Infectious Disease and Microbiology – Paragonimus Species

Overview

Paragonimus species are trematode helminths (lung flukes) that cause paragonimiasis. Humans typically acquire infection by consuming raw or inadequately cooked freshwater crabs or crayfish containing infective metacercariae.

Although many infections are asymptomatic, the lungs are the major site of disease. Symptomatic pulmonary paragonimiasis classically causes chronic cough, pleuritic chest pain, and hemoptysis. Because pulmonary imaging abnormalities can resemble tuberculosis, paragonimiasis is an important parasitic differential diagnosis for chronic pulmonary disease in patients with appropriate food and geographic exposures.

Classification

Genus: Paragonimus

Important species include:

• P. africanus

• P. kellicotti

• P. mexicanus (historically including P. peruvianus)

• P. skrjabini

• P. uterobilateralis

• P. westermani

Organism type: Trematode helminth

Group: Lung fluke

Disease: Paragonimiasis

Microbiologic Characteristics

Paragonimus species are:

• Trematodes

• Flatworm helminths

• Foodborne parasites

• Primarily pulmonary parasites

Adult flukes characteristically establish themselves in the:

Lungs

where they may become enclosed within inflammatory or fibrotic cystic lesions.

High-Yield Microbiology Pattern

Trematode

• ●

Lung fluke

• ●

Raw crab or crayfish

• ●

Hemoptysis

→ Think Paragonimus

Incubation Period

The interval between infection and the development of clinical manifestations is usually:

Prolonged

Symptoms may not appear until:

Several months after infection

This long interval can make the original food exposure less obvious when the patient eventually develops pulmonary symptoms.

Epidemiology

Paragonimiasis occurs in several regions of the world, with different species predominating geographically.

Africa

P. africanus and P. uterobilateralis

are found in:

Africa, particularly parts of Central and West Africa.

Americas

P. mexicanus

is associated with:

Central and South America

Asia

P. westermani and related species are important in:

Asia

P. westermani is the best-known human lung fluke and is particularly associated with East and Southeast Asia.

North America

P. kellicotti

occurs in:

United States and Canada

Human infection is uncommon but can occur after consumption of:

Raw or undercooked crayfish

Geographic Memory Aid

Africa

→ P. africanus

→ P. uterobilateralis

Americas

→ P. mexicanus

Asia

→ P. westermani

→ P. skrjabini

North America

→ P. kellicotti

Transmission

Humans acquire paragonimiasis primarily by eating infected:

Freshwater crabs

or

Freshwater crayfish

that have not been adequately cooked.

Infective Stage

The infective stage for humans is the:

Metacercaria

located within the crustacean intermediate host.

Life Cycle

Paragonimus eggs enter freshwater

↓

First intermediate host:

Freshwater snail

↓

Cercariae emerge from snail

↓

Cercariae infect second intermediate host:

Freshwater crab or crayfish

↓

Develop into:

Metacercariae

↓

Human eats raw or inadequately cooked crustacean

↓

Metacercariae excyst in intestine

↓

Immature flukes penetrate intestinal wall

↓

Migrate through abdominal cavity and diaphragm

↓

Enter:

Lungs

↓

Mature into adult lung flukes

↓

Produce eggs

↓

Eggs enter bronchi

↓

Coughed up in sputum

or

↓

Swallowed and passed in stool

High-Yield Life-Cycle Pattern

Snail

↓

Crab/crayfish

↓

Human

↓

Lungs

↓

Eggs in sputum

This is the classic Paragonimus sequence.

Paragonimiasis

The infection caused by Paragonimus species is:

Paragonimiasis

Many infections are:

Asymptomatic

When symptomatic disease develops, the lungs are the most characteristic site.

Pulmonary Paragonimiasis

The classic manifestations are:

• Chronic cough

• Pleuritic chest pain

• Hemoptysis

Patients may also experience:

• Dyspnea

• Fever

• Fatigue

• Recurrent pulmonary symptoms

High-Yield Clinical Triad

Cough

• ●

Pleuritic chest pain

• ●

Hemoptysis

after consumption of:

Raw crab or crayfish

→ Think Paragonimus

Pulmonary Imaging

Chest radiography or CT may demonstrate:

• Diffuse infiltrates

• Segmental infiltrates

• Cavitary lesions

• Ring-shaped cystic lesions

• Pulmonary nodules

• Pleural effusions

These abnormalities are not specific and may resemble other pulmonary infections.

Paragonimiasis vs. Tuberculosis

A particularly important diagnostic problem is confusion with:

Pulmonary tuberculosis

Both diseases may produce:

• Chronic cough

• Hemoptysis

• Pulmonary infiltrates

• Cavitary abnormalities

• Chronic constitutional symptoms

Important Distinguishing Clue

Chronic cough + hemoptysis + abnormal chest imaging

might initially suggest:

Tuberculosis

However:

Raw freshwater crab/crayfish exposure

• ●

Paragonimus eggs in sputum

→ Pulmonary paragonimiasis

High-Yield Exam Trap

Hemoptysis + lung cavity

does not automatically mean:

Tuberculosis

Ask about:

Raw or undercooked crustacean consumption

especially in endemic regions.

Extrapulmonary Paragonimiasis

Although the lungs are the major site of disease, Paragonimus can migrate abnormally and cause:

Extrapulmonary infection

Almost any organ may potentially be involved.

Cerebral Paragonimiasis

One of the most clinically important extrapulmonary manifestations is:

Central nervous system involvement

Ectopic migration into the brain can produce serious neurologic disease.

Possible CNS Manifestations

Patients may develop:

• Headache

• Seizures

• Focal neurologic deficits

• Other manifestations of intracranial inflammation or mass-like lesions

CNS disease may require both antiparasitic therapy and management of the associated inflammatory response.

Other Extrapulmonary Sites

Ectopic paragonimiasis may involve:

• Subcutaneous tissues

• Abdominal organs

• Pleura

• Central nervous system

• Other tissues

The manifestations depend on where migrating parasites become established.

Diagnosis

Diagnosis is based primarily on demonstrating:

Paragonimus eggs

in clinical specimens.

Sputum Examination

The classic diagnostic specimen is:

Sputum

Microscopic examination may demonstrate characteristic:

Paragonimus eggs

because eggs released from adult pulmonary worms can enter the bronchial tree.

Diagnostic Memory Aid

Lung fluke

→ Look in the SPUTUM

This is one of the most useful associations for exams.

Stool Examination

Eggs may also be detected in:

Stool

because sputum containing eggs may be:

Swallowed

↓

Eggs pass through gastrointestinal tract

↓

Detected in feces

Therefore:

Negative sputum does not necessarily exclude stool positivity, and vice versa.

Serology

Serologic tests are also available and may support the diagnosis, particularly when direct demonstration of eggs is difficult.

Serology may be especially useful in:

Extrapulmonary disease

where eggs may not be readily detected in sputum.

Eosinophilia

As with many tissue-invasive helminth infections:

Peripheral eosinophilia

may occur, particularly during migratory or active inflammatory stages.

Although supportive, eosinophilia is not specific for paragonimiasis.

Diagnostic Pattern

Raw crab/crayfish exposure

• ●

Chronic pulmonary disease

• ●

Hemoptysis

• ●

Eosinophilia

• ●

Eggs in sputum or stool

→ Paragonimiasis

Treatment

The source recommends:

Praziquantel 25 mg/kg orally every 8 hours for 3 days

Praziquantel is the principal antiparasitic treatment for paragonimiasis.

Treatment Schedule

Praziquantel

25 mg/kg

↓

Every 8 hours

↓

For 3 days

This is a classic treatment regimen associated with Paragonimus infection.

Bithionol

The source lists:

Bithionol

as an additional treatment.

The described regimen is:

15–25 mg/kg every 12 hours on alternate days for 10–14 doses

This represents an alternative/historical therapeutic approach.

CNS Disease

In patients with:

Cerebral paragonimiasis

the inflammatory response associated with antiparasitic treatment can be clinically important.

The source notes that:

Corticosteroids may be beneficial

in cases with CNS involvement.

Prevention

The most important preventive measure is:

Thoroughly cook freshwater crustaceans

especially:

• Crabs

• Crayfish

Raw, pickled, marinated, or inadequately cooked crustaceans may remain infectious if viable metacercariae are present.

Paragonimus vs. Clonorchis/Opisthorchis

Paragonimus

→ Raw crab/crayfish

→ Lungs

→ Cough and hemoptysis

→ Eggs in sputum or stool

Clonorchis / Opisthorchis

→ Raw freshwater fish

→ Bile ducts

→ Cholangitis and obstructive disease

→ Cholangiocarcinoma association

→ Eggs primarily detected in stool

Food Exposure Memory Aid

Crab/crayfish

→ PARAGONIMUS

Freshwater fish

→ CLONORCHIS / OPISTHORCHIS

Aquatic vegetation/watercress

→ FASCIOLA

Paragonimus vs. Tuberculosis

Paragonimiasis

→ Raw crab/crayfish exposure

→ Helminth infection

→ Eosinophilia may occur

→ Parasitic eggs in sputum

→ Praziquantel treatment

Tuberculosis

→ Mycobacterium tuberculosis

→ Acid-fast bacillus

→ Person-to-person airborne transmission

→ Acid-fast organisms/molecular evidence rather than helminth eggs

→ Requires multidrug antimycobacterial therapy

High-Yield Clinical Pattern

Patient from an endemic region

• ●

Raw freshwater crab or crayfish consumption

• ●

Months later develops chronic cough

• ●

Pleuritic chest pain

• ●

Hemoptysis

• ●

Cavitary or cystic lung lesions

→ Think Paragonimus

High-Yield Diagnostic Pattern

Hemoptysis

• ●

TB-like chest imaging

• ●

Eosinophilia

• ●

Raw crustacean exposure

• ●

Operculated trematode eggs in sputum

→ PARAGONIMIASIS

Exam Essentials

Genus: Paragonimus

Type: Trematode helminth

Common name: Lung fluke

Disease: Paragonimiasis

Important species: P. westermani, P. kellicotti, P. mexicanus, P. africanus, P. uterobilateralis, and P. skrjabini

Incubation: Usually prolonged, often several months

Transmission: Eating infected freshwater crabs or crayfish

Infective stage: Metacercaria

First intermediate host: Freshwater snail

Second intermediate host: Crab or crayfish

Major organ: Lung

Classic symptoms: Cough + pleuritic chest pain + hemoptysis

Imaging: Infiltrates, cavities, ring cysts, nodules, and pleural effusions

Important mimic: Pulmonary tuberculosis

Extrapulmonary disease: Possible in many organs

Important extrapulmonary site: CNS

Classic diagnosis: Eggs in sputum

Additional diagnostic specimen: Stool

Serology: Available and supportive

Treatment: Praziquantel 25 mg/kg orally every 8 hours for 3 days

Alternative source treatment: Bithionol

CNS disease: Corticosteroids may be useful for inflammatory complications

Prevention: Thorough cooking of freshwater crabs and crayfish

Key clinical pearl: Think of Paragonimus whenever chronic cough, pleuritic chest pain, and hemoptysis develop after consumption of raw or undercooked freshwater crab or crayfish. Pulmonary disease can closely mimic tuberculosis, but the classic clue is detection of Paragonimus eggs in sputum; praziquantel is the treatment of choice.