Paragonimiasis

Paragonimiasis is a foodborne trematode infection that primarily affects the lungs. The trematodes are of the Paragonimus genus and there are eight species that can cause significant infection in humans.¹ Paragonimus westermani is the most common of these and it is also known as the oriental lung fluke.²

Life cycle¹

• Humans become infected by eating raw infected crustaceans, eg crabs, crayfish, or from eating raw/undercooked pork.

• Larval flukes develop in the human intestine and penetrate the intestinal wall, entering the abdominal cavity. They then travel to the liver or the abdominal wall and mature further.

• Adult flukes are then able to penetrate the diaphragm and travel to the pleural space and lungs.

• The adult flukes live in human lungs and deposit eggs into the bronchi. This cycle from ingestion to maturity in the lungs takes 5-6 weeks. The adult flukes can live in the lungs for around 20 years.²

• Humans can then expel the eggs either through coughing or by swallowing the eggs and passing them in faeces.

• The eggs can develop in water until they reach a stage where they are able to invade an intermediate host, which is a species of freshwater snail.

• The eggs mature further, emerge and are then able to invade another intermediate host, the crustaceans. And so the cycle is maintained.

• Animals including pigs, dogs, and feline species can also harbour Paragonimus westermani.²

Epidemiology

• Paragonimus spp. are distributed throughout the Americas, Africa and Southeast Asia.²

• An estimated 20 million people are infected worldwide.¹

• It should be considered in returning travellers and migrants.³

Presentation¹

About 1 in 5 people has asymptomatic infection.

Acute phase (can last several weeks)

• Initially there may be abdominal pain, diarrhoea, and urticaria.

• This can be followed by fever, cough, dyspnoea, chest pain, malaise, and sweats.

• Hepatosplenomegaly can occur.²

Chronic phase

• Pulmonary symptoms (usually begin 6 months after infection):
  

  • Symptoms can be mistaken for tuberculosis, chronic bronchitis or bronchiectasis.

  • Dry cough initially followed by a productive cough with rusty sputum.

  • Chest discomfort may be present.

  • Shortness of breath on exertion.

  • Wheeze may be present.

  • Haemoptysis can sometimes be life-threatening.

  • Finger clubbing may be present.

• Extrapulmonary symptoms:
  

  • Flukes or eggs can travel to other sites. However, when this takes place completion of the life cycles is not achieved, because the eggs laid cannot exit these sites.² Symptoms can be divided into:
    

    • Cerebral - the most common extrapulmonary form; resembles meningo-encephalitis acutely; headache, vomiting and seizures can occur in the chronic phase with neurological signs, including facial palsy and hemiplegia.

    • Abdominal - cysts can form in the liver, spleen, intestinal wall, peritoneum or mesenteric lymph nodes. There may be abdominal pain, a palpable abdominal mass and bloody diarrhoea. Renal involvement can cause haematuria and eggs are sometimes found in the urine.

    • Subcutaneous tissues - subcutaneous nodules can contain immature flukes; abscesses and granulomas can form.

    • Miscellaneous - muscle, the testis, the ovary and the spinal cord can become involved.

Differential diagnosis

• Pulmonary paragonimiasis may be mistaken for tuberculosis⁴⁵ and in those who have been in an endemic area, it should be considered if there is not a firm diagnosis of tuberculosis and there is failure to respond to antituberculous treatment.

• Other differential diagnoses include:
  

  • Acute or chronic bronchitis

  • Bronchiectasis

  • Pneumonia

  • Asthma

  • Aspergillosis

  • Histoplasmosis

  • Whipworm infection

Investigations

Egg detection and antibody tests are standard, the latter being preferred due to low rates of egg detection. However, eggs are not present until 2 to 3 months after infection.²

• Egg detection:¹

  • Sputum, faeces, pleural fluid, cerebrospinal fluid, or pus are examined for worms or eggs.

  • Biopsies from lung, brain, subcutaneous or abdominal nodules or cysts may also reveal eggs or worms.

  • Multiple specimens may be needed before eggs are detected.

  • Try stool examination in children as they tend to swallow sputum.

• Serology:
  

  • Enzyme-linked immunosorbent assay (ELISA) has been developed to look for the presence of antibodies against lung flukes.⁶

• Other blood tests:
  

  • FBC: white blood count may be normal or slightly raised but there is usually an eosinophilia.¹

• Imaging:
  

  • Changes may be seen on CXR (eg cavitating lesions, nodules, fibrosis, ring shadows) but aren't specific.

  • CT or MRI scanning can be helpful, especially for cerebral infection.¹⁷

• Other tests:
  

  • Skin testing is a useful epidemiological tool.¹⁸

  • Lumbar puncture may be carried out if cerebral infection is suspected.

  • Pleural aspiration: eggs are rarely detected.¹

  • Lung biopsy specimens can contain adult worms or eggs.¹

Management

• Praziquantel and triclabendazole are the two drugs recommended to treat paragonimiasis. Praziquantel is the most commonly used and has a cure rate of 80-90%.¹

• Bithionol is used as an alternative treatment.²

• If there is extrapulmonary disease, lesions need surgical excision.¹ Laparoscopic approaches may be used.⁹

• If there is cerebral involvement, medical control of seizures may be required.

Complications¹

• Untreated infection can lead to interstitial pneumonia, bronchitis, and bronchiectasis.

• Other complications include bronchopneumonia, lung abscess, pleural effusion, or empyema.

• Cerebral complications include seizures and coma.

Prognosis

• This is good with the correct treatment.

• Untreated cerebral paragonimiasis has a mortality rate of 5%.¹

• Untreated pulmonary disease may be self-limiting, with lesions resolving in 5 to 10 years in light infections.¹

• Cerebral infections may result in long-term seizures.¹

Prevention

• In endemic areas, shellfish and pork must be well cooked.

• Education and changes in habit may help prevention.¹⁰