Author Fred Lewis
Introduction
Cercarial exposure of mice by tail exposure to S. mansoni offers significant advantages over other means of cercarial exposure. Skin penetration is the natural route of cercarial infection. In addition, there is no need to anesthetize the mice, which are secured by restrainers; and an investigator can obtain a relatively accurate estimate of the percentage of cercarial penetration by counting cercariae remaining in the suspension after the mouse’s tail has been removed.
Equipment
Mouse restrainers (see Follow-up comments, below)
Test tube racks of appropriate height to hold tubes of cercarial suspensions
Micropipette (Eppendorf 200 µl+) and tips
Materials and reagents
Aged tap water
12 x 75 mm glass or plastic test tubes
Adhesive tape
4” x 4” gauze pads
Procedure
· Pipette a pre-calculated number of cercariae into the 12 x 75 mm tubes, then fill the tube with aged water to within approximately 10 mm of the rim.
· Pick up a mouse by its tail and gently, but quickly pull it backward into the restrainer (photo – mouse restrainer) until it is completely inside the restrainer and the tail is completely through the opening in the bottom.
· Hold the tail securely with the fingers of one hand while tightening the lid of the restrainer with the other hand. Be careful not to cramp the mouse in the restrainer, but secure it sufficiently that it cannot escape.
· With the mouse in the restrainer, attach two small pieces of adhesive tape to anchor the tail in place.
· Wipe the tail clean with a gauze pad moistened with aged tap water, then insert the tail into the cercarial suspension, resting the restrainer on the top of the rack (photo – exposure setup).
· After 30-45 minutes remove the mouse, release it from the restrainer and return it to its cage.
Follow-up comments/recommendations
One can count the number of non-penetrating cercariae by emptying the contents of the test tube into a counting dish after the mouse tail has been removed. One can usually obtain a reasonable estimate by counting the contents from 4-5 randomly chosen tubes. Some cercariae may be intact (with a tail), whereas detached bodies may also be apparent. Cercarial tails will be numerous, but a count of the tails is an unreliable indicator of penetration success, since many of the detached tailsmay be caught up in the hair or tail skin of the mouse. Under the best of conditions, one can expect a penetration rate of at least 95%, or higher. A penetration rate of less than 90% may indicate a problem in the cercarial pool, or from some other source.
References
Stirewalt, M.A. and Bronson, J.F. 1955. Description of a plastic mouse restraining case. Journal of Parasitology 41: 328.
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.
