Neglected tropical diseases: Paragonimiasis

Paragonimiasis, or lung fluke disease, is caused by infection with a number of species of trematodes belonging to the genus Paragonimus. The most common are: P. westermani, P. heterotremus and P. philippinensis in Asia (China, the Democratic People’s Republic of Korea, the Republic of Korea, the Lao People’s Democratic Republic, the Philippines, Thailand, Viet Nam and other east Asian countries); P. africanus and P. uterobilateralis in western and central Africa; P. caliensis, P. kellicotti and P. mexicanus in north, central and south America. Adult flukes may measure up to 20 mm x 10 mm.

Paragonimus spp. is a common parasite of crustacean-eating mammals such as tigers, leopards, domestic cats, dogs, mongooses, opossums and monkeys (reservoir final hosts). The adult flukes live in the lungs and lay eggs that are coughed up through the airways and either expectorated in the sputum or swallowed and defecated. When they reach freshwater, the eggs develop into miracidia that penetrate various species of aquatic snails, where they further develop and reproduce asexually, giving rise to cercariae (larvae).

Cercariae released into water swim to penetrate suitable species of freshwater crabs, crayfish and other crustaceans and encyst the gills, liver and muscles as metacercariae. When such animals are eaten, the metacercariae hatch in the intestine: young worms penetrate the intestinal wall and the peritoneum, then the diaphragm and the pleura; they finally reach the lungs, where they live in pairs surrounded by a capsula, thus completing the cycle.

Humans may substitute reservoir hosts in the transmission cycle when they eat raw, salted, pickled, smoked, marinated, dried, partially cooked or poorly processed crustaceans, thus ingesting the metacercariae.

In humans, the earliest stages of paragonimiasis may present an elusive clinical picture, and be asymptomatic or scarcely symptomatic. Conversely, when worms reach the lungs, symptoms may be significant and typically include chronic cough with blood-stained sputum; chest pain with dyspnoea and fever; pleural effusion and pneumothorax are possible complications.

Symptoms and signs mimic those of tuberculosis, and paragonimiasis should always be suspected in patients with tuberculosis who are non-responsive to treatment. Ectopic paragonimiasis may result from erratic migration of the juvenile worms: the most frequent locations include the abdominal cavity and subcutaneous tissues and, most frequently, the brain: cerebral paragonimiasis is a severe condition that may be associated with headache, visual impairment and epileptic seizures.

Diagnosis of paragonimiasis is suspected on the basis of the clinical picture, on the anamnestic recall of consuming raw crustaceans, on the detection of eosinophilia, and on typical findings of ultrasound, X-ray, computed tomography or magnetic resonance imaging scans. Tests to rule out tuberculosis should always be conducted. Confirmation of diagnosis relies on different types of diagnostic techniques:

• parasitological techniques to detect Paragonimus eggs in sputum or stool samples.; the cost and sensitivity of these techniques may vary according to the type of technique used; they can only be employed once worms have reached the lungs and started laying eggs; some quantify the intensity of infection (allowing an estimation of the severity of the infection);
• immunological techniques to detect worm-specific antibodies in serum samples or worm-specific antigens in serum or stool samples; these techniques are usually more sensitive than the commonly used parasitological techniques; detection of antibodies does not distinguish between current, recent and past infections; their ability to quantify intensity of infection is disputed; these techniques are still at an experimental stage;
• molecular techniques such as the polymerase chain reaction are also still at an experimental stage.

Triclabendazole, 20 mg/kg, in two divided doses of 10 mg/kg, to be administered on the same day, and praziquantel 25 mg/kg of body weight, 3 times a day for 3 days, are both WHO-recommended medicines for treatment of paragonimiasis. The former is preferred for the simplicity of its regimen, which ensures higher compliances to treatment.

The most basic public health measure that should be implemented is making triclabendazole or praziquantel available at peripheral health centres in all endemic areas for clinical management of confirmed cases.

In areas where cases appear to be clustered, treatment should be also offered to people with suspected paragonimiasis. Suspected cases are defined as individuals coming from an endemic district with a history of consuming raw crustaceans who present with any of the following characteristics:

• cough lasting for more than 3 weeks;
• bloody or rusty sputum;
• clinically or radiologically diagnosed tuberculosis with a negative sputum smear (smear-negative tuberculosis);
• poor or no response to tuberculosis treatment.

In communities and villages where cases of paragonimiasis appear to be significantly clustered, mass drug administration with triclabendazole should also be considered. The recommended regimen is 20 mg/kg of body weight in a single administration.

Complementary interventions such as information, education and communication on safe food practices, improved sanitation and veterinary public health measures should also be implemented in order to decrease rates of transmission.