Shedding cercariae from patent snails

Introduction
After cercariae mature within the snail, they are then liberated (shed) into the surrounding water. In field conditions, cercariae typically emerge in greatest numbers in the daytime. In the laboratory and for experimental purposes, investigators can adjust lighting conditions to take advantage of maximal release of cercariae at a time of the investigator’s choosing. Some cercariae will still emerge in the dark. The following procedure works well for both B. glabrata and B. t. truncatus.

Equipment
Incubator or modified room that can hold a steady temperature around 26ºC
Incubator fitted with a strong light course

Materials and reagents
Featherweight forceps (photo – snail forceps)
200 ml beakers
Aged tap water
Filtration screen apparatus (if available) consisting of a 300 ml funnel with glass support (Kontes Glass) and a 47 mm diameter stainless steel support screen (photos – cercarial screen and assembled cerc filter)

Procedure

• Using featherweight forceps, remove snails from the maintenance incubator, and place them in a 200 ml glass beaker with 50 ml aged tap water.
• Place the beaker under a strong light source (e.g., in a 26ºC incubator) for 1-2 hours, taking care not to overheat the snails.
• Remove the snails, and place them back into the maintenance incubator.
• Pour the cercarial suspension through the filter apparatus into a clean beaker.

Follow-up comments/recommendations
The filtration apparatus and stainless steel filter used in this procedure are ideal for filtering out large particulate material, e.g. snail feces, from the cercarial suspension.

References
Lewis, F.A., Stirewalt, M.A., Souza, C.P., and Gazzinelli, G. 1986. Large-scale laboratory maintenance of Schistosoma mansoni, with observations on three schistosome/snail host combinations. Journal of Parasitology 72: 813-829.

Tucker, M. S., Karunaratne, L. B., Lewis, F. A., Frietas, T. C., and Liang, Y-S. 2013. Schistosomiasis, in Current Protocols in Immunology 19.1.1-19.1.57, John Wiley and Sons, Inc., (R. Coico, Ed).  Published online November 2013 in Wiley Online Library (wileyonlinelibrary.com). doi: 10.1002/0471142735.im1901s103.