Fascioliasis in humans: the pathways of infection and the cycle of development

The main source of infestation for humans is sheep and cattle. A person becomes infected when drinking contaminated adolescaria water, eating vegetables and greens (usually watercress salad). Fasciolosis is widespread in almost all regions of the world, but more widely in countries of Asia, Africa and South America. In Europe, the largest number of cases of fascioliasis have been reported in Portugal and France. In some regions of Russia and in the CIS countries sporadic cases are recorded, occasional outbreaks sometimes occur in Central Asia and Transcaucasia.

F. Hepatica parasitizes the bile ducts of the liver of many herbivores and occasionally in humans. In a day, one individual lays an average of 25 thousand eggs. The main source of infection is invasive herbivorous agricultural animals, mainly cattle. Man plays a big role in the epidemic process of fascioliasis, since he is only an occasional host of the parasite.

Fasciola eggs in winter in reservoirs and pastures persist up to 2 years. The larvae of parasites in the body of the mollusks can overwinter and emerge from them in the early spring. Adolescaria at 100% relative humidity tolerate temperature fluctuations in the range from -18 to +42 ° C; in conditions of relative humidity of 25-30% they die at 36 ° C. In dry hay, the incrusted adolescaria can survive for several months, while in wet soil and water they remain viable up to 1 year.

A person becomes infected when eating wild plants (watercress watercress, kok-sagyz, wild onion, sorrel) growing in standing or slowly flowing water bodies, as well as in wet pastures where invasive mollusks can live. You can catch and when you drink water from contaminated by adolescaria water reservoirs or swimming in them, as well as when eating ordinary vegetable vegetables (salad, onions) for watering which used water from such sources. The peak of infection occurs in the summer months.

The cycle of development of fascioliasis

Together with feces, eggs enter the external environment. Their further development occurs in water. At a temperature of 20-30 ° C, miracidia develop in eggs in two weeks. At lower temperatures the development of eggs slows down, at a temperature above 40 ° C miracidia perish. In the dark, eggs develop faster, but miracidia do not come out of them. Life expectancy of miracidia in water is no more than 2-3 days. Having penetrated the internal organs of gastropods Lymnea truncatula and other species of Lymnea, miracidia are transformed into sporocysts, in which redias develop. In the redias, either the second generation of redids or the caudate cercariae are formed. They leave the mollusk 2-3 months after infection and within the next 8 hours are encysted, attached to aquatic plants or to a surface water film. The injected larva of the hepatic fluke - adolescaria - enters the gastrointestinal tract of the final host by drinking water from water bodies, eating vegetation from flood meadows, or using contaminated water for domestic purposes (washing fruits and vegetables, watering beds, etc.). Once in the intestine of the final host with food (water and water plants) or with water, the adolescaria is excised, and the larvae penetrate the intestinal wall, migrate into the abdominal cavity, and then through the capsule and the parenchyma of the liver into the bile ducts. The second way of migration is hematogenous, through the intestinal veins into the portal vein, and then into the bile ducts of the liver. From the moment adolescariae enters the organism of the definitive host before the development of the sexually mature stage, 3-4 months pass. The life span of fasciolae in the human body is 5 years or more.

Moving along the hepatic tissue, fasciolae damage capillaries, parenchyma, bile ducts. Shapes are formed, which later turn into fibrous cords. Sometimes fasciolae flow by current of blood into other organs, most often into the lungs, where they are encapsulated and perish, not reaching puberty. In addition, young fasciolae bring from the intestine into the liver a microflora that causes the decay of stagnant bile, which causes an intoxication of the body, the formation of microabscesses and micro-necrosis.

Fasciola, caused by Fasciola hepatica, is common in most countries of the world. Especially often it occurs in those regions of Europe, Middle East, South America and Australia, where livestock is developed. Most sporadic cases are recorded, but outbreaks in hundreds of people have been reported in France, Cuba, Iran and Chile. Outbreaks of fascioliasis in the Baltic States, Uzbekistan and Georgia are described. The largest known outbreak of fascioliasis occurred in Iran in 1989, when more than 10,000 people became ill, including about 4,000 children. In the last century in Peru, the schoolchildren's incidence in some villages reached 34%.

The most intensive foci associated with F. Gigantica are noted in Vietnam and in other countries of Southeast Asia, the Hawaiian Islands and in some countries of tropical Africa.