Echinostome

Echinostoma revolutum

ORDER

Echinostomida

FAMILY

Echinostomidae

TAXONOMY

Echinostoma revolutum Froelich, 1802.

OTHER COMMON NAMES French: Fchinostome.

PHYSICAL CHARACTERISTICS

Adult echinostomes range from 0.24-1.3 in (6-30 mm) long and 0.02-0.06 in (0.6-1.6 mm) wide. Echinostomes have a tegument (outer surface) carrying spines or papillae (small rounded projections) and an anterior collar consisting of 37 spines arranged in a characteristic pattern. The ventral sucker is slightly behind the oral sucker. The uterus takes up much of the front half of the body and two testes follow, one after another, in the posterior half. The comparatively small ovary is located in the center of the animal.

DISTRIBUTION

Europe and Asia.

HABITAT

Echinostome eggs hatch in freshwater. The first intermediate hosts are lymnaeid snails. Common snail hosts in the United States and Europe belong to the genus Lymnaea. Second intermediate hosts include frogs, mussels, turtles, and snails; definitive hosts include various waterfowl, chickens, and other birds. Flukes attach to the mucosa or the ileum.

BEHAVIOR

Echinostome eggs hatch in nine to 12 days in freshwater habitats. The miracidia can survive for only six to eight hours, so these fast swimmers must find snail hosts quickly. Penetration typically occurs along the mantle edge or foot of the snail. After developing into sporocysts, the eggs then undergo two or three generations of asexual reproduction as rediae, finally resulting in the development and release of cercariae. The progress from miracidia to cercariae takes about a month. The cercariae then swim and/or crawl to a second snail, a fingernail clam, a tadpole, or a silurid fish. The larval forms of this fluke use chemotaxis (orientation toward or away from a chemical stimulus) to detect snail slime trails and find their first intermediate hosts. They also engage in a searching method that uses such environmental stimuli as light and gravity. The cer-cariae burrow into snails and clams, where they encyst in soft tissue; but enter tadpoles and fish through the cloaca, eventually encysting in the kidneys.

Birds then devour the infected snails, clams, tadpoles, or fishes, and become the definitive hosts of echinostomes. Humans who eat raw or undercooked frogs also become definitive hosts. The metacercariae migrate to the cecum, small intestine, or rectum, and attach themselves to these organs with oral and ventral suckers. The hermaphrodites mature in the host's digestive tract, engage in self-fertilization, and lay eggs. In the laboratory, adult echinostomes live four to eight weeks. The eggs are transmitted to the environment via the feces.

FEEDING ECOLOGY AND DIET

As with other trematodes, these parasitic flukes depend on host species to supply their nutrition. These species include Lymnea or Planorbis snails as first intermediate hosts; snails or tadpoles as second intermediate hosts; and ducks, geese, chickens, partridge, pigeons, or humans as definitive hosts.

REPRODUCTIVE BIOLOGY

Echinostomes reproduce asexually as larvae, and sexually as adults. The yellowish eggs range from 0.0034-0.0044 in (88-113 pm) in length and about 0.024 in (61 pm) in width; the flukes may produce as many as 3000 per day. The miracidia have four rows of epidermal plates and papillae on the body. The cercariae have a 37-spined anterior collar like that of the adults, and a robust unforked tail. Mature flukes are hermaphrodites and self-fertilize.

CONSERVATION STATUS Not listed by IUCN.

SIGNIFICANCE TO HUMANS

People who eat raw snails or frogs may become infected with this parasite. Symptoms appear about two to three weeks later. Hemorrhagic enteritis (inflammation of the intestines) may result from severe infections. Milder infections typically cause weakness and weight loss. ♦

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