The effects of temperature and light on the survival, development, and infectivity of Plagiorchis elegans eggs

Plagiorchis elegans eggs were incubated at 0, 4, 10, 20, or 30 C under a 12 -hr:12-hr (L:D) photoperiod for 120 days. Approximately one-quarter of the eggs had already initiated the process of embryonation when passed with the feces of the hamster (Mesocricetus auratus), the experimental definitive h ost. Eggs failed to embryonate at 0 C and died within 2 days. Incubation at 4 C allowed full embryonation, but the mean number of embryonated eggs per day (1.32 +/- 0.15) and the mean number of eggs available over the course of incubation (egg days) (219.00 +/- 2.24) remained low. These values incre ased progressively as incubation temperatures rose and reached levels of 3. 59 +/- 0.30 and 1,467.50 +/- 4.56, respectively, at 20 C. Although incubati on at 30 C further increased the mean number of eggs per day (5.45 +/- 0.56 ), the mean number of egg days declined sharply to 735.25 +/- 4.71, suggest ing that elevated temperatures enhanced embryonation but lowered the surviv al of embryonated eggs. This was also reflected in the infectivity of eggs over time. Embryonated eggs incubated at 10 and 20 C remained infective sig nificantly longer than eggs incubated at higher or lower temperatures. Incu bation in constant light yielded significantly lower mean numbers of embryo nated eggs per day (1.86 +/- 0.09) and mean number of egg days (96.25 +/- 0 .99) than incubation in constant darkness (2.23 +/- 0.17 and 701.50 +/- 2.8 6 eggs, respectively) but did not affect the infectivity of embryonated egg s. The data suggest that at moderate temperatures and shielded from light, fully embryonated eggs of P. elegans may survive in the aquatic environment for a period of months. Such eggs may serve as a source of infection for p opulations of Stagnicola elodes and ensure the sustained production of the highly entomopathogenic cercariae required for mosquito control.