The genus Theromyzon includes free-living leeches that search for bird hosts using preen-gland secretions and a heat gradient Sawyer 1972, 1986). The reports of Bartonek & Trauger (1975) and Trauger & Bartonek (1977) may have involved a species other than T. rude. The latter authors reported Theromyzon rude and Placabdella ornata parasitizing high percentages of hundreds of individuals of various species of ducks (e.g. 36% of 396 Lesser Scaup examined) in in a study area north of Yellowknife. The leeches attached to the inside of the nasal chamber, to the eyes beneath the nictitating membrane, and to parts of the body (legs, feet and cloaca). The leeches become so engorged with blood that they extrude from the nasal cavities through the nares (nostrils) on top of the bill and are clearly visible with binoculars at a distance (Trauger & Bartonek 1977, Fig. 3). It was later indicated that more species of waterfowl had been reported as parasitized by the Theromyzon leeches in NWT than anywhere else in North America (Trauger & Bartonek 1977).