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Leeches were used as part of primitive medicine on Earth, and were often referred to when disparaging apparently outdated medical techniques. In 2374, the EMH Mark II aboard the USS Prometheus remarked to The Doctor that medical science had evolved beyond "scalpels or leeches" when the latter failed to recognize a thrombic modulator. (VOY: "Message in a Bottle") In 2377, he compared the primitive technology on Dinaali hospital ship 4-2 to leeches. (VOY: "Critical Care")