Morphological features of the parasite
The adult lung fluke has an oval shape and reddish-brown coloration, reaching 7.5 to 12 mm in length and 4–6 mm in width. The helminth's body is reliably protected by a dense tegument and musculature body wall.
The internal structure is characterized by the absence of a true body cavity—this is a typical parenchymatous type of organization. The digestive system begins with the oral cavity, which passes into the pharynx and esophagus. This is followed by two convoluted, unbranched intestinal tracts that end blindly. The circulatory and respiratory systems of the worm are completely absent, and metabolic waste excretion is carried out via the protonephridial type. The nervous system is represented by an orthogon type structure.
The parasite is a hermaphrodite. Its reproductive system has a complex structure:
- Lobed ovaries and uterus lie on either side of the ventral sucker.
- Two lobed testes are located in the posterior part of the body.
- Highly developed vitelline glands are located on the sides of the body, necessary for egg formation.
Life cycle and transmission routes
The development of the helminth proceeds with a change of several hosts. Definitive hosts include humans, domestic animals (dogs, cats, pigs), and various wild carnivorous animals.
The cycle includes two intermediate hosts:
- First intermediate host — freshwater snails belonging to the genus Melania.
- Second intermediate host — freshwater crayfish and crabs.
The mechanism of human infection is triggered by eating thermally unprocessed meat of infected crabs and crayfish. The danger is posed by metacercariae located in their tissues — the infective stage of the parasite for the definitive host.
Pathogenesis and clinical complications
Entering the human body, the parasite causes significant harm to internal organs. The basis of pathogenesis includes:
- Direct mechanical damage to lung tissues during the life activity and migration of the helminth.
- Formation of fibrous cysts around the parasites in the lung tissue.
- Development of pronounced toxic-allergic reactions in response to the worm's metabolic products.
The most severe complication is the ectopic localization of the helminth. In cases where the lung fluke penetrates the brain structures, the patient rapidly develops neurological disorders and symptoms of meningitis.
Diagnostic methods and prevention
Diagnosis requires an integrated approach. Laboratory diagnostics is based on searching for helminth eggs in biological fluids:
- The main method is the detection of eggs in the patient's sputum.
- Additionally, eggs can be found in feces (this happens if the patient swallows coughed-up infected sputum).
Instrumental studies are widely used to assess the degree of organ damage and identify fibrous cysts:
- Radiography.
- Computed tomography (CT).
- Magnetic resonance imaging (MRI).
Prevention of the disease is simple and consists of strict adherence to culinary standards: it is necessary to completely exclude the consumption of raw and poorly cooked freshwater crayfish and crabs.
Briefly: answers to topical questions
In which structures of the respiratory system of the definitive host do maritae of the lung fluke localize?
In the respiratory system of the definitive host, lung fluke maritae localize in the lung tissue, where fibrous paragonimiasis cysts form around them.
The localization process is accompanied by the following changes:
- Settling — after migration, immature parasites settle and mature directly in the lungs.
- Inflammation — an inflammatory reaction develops around the helminth with active proliferation of connective tissue.
- Paragonimiasis cyst — after 2–3 months, a fibrous cavity the size of a hazelnut is formed.
- Cyst contents — inside are the maritae (parasites) themselves, their eggs, and pus.
To remember the cycle, use the "MRC" rule: Molluscs (Melania) → Crayfish/crabs → Definitive host (Human/carnivores).
Class and exam questions
Why can lung fluke eggs be found in a stool test?
Eggs enter the gastrointestinal tract and then the feces in cases where the patient coughs up infected sputum from the lungs and swallows it.
At what stage of development is the parasite dangerous to humans?
The infective stage for the definitive host (human) is metacercariae, which localize in the tissues of freshwater crayfish and crabs.
Does the lung fluke have a circulatory system?
No, this helminth completely lacks circulatory and respiratory systems, and the body cavity is filled with parenchyma.
What else to study on this topic
- Features of the structure of the tegument and musculature body wall of flukes
- Protonephridial type of excretory system: mechanism of action
- Orthogon: structure of the nervous system of flatworms
- Mechanisms of fibrous cyst formation in the lungs
- Toxic-allergic reactions in helminthiases
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