Identification of an Opisthorchis viverrini-like liver fluke from Myanmar: Metacercariae recovered from snakehead fish and adults from an experimentally infected hamster

A novel fluke species, morphologically resembling Opisthorchis viverrini, was discovered in the Yangon Region, Myanmar. Metacercariae were found in the muscle tissue of 2 snakehead fish species, Mi<Channa lucius and Channa striata. The metacercariae were elliptical (average 165×122 μm, n=10), having 2 suckers and a large excretory bladder. Experimental infection of a hamster resulted in the recovery of adult flukes from the bile ducts at day 50 post-infection. Adult flukes were lanceolate, with dimensions of 3.8-6.0 (average 5.1) mm by 1.1-1.2 (1.2) mm (n=10). Morphological characteristics included an oral sucker slightly smaller than the ventral sucker, a long uterus not extending anterior to the ventral sucker, vitelline follicles reaching the anterior ovary level, and 2 deeply lobed testes (4 lobes anteriorly, 5 posteriorly). Eggs were ovoid to elliptical, measuring 26.3-27.5 (26.9) μm in length and 13.8-15.0 (14.6) μm in width (n=10). Despite the close morphological resemblance to O. viverrini, our specimens displayed significant differences, including a more spiraled seminal vesicle, a longer uterus occupying 50%-60% of the body length, a less distinct grouping of vitelline follicles (forming 7-8 groups on each side in O. viverrini), vitelline follicles restricted to the ovary level (beyond the posterior end of the ovary or at anterior testis level in O. viverrini), and smaller metacercariae. We tentatively categorize our specimens as an O. viverrini-like liver fluke potentially parasitizing mammals or birds, highlighting the need for further research. Rigorous molecular analyses and identification of natural definitive hosts are essential to clarify the taxonomic status of this novel parasite.