The Lung Flukes
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p699-700
2025-11-10
166
GENERAL CHARACTERISTICS
The genus Paragonimus contains several species known to infect humans. Paragonimus westermani is the most common and widely distributed lung fluke. The adult worms live in the lungs and produce eggs that may be present in sputum, or if expectorated and swallowed, may be present in feces. Like other trematodes, a fresh water snail is required as an intermediate host.
EPIDEMIOLOGY AND LIFE CYCLE
Paragonimus is found primarily in the Far East (China, Japan, Korea, Manchuria, Papua New Guinea, and South east Asia), and certain species (such as P. mexicanus) are found in areas of Mexico and South America. Reservoir hosts for P. westermani include dogs and cats, and those for P. mexicanus include domestic and wild pigs, and dogs. Species of Paragonimus may also be found in other fresh water crab- or crayfish-eating mammals.
The adult worms, encapsulated in the lungs, produce eggs that leave the lung via the bronchioles, stimulating a cough response. The eggs are then swallowed and eventually excreted in the feces.* (*Egg size varies with species from approximately 80 to 120 µm long and 45 to 70 µm wide). The free-swimming miracidium is released from the egg in freshwater and enters the snail host where it develops into a redia and then a cercariae, which leaves the snail and enters the water. The cercariae then enter a second intermediate host, a crab or crayfish, where they encyst and develop into metacercariae. The metacercaria is the infective stage for humans. When infected freshwater crabs and crayfish are eaten raw or undercooked, the metacercariae will excyst in the duodenum and then migrate through the intestinal wall, and eventually through the diaphragm and into the lungs where they encapsulate (usually in pairs) and mature (see Figure 1).
Fig1. Life cycle of the liver and lung flukes.
PATHOGENESIS AND SPECTRUM OF DISEASE
Light infections may be asymptomatic. The migration of the metacercariae through muscle and tissue may cause local pain and immune response to tissue damage. In the lungs, the immune response causes infiltration of eosinophils and neutrophils. Serum IgE levels are usually elevated. Eventually the adult worms are encapsulated in a granuloma. Presence of the worms in the lungs usually results in a chronic cough, with possible production of blood-tinged sputum. The cough provides a mechanism to transport eggs up into the throat where they are swallowed and then may be excreted in the feces. The larvae of P. mexicanus may migrate to other areas of the body, frequently causing the formation of subcutaneous or lower abdominal nodules. The larvae of Paragonimus may even enter the brain (rarely), where they can cause severe damage.
LABORATORY DIAGNOSIS
The adult worms of Paragonimus vary in size, 10 to 25 mm × 3 to 5 mm, and are a reddish-brown color. The eggs of P. westermani measure 80 to 120 µm × 45 to 60 µm, and those of P. mexicanus are approximately 80 µm × 40 µm. The eggs are unembryonated when they leave the body, operculated with opercular shoulders, thick shelled, and brownish-yellow. The eggs of Paragonimus are similar to those of Diphyllobothrium (freshwater fish tapeworm), but may be distinguished by the operculum, opercular shoulders, and thickened shell at the end opposite the operculum.
Paragonimus eggs (see Table 1) may be recovered from sputum, and occasionally in feces using a sedimentation concentration method. The eggs may be observed in a wet mount (with/without iodine stain) (Figure 2). Charcot-Leyden crystals may also be observed in sputum or lung tissue specimens. Charcot-Leyden crystals are slender and pointed at both ends. The crystals normally appear colorless and stain purplish to red with trichrome. Elevated levels of eosinophils in whole blood and elevated IgE levels in serum may be present. Lesions in the lungs may be observed in x-ray. There is also serologic testing available in the United States for diagnosis of P. westermani. The Division of Parasitic Disease at the Centers for Disease Control and Prevention (CDC) per forms serum IgG EIA and immunoblot testing, and EIA serum and cerebrospinal fluid (CSF) IgG antibody testing is performed at private reference laboratories; cross reactivity with other species and trematodes may occur.
Table1. Characteristics of Liver and Lung Trematodes
Fig2. Paragonimus westermani egg. (Photo courtesy Dr. Henry Travers, Sioux Falls, S.D.)
TREATMENT AND PREVENTION
The drug of choice for treatment of Paragonimus infections is praziquantel given three times a day for 2 days. Alternative treatments are bithionol (which may have mild side effects such as skin rash) or a drug that has not yet been approved in the United States, triclabendazole.
Human infection can be prevented by not eating pickled, raw, or undercooked crabs and crayfish. Care should also be taken to properly clean utensils used in the preparation of these foods. Improvement of sanitary conditions and practices may also help to reduce the prevalence of these infections.
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