Opisthorchiasis - Causes and Pathogenesis

Causes of opisthorchiasis

The cause of opisthorchiasis is Opistorchis felineus (cat fluke) which belongs to the type of flatworms (trematodes), the class of flukes. It has a flat elongated body 8-14 mm long and 1-3.5 mm in diameter; it is equipped with two suckers - oral and abdominal. Opisthorchis are hermaphrodites. The eggs are pale yellow, almost colorless, with a smooth double-contour shell that has a lid on a slightly narrowed pole and a small thickening at the opposite end. The size of the eggs is 23-24x11-19 microns.

The causative agent of opisthorchiasis has a complex development cycle. In addition to the final hosts, it has two intermediate and an additional host. In definitive (main) hosts, the helminth parasitizes in the sexually mature stage of its development. From the bile ducts, gall bladder and pancreatic ducts of humans and carnivorous mammals (cats, dogs, foxes, arctic foxes, sables, wolverines, domestic pigs, etc.), the parasite eggs penetrate the intestines along with bile and then enter the environment. Further development takes place in water bodies, where opisthorchiasis remains viable for up to 6 months and is swallowed by the first intermediate host - a freshwater mollusk of the genus Codiella. In the body of which a number of transformations occur: a miracidium emerges from the egg, forming a sporocyst in which rediae are formed. giving birth to a large number of larvae of the next stage (cercariae). The latter leave the mollusk and penetrate the muscles of the second intermediate host - fish of the carp family (ide, Siberian dace, tench, European roach, chub, rudd, carp, carp, barbel, bream, white bream, chub, asp, bleak), where the cercariae turn into metacercariae, which become invasive after 6 weeks. Fish infected with opisthorchis metacercariae are a source of infection for humans and many carnivorous animals.

In the stomach and duodenum of the final host, metacercariae are excysed. Under the action of gastric juice, the fish tissue and connective tissue capsule are digested, and under the action of duodenal juice, the metacercariae are released from the inner shell. Having a positive chemotaxis to bile, the parasites find the openings of the bile duct and through the common bile duct penetrate the bile ducts and gall bladder, and sometimes the pancreas. After 3-4 weeks after infection, the helminths reach sexual maturity and after fertilization begin to release eggs. The life expectancy of opisthorchiasis reaches 15-25 years.

O. felineus eggs are stable in the environment: they remain viable for about a year in fresh water. Opisthorchis larvae die when whole fish is boiled after 20 minutes, and in minced fish - after 10 minutes from the start of boiling. When salting fish, the larvae die after 4-7 days. Hot smoking is fatal for the pathogen, but cold smoking does not destroy it.

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Pathogenesis of opisthorchiasis

After eating the infested fish, the metacercariae enter the stomach and duodenum, and after 3-5 hours they reach the intrahepatic bile ducts - their main habitat in the body of the final host. In 20-40% of infected individuals, opisthorchiasis is found in the pancreatic ducts and gall bladder. During migration and further development, they secrete enzymes and metabolic products that have a sensitizing and direct toxic effect on the body.

In the dynamics of the invasive process in opisthorchiasis, two phases are distinguished - early (acute) and late (chronic).

• The pathogenesis of the early stage is based on toxic-allergic reactions of the body to metabolites secreted by the larvae during their migration and maturation, as well as to the antigens of the latter. At this stage, increased permeability of the vessels of the liver and pancreas; productive vasculitis; eosinophilic infiltration of the stroma of organs, their edema; proliferation and desquamation of the epithelium of the bile ducts. Eosinophilic infiltrates are formed in the gastrointestinal tract (in the duodenum, liver, lungs, etc.).
• In the chronic stage, toxic-allergic reactions persist, but the main pathological changes are caused by the activity of opisthorchiasis, which with their suckers and spines have an irritating and damaging effect on the wall of the bile and pancreatic ducts, gall bladder, causing an inflammatory and regenerative-hyperplastic reaction with the development of cholangitis and pericholangitis, leading to fibrosis of the organs. Clusters of parasites and their eggs slow down the flow of bile and pancreatic juice. Hyperplastic and inflammatory processes lead to the development of strictures in the terminal part of the common bile and cystic duct, contribute to the addition of bacterial infection and the formation of stones in the bile ducts and pancreatic duct. Long-term invasion can end in cirrhosis of the liver. It is often accompanied by gastroduodenitis (even erosive-ulcerative).

Proliferative processes in opisthorchiasis, considered a precancerous condition, in combination with the action of exogenous carcinogens can lead to the development of cholangiocarcinoma. In Western Siberia, where the level of opisthorchiasis prevalence is high, the incidence of cholangiocarcinoma is 10-15 times higher than in other populations.

The early immune response to opisthorchiasis is accompanied by a 10-12-fold increase in the level of total IgM with a maximum at 2-3 weeks and a decrease in their concentration after 6-8 weeks, when an increase in the IgG content is noted. Subsequently, the concentration of antibodies falls below threshold values, which creates conditions for reinvasion and long-term parasitism of opisthorchiasis in the body. Immunosuppression accompanying the invasion reduces resistance to other infections, contributes to the severe course of shigellosis and other intestinal infections, often provokes chronic carriage of bacteria in patients with typhoid fever, aggravates the course of viral hepatitis with severe cholestasis, frequent exacerbations and relapses.