The problem the leech solves is venous congestion. When a surgeon reattaches tissue, small arteries can often be repaired but the thin-walled veins are harder to reconnect. Blood arrives and cannot leave, the flap swells, pressure rises and the tissue dies. A leech feeds for perhaps twenty to forty-five minutes, then the wound continues to ooze for hours afterwards, decompressing the flap.
The prolonged bleeding is chemical. Leech saliva contains hirudin, a direct thrombin inhibitor, plus vasodilators and a spreading factor. The United States Food and Drug Administration cleared leeches for marketing as a medical device in 2004, and hospital-supplied leeches come from controlled farms rather than ponds.
The risk is infection. Leech gut carries Aeromonas bacteria that aid blood digestion, and patients are generally given prophylactic antibiotics. Historic bloodletting, by contrast, had no sound basis, and the modern use should not be read as a vindication of it. Optimal antibiotic choice and application protocols are still debated.