Paragonimiasis (Lung fluke disease): The Neglected Tropical Disease Spread by Food

Paragonimiasis is a foodborne parasitic disease caused by several species of lung flukes belonging to the genus Paragonimus. The most common cause of human infection is P. westermani, also known as the oriental lung fluke [3].

Human infections of paragonimiasis occur when people eat inadequately cooked or pickled crab or crayfish that harbor the parasite’s larvae [2].

The disease is characterized by a range of clinical symptoms, including chronic cough, chest pain, hemoptysis (coughing up blood), abdominal pain, fever, and cough [1,2].

Lung fluke infections are transmitted by eating raw or undercooked crabs or crayfish, including pickled crabs. Paragonimiasis does not spread from person to person [2].

Paragonimus species can be found worldwide. However, most human infections are found in Southeast Asia, Africa, and South America [4].

Over 40 different types of the Paragonimus parasite have been identified, but most human infections are caused by P. westermani, which is found mainly in Southeast Asia and Japan [5].

Other commonly found Paragonimus species include:

• P. heterotremus and P. philippinensis in Asia
• P. africanus and P. uterobilateralis in western and central Africa
• P. caliensis, P. kellicotti, and P. mexicanus in Central and South America [6].

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The lifecycle of Paragonimus is one of resilience and adaptation. It begins in the lungs of mammals—humans, cats, dogs, and even monkeys—where adult flukes settle and lay eggs. The mammals act as the reservoir hosts for the parasite. Here, the adult worms produce eggs that are excreted through sputum (a thick mucus produced in the lungs) or feces [3, 7].

These unembryonated eggs are either coughed out or swallowed, eventually making their way into the environment through the sputum or feces [3, 7].

When these eggs find their way to a water source, they hatch into free-swimming ciliated larvae called miracidia that target aquatic snails as their first intermediary hosts. Inside a host snail, the miracidia transform into cercariae, free-swimming larvae with a forked tail that can swim in water.

These cercariae are then released into the water and they find freshwater crabs or crayfish where they encyst (get enclosed in a cyst), and wait for a mammalian host. This is their infective stage [3, 7].

How does the Paragonimus parasite infect humans?

Humans and other mammals become infected by consuming these crustaceans raw or undercooked. Various feline species can also harbor the lung fluke parasite. Once ingested, the larvae penetrate the intestinal wall and migrate to the lungs, where they mature and continue the cycle [3, 7].

A Paragonimus infection usually infects the lungs but can infect the skin, brain, and other organs as well [2].

How long does the lung fluke life cycle take?

This entire process, from ingestion to maturation, can take 2–3 months, with infections sometimes lasting up to 20 years [7].

In the early stages of a paragonimiasis infection, humans may be asymptomatic or have mild symptoms.

Acute phase

• Diarrhea
• Abdominal pain
• Fever
• Cough
• Urticaria (hives; an itchy rash)
• Eosinophilia (high count of eosinophils, a type of white blood cell) [2, 3].

Chronic phase

• Pulmonary symptoms arise, including chronic cough, blood in the sputum, chest pain, and shortness of breath [2,3].

Pulmonary abnormalities are a significant clinical presentation during this phase.

If the parasite reaches the brain, it can lead to cerebral paragonimiasis, resulting in symptoms such as seizures, motor dysfunction, and vision problems [2,3].

Paragonimiasis is diagnosed by detecting Paragonimus eggs in mucus that has been coughed up, as well as stool, lung fluid, and skin tissue samples, depending on the type of infection [8].

A healthcare provider may request blood tests, chest X-rays, and other imaging tests to learn more.

A lung infection of paragonimiasis (pulmonary paragonimiasis) can be confused with tuberculosis (TB), so laboratory tests must be conducted to accurately identify the source of infection [4, 8].

Antiparasitic medications, mainly praziquantel or triclabendazole, are the gold standard for paragonimiasis treatment. Studies show that treatment with praziquantel is effective in more than 90% of lung fluke infections [9].

The best way to prevent parasitic infections like paragonimiasis is to avoid eating raw, undercooked, or pickled seafood, including freshwater crabs [1].

Despite the significant public health burden of paragonimiasis, several challenges hinder effective disease control and research.

Lack of effective diagnostic resources

One major challenge is the lack of effective diagnostic tools, especially in regions without sufficient resources. Current diagnostic methods rely on microscopic examination of sputum and stool samples, which can be time-consuming and require specialized expertise.

As a result, there is a pressing need to develop more sensitive and specific diagnostic tests, such as serological assays, that are cost-effective and can help improve disease detection and treatment [8, 10].

Need for more research on transmission

Another challenge is the limited understanding of the epidemiology and transmission dynamics of paragonimiasis.

Further research is needed to identify the key factors contributing to the spread of the disease, including the role of intermediate hosts like snails and crustaceans and the impact of environmental factors such as climate change [8, 10, 11].

Better treatment options

Additionally, there is a need for more effective treatment options for paragonimiasis. Current treatments rely on anthelmintic medications like praziquantel, which can have variable efficacy and may not be effective against all species of Paragonimus. Further research is needed to develop new and more effective treatments, including combination therapies and novel compounds.

Overall, paragonimiasis is a significant public health concern that requires further research and attention. Addressing the challenges in disease control and research will be critical to reducing the burden of this foodborne parasitic disease and improving the well-being of affected communities.

1. What is the incubation period of paragonimiasis?

65-90 days, which is 2-3 months [3].

2. What are the complications of paragonimiasis?

Complications of paragonimiasis typically involve the lungs and can be quite severe. Common issues include lung damage, secondary bacterial infections, and various respiratory problems [9, 12].

In some rare cases, the adult worms can migrate to other organs and tissues, such as the brain and striated muscles, leading to more severe manifestations [9, 12].

If left untreated, paragonimiasis can result in long-term health consequences, including chronic respiratory problems and an increased risk of secondary infections [9, 12].

3. Is paragonimiasis curable?

Yes, paragonimiasis is curable. If left untreated, cerebral paragonimiasis may lead to death in 5% of cases.

4. Who is most at risk of getting paragonimiasis?

People who are at risk of getting paragonimiasis are those who eat raw, undercooked, or pickled crabs or crayfish. Other high-risk individuals include those with a history of asthma, chest pain, and lesions in the lung [13].

5. What is a lung fluke?

A fluke, also called a trematode, is a type of parasitic flatworm. The most common species include P. westermani, which causes a human infection of paragonimiasis, and P. kellicotti, which primarily infects dogs. However, rare human infections of P. kellicoti have been recorded [14].

6. How do lung flukes infect humans?

Lung flukes infect humans when they consume undercooked or raw crabs or crayfish containing the parasite’s larvae. Once ingested, the larvae move from the intestines through the diaphragm to the lungs, where they mature into adult flukes. In the lungs, they lay eggs, completing the life cycle and causing paragonimiasis [14].

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