Life History of Manson's Blood Fluke (Schistosoma mansoni). I. Extramammalian Phase of the Cycle. ∗

The work of which this contribution forms a part was carried on in Puerto Rico, where Manson's blood fluke infection is an important and extensive clinical entity. The lateral-spined eggs of the parasite were obtained from the feces of human and experimental animals. The stools were washed and concentrated either by centrifugation or by sedimentation. The numbers of these eggs obtained from a 24-hour specimen varied not only with the age of the infection in the mammalian host but also fluctuated from day to day. The majority were viable, and contained ciliated larvae (miracidia), which were studied for detailed morphological features, including the number and pattern of the epithelial plates.

Hatching was found to be due to several factors, including the maturity and vitality of the miracidium, the tonicity of the medium and mechanical agitation of the eggs. Some larvae hatch immediately after washing and concentration but the majority require from 12 to 24 hours. After swimming around for several minutes in or near the bottom sediment the miracidia rise to the top of the water level and swim energetically in the top inch of the water. Under laboratory conditions their length of free-living existence was not over 24 hours, usually considerably less. If specimens of laboratory bred snails of the species Helisoma (Planorbina) guadeloupense, the appropriate intermediate host in Puerto Rico, are placed in a pan containing miracidia hatched from washed viable eggs of S. mansoni, they are soon attacked by the miracidia, which are attracted to the soft parts of the snail, particularly the tentacles. Some of these larvae secure a permanent hold on the tissues and bore their way inward. The compatibility of this host-parasite relationship was confirmed by the high percentage of snails which usually became infected (75–100%). Exposed snails were sacrificed at suitable intervals and dissected to determine the stage of development of the intramolluscan phase and the position of the parasite in the snail's body.