Lung flukes (Paragonimus species)

 

   The genus Paragonimus contains more than 30 species that have been reported to cause infections in animals and humans. Among these, approximately 10 species have been reported to cause infection in humans, of which P. westermani is the most important. P. westermani, also known as the Oriental lung fluke is most common in China, Korea, Thailand, Philippines, and Laos. Isolated endemic foci
have also been reported from the states of Manipur, Nagaland, and Arunachal Pradesh in India.

  A low prevalence has been reported from African countries of Cameroon and Nigeria, where infections with Paragonimus africanus and Paragonimus uterobilateralis were reported. Humans are infected by eating raw or partially cooked crab or crayfish or crabs soaked in wine as a food delicacy or by drinking juice from raw crabs or crayfish as a part of a food habit. It inhabits parenchyma of the lung close to bronchioles in humans, foxes, wolves, and various feline hosts (e.g, lions, leopards, tigers, cats). Paragonimus species belong to the family ‘Troglotrematidae’ and they possess the

following striking features:

- The adult worm is reddish brown fluke.

- The body of adult worm is thick, fleshy, and ovoid in shape

- The tegument is spiny or scaly

- Have weak suckers

- Testes lie side by side

- Uterus is short with a few tight uterine coils forming a ‘rosette’

- Extensive vitellaria in lateral fields



- Cercariae are microcercous xiphidiocercariae

- The eggs are ovoid, brownish yellow, thick shelled and operculated.

 Life cycle and Transmission

  The infection is typically transmitted via ingestion of metacercariae contained in raw freshwater crabs or crayfish. Additionally, consumption of the raw meat of paratenic hosts (e.g, omnivorous mammals) may also contribute to human  infection. Freshwater snails and crabs are first and second intermediate hosts of Paragonimus species, respectively. In the duodenum, the cyst wall is dissolved, and the metacercariae are released.

  The metacercariae migrate by penetrating through the intestinal wall, peritoneal cavity, and, finally, through the abdominal wall and diaphragm into the lungs. There, the immature worms finally settle close to the bronchi, grow, and develop to become sexually mature hermaphrodite worms.


    Adult worms begin to lay the eggs, which are unembryonated and are passed out in the sputum. However, if they are swallowed, they are excreted in the feces. The eggs develop further in the water. In each egg, a ciliated miracidium develops during a period of 2-3 weeks. The miracidium escapes from the egg and penetrates a suitable species of snail (first intermediate host), in which it goes through a generation of sporocysts and 2 generations of rediae to form the cercariae. The cercariae come out of the snail, invade a freshwater crustacean (crayfish or crab), and encyst to form metacercariae. When ingested, these cause the infection, and the cycle is repeated.

Pathology and clinical symptoms

a. Acute manifestations: Acute pulmonary infection is characterized by low-grade fever, cough, night sweats, chest pain, and blood-stained rusty-brown sputum.

b. Chronic manifestations: Lung abscess or pleural effusion develops in individuals with chronic infections. Fever, haemoptysis, pleurisy pain, dyspnea and recurren attacks of bacterial pneumonia are the common symptoms. The condition mimics pulmonary tuberculosis.

c. Fever, headache, nausea, vomiting, visual disturbances, motor weakness, and localized or generalized paralysis are the symptoms of cerebral paragonimiasis.

Diagnosis

Diagnosis is based on finding the characteristic eggs in brown sputum. The eggs can also be found in the faeces due to swallowing sputum. A chest x-ray may show cystic shadows and calcification. Serological tests, in particular, the ELISA method, are useful diagnostic tests.

Epidemiology

The epidemiology of the disease (Paragonimiasis) depends on one of the following

- Presence of appropriate snail, crab and mammalian reservoir hosts in the area

- Pollution of snail habitats with sputum and faeces of man as well as natural mammalian reservoir hosts infected with the parasite

- Consumption of metacercariae through eating of raw or undercooked crabs or through contamination of the fingers and cooking utensils with metacercariae while cleaning

Control

- Proper cooking of crabs before consumptions

- Proper waste disposal