Location of Bulinus (Physopsis) globosus by Miracidia of Schistosoma haematobium

Abstract

Young Bulinus (Physopsis) globosus snails were exposed to miracidia of Schistosoma haematobium under various conditions of mutual dispersion to investigate host location mechanisms. There is little variation in infectivity of miracidia taken from different human hosts on different days. Exposure of one snail to one miracidium in cylindrical vessels with diameters ranging from 2.3 to 29.0 cm filled to 0.5 cm depth resulted in infection of about 42% of snails; this percentage did not increase if three snails were used in each exposure. With the snails confined under small perspex cages, the percentage infected by single miracidia rose significantly. It is suggested that this was the result of a chemotaxis, and experiments using a simple Y-maze demonstrated an initial response some distance from the snail. In a long trough miracidia were found to range over 294 cm in still water; in water flowing at 15 cm/sec 10 miracidia per snail produced no infections. At 7.5 cm/sec 6.7% of snails became infected. The introduction of a weir to cause turbulence increased infection. It is suggested that miracidia of S. haematobium are unable to infect snails in moderately fast-flowing water unless turbulence occurs near the snails. The success of the free swimming miracidium in finding its host is an important feature in the bionomics of schistosomes. Selective pressures must have evolved the most efficient means of finding and infecting the appropriate snail. It is therefore reasonable to presume that in the miracidium specific mechanisms exist adapted to the behavior and physiology of the appropriate intermediate host. Faust (1924) pointed out a distinct difference between the behavior of miracidia of Schistosoma japonicum (Mollendorf) and those of S. haematobium (Weinland) derived from the northern African snail Bulinus truncatus (Audouin), and suggested that this was an adaptation to the habits of the different intermediate host snails. Wright (1959a) in discussing host location by trematode miracidia has presented arguments suggesting the presence of a chemical attraction between the host snail Biomphalaria (= Australorbis) glabrata (Say) and the miracidia of S. mansoni Sambon. Recently, Maclnnis (1965) showed that miracidia of S. mansoni react positively to chemostimulation in the form of either the appropriate host snails, or extracts of these snails, or to small agar pyramids impregnated with chemicals such as butyric, glutamic, or sialic acids. He reports Gottschalk (1960) as stating that sialic acids are known to be components of mucopolysaccharides, substances which may be present in the mucus of snails Received for publication 16 February 1968. (Wright, 1959b); similarly, Etges and Decker (1963), using a four-arm maze, concluded that S. mansoni miracidia were attracted to B. glabrata. However, Chernin and Dunavan (1962), investigating infection of B. glabrata by miracidia of S. mansoni in various sized vessels, concluded that attraction between the miracidium and the snail had not been demonstrated unequivocally. Host location in running water has been investigated by Webbe (1966) using a trough and the miracidia of S. mansoni with Biomphalaria sudanica tanganyicensis (Smith). He concluded that water flow may enhance the scanning capacity of miracidia because he obtained high infection rates in caged snails exposed to miracidia in water flowing at velocities of 0.5 to 3.5 ft/sec (15 to 105 cm/sec). The work reported here concerns the ability of S. haematobium miracidia to infect Bulinus (Physopsis) globosus (Morelet) under different conditions of mutual dispersion in still and flowing water. Many of Chernin and Dunavan's experiments have been repeated and acknowledgment for these is duly tendered. MATERIALS AND METHODS Snails were laboratory-bred B. globosus. They were exposed when the shell height was between 3.5 to 5.0 mm; i.e., between 4 and 8 weeks old. Parent colonies and young snails were maintained at room temperature in the laboratory. Unless otherwise stated, all exposures of snails to miracidia were carried out in the dark and in constant temperature cabinets regulated to 22 ? 0.5 C. In this way extraneous stimuli were elim-