Morphology of the adult form (marita)
The adult worm (marita) has a characteristic oval, slightly flattened shape. Its anatomical structure is adapted to a parasitic lifestyle:
- Fixation organs: At the anterior end of the body there is an oral sucker, followed by a muscular pharynx. Approximately in the center of the body lies the ventral sucker, serving as the main organ for attachment to the host tissues.
- Digestive system: Represented by two intestinal branches. An important distinctive feature is that the branches terminate blindly and reach the posterior part of the parasite's body.
- Hermaphroditic reproductive system: The organs are distributed in a specific way. Asymmetrically to the side of the ventral sucker lies the lobed ovary, and on the opposite side is the convoluted uterus. In the posterior third of the body, there are two branched testes. Along the entire length on the lateral parts of the body, vitellaria (yolk glands) are strongly developed.
- Excretory system: Consists of a large central canal that runs exactly in the middle between the two testes.
Life cycle and change of hosts
The causative agent of paragonimiasis is a biohelminth, and its life cycle proceeds with the alternation of three hosts.
- Definitive hosts: Humans, as well as wild and domestic carnivorous animals (pigs, dogs, cats). Sexually mature individuals localize in the lung tissue, where they live in pairs inside cysts communicating with the bronchial lumen.
- First intermediate host: Freshwater gastropod mollusks (specifically of the genera Melania libertina, Ampullaria luteosoma, and others). Within their bodies, the parasite undergoes stages of parthenogenesis: from the invading miracidium, a sporocyst is formed, followed by rediae and, finally, cercariae.
- Second intermediate host: Freshwater crabs (genera Potamon, Eriocheir) and crayfish (genera Procambarus, Cambaroides). Cercariae emerging from the mollusk actively swim in water, penetrate through the body integuments into crustaceans, encyst, and transform into the infective stage — the metacercaria. This process takes about 1.5 months.
Mechanism of infection and migration in the body
For further development, the parasite's eggs must necessarily enter the external environment and a water body. They are excreted from the definitive host's body along with sputum (during coughing). If sputum is swallowed, the eggs pass through the intestine unhindered and are excreted with feces. When eggs enter water at a temperature of 27 °C, a ciliated larva — the miracidium — develops in them.
Human infection occurs via the fecal-oral mechanism (alimentary route). The transmission factor is the consumption of infected crayfish and crabs that were eaten raw, dried, or underwent insufficient heat treatment.
Stages of migration in the human body:
- Excystation occurs in the intestine — metacercariae shed their membranes.
- Larvae penetrate through the intestinal wall into the abdominal cavity.
- Perforation of the diaphragm occurs, after which the helminths enter the pleural cavity and lung tissues.
- Fibrous cysts the size of a hazelnut form around the parasites in the lungs. The worms reach sexual maturity 5–6 weeks after the moment of infection.
Epidemiological features
Paragonimiasis has a clearly pronounced natural focality. The disease is most widespread in Southeast Asian countries such as China, Korea, and Japan, and is also found in South America.
The source of infection is infested definitive hosts: sick people, pigs, domestic pets (dogs, cats), and wild carnivorous animals. The main routes of transmission are foodborne and waterborne. It is important to remember that the only dangerous (infective) stage for humans is precisely the metacercaria located in the body of crustaceans, whereas eggs, miracidia, or cercariae do not directly infect humans.
To memorize the stages of parthenogenesis in the mollusk body, use the abbreviation "SRC" — Sporocyst, Redia, Cercaria. And the arrangement of the internal organs of the marita is easy to remember by the "lateral neighbors" rule: the ovary and uterus lie on different sides of the ventral sucker, and the testes are always posterior.
Class and exam questions
Which stage of the Paragonimus westermani life cycle is infective for humans?
The infective stage for humans is the metacercaria. Infection occurs upon ingestion of metacercariae along with the meat of freshwater crayfish and crabs.
How do lung fluke eggs leave the definitive host's body?
Eggs enter the external environment predominantly with sputum during a cough. They can also be excreted with feces if the patient swallows infested sputum.
What path does the larva take inside the human body to the site of its final localization?
Having freed itself from the membrane in the intestine, the larva pierces its wall, enters the abdominal cavity, then bores through the diaphragm and penetrates the pleural cavity and lungs.
Where exactly do adult worms parasitize in the human body?
Maritae localize in the bronchial branches of the lungs, where they live in pairs inside formed fibrous cysts the size of a hazelnut.
What else to study on this topic
- Morphological differences in the structure of lung fluke eggs from eggs of other trematodes.
- Species diversity and ecology of intermediate hosts (mollusks of the genera Melania, Ampullaria, and crustaceans).
- The influence of temperature factors (27 °C) on the rate of miracidium development in the aquatic environment.
- Mechanisms of fibrous capsule formation in lung tissue around a pair of parasites.
- Pathogenesis of tissue damage during the complex migration of larvae from the intestine through the diaphragm.
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