Development of the Metacercaria and Adhesive Organ of Mesostephanus yedeae Dennis and Penner, 1971 (Trematoda: Cyathocotylidae), and their Effects on Host Tissues

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Abstract
Development of the infective metacercaria of Mesostephanus yedeae Dennis and Penner, 1971 included: (a) ceicarial penetration and encystment in striated muscles, brain, liver and eye of Poecilia reticulata Peters; (b) a quiescent phase of nine days when no observable morphological change occurred; (c) a period of morphogenesis when an adhesive organ and a posterior body segment were differentiated. The time required for development to the infective metacercaria was delayed by decreasing the temperature of the fish host environment. The quiescent phase exhibited the greatest effects resulting from lowered temperature. Radial muscles and glands of the adhesive organ originated from parenchymal beta cells. The posterior appendage issued as a dorso-posterior evagination of the metacercarial body. Effects of the metacercaria on host tissues resulted in formation of a fibrous capsule in striated muscles, a granulomatous-like response in liver tissue, and necrosis of nervous tissue of the retinal complex which terminated in blindness of the infected eye. Effects of the adhesive organ on duodenal villi of the definitive host included “tearing” of the lamina propria and “pinching off” of pieces of duodenal epithelium.