Infection with the carcinogenic human liver fluke, Opisthorchis viverrini

Abstract

Throughout Southeast Asia there is a strikingly high incidence of cholangiocarcinoma (CCA--hepatic cancer of the bile duct epithelium), particularly in people from rural settings in Laos and Northeast Thailand who are infected with the liver fluke, Opisthorchis viverrini, one of only three carcinogenic eukaryotic pathogens. More ubiquitous carcinogenic microbes, such as Helicobacter pylori, induce cancer in less than 1% of infected people, while as many as one-sixth of people with opisthorchiasis will develop CCA. The mechanisms by which O. viverrini causes cancer are multi-factorial, involving mechanical irritation from the activities and movements of the flukes, immunopathology, dietary nitrosamines and the secretion of parasite proteins that promote a tumourigenic environment. Genomic and proteomic studies of the liver fluke secretome have accelerated the discovery of parasite proteins with known/potential roles in pathogenesis and tumourigenesis, establishing a framework towards understanding, and ultimately preventing, the morbidity and mortality attributed to this highly carcinogenic parasite.

Figure 1. Images depicting cultural practices that result in liver fluke infection

A. Local creeks in North-East Thailand that harbour fish rife with O. viverrini metacercariae. B. Thai Farmer net fishing. C. Infected fish catch. D. Fermented fish that gives consumers the carcinogenic combination of O. viverrini and nitrosamines. E. The fermented fish dish, pla-ra.

Figure 2. Life Cycles of O. viverrini and C. sinensis.

Embryonated eggs are discharged in the biliary ducts and in the stools (stage 1). Eggs are ingested by a suitable snail intermediate host (2); there are more than 100 species of snails that can serve as intermediate hosts. Each egg releases a miracidium (2a), which morphs through several developmental stages (sporocyst [2b], redia [2c], to become a cercaria [2d]). Cercariae are released from the snail and after a short period as free-swimming larvae in fresh water, they penetrate the flesh of freshwater fishes, such as Cyclocheilichthys armatus or Puntius leiacanthus, where they encyst as metacercariae (3). Humans are infected through ingestion of undercooked, salted, pickled, or smoked freshwater fishes (4). After ingestion, the metacercariae excyst in the duodenum (5) and ascend the biliary tract through the Ampulla of Vater. Maturation to adulthood (right, O. viverrini; left, C. sinensis) takes approximately one month (6). The adult flukes reside in the small and medium-sized bile ducts. In addition to humans, carnivorous animals can serve as reservoir hosts .

Figure 3. Hypothesized pathways of pathogenesis of opisthorchiasis-associated cholangiocarcinoma

The liver fluke Opisthorchis viverrini induces damage to the bile duct tissue via at least three distinct pathways: (1) mechanical damage to biliary epithelia caused by feeding parasites (yellow arrows); (2) inflammation-induced immunopathology, particularly due to reactive oxygen intermediates (ROI) and nitric oxide (NO) (blue arrows); (3) direct effects of fluke secreted proteins on biliary epithelia including cell proliferation induced by parasite-derived growth factors (red arrows). These pathways converge to result in oxidative DNA damage and excessive, chronic cell proliferation. Damaged DNA/genes after successive replications become fixed, leading to malignant transformation of the transformed cholangiocytes. Adapted from .

Figure 4. Opisthorchis in a liver section

The ventral sucker (S) of the liver fluke (Ov) is attached to the bile duct epithelium (E) . Note the infiltration of eosinophils into the site of inflammation.

Figure 5. Immunolocalisation of Ov-GRN-1 in histological sections of adult O. viverrini in the bile ducts of experimentally infected hamsters

The left panel was probed with IgG purified from control normal mouse serum (NMS); the right panel was probed with anti-Ov-GRN-1 IgG. The bottom panel shows a liver section from an uninfected hamster that was probed with anti-Ov-GRN-1 IgG. All three sections were stained with peroxidase staining revealed as a brown/rust colored deposit and Mayer’s Haematoxylin counterstained the nuclei in blue. Red arrows highlight the regions within the O. viverrini parasite and bile duct tissue that stained positive for Ov-GRN-1 .