Authors Fred Lewis and Yung-san Liang
There are several invertebrates that can interfere with the growth of Biomphalaria glabrata and/or the development and release of cercariae from the infected snails in a laboratory setting. Among the most common are rotifers, ostracods and oligochaetes. Field-collected snails may harbor a wide variety of contaminants, some of which, if left unchecked, may have a deleterious effect on a laboratory-maintained snail colony. A variety of techniques have been suggested to control or eliminate these contaminants, but in many cases one may have to completely restart the snail colony with uncontaminated snails or egg masses. This section will discuss control methods that our laboratory has used to limit the populations of contaminating rotifers, ostracods and oligochaetes.
Equipment, materials and reagents
Peristaltic pump with tubing and 20-22 gauge needle
or dental water pic
Aged tap water
Rotifers are frequent contaminants in aquaria, and many will adhere to the shells of B. glabrata, forming a mat of organisms in the center whorl (photo – rotifers on shell). From published (and unpublished) data, we know that at least one species of the bdelloid rotifers emits a small molecular weight component that can cause a reversible paralysis of S. mansoni cercariae and limit cercarial release from patent snails. If rotifers are found as a contaminant of the snails in a laboratory, limiting their numbers is essential if active cercariae are a necessary part of the lab’s experimental procedure. Rotifers can be removed from the shells of B. glabrata by mechanical means.
· One effective removal method of rotifers is to direct a stream of aged tap water onto the surface of the snail’s shell. The force of the stream is provided by a perfusion pump attached with tubing and a 20-22 gauge needle (or by a commercial dental water pic). If the snails are shedding cercariae, one obviously must take care to shield oneself from any spray.
· Alternatively, one may use a Q-tip to swab the snail shell surface to reduce the rotifers to manageable levels. Whatever the means of mechanical removal used, the procedure should be repeated whenever rotifers are observed building up again on the shells. Periodic examination of snail shell surfaces under a dissecting microscope is highly recommended to prevent rotifer problems.
There are several reports in the literature about the effect of ostracods on snails. These commensals tend either to attack the bodies of the snails or to disturb them enough to cause them to withdraw into their shells and prevent the snails from feeding. Ostracod eggs can be harbored in the snail and passed through the intestines. Continuous removal of snail feces may eventually eliminate the problems caused by these organisms.
Little is known about the interaction of oligochaetes and Biomphalaria spp., but they are frequently found as contaminants of snails collected from the field. Michaelson reported that the oligochaete, Chaetogaster limnaei, had a dramatic effect on infection of the snail, but that they could be eliminated by immersing the snail in 1% urethane for 10-20 minutes.
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.
Stirewalt, M.A. and Lewis, F.A. 1981. Schistosoma mansoni: effect of rotifers on cercarial output, motility and infectivity. International Journal of Parasitology 11: 301-308.
Michaelson, E.H. 1964. The protective action of Chaetogaster limnaei on snails exposed to Schistosoma mansoni. Journal of Parasitology 50: 441-444.
Liang,Y-S, van der Schalie, H., and Berry, E.G. 1973. Transmission of ostracods in snails. Malacological Review 6: 66.