The initiation of further development is fundamental to the
infectious processes of parasitic nematodes. We have examined
early developmental activation of Trichinella spiralis
larvae during host invasion, with particular emphasis on the timing
of events. Using a novel approach, we have observed changes in
tissue-specific transcriptional activity in live larvae during
the infectious process with the fluorescent nucleic acid dyes SYTO12
and acridine orange. Simultaneously, the metabolic
switch from anaerobic metabolism, characteristic of the infective
stage, to aerobic metabolism, as found in the enteral
stages, was tracked by measuring activities of the key regulatory
enzymes phosphoenolpyruvate carboxykinase and
pyruvate kinase, as well as isocitrate dehydrogenase (NADP) activity,
and used as a co-indicator for developmental
activation. Both metabolic enzyme activities and transcription patterns
were found to change in response to host death,
liberation from the nurse cell, and exposure to components of the
host stomach environment. The results give a clear
indication that the activation processes of T. spiralis
infective larvae occur at a much earlier time than previously thought,
and are stimulated upon liberation of the larvae from the nurse cell inside
the host stomach.