Migration, development, and condition of Trichobilharzia ocellata (Trematoda: Schistosomatidae) in homologous challenge infections

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Abstract
Ducks previously infected with Trichobilharzia ocellata were challenged by exposure to conspecific cercariae. Comparisons were made among worms recovered from teased or squashed tissue or located in sections, after first, second, or multiple challenges. Contrasting with 24 h clearance in initial infections, schistosomula were retained in the skins of challenged birds, mainly in the stratum corneum, for up to 7 days. Parasites reached the lungs as early as day 1 in first and second challenges but more than 50% of these were adhering to bronchial walls in mucoid material. After third challenge, worms were seen in the lung on day s 3–7 but 35–83% of these were coated with lymphoid cells. Schistosomula were found in the livers of 16/19, 4/15, and 2/20 birds examined after first, second, and multiple challenges respectively, most being in sinusoids surrounded by lymphocytes. No worms assignable to challenge infections were found in the hosts' intestinal veins. Parasites which survived 7 days in challenge infections reached lengths of about 300 μm, about 10–15% of the normal value. Those from 16/19 birds retained cercarial features such as tegumental spines, eyespots, and head organs.

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