Digenean trematodes in the strigeid genus Nematostrigea are parasites of various birds, most often ospreys (Pandion haliaetus). Nematostrigea currently contains 2 species, Nematostrigea serpens and Nematostigea hepatica. Nematostrigea serpens is divided into 2 subspecies: N. serpens serpens from the Palearctic and Indomalayan realms and N. serpens annulata from the Nearctic realm. In the present work, we studied the type material of N. serpens annulata and collected new specimens from an osprey in Wisconsin close to the type locality. The original description and illustration of N. serpens annulata were incomplete. Herein we provide a detailed redescription and new illustrations of N. serpens annulata on the basis of the type material as well as newly collected and sequenced specimens. Nematostrigea serpens annulata has a distinct tegumental thickening near the mid-length of the genital cone, which is absent in its congeners. Partial sequences of the large ribosomal subunit (28S) DNA and cytochrome c oxidase subunit 1 (COI) mitochondrial DNA gene sequences were generated from our specimens of N. serpens annulata. Comparisons with previously published sequences of N. serpens serpens from the Palearctic demonstrated 2.2% difference in 28S and 3.7% in COI. On the basis of morphological and molecular comparisons, we elevate N. serpens annulata to species status and recognize it as Nematostrigea annulata Dubois and Rausch, 1948.

鳖属的双殖吸虫是各种鸟类的寄生虫，最常见的是鱼鹰（Pandion haliaetus）。Nematostigea目前包含2种，Nematostigea serpens和Nematostigea hepatica。Nematostrigea serpens分为 2 个亚种：来自古北界和印度马拉雅地区的N. serpens serpens和来自近北界的N. serpens annulata 。在目前的工作中，我们研究了N. serpens annulata的模式材料，并从威斯康星州靠近模式产地的一只鱼鹰中收集了新的标本。对N. serpens annulata的原始描述和插图并不完整。在此，我们根据模式材料以及新收集和测序的标本对N. serpens annulata进行了详细的重新描述和新插图。Nematostrigea serpens annulata在生殖锥中部附近有明显的皮膜增厚，这是其同类中所不存在的。大核糖体亚基 ( 28S ) DNA 和细胞色素c氧化酶亚基 1 ( COI ) 线粒体 DNA 基因序列的部分序列是从我们的环蛇蛇标本中产生的。与之前发表的古北界蛇蛇序列比较表明， 28S存在 2.2% 的差异， COI存在 3.7% 的差异。在形态学和分子比较的基础上，我们将N. serpens annulata提升到物种地位，并将其识别为Nematostrigea annulata Dubois 和 Rausch，1948。