Shoukry N M, el-Assal F M, Soluman G N, Mansour N S
Department of Zoology, Faculty of Science, Cairo University, Giza, Egypt.
J Egypt Soc Parasitol. 1997 Aug;27(2):317-29.
Susceptibility of snail vectors to schistosome infection varies between geographical areas, populations in the same area and among individuals in the same population. It is also affected by biological factors of which are snail size (age), number and age of miracidia exposed to water, temperature, illumination and darkness. Our preview studies indicated that snails 4-6 mm in diameter exposed to 5 freshly hatched miracidia under light and in water at 25 degrees C are optimum conditions for infection of Biomphalaria alexandria from various localities in Egypt with Schistosoma mansoni (TBRI strain maintained in hamsters). In the present study, groups of 16 snails from three successive generations from positive and negative laboratory bred B. alexandrina from Giza, Alexandria, Menoufia, Cairo and Minia were exposed to S. mansoni miracidia (TBRI Strain) from Giza under the above mentioned optimum conditions. From the 20th day infection snails were checked from cercariae shedding to estimate the prepatent period and their number from each snail light for one hour. B. alexandrina from the various localities in Egypt were susceptible to S. mansoni (TBRI strain) infection. Snails from Alexandria were the most susceptible, followed by those from Cairo, Giza, Menoufia and Minia. Snails from positive generations were more susceptible to infection than those from negative ones. However, there has been a decline in the susceptibility of snails from one generation to the next. The prepatent period for shedding of cercariae was 27 days in case of snails from positive generations and it was extended to 38 days among snails from negative ones. It can be concluded that: (1) There are different levels of susceptibility between B. alexandrina from the various localities in Egypt and infection with S. mansoni from Giza. (2) Unsusceptibility (refractory character) of B. alexandrina to S. mansoni infection could be a heritable character like susceptibility. Consequently, it would be beneficial to select actively resistant snails and mass culture them to increase the proportion of alleles for insusceptibility as a possible mean for biological control of schistosomiasis in natural population.
钉螺媒介对血吸虫感染的易感性在不同地理区域、同一区域的不同种群以及同一种群的不同个体之间存在差异。它还受到一些生物学因素的影响，这些因素包括钉螺大小（年龄）、接触水中毛蚴的数量和年龄、温度、光照和黑暗。我们之前的研究表明，直径4 - 6毫米的钉螺在25摄氏度的光照和水中接触5只刚孵化的毛蚴，是埃及各地的亚历山大双脐螺感染曼氏血吸虫（在仓鼠体内维持的TBRI株）的最佳条件。在本研究中，将来自吉萨、亚历山大、米努夫、开罗和明亚的实验室培育的阳性和阴性亚历山大双脐螺连续三代的每组16只钉螺，在上述最佳条件下暴露于来自吉萨的曼氏血吸虫毛蚴（TBRI株）。从感染第20天起，检查钉螺是否有尾蚴逸出，以估计潜伏期，并在每只钉螺光照1小时的情况下统计尾蚴数量。埃及各地的亚历山大双脐螺对曼氏血吸虫（TBRI株）感染均易感。来自亚历山大的钉螺最易感，其次是来自开罗、吉萨、米努夫和明亚的钉螺。阳性代的钉螺比阴性代的钉螺更易感染。然而，从一代到下一代，钉螺的易感性有所下降。阳性代钉螺尾蚴逸出的潜伏期为27天，而阴性代钉螺的潜伏期延长至38天。可以得出以下结论：（1）埃及各地的亚历山大双脐螺对来自吉萨的曼氏血吸虫感染的易感性存在不同水平。（2）亚历山大双脐螺对曼氏血吸虫感染的不敏感性（抗性特征）可能与易感性一样是一种可遗传的特征。因此，积极选择抗性钉螺并进行大量养殖，以增加不敏感性等位基因的比例，可能是自然种群中血吸虫病生物防治的一种手段。
