known as hirudin, which prevents blood from clotting and allows the leech to draw a blood meal.
The amount of blood consumed is negligible, and leeches are not known to transmit diseases to humans, unlike mosquitoes or ticks. The primary risk is that the open wound, which may bleed for several hours due to the lingering hirudin, is susceptible to a secondary bacterial infection.
This secondary infection is often caused by bacteria, such as Aeromonas hydrophila, which naturally live in the leech’s gut. The risk of infection increases if the bite site is scratched or if the leech is improperly removed, causing it to regurgitate its stomach contents into the wound. Allergic reactions are extremely rare, but the bite site may remain itchy, red, or slightly swollen for a few days.
Safe Removal and Prevention Methods
If a leech is found attached, the recommended removal method focuses on breaking the suction without causing the leech to regurgitate. The safest technique involves using a fingernail, thin plastic, or a credit card to gently slide under the rim of the smaller, oral sucker. Once the suction is broken, the leech will typically detach its larger rear sucker, allowing it to be flicked away.
It is important to avoid common but ineffective removal techniques like applying salt, fire, or chemicals, or simply pulling the leech off. These harsh methods cause the leech stress, increasing the likelihood of it vomiting digestive tract contents into the wound and raising the risk of bacterial infection. After removal, the wound should be thoroughly cleaned with soap and water and a sterile bandage applied. Prevention involves avoiding areas known to have high leech populations, such as wading in stagnant, shallow water, or wearing protective clothing.