New Species of Leech Found Near Washington, DC

There’s a new bloodsucker in town — and it has been lurking in plain sight for decades.

Scientists have discovered a previously unrecognized species of medicinal leech from specimens collected in southern Maryland, just less than 50 miles from the National Museum of Natural History in Washington, DC — one of the world’s largest libraries of biodiversity.

The discovery prompted Smithsonian scientists to search through marshes and museum collections that ultimately revealed that the leech, now named named Macrobdella mimicus, has long occupied a range that stretches throughout the Piedmont region of the eastern United States, between the Appalachian Mountains and the Atlantic Coast, according to a press release.

“A discovery like this makes clear just how much diversity is out there remaining to be discovered and documented, even right under scientists’ noses,” said Anna Phillips, the museum’s curator of parasitic worms and lead author of an article describing the new species published in the Journal of Parasitology.

Leeches are parasitic worms, many of which feed on the blood of their hosts. In the 1700s and 1800s, physicians used leeches to treat a wide range of ailments, believing that by ridding a patient’s body of bad blood, the parasites could cure headaches, fevers and other conditions, according to the Smithsonian. Any leech that readily feeds on humans is considered a medicinal leech, although in North America most leeches used for bloodletting were imported from Europe, leaving native species relatively undisturbed.

During a 2015 field expedition to the explore the diversity of medicinal leeches in North America, Phillips had collected several orange-spotted, olive-green leech specimens from a Maryland swamp. She and her team assumed they belonged to a familiar species called M. decora, a leech that is thought to live throughout a large swath of the northern United States. But DNA sequencing revealed otherwise.

A team of scientists led by Anna Phillips (pictured), the Smithsonian’s curator of parasitic worms, describe the Macrobdella mimicus in the Journal of Parasitology. (Photo Credit: Paul Fetters for Smithsonian)

Examining the specimens’ genomes at key regions used for species identification, Ricardo Salas-Montiel, a graduate student at the National Autonomous University of Mexico, found significant differences from the DNA of M. decora, the Smithsonian said in the statement.

Aside from the molecular discrepancy, scientists also found a physical difference that distinguished them from M. decora. “Like M. decora, the new leeches have multiple reproductive pores along the bottom of their bodies, known as gonopores and accessory pores. In the new leeches, the gonorpores and accessory pores were located in a different position relative to each other,” the Smithsonian said.

Another expedition led to the team finding more leeches from South Carolina that shared the same accessory pore positioning. “Then we sequenced [their DNA], and they all came out more closely related to the leeches we had found in Maryland than to anything else known to science,” Phillips said.

Phillips quickly retrieved dozens of North American leeches stored in the Smithsonian’s parasite collection and examined their accessory pores. “All of a sudden, I started finding these things everywhere,” she said.

The National Museum of Natural History in Washington, DC. (Photo Credit: George Rose / Getty Images)

Leeches with the unique pore positioning had been found in locations from northern Georgia to Long Island and preserved in the museum’s collection for years. The oldest, Phillips said, dated back to 1937.

Phillips also scoured parasite collections at the North Carolina Museum of Natural Sciences and the Virginia Museum of Natural History, pinpointing additional places where the leech had been found in the past. She and her team also found fresh specimens in Georgia and North Carolina and used DNA sequencing to confirm their close relationship to the others.

The historical record from the museums’ collections, with specimens spanning 63 years, adds another critical layer of information, confirming that the species was not recently introduced to the area and does not represent a newly evolved species.

“It’s been here this whole time,” Phillips said. “We just hadn’t looked at it in this new way.”

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