Metagonimiasis

One of the varieties of helminths that affect the small intestine of humans is the flatworm metagonimus, which belongs to the class of parasitic flukes (trematodes).

Epidemiology

It is widespread as an endemic or potentially endemic helminth in almost two dozen countries, primarily in East Asia (China, Japan, Korea, Indonesia), as well as in the Balkans, Spain and the Russian Far East.

Outside endemic areas, metagonimus can be acquired by eating undercooked contaminated fish imported from these areas.

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Causes metagonimus

All species of this parasite (M. yokagawai, M. takahashi, M. miyatai, M. ovatus, Heterophyes yokogawai, etc.) were discovered by Japanese and Korean researchers at about the same time (in 1912-1932) and are identical in their morphology, size (with minor variations), life cycle, and even geographic distribution of intermediate hosts. Therefore, in the International Code of Zoological Nomenclature (ICZN), they are recognized as synonyms, and the main species name is Metagonimus yokogawai.

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Pathogenesis

Metagonimus is a biohelminth, meaning that its life cycle requires an intermediate host, and this parasite has two. The first of these is freshwater gastropods - snails of species such as Semisucospira libertine, Scoreana, Tarebia lateritia and Thiara granifera, found from Madagascar to Hawaii, including most countries in Southeast Asia.

The second intermediate host is freshwater fish (trout, carp, crucian carp, etc.), and the final host (in whose intestines the parasite matures to the state of a sexually mature individual) is a person, as well as various predators and birds that feed on fish. The incubation period is about two weeks, while invasions in the metacercariae stage can persist in the small intestine for more than a year.

The structure of the metagonimus depends on the stage of its life cycle: egg – miracidium – sporocyst – redia – cercaria – metacercaria – adult (sexually mature) worm.

An adult Metagonimus has an elongated, leaf-shaped body (like most trematodes), 1-2.5 mm long and 0.4-0.75 mm wide. The body is filled with parenchyma and covered with a protective layer – the syncytial tegument, which performs secretory and absorption functions (the worm absorbs nutrients with its entire surface). The parasite attaches to the intestinal mucosa with suckers – ventral (deviated to the right of the midline) and oral (in the cranial region). A pair of ganglia in the anterior part of the body and sensory receptors serve as the worm's brain and nervous system.

This parasite is a hemaphrodite, meaning it reproduces by internal fertilization, for which it has ovaries, testicles, and a uterus filled with eggs, which is the worm's largest organ.

The eggs (26-28 microns long and 15-17 microns wide) laid in the intestines of the main host have a hard, transparent shell of a yellow-brown color. The eggs have a kind of "hatch" in the form of a clear split line between the body and the lid, which facilitates the exit of the first larval stage - miracidia. And this happens when the eggs are outside the host's body and end up in the water along with excrement.

From some of the eggs, while still in the water, oval active larvae of the metagonimus emerge – miracidia, capable of moving, as they are equipped with cilia (cilia); in addition, they have their own germ cells for further asexual reproduction, and for access to the body of the snails – a dense process-proboscis.

The eggs swallowed by the mollusks open in their stomach and intestines, releasing larvae, and the already free miracidia make their way into the liver of the snails with their proboscis. But not for food: they exist on a previously accumulated supply of glycogen. Their task is to remove the shell and move to the stage of a passive larva in the form of an elongated cavity. These are the mother sporocysts, in which daughter sporocysts - rediae (having muscles, a mouth and intestines) begin to develop from the germ cells.

In turn, rediae transform into cercariae, the active larval form. Since these larvae leave the mollusk and enter the water, the structure of the metagonimus at this stage allows it to move in search of a second intermediate host, a fish. In the gills and flesh of fish, the cercariae become dormant and form double-shelled cysts, encapsulating them into metacercariae. And this stage is infectious for the final host, a human: once in the intestine, the metacercariae attach to its walls and develop to the stage of an adult parasite, and a new life cycle of the worm begins, which lays new eggs.

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Symptoms metagonimus

Helminthiasis caused by metagonimus is called metagonimiasis in parasitology. Often the invasion is asymptomatic, or such key symptoms of metagonimus as diarrhea and abdominal pain are taken for a common intestinal disorder.

It should be borne in mind that in addition to this, the symptoms of metagonimus are manifested by nausea, general malaise with lethargy and a feeling of loss of strength, deterioration of appetite up to its complete loss.

Invasion by this helminth is accompanied by the production of antigens by the parasites, which leads to sensitization of the human body and the development of an immune response (synthesis of IgE antibodies by mast cells) in the form of eosinophilia - an increase in the number of protective granulocyte leukocytes (eosinophils) in the peripheral blood.

Against this background, the lymph nodes may enlarge and become painful, the liver and spleen may enlarge (hepatosplenomegaly), headaches and muscle pain may occur, the face may swell, and the skin may become covered in hives.

In extreme cases, when metagonimus burrows deeply into the mucous membrane, the eggs they lay can enter the bloodstream or lymphatic system and end up in the brain, spinal cord, or heart. Granulomas can form around the egg, causing seizures, neurological disorders, or heart failure.

In children, undetected, latent chronic metagonimiasis can lead to growth retardation and developmental delays.

Therefore, the prognosis for this helminthiasis largely depends on timely diagnosis and treatment, although, according to experts, the duration of the disease does not exceed one year.

Diagnostics metagonimus

Metagonimiasis is diagnosed based on a stool analysis that reveals metagonimiasis eggs, as adult worms can only be seen in the stool after drug therapy.

However, accurate diagnosis of metagonimus solely by microscopic identification of eggs in the stool is difficult because the eggs of most trematodes are similar in size and morphology, and the eggs of Metagonimus yokogawai are indistinguishable from those of fluke flatworms such as H. heterophyes, Clonorchis sinensis, Opisthorchis felineus, or Opisthorchis viverrini.

A common diagnostic procedure is a general blood test for eosinophils. An abnormal blood eosinophil count is considered to be 500 or more eosinophils/µl. Moderate eosinophilia is diagnosed with 1500-5000 eosinophils/µl, and severe eosinophilia with more than 5000.

It is also important to find out whether the patient has visited regions where metagonimus is endemic and whether he has eaten fish.

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Treatment metagonimus

Standard treatment for metagonimus is with one of two anthelmintic drugs: Praziquantel or Niclosamide.

The drug Praziquantel (other trade names - Azinox, Biltricid, Cezol, Cystricide) in tablets of 0.6 g for oral use is taken once a day at a dose of 0.04 g per kilogram of body weight. The tablets are taken whole, during meals, with water. The second scheme: 0.02 g per kilogram - twice during the day (with an interval of 4-5 hours).

Side effects of Praziquantel include nausea and vomiting, abdominal pain, headache, skin itching and rash, slight hyperthermia of the body and drowsiness. This drug is not allowed to be used by pregnant and lactating women.

The drug Niclosamide (Biomesan, Fenasal, Cestocide, etc.) in tablets of 0.25 g should be taken by adults and children over 12 years old, 8-12 tablets within 24 hours: 2 tablets every 4 hours. Children 5-12 years old, 2 tablets at intervals of 5-6 hours: children 2-5 years old - 2 tablets per day (morning and afternoon). The course of treatment is 4 days. At the same time, it is necessary to use saline laxatives, as well as eat light food in liquid form. Side effects of this drug include skin itching and dyspepsia, contraindications include gastric ulcer, kidney and liver dysfunction, low hemoglobin levels in the blood and the gestation period.

Prevention

The simplest prevention of metagonimus infection involves thoroughly cooking raw fish and following hygiene rules (including after handling the fish before cooking it). In countries where metagonimus is endemic, this is difficult to do because many people in Southeast Asia eat raw or pickled fish as part of their traditional cuisine.

According to the Korean Journal of Parasitology, Metagonimus is the most common intestinal parasite in Korea, Japan, China and Hong Kong due to the frequent consumption of raw fish.

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