VPA 211: General Veterinary Parasitology and Helminthology (3+1)

DEVELOPMENT IN THE INVERTEBRATE HOST

• Eggs hatch in about 10 - 12 days at 26^ºC. (depends on temperature) release miracidium.
• Miracidium is broad anteriorly, with small papiliform protrusion, tegument is ciliated and has a pair of eyespot. (Within 3 hrs it should reach the snail.)
• Miracidium actively penetrate into acquatic snail (L.acuminata), casting off its ciliated covering and develops into sporocyst. Each sporocyst gives rise to 1 - 6 rediae by polyembryony.
• Redia has a circular thickening behind the level of pharynx and a pair of blunt process at the beginning of the posterior quarter. They are 1 - 3 mm long.
• Usually redia produce cercaria (daughter redia may develop under unfavorable condition).
• Cercaria is Gymnocephalus, there is no spine and eyespot but dark granular cystogenous glands in the lateral part of body and long tail are present.
• Time taken for development of cercaria in the snail is 5-7 weeks under favourable condition. Within few minutes to 2 hrs of the release of cercaria from snail settles on grass blades / other plants just below the water level (submerged vegetation) casting off its tail and secrete a covering by the cystogenous gland froms the spherical cyst is called as metacercaria.
• Small number may encyst on the surface of the water and sink into the bottom of the water. Metacercaria is infective to D/H. They are pearly / milky white in colour with pinhead size.
• D/H acquire infection by ingestion of metacercaria along with herbage / drinking water.

Note: Infection of snail with one miracidium can produce over 600 metacercaria. They survive on the herbage for 4 – 6 weeks at R.H 70%. It survive for 270 – 340 days on moist hay for 8 months and on silage for 35 – 57 days. Under normal condition 3 – 6 months but die quickly in hot and sunny days. This process of one embryo giving rise to numerous offsprings is unique to trematode and is known as paedogenesis.

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