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Echinostomiasis

[Echinostoma spp.]

Causal Agents

Trematodes in the genus, Echinostoma. The genus is worldwide, and about ten species have been recorded in humans, including E. hortense, E. macrorchis, E. revolutum, E. ilocanum and E. perfoliatum. .


Life Cycle

lifecycle

Many animals may serve as definitive hosts for various echinostome species, including aquatic birds, carnivores, rodents and humans. Unembryonated eggs are passed in feces The number 1 and develop in the waterThe number 2. The miracidium takes on average 10 days to mature before hatchingThe number 3 and penetrating the first intermediate host, a snailThe number 4. Several genera of snails may serve as the first intermediate host. The intramolluscan stages include a sporocystThe number 4a, one or two generations of rediaeThe number 4b, and cercariaeThe number 4c. The cercariae may encyst as metacercariae within the same first intermediate host or leave the host and penetrate a new second intermediate hostThe number 5. Depending on the species, several animals may serve as the second intermediate host, including other snails, bivalves, fish, and tadpoles. The definitive host becomes infected after eating infected second intermediate hosts The number 6. Metacercariae excyst in the duodenumThe number 7 and adults reside in the small intestineThe number 8 .

Geographic Distribution

Worldwide, but human cases are seen most-frequently in southeast Asia and in areas where undercooked or raw freshwater snails, clams and fish are eaten.

Clinical Presentation

Catarrhal inflammation often occurs due to the penetration of the sharp-spined collar into the intestinal mucosa. In heavy infections, nausea, vomiting, diarrhea, fever and abdominal pain may occur.

Echinostoma spp. egg in wet mounts.

 

Eggs of Echinostoma spp. have a wide size range depending on the species and may range from 80-135 µm long by 55-80 µm wide. They have an inconspicuous operculum and the abopercular end is often thickened. The larger eggs are very similar to Fasciola and Fasciolopsis. Eggs are passed unembryonated in feces.
Figure A

Figure A: Egg of Echinostoma sp. in an unstained wet mount of stool. Image taken at 400x magnification.

Echinostoma spp. adults.

 

Adults of Echinostoma spp. are much longer than wide and measure about 2-10 mm long by 1-2 mm wide, depending on the species. The oral sucker is surrounded by a collar of spines, the number of which varies among species. The oral and ventral suckers are located fairly close to one another. A single ovary is situated near the large, paired testes. Adults reside in the small intestine of the definitive host.
Figure A

Figure A: Adult of E. revolutum, stained with carmine. Structures illustrated in this figure include: oral sucker (OS), armed collar (CL), cirrus sac (CS), ventral sucker, or acetabulum (AC), uterus containing eggs (UT), ovary (OV), paired testes (TE), and vitelline glands (VT). This species has been recorded from humans in Taiwan.

Echinostoma sp. in tissue, stained with hematoxylin and eosin (H&E).

 

The following images were taken from an adult echinostome removed from a colon polyp during routine colonscopy.
Figure A

Figure A: Adult Echinostoma removed during a colonoscopy, stained with hematoxylin and eosin (H&E).

Figure B

Figure B:Higher magnification of the anterior end of the specimen in Figure A. Notice the acetabulum (ventral sucker, AC).

Figure C

Figure C:Higher magnification of the posterior end of the specimen in Figure A. Notice the vitelline glands (VT) and lobed testes (TE).

Figure D

Figure D:Higher magnification of the specimen in Figures A-C. Shown here are eggs (EG) within the size range for Echinostoma spp. (roughly 100 micrometers in length, taking into account they are sections and may not be cut in a perfect horizontal plane).

Intermediate hosts of Echinostoma spp.

 

Like all trematodes, Echinostoma spp. require a snail as a first intermediate host. The second intermediate host may also be a snail, sometimes the same individual snail that served as the first intermediate host. Due to the large geographic distribution of echinostomes, and the many species present, there are many species of snails that may serve as first or second intermediate hosts. Other second intermediate hosts include bivalves, fish and tadpoles.
Figure A

Figure A: Lymnaea sp. This snail genus has been recorded as a second intermediate host for E. malayanum. Image courtesy of Conchology, Inc, Mactan Island, Philippines.

Figure B

Figure B: Radix sp. This snail genus has been recorded as a first intermediate host for E. hortense and a second intermediate host for E. cinetorchis. Image courtesy of Conchology, Inc, Mactan Island, Philippines.

Figure C

Figure C: Gyraulus sp. This snail genus has been recorded as an intermediate host for E. cinetorchis. Image courtesy of Conchology, Inc, Mactan Island, Philippines.

Figure D

Figure D: Viviparus sp. This snail genus has been recorded as a second intermediate host for E. cinetorchis and E. hortense. Image courtesy of Conchology, Inc, Mactan Island, Philippines.

Figure E

Figure E: Corbicula sp. This bivalve genus has been recorded as a second intermediate host for E. lindoense. Image courtesy of Conchology, Inc, Mactan Island, Philippines.

Laboratory Diagnosis

Microscopic identification of eggs in the stool. Because the eggs are large, careful measurements must be taken to avoid confusion with the eggs of Fasciola or Fasciolopsis. Species-level identification cannot be done based on egg morphology and adults are needed for a definitive diagnosis.

Morphology

More on: morphologic comparisons with other intestinal parasites.

Treatment Information

There are reports of successful removal of Echinostoma flukes from the gastrointestinal tract. Praziquantel* is the anthelminthic drug most commonly used for treatment.

* This drug is approved by the FDA, but considered investigational for this purpose.

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DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit www.cdc.gov/parasites/.

  • Page last reviewed: May 3, 2016
  • Page last updated: May 3, 2016
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