This video from the Shedd Aquarium in the USA says about itself:
27 April 2015
Shedd researcher Solomon David took this video of aquatic leeches at an agricultural ditch-wetland junction near the Bay of Green Bay. According to him, “I’ve seen many fishes at this site over past years, but never leeches (here or anywhere else) in numbers like this. They were at this site for at least two days.”
Freshwater wetlands from Georgia to New York are home to a previously unrecognized species of medicinal leech, according to scientists. The new species was first identified from specimens collected in southern Maryland less than 50 miles from Washington, D.C., prompting a search through marshes and museum collections that revealed that the leech has long occupied a range that stretches throughout the eastern United States: here.