eMedicine Specialties > Pediatrics: General Medicine > Parasitology
Paragonimiasis
Updated: Apr 10, 2009
Introduction
Background
Trematodes of the Paragonimus genus cause paragonimiasis, a parasitic disease that strikes carnivores, causing a subacute to chronic inflammatory disease of the lung. Of the 10 or more Paragonimus species that are human pathogens, only 8 cause significant infections in humans.
The first case described in humans was at autopsy in Taiwan in 1879, when adult flukes were found in the lung.1 The most common is the Oriental lung fluke, Paragonimus westermani. The adult trematode is reddish-brown and ovoid. Adults have 2 muscular suckers, an oral sucker situated anteriorly and a ventral sucker at mid body on the ventral surface. The eggs, golden brown and asymmetrically ovoid, have a thick shell with an operculum.
This micrograph depicts an egg from the trematode parasite Paragonimus westermani. Eggs range in size from 68-118 µm x 39-67 µm. They are yellow-brown and ovoidal or elongated, with a thick shell. They are often asymmetrical, with one end slightly flattened. At the large end, the operculum (ie, lid or covering) is visible. Photo courtesy of The Centers for Disease Control and Prevention.
Pathophysiology
The life cycle of these flukes involves 2 intermediate hosts plus humans. Its complex life cycle involves 7 distinct phases: egg, miracidium, sporocyst, redia, cercaria, metacercaria, and adult. Adult flukes live in human lungs and deposit eggs into the bronchi. Eggs are expelled either by coughing or by being swallowed and passed in human feces. Eggs then develop in water for 2-3 weeks and ultimately release miracidia, which invade the first intermediate host (ie, a specific species of fresh water snail). These miracidia develop through sporocyst and rediae stages into cercariae. The cercariae emerge and invade the second intermediate host (ie, a crustacean such as crabs or crayfish), in which they become metacercariae.
This is an illustration of the life cycle of Paragonimus westermani, one of the causal agents of paragonimiasis. Photo courtesy of The Centers for Disease Control and Prevention.
When humans ingest raw infected crustaceans, larval flukes develop in the small intestine and penetrate the intestinal wall into the peritoneal cavity 30 minutes to 48 hours after excysting. They then migrate into the abdominal wall or liver, where they undergo further development. Approximately 1 week later, adult flukes reenter from the abdominal cavity and penetrate the diaphragm to reach the pleural space and lungs. The eggs may then be expectorated or swallowed. If these eggs reach a water source, the life cycle will start over again.2 Flukes mature, a fibrous cyst wall develops around them, and then egg deposition starts 5-6 weeks after infection. Lung flukes may live 20 years or more. In Japan, transmission has also occurred following human ingestion of raw pork from wild pigs that contained the juvenile stages of Paragonimus species.
Frequency
United States
Generally, small numbers of cases have occurred in immigrants from endemic areas; however, the first case of paragonimiasis was reported in the United States in 1986 in a nonimmigrant adult. It is an important infection to consider in Southeast Asians who have settled in various areas of the United States.3
International
Paragonimus species are endemic to Southeast Asia, Latin America (most commonly in Peru), and Africa (most commonly in Nigeria). Paragonimiasis is less commonly found in West Africa and Central and South America.2 An estimated 22 million people are infected worldwide. Prevalence of infection in endemic areas ranges from 0.1-23.75%.
Mortality/Morbidity
Death may occur during the acute phase of infection. For those who survive the acute phase, spontaneous recovery usually occurs within 1-2 months, but symptoms may recur intermittently over several years. Complications of untreated heavy infection include interstitial pneumonia, bronchitis, and bronchiectasis. Secondary complications may include bronchopneumonia, lung abscess, pleural effusion, or empyema. Untreated cerebral paragonimiasis has a mortality rate of approximately 5%.
Race
Paragonimiasis is most common in Asians, Africans, and Hispanics.
Sex
Prevalence of infection is higher among females. An increase in infection in men, most notably those who are middle aged, because of their traditional culinary habits, has been observed in Japan.
Age
Prevalence reportedly increases with age and peaks in older adolescents and young adults; prevalence then declines progressively with age. By the sixth decade of life, prevalence is less than 25% of its peak in young adulthood.
Clinical
History
About 20% of patients with paragonimiasis are asymptomatic. Abdominal pain, diarrhea, and urticaria occur during the acute phase, which corresponds to the period of invasion and migration of immature flukes. These initial symptoms are followed a few days later by fever, cough, dyspnea, chest pain, malaise, and sweats. The acute phase usually persists for several weeks. During the chronic phase, manifestations may be pulmonary or extrapulmonary. Chronic pulmonary symptoms consist of dry cough followed by a cough productive of tenacious and rusty or golden sputum. Pulmonary symptoms begin approximately 6 months after infection and are often mistaken for symptoms of tuberculosis (TB). The American College of Chest Physicians has established clinical practice guidelines for chronic due to TB and other infections.4
Eosinophilia and lack of fever suggest the true diagnosis. Peripheral eosinophilia is present in as many as 25% of patients.5 Patients frequently report vague chest discomfort, dyspnea on exertion, or wheezing. Life-threatening hemoptysis may occur in some cases. Extrapulmonary paragonimiasis can be divided into cerebral, abdominal, subcutaneous, and miscellaneous forms of the disease.
Extrapulmonary paragonimiasis can occur either from the migration of young or mature flukes to various organs or from eggs that enter the circulation and are carried to the following sites:
- Liver
- Spleen
- Kidney
- Brain
- Intestinal wall
- Peritoneum
- Mesenteric lymph nodes
- Muscle
- Testis/ovary
- Subcutaneous tissues
- Spinal cord
Although cerebral paragonimiasis occurs in fewer than 1% of symptomatic patients, it is the most common extrapulmonary site of infection and is responsible for 50% of all extrapulmonary disease.1 Moreover, it is seen in as many as 25% of patients requiring hospitalization. This form of the disease is also particularly common in children. Early symptoms resemble meningoencephalitis and may persist 1-2 months. Chronic phase symptoms include headache, vomiting, seizures, or weakness.
Eggs and worms may also cause surrounding cysts, abscesses, or granulomas to form. Cysts may occur in the intestinal wall, liver, spleen, abdominal wall, peritoneal cavity, or mesenteric lymph nodes. Symptoms may include bloody diarrhea or abdominal pain.
Physical
Physical findings are usually not impressive in pulmonary paragonimiasis, but may include the following:
- Clubbing of the fingers occasionally occurs.
- Lungs are normally clear but rales, egophony, or dullness to percussion may occur with complications such as pneumonia or pleural effusion.
- Cough begins as dry and progresses to productive with blood-tinged sputum.1 The late clinical picture is similar to chronic bronchitis or bronchiectasis with profuse expectoration, pleuritic chest pain, dyspnea, cough, and occasional hemoptysis.
- Signs of cerebral paragonimiasis include facial palsy, hemiplegia, seizures, and paraplegia.
- Ocular signs include impaired visual acuity because of optic atrophy, papilledema, and hemianopsia.
- Spinal involvement may produce monoplegia, paraplegia, lower extremity paresthesias, or sensory loss.
- Findings in cases of abdominal involvement may include palpable masses.
- Hematuria may be observed with kidney involvement, and eggs may sometimes be detected in the urine.
- Patients with subcutaneous paragonimiasis can present with migratory swelling or subcutaneous nodules containing immature flukes. These firm, mobile, and tender subcutaneous nodules are often found in the lower abdominal and inguinal region.
- Scrotal paragonimiasis may mimic epididymitis or an incarcerated hernia.
Causes
Factors that facilitate the life cycle of the flukes and subsequent transmission of infection to humans include the following:
- Large numbers of reservoir and intermediate hosts
- Behaviors such as spitting
- Culinary habits
In Asia, raw and undercooked crab or crayfish are popular foods. In Korea and Japan, raw crayfish are used to treat measles, diarrhea, and skin conditions. Some tribes in Africa eat raw crustaceans to cure infertility. Peruvians eat raw crab with vegetables and lemon juice. Paragonimiasis may also be acquired by consuming raw meat from a paratenic host that contains young flukes (eg, wild boar as "sashimi"). Infection may also be transmitted via contaminated kitchen utensils (eg, cutting boards, knives) or from cloths used to squeeze and strain juices from crabs for the preparation of soup.
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| References |
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References
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Further Reading
Keywords
paragonimiasis, Paragonimus, Paragonimus westermani, P westermani, Oriental lung fluke, parasitic infection, trematodes, paragonimiasis, inflammation of the lung, treatment, diagnosis, pneumonia, bronchitis, bronchiectasis, bronchitis, bronchiectasis, pleural effusion, empyema, abdominal pain, diarrhea, urticaria, facial palsy, seizures, optic atrophy, hematuria, hernia
