bubble_chart Overview

Paragonimiasis is primarily caused by the mechanical injury from the migration and residence of juvenile or adult worms in human tissues and organs, as well as the pathological immune responses triggered by their metabolites.

bubble_chart Epidemiology

Paragonimus westermani is widely distributed, with reports from Japan, Korea, Russia, the Philippines, Malaysia, India, Thailand, as well as Africa and South America. In China, it is found in 23 provinces and regions, including Shandong, Jiangsu, Anhui, Jiangxi, Zhejiang, Fujian, Guangdong, Henan, Hubei, Hunan, Sichuan, Guizhou, Guangxi, Yunnan, Taiwan, Gansu, Shaanxi, Shanxi, Hebei, Liaoning, Jilin, and Heilongjiang.

Patients and reservoir hosts are the Bingchuan sources of infection. Reservoir (paratenic) hosts include domestic animals (such as dogs and cats) and some wild carnivores (such as tigers, leopards, wolves, foxes, leopard cats, civets, and raccoon dogs). In certain areas, like Kuandian County in Liaoning, dogs are the primary source of pestilence. In most regions, however, wild animals are more significant. Overall, these infected animals play a more critical role in the epidemiology of Paragonimus westermani infection. Infected wild animals in remote areas where humans and livestock rarely venture constitute natural foci of the disease.

In endemic areas, there is a habit of eating raw or undercooked freshwater crabs or crayfish. In some mountainous regions, crabs are consumed raw, pickled, marinated, roasted, or boiled. Pickling and marinating do not kill the metacercariae in the crabs, effectively equating to raw consumption, which is the most dangerous method. Roasting or boiling often fails to kill all metacercariae due to insufficient time, resulting in undercooked consumption, which still carries a risk of infection. In northeastern China, crayfish tofu and crayfish paste are local delicacies, but these cooking methods do not kill the metacercariae, leaving the food contaminated with large numbers of live metacercariae, posing a significant risk. Additionally, utensils contaminated with live metacercariae, or the death of intermediate hosts causing metacercariae to detach and pollute water sources, can also lead to infection. There is also evidence that dogs can develop adult worms after being infected by cercariae. Therefore, drinking untreated water contaminated with metacercariae or cercariae may also result in infection. In recent years, reports have identified wild boars, pigs, rabbits, rats, chickens, spiny-bellied frogs, birds, and other animals as paratenic hosts for Paragonimus westermani. Consuming raw or undercooked meat from these paratenic hosts may also lead to infection.

bubble_chart Pathogen

Paragonimus westermani

bubble_chart Pathogenesis

The pathogenicity of Paragonimus westermani is mainly caused by the mechanical injury from the migration and residence of larvae or adult worms in human tissues and organs, as well as the immune pathological reactions induced by their metabolites.

bubble_chart Pathological Changes

According to the pathological process, it can be divided into acute and chronic stages. **Acute Stage** This is mainly caused by the migration and wandering of juvenile worms. Symptoms appear from a few days to about one month after ingesting metacercariae, with severe infections showing symptoms as early as the second day. After the metacercariae cause scrotal swelling, the juvenile worms penetrate the intestinal wall, leading to intestinal bleeding. They repeatedly wander in the abdominal cavity and abdominal wall, particularly as most juvenile worms migrate across the liver surface or through liver tissue, causing localized liver hemorrhage and necrosis.

**Chronic Stage** The lesions caused by juvenile worms entering the lungs can generally be divided into:

**Abscess Stage** Primarily due to tissue destruction and hemorrhage caused by worm migration. Macroscopically, the lesions appear as cavities or tunnels filled with blood, sometimes with visible worms. Subsequently, inflammatory exudation occurs, containing neutrophils and eosinophils. Then, granulation tissue forms around the lesion, creating a thin membrane-like abscess wall, which gradually develops into an abscess. X-rays show infiltrative shadows with blurred edges and indistinct boundaries. When accompanied by pleural effusion, the costophrenic angle becomes blunt.

**Cyst Stage** Due to exudative inflammation, a large number of cells infiltrate and aggregate, eventually dying and undergoing liquefaction. The abscess contents gradually turn into a reddish-brown viscous fluid. Microscopically, necrotic tissue, Charcot-Leyden crystals, and numerous eggs can be observed. The cyst wall thickens due to extensive granulation tissue proliferation, appearing macroscopically as well-defined nodular worm cysts resembling purple grapes. X-rays show clearly defined nodular shadows, sometimes with fluid levels. If the worms leave the cyst and migrate elsewhere to form new cysts, these cysts may interconnect. X-rays may reveal multilocular cystic shadows.

**Fibrous Scar Stage** After the worms die or migrate elsewhere, the cyst contents are expelled through the bronchi or absorbed, and the lesion is filled with granulation tissue, undergoing fibrosis and eventually forming a scar. X-rays show indurated or cord-like shadows.

The above **late-stage [third-stage]** lesions are often observed simultaneously within the same organ.

bubble_chart Clinical Manifestations

After scrotal swelling, the juvenile worms penetrate the intestinal wall, causing intestinal bleeding. They migrate repeatedly through the abdominal cavity and abdominal wall, particularly as most juvenile worms traverse the liver surface or pass through liver tissue, leading to localized liver hemorrhage and necrosis. The systemic symptoms during this stage can range from mild to severe. Mild cases may only present with nonspecific symptoms such as loss of appetite, lack of strength, weight loss, and low-grade fever. Severe cases, however, manifest acutely with pronounced toxic symptoms, including high fever, abdominal pain, and diarrhea.

bubble_chart Auxiliary Examination

Pathogen Diagnosis

⑴ Examination of sputum or fecal eggs: Detection of Paragonimus eggs can confirm the diagnosis.

⑵ Biopsy: Surgical removal of subcutaneous masses or nodules may reveal juvenile worms or typical pathological changes.

Immunological Tests

⑴ Intradermal test: Commonly used for screening, the positive coincidence rate can be as high as over 95%, but false positives and false negatives often occur.

⑵ Enzyme-linked immunosorbent assay (ELISA): Highly sensitive, with a positive rate of 90–100%.

⑶ Circulating antigen detection: Recently, the antigen spot test (AST-ELISA) has been applied to directly detect circulating antigens in serum, with a positive rate of over 98%, and it can also serve as an efficacy evaluation.

bubble_chart Diagnosis

The detection of Paragonimus eggs can confirm the diagnosis. Additionally, complement fixation tests, metacercariae membrane tests, paper disc solid-phase radioimmunosorbent assays, immunoelectrophoresis, agar double diffusion, indirect hemagglutination tests, and indirect charcoal agglutination tests have been used for diagnosing paragonimiasis. Recently developed techniques such as hybridoma technology, immunoblotting, and the biotin-avidin system have also begun to be applied.

bubble_chart Treatment Measures

Publicity and education are the most important measures to prevent this disease. It is advised to provide cooked food or avoid eating raw freshwater crabs and crayfish, and to refrain from drinking untreated water.

Commonly used treatment drugs include: bithionol, which primarily targets the reproductive organs of the parasite; and praziquantel, which offers advantages such as high efficacy, low toxicity, and a short treatment course.

bubble_chart Cure Criteria

After thorough treatment, the absence of clinical symptoms within 2 to 4 months and no detection of parasite eggs in stool examinations indicate a cure.

bubble_chart Prognosis

No sequelae, but reinfection should be prevented.

bubble_chart Prevention

Provide cooked food or avoid eating raw freshwater crabs and crayfish, and do not drink untreated water.

bubble_chart Complications

abscess, cyst

bubble_chart Differentiation

Easily misdiagnosed as subcutaneous nodules of the lung or pneumonia