CLONORCHIS SINENSIS: CHARACTERISTICS, MORPHOLOGY AND LIFE CYCLE - BIOLOGY - 2025

Clonorchis sinensis is the scientific name for the tissue / intestinal parasite called Chinese liver fluke. From the taxonomic point of view it belongs to the kingdom animalia, phylum platyhelminthes, class trematoda, subclass digenea, order plagiorchiida, family opisthorchiidae, genus clonorchis, species sinensis.

This parasite is considered a zoonosis because its evolutionary cycle does not consider the human as the main host, being able to complete its entire cycle without its participation. For this reason it is considered that man is accidentally infected.

By Banchob Sripa, Sasithorn Kaewkes, Paiboon Sithithaworn, Eimorn Mairiang, Thewarach Laha, Michael Smout, Chawalit Pairojkul, Vajaraphongsa Bhudhisawasdi, Smarn Tesana, Bandit Thinkamrop, Jeffrey M. Bethony, Alex Loukas & Paul J. Brindley, via Wikimedia

In addition, for this parasite to infect man, once it passes through their feces in the form of eggs, they are unable to directly infect another human, since it must first go through multiple complex stages of evolution within two intermediaries of aquatic life.

Clonorchia sinensis can reach humans through raw or undercooked food (fish) contaminated with metacercariae. The infection in man is called clonorchiasis and falls within the main foodborne trematodiasis.

Man is easily infected in those populations that have a habit of consuming raw freshwater fish meat, regardless of whether these were frozen, salted, smoked or prepared with pickled vinegar.

This has undoubtedly caused significant economic losses, mainly in the Asian continent, where the disease is limited, it is estimated that many disability-adjusted life years (DALYs) are lost each year.

Biological characteristics

The Chinese liver fluke (Clonorchis sinensis) is characterized by being a hermaphroditic trematode, that is, the adult worm has the ability to self-fertilize, since both sex organs are in the same individual, although sometimes cross fertilization occurs.

C. sinensis is considered an endoparasite because it lives in the bile ducts of the definitive host, which are generally domestic mammals such as rats, cats, dogs and pigs, and can also affect man.

The parasite can last from 20 to 50 years inside humans, staying alive in the body by feeding on the rich secretions of the mucosa of the bile ducts.

Another important characteristic is that its evolutionary cycle is complex, since it requires two intermediate hosts before it can infect the definitive host where the adult worm develops.

Morphology

They are located in the bile and feces of the infected mammal (definitive host). They have a cuboid shape, their size ranges from 26 to 30 µm long x 15 wide, they have a convex operculum through which the miracidium larvae come out, and a protrusion in the wide posterior pole that gives them an urn-like appearance. They are yellowish brown in color.

Larvae

The larval stage includes a continuous evolution of the parasite, which goes through several phases, which are miracidium, sporocyst, redia and cercaria.

Miracide

This larva hatches from the egg once inside the snail. It is oval in shape surrounded by cilia, which give it the ability to move.

Sporocyst or sporocyst

They are shaped like a sac where the redia will develop. It sticks to the snail's intestinal wall to absorb intraluminal nutrients.

Redia

This will continue its maturation process to give rise to approximately 250,000 cercariae.

Cercaria

They are shaped like tadpoles, with a head and a non-forked tail. Once it leaves the snail, it has 2 to 3 days to penetrate the second intermediate host (freshwater fish). If he does not succeed, he dies. Unlike other cercariae these cannot swim.

Metacercaria

The cyst is elliptical in shape and measures 0.16 to 0.20 mm. They have dark granules inside. The cyst forms within the second intermediate host.

Adult worm

This form of life develops in the definitive host from the metacercaria consumed in raw or semi-raw fish meat.

The adult worm is translucent, it can be 20 to 25 mm long and 3 to 5 mm wide. The shape of the worm is flattened similar to a leaf, being narrower in the front part and wider in the back part.

It has an oral and a ventral suction cup that function as a fixation organ. Their digestive tract is incomplete.

Much of your body is occupied by your reproductive system, which contains two deep globular testes and a single ovary.

Every day the adult hermaphroditic worm eliminates approximately 2000 eggs already embryonated in the bile duct, and through the bile reaches the feces where they are excreted into the environment.

Lifecycle

The life cycle begins when freshwater and slow-flowing water sources are contaminated with mammalian faeces mined from C. sinensis eggs.

These water sources can be rivers, lakes and streams, where intermediate hosts live.

The excreted eggs that contain inside the first larval stage (miracidium), are consumed by snails that can be of different genera and species, such as: Parafossarulus manchouricus, Alocinma longicornis, Bithynia fuchsianus, Melanoides tuberculata, Parafossarulus sinensis, Parafossarulus anomalospiralis, Semisulcospira cancellata, among others.

Egg hatching

The egg inside the snail hatches thanks to the snail's digestive enzymes, freeing the miracidium, which then moves inside it until it lodges in the hemocele and the digestive gland.

There it begins its evolution to the form of sporocysts, then it transforms in 17 days to a larva called redia and finally this originates a large number of cercariae.

Host death

The reproduction of the redias becomes so intense that it ends up killing the snail.

This is how the fences are free in the water. Then, since they are unable to swim, they hang head down on the surface of the water and drop to the bottom.

Later they rise again, repeating this movement until they find their second intermediate host, which is a freshwater fish.

Among the types of fish that can penetrate are Pseudorasbora parva, Ctenopharyngodon idellus, Cyprinus carpio, Hypophthalmichthys nobilis, Carassius auratus, among many others.

In reality, the number of freshwater fish genera and species that can be affected is quite high and most of them are traded as food in endemic areas.

It has also been known that some shrimp species can serve as secondary intermediate host.

Second host

Once the cercariae reach the second host, they only penetrate the head, freeing themselves from the tail. It becomes embedded in the muscle mass of the fish one hour after penetrating and in a period of approximately 20 days, they mature into the metacercaria form.

The infected fish or crustacean when eaten uncooked by a susceptible mammal will become infected with the metacercariae of C. sinensis.

The metacercaria enters the digestive system of the definitive host and the larva will be released in the duodenum, which will subsequently ascend in 1 or 2 days through the common bile duct, from there to the ramifications of the second order bile ducts and in 30 days mature into the adult worm stage, where they begin to lay 2,000 to 4,000 eggs per day.

The final hosts that serve as a reservoir can be domestic or wild animals, including dogs, cats, rats, pigs, weasels, badgers, among others.

Pathogeny

Damage from bile duct infection

The adult worm can settle in the bile ducts for years. When the infection is mild, it can go unnoticed, but when the parasite load is high, the presence of the adult C. sinensis worm can cause different types of damage.

The first is related to the physical obstruction that can generate stasis and gallstones, inflammation with epithelial hyperplasia, adenoma formation, and even fibrosis of the tissues that surround the bile ducts.

If the worms migrate to the pancreatic ducts, they can obstruct them and cause acute pancreatitis.

Preparation of metabolic products

The second way of causing damage has to do with the production of metabolic products, which promote prolonged inflammation, generating hepatobiliary abnormalities.

Accumulation of dead worms

The accumulation of dead worms in the lumen of the bile duct causes secondary bacterial cholangitis that results in complications such as: bacteremia, endotoxic shock and hypoglycemia.

Others

Also, C. sinensis has been linked as a risk factor for the development of a type of bile duct cancer (cholangiocarcinoma).

Likewise, the presence of cirrhosis and decreased liver function have been reported in this parasitosis, very similar to what occurs with infection with hepatitis B and C.

Therefore, the coinfection of C. sinensis with any of these pathogens will increase the risk of a second type of cancer (hepatocellular carcinoma).

That is why C. sinensis is classified as a group I biocarcinogen.

Symptoms of contagion

Sometimes parasitosis can go asymptomatic for long periods of time. Other people may manifest nonspecific symptoms such as fatigue, anorexia, nausea, vomiting, loose stools, intermittent diarrhea, weight loss, abdominal discomfort, epigastric pain, biliary inflammation, among others.

In more severe cases where the parasite load is higher, fever, chills, leukocytosis with eosinophilia, mild jaundice, portal cirrhosis syndrome, and hepatomegaly may appear.

Treatment

The drugs of choice are Praziquantel or Albendazole to treat Clonorchis sinensis infection.

Praziquantel

It is a derivative of pyrazinoisoquinoline. This medicine works by altering the permeability of calcium in the parasite's membrane, causing the paralysis and death of the adult worm, to then be expelled by the bile flow into the intestine and expelled through the feces.

The recommended dose is 25 mg / kg, 3 times at 5 hour intervals in a day.

The treatment success range is 83 to 85%.

Albendazole

Methyl 5- (propylthio) -2-benzimidazolecarbamate inhibits the polymerization and assembly of microtubules by binding to tubulin once the cells of the worm's integument and intestine have degenerated, which paralyzes and kills the worm.

In patients with body weight 60 kg or more, the dose is 400 mg twice a day, taken with meals.

In patients with body weight below 60 kg, the dose is 15 mg / kg / day in two divided doses. Take with meals. Important, do not exceed the maximum total daily dose of 800 mg.

28-day cycles should be performed followed by a 14-day rest period without the drug, for a total of 3 cycles.

The success rate is similar to praziquantel.

Diagnosis

The diagnostic test par excellence for detecting C. sinensis eggs is the serial stool examination, although duodenal aspirates can also be analyzed.

Care must be taken, since C. sinensis eggs are very similar to those of Opisthorchis, so special attention must be paid to their microscopic characteristics.

The ELISA and PCR test are also available to detect antigens or DNA respectively of C. sinensis eggs in the patient's feces.

All of these tests are only useful if the worms are alive, otherwise no eggs will be found in the stool.

As complementary laboratory tests, a complete hematology can be performed to detect leukocytosis with eosinophilia, and measure alkaline phosphatase, which is usually elevated.

Finally the computed tomography, as well as the liver ultrasound can reveal abnormal results.

epidemiology

The main endemic areas of this parasite include South China, Korea, Japan, Taiwan, the Vietnam River Valley and a part of Russia.

12.49 million people are infected with C. sinensis in western China, with Guangdong province having the highest prevalence with 16.4% infection.

The death rate is 1 in 5 cases.

Prevention

Prevention is summarized in the proper cooking of freshwater fish and the good disposition of excreta.

References

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