Every year, tens of millions of people worldwide (particularly in tropical Asia) suffer the effects of clonorchiasis and opisthorchiasis, conditions caused by infections with liver flukes of the family Opisthorchiidae. Exactly which condition the victim is suffering from depends on just which species of flukes they find themselves infected with, but there is little immediate difference between the clinical symptoms of either. Issues arising from clonorchiasis include fever, jaundice, diarrhoea and malnutrition. Long-term or heavy infections may result in cirrhosis, pancreatitis or even cancer (King & Scholz 2001). But just what is responsible for these debilitating illnesses?
Flukes are a diverse group of endoparasitic flatworms that reach maturity in association with vertebrates. As with other parasite lineages, different fluke species prefer different hosts and infect different parts of the host's system. Many have complex life cycles involving multiple larval stages and the successive infection of up to three distinct hosts on the way to maturity. Opisthorchiidae have such a three-host life cycle; their adult (or 'definitive') hosts span the gamut of vertebrates from fish to birds to mammals. Opisthorchiids in the strict sense are invariably associated with the liver of these hosts, taking up residence in the bile duct and gall bladder (however, phylogenetic studies have indicated that the closely related Heterophyidae, which infect the intestine, are probably paraphyletic with regard to opisthorchiids and the two families may be merged into an expanded Opisthorchiidae—Thaenkham et al. 2012). When mature they are elongate and flattened with the mouth near the front of the body surrounded by a sucker for attachment to host tissue. A second sucker is present on the underside of the body not too far behind the first (Dawes 1956).
Like other internal parasites, liver flukes are incredibly fecund. A female of Clonorchis sinensis, one of the main opisthorchiid species of concern to humans (yes, flukes reproduce sexually; I'll allow a moment for the disgusting implications to fully sink in), may produce up to 4000 eggs in a single day. These eggs are released into the host's digestive system, passing out in the faeces. They do not hatch until after they are ingested by the first larval host, an aquatic snail (many sources will say a freshwater snail but at least one opisthorchiid genus, Delphinicola, paratises marine dolphins so presumably has a correspondingly marine gastropod host). The egg hatches into a ciliated larva called a miracidium that over the course of the next few hours will find a likely spot in the snail's gut to develop into the next larval stage, the sporocyst. The sporocyst is immobile and mouthless, and feeds by absorbing nutrients directly from the host tissue. It also contains a mass of germ tissue that develops into multiple individuals of the next larval stage, the redia, that are released from the parent sporocyst after a couple of weeks or so. The rediae are worm-like and mobile, chomping their way through suitable sections of host tissue. They also develop multiple individuals of the next stage within them just as the sporocysts did. In this way, a single egg may eventually result in an exponentially increased number of larvae.
The next larval stage is called the cercaria. In opisthorchiids, the cercariae look a bit like tadpoles with a dorsoventrally finned tail. I haven't found exactly how opisthorchiid cercariae are released into the water column but in other flukes they may be released with the discharge from the abcess or cyst that forms as the rediae feed on their host, or escape from the host tissue after the snail dies as a result of its infection. The cercaria is a dispersive stage that seeks out the next host in the life cycle. This they do by hanging head-down in the water column and allowing themselves to slowly sink until disturbed by contact with a potential host or water-currents created by its movement. At this point the cercaria rapidly swims upwards before allowing itself to sink again, hopefully onto the new hosts skin. The cercaria will then dig its way into the host's muscle tissue and transform into the last larval stage, a cyst called a metacercaria. Opisthorchiid cercariae most commonly attach themselves to some kind of fish but they are a bit less picky about their host than the other stages in their life cycle; opisthorchiid metacercariae have also been found in crustaceans and have been shown in the laboratory to even be capable of infecting mammals (specifically guinea pigs).
The developing liver fluke reaches its definitive host when the second larval host is eaten. A young fluke hatches from the metacercaria inside the definitive host's gut and make their way to the liver which they find by detecting the traces of its chemical products and/or by detecting the physical track of the bile duct. There they will mature into fully adult flukes, all ready to begin the cycle again (by doing the nasty in some poor sod's gall bladder).
The economic impact of opisthorchiids around the world is estimated to amount to hundreds of millions of dollars each year. Unfortunately, as with many other illnesses more widespread in developing nations, there still remains a lot to be learned about their control. Cooking fish before consumption to kill metacercariae is one of the more obvious methods, though it should be noted that metacercariae can be devillishly difficult buggers to kill. Installation of sanitation and sewerage systems can also help by reducing the chance of egg-carrying faeces to make it into water bodies, though medically significant opisthorchiids may also infect animals other than humans such as cats, dogs or pigs. For now, it looks like liver flukes will be with us for some time.
Dawes, B. 1956. The Trematoda, with special reference to British and other European forms. University Press: Cambridge.
King, S., & T. Scholz. 2001. Trematodes of the family Opisthorchiidae: a minireview. Korean Journal of Parasitology 39 (3): 209–221.
Thaenkham, U., D. Blair, Y. Nawa & J. Waikagul. 2012. Families Opisthorchiidae and Heterophyidae: are they distinct? Parasitology International 61: 90–93.