In a reaction to the paper ‘Opisthorchis viverrini-like liver fluke in birds from Vietnam: morphological variability and rDNA/mtDNA sequence confirmation’, by Dao et al. (Reference Dao, Nguyen, Victor, Gabriël and Dorny2013), Nawa, Doanh and Thaenkham comment on the validity of the conclusions of that paper and suggest that the flukes recovered from ducks in Central Vietnam are specimens of Opisthorchis parageminus.

Indeed, in the book Fauna of Vietnam. vol. 8, written in Vietnamese by Le (Reference Le2000), three avian Opisthorchis spp. were reported from ducks in North Vietnam, i.e. O. cheelis Lal, 1939, O. longissimus Linstow, 1883 and O. parageminus Oshmarin, Reference Oshmarin, Oshmarin, Mamaev and Lebedev1970. Consequently, we acknowledge the remark made by Nawa et al. (Reference Nawa, Doanh and Thaenkham2013) that our observation is not the first report of an Opisthorchis sp. in birds in Vietnam. However, it is the first report of an Opisthorchis sp. in South Vietnam that is part of the greater Mekong sub-region, which is endemic for O. viverrini, in contrast to North Vietnam.

Looking at the morphological description of O. parageminus by Le (Reference Le2000) and Oshmarin (Reference Oshmarin, Oshmarin, Mamaev and Lebedev1970) (the latter description was based on an earlier report by Le in 1968), there are some morphometric similarities with the liver flukes that we collected, i.e. the greater size of the testes and the length of the vitelline glands, but also some disparities in morphology. Le (Reference Le2000) describes the presence of spines on the cuticula of worms referred to as O. parageminus, while none of the specimens that we examined have spines. It would be interesting to perform polymerase chain reaction (PCR) and DNA sequencing analyses on morphologically identified O. parageminus samples to examine how they relate to O. viverrini (and other Opisthorchis spp. for that matter). However, no such data are currently available.

Referring to the phylogenetic trees of the partial sequences of ITS2 and CO1, Nawa et al. in their reaction contest our conclusion that our isolates be classified as O. viverrini-like because they also show close genetic resemblance to O. lobatus, an avian fluke. They even suggest including O. lobatus as an intra-species variant in the O. viverrini clade.

In our paper we point out some morphometric differences between our isolates from ducks and O. viverrini isolated from cat, human and hamster. We refer to reports that show the influence of host species on morphometric characteristics in other trematodes. We believe that the high identity of the ITS-2 partial sequence of our VN1 and VN2 isolates to O. viverrini justifies our conclusion to classify the duck flukes as O. viverrini-like.

Molecular studies, using DNA sequences, have not always simplified matters but have certainly highlighted taxonomic difficulties. It is clear that molecular studies are needed to confirm the validity of previously described Opisthorchis species that were based on morphological differences only. Genetic polymorphism has been demonstrated within O. viverrini (Sithithaworn et al., Reference Sithithaworn, Nuchjungreed, Srisawangwong, Ando, Petney, Chilton and Andrews2006) but has, until now, not been associated with morphological differences.

We acknowledge that exchange of sequence data (and isolates) between laboratories would be beneficial to this type of research. Therefore, we have now made available the ITS2 and CO1 sequence data of our isolates in GenBank, with accession numbers KF577570 and KF577571 for VN1 and VN2 (ITS2), respectively, and KF577572 and KF577573 for VN11 and VN22 (CO1), respectively. Additionally, we agree that there is a need for experimental infections in different host species to better understand the epidemiology of liver flukes in the greater Mekong region.