Plagiorchis elegans: Requirements for Metacercarial Development to Infectivity, and Conditions Required for Excystment

The temperatures and times required for the development of Plagiorchis elegans (Digenea: Plagiorchiidae) metacercariae from encystment to infectivity were studied in an experimental host, Aedes aegypti (Diptera: Culicidae). The ability of metacercariae to excyst was evaluated in vitro and was equated with infectivity to the definitive host. Development of metacercariae to infectivity followed a sigmoidal curve at temperatures between 15 and 30°C. Rates of development increased significantly with temperature. At 15°C metacercariae first excysted 72 hr postinfection (PI); at 30°C this occurred as early as 12 hr PI. Excystment reached 80% after 132 hr at 15°C and 60 hr at 30°C. A minimum of 8 hr of contact with the host was required for subsequent development to infectivity in vitro. Excystment of infective metacercariae was temperature dependent. Less than 6% of metacercariae excysted at <30°C, whereas >80% excysted at >37°C. Such temperature requirements may explain in part why adult P. elegans are principally parasites of homeothermic animals.