"Comparative Studies on the Susceptible and Non-Susceptible Biomphalaria alexandrina the Intermediate Snail Host of Schistosoma mansoni in Western Saudi Arabia"

Schistosomiasis is an important parasitic disease that infects humans. Among the main species of schistosomes infecting humans is Schistosoma mansoni and S. haematobium. Snails of various genera, such as Biomphalaria alexandrina and Bulinus truncates, act as intermediate hosts and play a major role in the transmission of schistosomes. Transmission from human to snail is under the influence of the ciliated miracidium larvae that hatch from the eggs of the parasite voided in the faeces or urine. After a period of asexual multiplication in the snail, a second aquatic larval stage, the cercaria, emerges and infects human by direct penetration of the skin. The present work was carried out on Biomphalaria alexandrina snails, the intermediate host of Schistosoma mansoni. Snails were collected from various sites in south western Saudi Arabia. Their progeny were reared in the laboratory under standard conditions. Different biological experiments were carried out to determine the susceptibility level in the various populations of snail. The miracidia were obtained by isolating viable ova of the hatching parasite from faeces of infected individuals (human and mice). Different sets of regulated experimental infection of snails with the miracidia of Schistosoma mansoni were preformed. The effect of miracidial density, water temperature, miracidial age, size of snails, as well as light and darkness were investigated. The study revealed that the infection rate increased as the number of miracidia used increased (1, 60%; 2, 60%; 3, 70%; 5, 80%; 15, 90%; and 20, 90%). No significant increase of infection rate was achieved at higher water temperatures (at 15°C: 65% with newly hatched miracidia; 75%, with 15 minutes miracidia; 85% with 30 minutes miracidia; 95% with 45 minutes miracidia and 100% with 60 minutes miracidia. At 20°C: 75% with newly hatched miracidia; 80%, with 15 minutes miracidia; 90%, with 30 minutes miracidia; 95% with 45 minutes miracidia and 100% with 60 minutes miracidia). Similar results were obtained at 25°C and 30°C. However, the infection rate decreased in relation to snail size increase. At snail size 2-4 mm the infection rate was 90%, the rate decreased to 85% at 4-6 mm and 6-8 mm size, while at 8-10 mm size the rate was 80%. At 10-12 mm and 13-15 mm size the rate dropped to 75%. Light and dark conditions had a significant difference on the infection rate (40% in darkness and 90% in light). Laboratory observations also indicated that some of the snails exposed to infection with miracidia of the respective parasite developed infection and produced cercariae, while the rest remained uninfected.