# Zoonotic human liver flukes, a type 1 biocarcinogen, in freshwater fishes: genetic analysis and confirmation of molluscan vectors and reservoir hosts in Bangladesh

## TL;DR

This study finds that human liver flukes are common in freshwater fish in Bangladesh, posing a health risk due to their presence in commonly consumed fish and their spread through intermediate hosts like snails and domestic animals.

## Contribution

The study confirms the presence of multiple zoonotic liver fluke species in Bangladeshi freshwater fish and identifies molluscan vectors and reservoir hosts using molecular techniques.

## Key findings

- Eleven species of freshwater fish were found infected with metacercariae of human liver flukes.
- Clonorchis sinensis had the highest infection rate among detected flukes.
- Bithynia snails and domestic animals like dogs, cats, and ducks were identified as reservoir hosts.

## Abstract

Opisthorchiid flukes, particularly Opisthorchis viverrini, Opisthorchis felineus, Clonorchis sinensis, and Metorchis spp. are the most common fish-borne zoonotic human liver flukes (hLFs). Liver fluke infections are more prevalent in resource-deprived and underprivileged areas. We herein estimated the prevalence of the metacercariae (MC) of major hLFs in common large freshwater fishes (lFWF) marketed for human consumption from some selected areas of Bangladesh along with detection of their molluscan vectors and reservoirs.

The current status of fish-borne zoonotic hLF infections in lFWF was investigated along with their molluscan vectors and mammalian reservoir hosts in Mymensingh and Kishoreganj in Bangladesh from July 2018–June 2022 using conventional and multiple molecular techniques, such as PCR, PCR-restriction fragment length polymorphism (RFLP), sequencing, and bioinformatic analyses. The infection rate of fishes was analyzed using the Z-test and the loads of MC were compared using the chi-squared (χ2) test.

The MC of C. sinensis, Opisthorchis spp., and Metorchis spp. were detected in 11 species of common and popular lFWF. In lFWF, the estimated prevalence was 18.7% and the mean load was 137.4 ± 149.8 MC per 100 g of fish. The prevalence was the highest (P < 0.05) in spotted snakehead fishes (Channa punctata, 63.6%). The highest rate of infection (P < 0.05) was observed with the MC of C. sinensis (11.8%). Metacercariae were almost equally (P > 0.05) distributed between the head and body of fishes. The infection rate was slightly higher in cultured (19.6%) fishes. The MC of C. sinensis, O. felineus, O. viverrini, and Metorchis orientalis in fishes were confirmed using PCR, PCR-RFLP and bioinformatics. The cercariae of opisthorchiid (Pleurolophocercus cercariae) flukes were only recovered from Bithynia spp. (3.9%, 42 out of 1089). The ova of hLFs from dogs (4.3%, 5 out of 116) and cats (6.0%, 6 out of 100), and adult flukes (M. orientalis) from ducks (41.1% 113 out of 275) were detected.

The MC of hLFs are highly prevalent in fresh water fishes in Bangladesh. Reservoir hosts, such as street dogs, cats, and ducks carried the patent infection, and residents of Bangladesh are at risk.

The online version contains supplementary material available at 10.1186/s40249-024-01209-0.

■ Human liver flukes are fish-borne zoonotic trematodes prevalent in poor population.

■ Eleven species of common fresh water fishes were infected with metacercariae.

■ Clonorchis sinensis, Opisthorchis felineus, O. viverrini and Metorchis orientalis were confirmed.

■ Pleurolophocercus cercariae were recovered from Bithynia spp.

■ Ova of human liver flukes from dogs, and cats, and adult flukes from ducks were detected.

The online version contains supplementary material available at 10.1186/s40249-024-01209-0.