What is the life cycle of the hepatic trematode? This flatworm belongs to the parasites of the trematode family, which cause a liver disease such as fascioliasis. The disease is the result of a large number of immature helminths entering the liver or bile ducts. Intermediate owners can be all herbivores, as a rule, this applies to cattle. The most pathogenic parasite is for sheep. The life cycle of the hepatic trematode in comparison with other helminths is a rather complicated process, where one of the intermediate carriers can be a water snail.
Liver fluke cycle
The life of the parasite begins from the moment when the female lays eggs in the liver of an infected person, which are discharged in the biliary tract and excreted naturally. Once in water, they become embryos from which larvae develop. The life cycle of the hepatic trematode at this stage includes the search for an intermediate host in the form of a water snail. Having found a suitable carrier, the larva develops into cercaria, for which the presence of a large tail is a characteristic feature. Under optimal conditions, the development process in the cochlea can be completed in a period of 5 to 7 weeks.
When exposed to aquatic vegetation, the parasite forms cysts called metacercariae, which, unlike cercariae, have rigid outer walls of the cyst and can survive for long periods in a humid environment. When using raw freshwater plants, they can enter the human or animal body. Through the wall of the intestine, just as a
bovine tapeworm does
, the hepatic trematode enters the abdominal cavity. Having reached the liver, he begins to feed on its cells.
This occurs only a few days after the first contact with the parasite. Usually, the larva spends several weeks on food, after which it moves to the bile ducts, where further development of the hepatic trematode occurs until complete adulthood. In time, this can take about three months. Adults grow to about 3 cm in length and 1 cm in width. Mature females can produce up to 25,000 eggs per day.
The parasite infects the liver
An early stage of infection can be diagnosed by making a blood test for antibodies. In the chronic stage of fascioliasis in the large bile ducts, inflammation of the liver and obstruction of the bile fluid occurs. What effect does the life cycle of the hepatic trematode have on the human body? The larval migration time is considered the acute phase of the disease, which lasts several weeks and may include the following symptoms: diarrhea, eosinophilia, fever, nausea, abdominal pain, and vomiting. Chronic fascioliasis is characterized by the progressive development of symptoms such as anemia (a decrease in the number of red blood cells), swelling due to excess fluid, indigestion in the form of diarrhea or constipation, and cachexia (exhaustion, weight loss, fatigue, weakness, loss of appetite, and so on).
Risk groups
The life cycle of the hepatic trematode involves the presence of several intermediate carriers. At risk are cattle, sheep, goats, horses, dogs, cats, as well as people. Representatives of flat parasitic worms are found all over the world, most often their habitat is humid areas with a temperate climate. Hepatic trematode is one of the most common and harmful helminths affecting the liver, usually grazing ruminants. In endemic regions, the degree of infection increases; deadly infection is for sheep. The prevalence and incidence rate in a particular region strongly depends on environmental and climatic conditions, for example, the habitat of intermediate hosts and wild mammals, wintering parasites in the environment and others. Horses, dogs, and cats can also be infected, especially in rural areas.
Danger to humans
Can a host of hepatic trematode in the form of, for example, an infected cow, become a source for human infection? Usually not. A sick animal is not able to infect humans, either directly through contact, or by eating meat, milk or blood, or through feces. But experimental studies show that people can become infected by eating a raw liver, provided that the tissues are infected with parasites at an immature stage of development. The same goes for pets. Usually people become infected by swallowing water contaminated with cercariae. It is generally accepted that adult helminths and their eggs are not dangerous to humans.
Hepatic fluke: structure
Adults have a flat body of oval shape from 3 cm long and 1.5 cm wide. The color of the parasite varies from pink-gray to dark red. There are two suction cups on the ventral side. The surface of the body is covered with numerous spikes. External signs of segmentation are absent. The mouth ends in the throat, which is a muscle tube with the ability to absorb.
The digestive system is one-sided, without anus, that is, the only opening is the mouth), but branched, ending in several blind ducts. As for the reproductive function, the development of hepatic trematode involves the formation of genitals of both sexes, which means that the parasites are hermaphrodites, that is, they have male and female genital organs. The size of the eggs of the worms is approximately 80 by 140 micrometers, mainly oval and yellowish or greenish in color.
Harm to the body
The hepatic trematode damages the bile ducts, causing serious harm to its owners. Young parasites migrate through the liver tissue, destroy them and cause bleeding. Small spines on the surface of the worms irritate the tissues and lead to their inflammation. As a result of this, cell death and fibrosis occur, that is, excessive formation of connective tissue, which replaces dead liver cells, thereby impairing the normal functioning of the organ.
The liver greatly increases in size and becomes very sensitive. Some flukes can cause walnut-sized cysts. The bile ducts also increase. All this leads to general intoxication of the body and a decrease in immunity, which, in turn, causes infection with secondary bacteria, mainly due to the general weakness of the host.
Symptoms and diagnosis
The infection has no characteristic and easily recognizable signs. The main symptoms are associated with inflammation of the liver (hepatitis) and bile ducts (cholangitis), which can be caused by completely different reasons. Other vital organs are usually not affected. Diagnosis of the disease is complicated by the fact that even the presence of several tens of adult flukes in the body may not cause acute clinical signs, except for general weakness and decreased productivity. The diagnosis can be confirmed by the detection of helminth eggs in the feces.
Geography of the parasite
To successfully complete the life cycle of the hepatic trematode, at least two hosts are needed. The role of one of them is played by freshwater snail species. The choice of secondary carrier is very wide, it includes many herbivorous mammals, including humans. Having spread to other continents from Europe by exporting livestock, this viable parasite has adapted to new hosts such as camels in Africa and South America and marsupials in Australia. The hepatic trematode easily adapts to the new geographical environment, finding intermediate hosts, including plant origin, in new areas.