The structure of the male reproductive system of Schistosoma margrebowiei as viewed by light, scanning and transmission electron microscopy is described. The cirrus tube opens at the genital pore without the presence of a cirrus and is spongy and leaf-like. The structure of the germinal and non-germinal cells of the testes are described. The spermatozoon consists of a head, without an acrosome, and a flagellum with a 9+0 arrangement of microtubles in the axial filament. A large mass of mitochondria is situated in the anterior end of the head and cortically arranged longitudinally running microtubles are present. The apical region of the epithelial cells of the vas deferens extends into the lumen of the duct forming convoluted lamellae. This arrangement, together with the presence of dense bodies within the cells, suggests that these cells have a secretory function.

The cercariae of Schistosoma margrebowiei showed two peaks of emergence from Bulinus natalensis in a 12 h light/dark cycle. Peak emission occurred at 0700 h (one hour after the onset of light) and at 1900 h (one hour after the onset of darkness). Both peaks were of equal magnitude and were maintained during constant illumination indicating that the rhythm is innate. Delaying or advancing the timing of the dark period did not affect the timing of these two peaks. Following a five minute dark treatment elevation in cercarial output resulted irrespective of when the treatment was applied. Subjecting snails to various intensities of light only resulted in an elevation in output when a sudden change in intensity from 0 to 360 Lux or the reverse was applied. No response was seen to a gradual change in light intensity although the parasite could detect a change in light from 1 to 0 Lux. These responses appear to optimize the chances of host parasite contact.

Three age groups of Bulinus natalensis, immature, mature but not yet egg-laying and mature egg-laying, were infected with miracidia of Schistosoma margrebowiei. The growth of infected, exposed but showing no signs of infection, and uninfected control groups, were examined at weekly intervals for ten weeks post exposure. Snails exposed to infection when immature, even in the group where no patent development ensued, showed a statistically significant reduction in growth and this was evident as early as 2–5 weeks post exposure. When infection occurred at the stage prior to egg-laying a significant reduction in growth was seen but only in the group which developed a patent infection. This was also the case for the egg-laying group although the reduction in growth was only significant in the final three weeks of the experiment. Thus in all three age groups, growth rate was reduced and the infected snails were significantly smaller at the end of ten weeks compared with the controls. The reasons for these effects are discussed.

Transmission electron microscopy shows that the uterus of female Schistosoma margrebowiei possesses the same ultrastructure as that of the tegument but lacks spines and sense organs. It does not possess secretory cells and opens at the gonopore which by scanning electron microscopy was seen to be composed of numerous leaf-like protrusions. The morphology of the ovary is comparable with that of other Digenea. Immature and mature ova possess cortically arranged granules and occur within the posterior zone of the ovary. Cilia and lamellae line the luminal surface of the oviduct and ootype, the lamellae running unidirectionally along the duct. Only a single type of secretory cell is seen within Mehlis' gland and this produces dense bodies which are associated with Goldi bodies. Narrow cytoplasmic channels supported by microtubules deliver these secretory bodies to the ootype. The vitelline duct is lined with cilia and lamellae and the vitelline gland contains four types of cells, S1, S2, S3 and S4. Calcareous corpuscles are found within mature S4 cells.

The effect of various concentrations of praziquantel at different time intervals post-treatment on the ultrastructure of Schistosoma margrebowiei using scanning and transmission electron microscopy has been examined. The major changes involved blebbing of the entire surface tegument of both sexes (although more marked in males) together with vacuolation of the basal membrane accompanied by the development of membraneous whorls. These effects were progressively more marked with increased concentration and time of exposure resulting in severe erosion of the tubercles and collapse of the sensory organelles. Exposure of the underlying tegumental tissue resulted and paralysis and contraction of the suckers and neck region was apparent. Disruption of the subtegumental musculature and the appearance of vacuolation and membraneous whorl formation were seen. The gastrodermis was similarly affected and the S4 cells of the vitelline gland showed protein disruption of the vitelline droplets. Host cells were seen adhering to the surface of the worms following drug treatment and the synergism between PZQ and the action of the hosts immune system has been discussed.

The in vitro transformation of the miracidium to the mother sporocyst of Schistosoma margrebowiei was initiated by placing the miracidium in mammalian physiological saline. The transformation occurs in stages: the cilia cease beating; the ciliated plates become detached from the intercellular ridges and underlying muscle layers; the intercellular ridges spread over the body surface eventually forming a new tegument; the sporocyst changes from an ovoid to a tubular shape in about 48 h at room temperature. The surfaces of the miracidium, sporocyst and cercaria of S. margrebowiei display stage-specific carbohydrates on their surfaces as indicated by lectin staining. Ricin120 stains the cilia alone of the miracidium whereas peanut agglutinin stains the larval surface except for the cilia. The intercellular ridges of the miracidium stain with concanavalin A and wheat germ agglutinin, and these lectins stain the entire surface of the mature mother sporocyst. The cercaria is the only larval stage which stains positively with asparagus pea lectin. Bulinus nasutus is incompatible with Schistosoma margrebowiei; the haemolymph of this snail contains an agglutinin which agglutinates a wide variety of mammalian erythrocytes including those of human ABO blood groups. The haemagglutinin titre of B. nasutus plasma is reduced after incubation with miracidia of S. margrebowiei indicating that the agglutinin is absorbed onto the surface of this larval stage but not that of the mother sporocyst or cercaria. The possible roles of agglutinins in host–parasite interactions together with the significance of the differences in the surface carbohydrates of the larval stages are discussed.

Previous studies of mating interactions between schistosome species from different evolutionary lineages in mice showed that mating is not random: heterospecific pairs form but, given the choice, the preference is for homospecific partners. Mating competition and change of mate were also demonstrated. Schistosoma mansoni is a medically important parasite of humans belonging to a different evolutionary group from S. margrebowiei, a parasite of bovines of veterinary importance. Experiments were designed to investigate the mating behaviour of these two species in mice. Analysis of the data showed a preponderance of intraspecific pairs over interspecific, demonstrating a specific mate preference system for both species. The existence of mating competition between these species was indicated. Males of both species were able to actively compete for paired females by pulling them away from their partners, thus effecting a change of mate. Overall, neither species appeared to be competitively dominant to the other, and the data suggest that where this is the case in sequential infections, the most successful species in terms of worm return and ability to form pairs will be the first species to infect the host.