The genus Opisthorchis consists of 53 nominal species, including two species pathogenic to humans – O. viverrini and O. felineus. Of these, 30 species are recorded as the parasites of birds, and the remaining species are parasites of mammals, reptiles, fish and molluscs (https://insects.tamu.edu/research/collection/hallan/test/Platyhelminthes/Family/Opisthorchiidae.txt). Among Opisthorchis species, O. viverrini is the most well-known species because of its close association with cholangiocarcinoma in humans (Mayer & Fried, Reference Mayer and Fried2007; Sripa et al., Reference Sripa, Brindley, Mulvenna, Laha, Smout, Mairiang, Bethony and Loukas2012). Cats, dogs and various fish-eating mammals, including humans, are the definitive hosts of O. viverrini (Kaewkes, Reference Kaewkes2003).

Recently, in the Journal of Helminthology, Dao et al. (Reference Dao, Nguyen, Victor, Gabriel and Dorny2013) reported Opisthorchis adult worms from the bile duct of domestic ducks (Anas platyrhynchos) and considered them as O. viverrini. The authors stated that Opisthorchis worms have never been reported in birds in Vietnam (Dao et al., Reference Dao, Nguyen, Victor, Gabriel and Dorny2013). However, the book Fauna of Vietnam, volume 8, written by Le (Reference Le2000), provided full information about three avian species of Opisthorchis – O. cheelis Lal, 1939, O. longissimus (Linstow, 1883) and O. parageminus Oshmarin, Reference Oshmarin, Oshmarin, Mamaev and Lebedev1970 – which were reported from birds in Vietnam. Among these, O. parageminus was reported first as the new Opisthorchis species found in domestic ducks (A. platyrhynchos) in Vietnam (Oshmarin, Reference Oshmarin, Oshmarin, Mamaev and Lebedev1970). Since Dao et al. (Reference Dao, Nguyen, Victor, Gabriel and Dorny2013) compared the morphology of their specimens with O. viverrini obtained from a cat in Vietnam (Le, Reference Le2000), the authors should have been aware of the presence of three avian Opisthorchis in Vietnam, which appeared in the same genus in the same reference book (Le, Reference Le2000).

When morphological and morphometric features of O. viverrini-like liver flukes from domestic ducks in Vietnam described by Dao et al. (Reference Dao, Nguyen, Victor, Gabriel and Dorny2013) were compared with those of O. viverrini, there are at least three differences between them: (1) O. viverrini has two deeply lobed testes, diagonally arranged and situated near the posterior extremity, whereas O. viverrini-like flukes of Dao et al. have two deeply lobed testes, which were much larger (two times) than those of O. viverrini and tandemly arranged near the posterior extremity; (2) the vitellaria of O. viverrini possess numerous follicles dispersed as several columnar groups in the lateral fields between ventral sucker and testes, whereas O. viverrini-like flukes of Dao et al. also have numerous follicles dispersed as several columnar groups in the lateral fields, which are longer than those of O. viverrini, close to or extending anteriorly to the ventral sucker; and (3) the excretory bladder of O. viverrini is a long, S-shaped sac-like tube running between the two testes, whereas that of O. viverrini-like flukes by Dao et al. possess a thin, straight tube-like excretory bladder located behind the testes. The morphological characteristics of O. viverrini-like liver flukes from ducks by Dao et al. (Reference Dao, Nguyen, Victor, Gabriel and Dorny2013) are more compatible with those of O. parageminus, which have also been collected from ducks from Vietnam (Oshmarin, Reference Oshmarin, Oshmarin, Mamaev and Lebedev1970; Le, Reference Le2000).

Moreover, in the phylogenetic trees of internal transcribed spacer 2 (ITS2) and cytochrome oxidase 1 (CO1) gene sequences provided by Dao et al. (Reference Dao, Nguyen, Victor, Gabriel and Dorny2013), the O. viverrini-like liver flukes from domestic ducks were closer to O. lobatus, which is an avian Opisthorchis (Thaenkham et al., Reference Thaenkham, Nuamtanong, Vonghachack, Yoonuan, Sanguankiat, Dekumyoy, Prommasack, Kobayashi and Waikagul2011), but were separated from O. viverrini by the presence of O. lobatus in both trees. Since Dao et al. (Reference Dao, Nguyen, Victor, Gabriel and Dorny2013) considered the specimens from ducks as O. viverrini in these phylogenetic trees, O. lobatus should also be included as the intra-species variant in the O. viverrini clade. It is regretful that the GenBank accession numbers for the ITS2 and CO1 sequence data of O. viverrini-like liver flukes of Dao et al. (Reference Dao, Nguyen, Victor, Gabriel and Dorny2013) are not available in the literature or in the DNA database. Access to the GenBank accession numbers of the ITS2 and CO1 sequence data of the isolates in question would also have been desirable.

To our knowledge, O. viverrini has never been found in birds. Since O. viverrini is a carcinogenic liver fluke, extreme caution is required to study its distribution in the intermediate and definitive hosts, especially in avian hosts such as ducks, or its geographical distribution. Whether O. viverrini can really parasitize domestic ducks could be confirmed by experimental infection of ducks with O. viverrini metacercariae from fish.