Meyer-Lassen J, Daffalla A A, Madsen H
Danish Bilharziasis Laboratory (WHO Collaborating Centre for Applied Medical Malacology and Schistosomiasis Control and Joint WHO/FAO/UNEP/UNCHS Collaborating Centre for Disease Vector Control in Sustainable Development, Charlottenlund.
Acta Trop. 1994 Dec;58(3-4):229-41. doi: 10.1016/0001-706x(94)90017-5.
Since the establishment of the Rahad Irrigation Scheme in 1979, the prevalence of schistosomiasis has increased to high levels in some villages despite the efforts of the Blue Nile Health Project to prevent this. Although a programme of focal snail control has been implemented since 1979, when the first intermediate host snails were detected, infected snails are abundant in human water contact near villages where transmission of both S. mansoni and S. haematobium takes place. The objective of the present study was to assess the feasibility and effectiveness of the focal snail control operations in irrigation canals, in terms of reductions in snail density and repopulation rates of treated sites. The efficacy of focal mollusciciding was evaluated by (1) evaluation of the routine molluscicide operation done by the Blue Nile Health Project, (2) experimental application of various dosages of the molluscicide and (3) study of snail repopulation rates. Efficiency was measured by mortality among caged snails placed in the target stretch and from pre- and post-treatment counts of relative snail densities. The short-time application of niclosamide can be very effective in reducing the density of the intermediate host snails in the target stretch of canal. An efficient exposure seems to be approximately 3 ppm (a.i.) for 30-40 min when water flow is slow, while 1 ppm is efficient when water velocity is very slow, i.e. less than 0.04 m S-1, and 0.6 ppm is efficient when the water is virtually stagnant. Turbidity seems not to impair the effect of the treatment and vegetation does not reduce the efficiency of niclosamide, provided the current speed is low and the chemical is applied to the entire width of the canal. However, the results also clearly demonstrate some of the shortcomings of the standard procedure of niclosamide application. A high water velocity and discharge will rapidly wash the chemical through the target canal stretch and the snails will be exposed only for a short period if the routine procedure is followed. In addition, the chemical will not mix sufficiently with water within standards of aquatic macrophytes. Recolonization of treated canal stretches is fast, especially during the irrigation period when water velocities are high and juvenile snails are abundant. Snail density may attain pretreatment levels three weeks after an effective application. Since the routine procedure is most effective at low current speeds, it is suggested that water velocity could be controlled during the application in coordination with the Irrigation Department.
自1979年拉哈德灌溉计划建立以来，尽管青尼罗河健康项目努力预防，但血吸虫病的流行率在一些村庄已升至很高水平。自1979年首次发现中间宿主钉螺后，虽然实施了局部钉螺控制计划，但在曼氏血吸虫和埃及血吸虫均有传播的村庄附近，人类与水接触区域仍有大量受感染的钉螺。本研究的目的是评估灌溉渠道局部钉螺控制行动在降低钉螺密度和处理区域再繁殖率方面的可行性和有效性。通过以下方式评估局部灭螺的效果：（1）评估青尼罗河健康项目开展的常规灭螺行动；（2）试验应用不同剂量的灭螺剂；（3）研究钉螺再繁殖率。通过置于目标河段的笼养钉螺死亡率以及处理前后相对钉螺密度计数来衡量效率。短时间施用氯硝柳胺可非常有效地降低目标河段灌溉渠道中间宿主钉螺的密度。水流缓慢时，有效暴露浓度似乎约为3 ppm（有效成分），持续30 - 40分钟；水流速度非常缓慢（即小于0.04 m S-1）时，1 ppm有效；水几乎静止时，0.6 ppm有效。浑浊度似乎不影响处理效果，并且只要水流速度低且化学药剂施用于渠道整个宽度，植被也不会降低氯硝柳胺的效率。然而，结果也清楚地表明了氯硝柳胺施用标准程序的一些缺点。如果遵循常规程序，高水流速度和流量会迅速将化学药剂冲过目标渠道河段，钉螺仅会短时间暴露。此外，在水生大型植物标准范围内，化学药剂与水混合不充分。处理后的渠道河段重新定殖很快，特别是在灌溉期，此时水流速度高且幼螺数量众多。有效施用三周后，钉螺密度可能达到处理前水平。由于常规程序在低水流速度下最有效，建议在施用期间与灌溉部门协调控制水流速度。
