Paragonimiasis

This is a food-borne trematode infection that primarily affects the lungs. The trematodes are of the Paragonimus genus and there are 8 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

• Humans eat raw infected crustaceans e.g. crabs, crayfish.
• 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.
• These adult flukes live in human lungs and deposit eggs into the bronchi. (They can live in the lungs for around 20 years.)²
• This cycle from ingestion to maturity in the lungs takes 5-6 weeks.
• Humans can then expel the eggs either through coughing or by swallowing the eggs and passing them out with their faeces.
• These eggs can develop in water until they reach a stage where they are able to invade an intermediate host, a species of fresh water snail.
• They mature further, emerge and are 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

• 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

About 1 in 5 people have 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.
• Extra-pulmonary 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 extra-pulmonary 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.

• Pulmonary paragonimiasis may be mistaken for tuberculosis^4,5 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 anti-tuberculous treatment.
• Other differential diagnoses include:
  • Acute or chronic bronchitis
  • Bronchiectasis
  • Pneumonia
  • Asthma
  • Aspergillosis
  • Histoplasmosis
  • Whipworm infection

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, CSF, 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:
  • White blood count may be normal or slightly raised but there may be an eosinophilia.¹
• Imaging:
  • Changes may be seen on chest X-ray (e.g. cavitating lesions, nodules, fibrosis, ring shadows) but aren't specific.
  • CT or MRI can be helpful, especially for cerebral infection.^1,7
• Other tests:
  • Skin testing is a useful epidemiological tool.^1,8
  • 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

• Praziquantel is the drug of choice to treat paragonimiasis.^1,2
• If there is extra-pulmonary disease, lesions need surgical excision.¹ Laparoscopic approaches may be used.⁹
• If there is cerebral involvement, medical control of seizures may be required.

• 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.

• 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

• In endemic areas, shellfish and pork must be well cooked.
• Education and changes in habit may help prevention.¹⁰