Life cycle of the pathogen and pathogenesis
The etiologic factor of the disease is the trematode Paragonimus westermanii. Definitive hosts can be not only humans, but also cats, dogs, pigs, tigers, and mongooses. The parasite develops with a change of hosts:
- Egg excretion. The diseased organism excretes helminth eggs mainly with sputum (less often with feces). For further development, they must enter an aquatic environment.
- Larval development. After about 4 weeks, a larva emerges from the egg. It penetrates the body of the first intermediate host, a freshwater mollusk.
- Formation of metacercariae. Having passed the developmental stage, cercariae leave the mollusk and invade additional hosts—freshwater crayfish or crabs—where they encyst, turning into metacercariae.
- Infection and migration. Upon eating raw crustacean meat, immature forms of the parasite emerge from cysts in the human small intestine. Next, they perforate the intestinal wall, ending up in the free abdominal cavity, after which they migrate to the diaphragm. Having passed through it (or through the mediastinum), the helminths enter the lungs.
In the lung tissue around the settled parasite, an inflammatory process begins. Connective tissue gradually proliferates, and after 2–3 months a specific paragonimial cyst forms, filled with pus, the parasites themselves, and their eggs. With the destruction of blood vessels and lung tissue, hematogenous metastasis to other organs is possible (multiple cysts in the brain are most commonly formed).
Clinical presentation
The symptoms of the disease directly depend on the current stage of development and the migration pathway of the helminth in the patient's body.
1. Abdominal syndrome
Characteristic of the early stage, when the parasite penetrates the intestinal wall and migrates through the abdominal cavity. It manifests as persistent enteritis, signs of an "acute abdomen," and a picture of acute hepatitis.
2. Pulmonary syndrome
Develops upon the invasion and maturation of the trematode in the respiratory system. Inflammation manifests as acute bronchitis, bronchopneumonia, or hemorrhagic pleuritis.
- Patient complaints: fever, chest pain, morning paroxysmal cough with purulent sputum, hemoptysis.
- Objective findings: auscultation reveals rales (predominantly moist).
- Intoxication syndrome: accompanied by tachycardia, dystrophic myocardial changes, and neurological disorders (dizziness, irritability, headache, fatigue).
3. Acute cerebral paragonimiasis
A complication of the pulmonary form, proceeding with the clinical picture of meningitis or meningoencephalitis. Severe headaches, dizziness, convulsive seizures with loss of consciousness (epileptic type), as well as severe visual disturbances, including visual field constriction and decreased visual acuity, are noted.
Diagnostic methods
Diagnostic search begins with anamnesis collection: it is important to establish the fact of staying in an endemic focus and consuming crab or crayfish meat. A number of laboratory and instrumental tests are used to confirm the diagnosis.
Laboratory diagnostics:
- Sputum analysis: macroscopically, the sputum is viscous and has a specific appearance ("chocolate," red-brown, or bright red). Microscopy reveals clusters of parasite eggs.
- Blood test: eosinophilia is detected, often lymphocytosis and hypochromic anemia.
- Analysis of effusion (obtained via laparoscopy/laparotomy): the exudate is hemorrhagic, fibrinous, or fibrinous-purulent in nature; immature forms of the helminth can be found under a microscope.
- Allergy skin test: intradermal test with a specific antigen. A positive reaction produces an erythema of 3–4 cm and an infiltrate of more than 1 cm, which is accompanied by severe itching and often lymphangitis.
- In the cerebral form, cerebrospinal fluid analysis (CSF examination) is mandatory.
Instrumental diagnostics: Lung radiographs show diffuse ring-shaped small- or large-focal shadows, which are most often localized in the middle lobe. Skull radiography and angiography are used to visualize CNS lesions (allowing precise localization of the cyst in the brain).
Principles of treatment
The mainstay of treatment is conservative therapy with anthelmintic agents. The drug of choice is bithionol (bithionol). It is prescribed orally at a total daily dose of 30–40 mg/kg, divided into 2–3 doses. The duration of the course is 10 days.
When severe complications develop, conservative measures may be insufficient, requiring surgical intervention:
- Lung resection is indicated when drug therapy fails, with recurrent pulmonary hemorrhages, persistent hemoptysis, and progressive exhaustion of the patient.
- Removal of brain cysts is performed when neurological and psychiatric symptoms worsen, if the use of bithionol has not brought results.
Briefly: answers to topical questions
Through which anatomical structures does the paragonimus larva migrate on its way to the lungs?
On its way to the lungs, the paragonimus larva emerges in the small intestine, perforates its wall, and enters the free abdominal cavity. Then it migrates to the diaphragm and penetrates the lungs through the diaphragm or through the mediastinum.
Class and exam questions
What transmission route is characteristic of paragonimiasis?
Humans are infected alimentarily by consuming freshwater crayfish and crabs that have not undergone sufficient heat treatment.
What does sputum look like in pulmonary paragonimiasis?
It is viscous and has a characteristic "chocolate," red-brown, or bright red color. Microscopy can reveal clusters of helminth eggs in it.
Why does paragonimiasis affect the brain?
Extrapulmonary forms arise due to hematogenous metastasis. When tissues and blood vessels in the lungs are destroyed, parasites and their eggs can enter the central nervous system with the bloodstream, forming cysts there.
In which cases is surgery required for paragonimiasis?
Surgical treatment (lung resection or removal of brain cysts) is indicated for persistent hemoptysis, pulmonary hemorrhages, worsening neurological symptoms, and ineffectiveness of conservative treatment.
What else to study on this topic
- Mechanisms of hematogenous metastasis of trematodes to the central nervous system
- Pathomorphology of a pulmonary paragonimial cyst
- Differential diagnosis of pulmonary paragonimiasis and tuberculosis
- Features of exudate composition in the abdominal stage of paragonimiasis
- Pharmacodynamics of bithionol in the treatment of tissue helminthiases
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