Opisthorchiasis: causes and pathogenesis

Causes of opisthorchiasis

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

The causative agent of the opisthorchiasis has a complex developmental cycle. In addition to the final, he has two intermediate and additional hosts. In definitive (basic) hosts, the helminth parasitizes in the sexually mature stage of its development. Of the bile ducts, gallbladder and ducts of the human pancreas and carnivorous mammals (cats, dogs, fox, arctic fox, sable, wolverine, domestic pig, etc.), parasite eggs along with bile enter the intestine and then enter the environment. Further development takes place in reservoirs where opisthores retain their viability up to 6 months and are swallowed by the first intermediate host, the freshwater mollusk of the genus Codiella. In the body of which there is a series of transformations: the miracidium emerges from the egg, forming a sporocyst, in which redias are formed. They produce a large number of larvae of the next stage (cercariae). The latter emerge from the mollusk and penetrate the muscles of the second intermediate host, the fish of the Cyprinidae family (Yaz, Siberian dace, tench, European roach, Chebak, reddish, sazan, carp, barbel, bream, guster, podost, asp, sting), where the cercariae turn in metacercariae, which after 6 weeks become invasive. Fish, struck by the metacercariae of the opisthorchus, is the source of infection of humans and many carnivores.

In the stomach and duodenum of the final host, excision of metacercariae occurs. Under the influence of gastric juice, fish tissue and connective tissue capsule are digested, and under the action of duodenal juice, the metacercaria is released from the inner shell. Possessing positive chemotaxis to bile, parasites find holes in the bile duct and through the common bile duct penetrate into the galleries and the gallbladder, and sometimes into the pancreas. After 3-4 weeks after infection, the helminths reach the mature state and after fertilization they start to secrete eggs. The lifespan of opisthorchs reaches 15-25 years.

O. Felineus eggs are stable in the environment: fresh water retains viability for about a year. Opisthorch larvae die when cooking fish in one piece after 20 minutes, in fish minced - 10 minutes after the beginning of boiling. When salting fish larvae die after 4-7 days. Hot smoking is disastrous for the pathogen, and the cold does not destroy it.

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Pathogenesis of the opisthorchiasis

After eating invasive fish, metacercariae enter the stomach and duodenum, and within 3-5 hours reach intrahepatic bile ducts - the place of their main habitat in the body of the final host. In 20-40% of infected individuals, opisthorchia is found in the ducts of the pancreas and gall bladder. In the process of migration and with further development, they secrete enzymes and metabolic products that exert a sensitizing and direct toxic effect on the body.

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

• The pathogenesis of the early stage is based on the toxic-allergic reactions of the body to the metabolites released by the larvae during their migration and maturation, as well as to the antigens of the latter. In this stage, an increased permeability of the blood vessels of the liver and pancreas is observed; productive vasculitis; eosinophilic infiltration of the stroma of organs, their swelling; proliferation and desquamation of the epithelium of bile ducts. In the digestive tract (in the duodenum, liver, lungs, etc.) eosinophilic infiltrates are formed.
• In the chronic stage, toxicological and allergic reactions persist, but the main pathological changes are due to the vital activity of the opisthorchus, which, with its suckers and spines, exert an irritating and damaging effect on the wall of the bile and pancreatic ducts, the gallbladder, causing inflammatory and regenerative hyperplastic reaction with the development of cholangitis and pericholangitis, leading to fibrosis of the organs. Accumulations of parasites and their eggs slow 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, promote the attachment of bacterial infection and the formation of concrements in the bile duct and duct of the pancreas. Long-term infestation may result in cirrhosis of the liver. Often it is accompanied by gastroduodenitis (up to erosive-ulcerative).

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

The early immune response in opisthorchiasis is accompanied by an increase in the level of total IgM by a factor of 10-12 with a peak at week 2-3 and a decrease in their concentration at 6-8 weeks, when an increase in IgG is noted. Further, the concentration of antibodies falls below threshold values, which creates conditions for re-invasion and prolonged parasitization of opisthorchms in the body. Immunosuppression, accompanying the invasion, reduces the resistance to other infections, contributes to the severe course of shigellosis and other intestinal infections, often provokes chronic bacteriocarrier in patients with typhoid fever, weighed down the course of viral hepatitis with severe cholestasis, frequent exacerbations and relapses.