Taenia taeniaeformis: Effectiveness of staining oncospheres is related to both temperature of treatment and molecular weight of dyes utilized

Abstract

Methods to determine viability of taeniid oncospheres following treatments with potential lethality have practical application in efforts to control transmission. Here we investigated several methods, in lieu of infectivity studies, to assess oncosphere viability and determine lethal temperature treatment regimens. In the first experiment, a standard treatment to exshell oncospheres with 0.5% hypochlorite was assessed for influence on oncosphere recovery of Taenia taeniaeformis eggs. Recovery of eggs and exshelled oncospheres decreased with increasing time in hypochlorite, which indicated that hypochlorite can damage eggs and oncospheres, translating into potential overestimation of lethality of experimental treatments. Losses in hypochlorite were accentuated when eggs were pretreated at 75 °C, but not lower temperatures, including 65 °C, indicating a sharp threshhold between 65 °C and 75 °C where eggs and oncospheres became hypersensitive to subsequent hypochlorite treatment. To further investigate this change in relation to temperature, non-vital (acridine orange, AO) and vital (propidium iodide, PI; trypan blue, TB) dyes were used to assess staining of oncospheres (exshelled or not) under conditions ranging from room temperature up to 95 °C. The behaviors of dyes as related to internal staining of oncospheres were described using non-linear regression and a sigmoid four-parametric model to determine the inflection point ( T 50). Each of the dyes differed significantly in T 50 estimates, e.g. AO (69.22 ± 0.53), PI (73.89 ± 0.52) and TB (79.43 ± 0.45). For these dyes, the T 50 increased in relation to the increasing molecular weight of the dyes. Collectively, the results suggested that barriers to chemical permeability exist in eggs that breakdown incrementally with increasing temperatures above 65 °C. This staining behavior and the likelihood that the temperatures involved are above a lethal threshhold clarify a basic limitation in the use of vital dyes to assess oncosphere viability. The results may be relevant to other Taenia spp.