tags: evolutionary biology, evolutionary biogeography, molecular biology, medicine, ectoparasite, orificial hirudiniasis, mucosal leech infestation, hirudinoids, leech, Tyrannobdella rex, public health, zoology, PLoS ONE, anatomy, phylogenetic analysis, taxonomy, bpr3.org/?p=52,peer-reviewed research, journal club
Figure 1. Mucosally invasive hirudinoid leeches. Known from a wide variety of anatomical sites including eyes (A) as in this case involving Dinobdella ferox (B), mucosal leech species, as in a case involving Myxobdella annandalei (C), more frequently feed from the nasopharyngeal surfaces of mammals (D). [larger (and more delightfully repulsive) picture.]
Most people are repulsed by leeches -- those spineless blood sucking animals that are not only ugly, but can, in extreme cases, pose a threat to the host's life. But most people are blissfully unaware that some species of leeches specialize in attacking mammalian mucous membranes -- those hairless, smooth and moist tissues that line the mouth, intestines, eyes and urinary and reproductive tracts (Figure 1).
As if most people don't have enough blood-suckers in their lives, a new species of mucous-membrane infesting leech was discovered in the nostril of a 9-year-old girl. She frequently bathed in lakes, rivers and streams in the Amazonian part of Peru and was distressed when she felt "a sliding sensation" in the back of her nose.
The girl's physician, Renzo Arauco-Brown, at the School of Medicine at the Universidad Peruana Cayetano Heredia in Lima, removed the leech and sent it to Mark Siddall, a leech expert and curator of Invertebrate Zoology at the American Museum of Natural History in New York City. Despite careful study, Dr Siddall and his colleagues were unable to place this specimen into any of the known leech families.
However, they did note that the specimen had eight very large teeth embedded in its jaw (Figure 2);
Figure 2. Comparative jaw morphology of Tyrannobdella rex. (A) Stereomicrograph of the single dorsal jaw of T. rex with large teeth. Scale bar is 100 µm. (B) Tyrannobdella rex anterior sucker exhibiting velar mouth and longitudinal slit through which the dorsal jaw protrudes when feeding. Scale bar is 1 mm. (C) Compound micrograph in lateral view of eight large teeth of T. rex. Scale bar is 100 µm. (D) Lateral view of jaw of Limnatis paluda illustrating typical size of hirudinoid teeth. Scale bar is 100 µm. [larger view].
Unlike any other leech species known to science so far, this new species is also unique because it has only one jaw. But possessing only one jaw apparently doesn't hamper its blood-sucking abilities.
"It uses it like a saw," said Dr Siddall. "It doesn't need a huge wound, because it has incredible suction power to get blood."
Embedded in that single jaw are eight very large teeth. Although its teeth are not more than 130 microns tall (roughly the width of a human hair), "that's at least five times as high as that of other leeches," explained Dr Siddall. "We named it Tyrannobdella rex because of its enormous teeth."
Tyrannobdella rex is Latin for "tyrant leech king."
Since researchers and medical doctors were alerted to this previously unidentified leech, two more cases were re-discovered, also from 1997, from different clinics in the western Amazon.
Unlike many leeches, which latch onto bodies from the outside, T. rex attaches itself to the mucous membranes inside the nose of its victims. The victims -- all young children (so far) -- described parasitization by this aptly-named new species as being quite painful.
"Every one of the people who were found with these in the clinical cases had a frontal headache," remarked Dr Siddall. "Their teeth are big, and these things hurt."
Hirudiniasis -- the habit of particular leech species for invading an orifice and feeding on mucous membranes -- has been reported for a variety of leech species in the Middle East, Africa and Asia. Because of this huge geographic range, scientists originally regarded this feeding habit "only as a loathsome oddity and not a unifying character for a group of related organisms," according to the research team. However, a closer look at the morphology, anatomy as well as the genetics reveals similarities between these widely distributed animals.
The morphology and anatomy of T. rex reveals it is less than 5 centimeters (two inches) long, and unlike most leeches, which seem to be bags of gametes, T. rex has extremely small genitalia (Figure 3);
Figure 3. Comparative internal and external anatomy of Tyrannobdella rex. (A) Whole body ventral view illustrating annulation, relative size of the caudal sucker and relative position of gonopores. (B) Eyespot arrangement illustrated dorsally. (C) Male and female median reproductive anatomy. [larger view].
"The width of the adult leech is about 1 centimeter, or about the width of my pinkie, while the genitalia are about one-tenth to one-fifth the width of a millimeter, or 100 to 200 microns," Dr Siddall explained. "The diameter of a blood cell is only about seven microns."
Several morphological and anatomical characters suggest that all of the leeches that specialize in attaching to mucosal membranes to feed on the host's blood are closely related.
Phylogenetic analyses of several genes further provide strong evidence for the close relationships between T. rex and the other mucous-membrane feeding leeches. This family tree indicates these leeches actually belong to a single group (Figure 4);
Figure 4. Single most parsimonious tree based on combined 18S rDNA, 28s rDNA, 12s rDNA, and COI datasets. The family Praobdellidae formed a well-supported monophyletic group of leeches that exhibits a predilection for mammalian mucosa. All groups received 100 percent bootstrap support and posterior probabilities of 1.00 except as noted on the tree. Branches are drawn proportional to amount of change. [larger view].
T. rex is most closely related to the Mexican leech, Pintobdella chiapasensis, which parasitizes the nasal passages of tapirs. Another close relative, known as the Terrible Ferocious Leech, Dinobdella ferox, feeds on mucous membranes in the rectum, vagina or upper airway in humans and other mammals.
The regular host for this particular T. rex remains unknown.
"We think the leech could feed on aquatic mammals, from their noses and mouths for example, where they could stay for weeks at a time," said graduate student Anna Phillips, first author on this paper who led the study.
Interestingly, unlike the newly described Peruvian T. rex and its better-known Mexican sister species, P. chiapasensis, D. ferox is an Old World leech found in throughout India and Taiwan. This tremendous geographic separation indicates that the common ancestor for all leeches evolved when all the world's continents were united into a single very large landmass of Pangaea before the supercontinent broke up, the researchers said.
"The earliest species in this family of these leeches no doubt shared an environment with dinosaurs about 200 million years ago when some ancestor of our T. rex may have been up that other T. rex's nose," remarked Dr Siddall.
There are between 600 and 700 species of leeches known to science, but Dr Siddall and his colleagues think there may be as many as 10,000 species throughout the world.
"Last week, we were up to our necks in water in Peru trying to find more," remarked the brave Dr Siddall.
Phillips, A., Arauco-Brown, R., Oceguera-Figueroa, A., Gomez, G., Beltrán, M., Lai, Y., & Siddall, M. (2010). Tyrannobdella rex N. Gen. N. Sp. and the Evolutionary Origins of Mucosal Leech Infestations. PLoS ONE, 5 (4) DOI: 10.1371/journal.pone.0010057