Morgan J A, Dejong R J, Snyder S D, Mkoji G M, Loker E S
Department of Biology, University of New Mexico, Albuquerque 87131, USA.
Parasitology. 2001;123 Suppl:S211-28. doi: 10.1017/s0031182001007703.
Schistosoma mansoni is one of the most abundant infectious agents of humankind. Its widespread distribution is permitted by the broad geographic range of susceptible species of the freshwater snail genus Biomphalaria that serve as obligatory hosts for its larval stages. Molecular phylogenetic studies suggest that Schistosoma originated in Asia, and that a pulmonate-transmitted progenitor colonized Africa and gave rise to both terminal-spined and lateral-spined egg species groups, the latter containing S. mansoni. Schistosoma mansoni likely appeared only after the trans-Atlantic dispersal of Biomphalaria from the Neotropics to Africa, an event that, based on the present African fossil record, occurred only 2-5 million years ago. This parasite became abundant in tropical Africa and then entered the New World with the slave trade. It prospered in the Neotropics because a remarkably susceptible and productive host, B. glabrata, was widely distributed there. Indeed, a snail similar to B. glabrata may have given rise to the African species of Biomphalaria. Schistosoma mansoni has since spread into other Neotropical Biomphalaria species and mammalian hosts. The distribution of S. mansoni is in a state of flux. In Egypt, S. mansoni has nearly completely replaced S. haematobium in the Nile Delta, and has spread to other regions of the country. A susceptible host snail, B. straminea, has been introduced into Asia and there is evidence of S. mansoni transmission in Nepal. Dam and barrage construction has lead to an epidemic of S. mansoni in Senegal, and the parasite continues its spread in Brazil. Because of competition with introduced aquatic species and environmental changes, B. glabrata and consequently S. mansoni have become less abundant on the Caribbean islands. Control of S. mansoni using praziquantel and oxamniquine has reduced global prevalence but control is difficult to sustain, and S. mansoni can develop tolerance/resistance to praziquantel, raising concerns about its future efficacy. Because of legitimate environmental concerns, snail control is unlikely to be an option in future control efforts. Global warming will impact the distribution of Biomphalaria and S. mansoni, but the magnitude and nature of the effects are poorly understood.
曼氏血吸虫是人类最常见的感染源之一。淡水螺类双脐螺属的易感物种地理分布广泛，它们是曼氏血吸虫幼虫阶段的必需宿主，这使得该寄生虫得以广泛传播。分子系统发育研究表明，血吸虫起源于亚洲，一种通过肺螺类传播的祖先物种在非洲定殖，并产生了端刺卵类群和侧刺卵类群，后者包含曼氏血吸虫。曼氏血吸虫可能是在双脐螺从新热带地区跨大西洋传播到非洲之后才出现的，根据目前非洲的化石记录，这一事件仅发生在200万至500万年前。这种寄生虫在热带非洲大量繁殖，然后随着奴隶贸易进入新大陆。它在新热带地区繁荣发展，因为那里广泛分布着一种极易感且繁殖力强的宿主——光滑双脐螺。实际上，一种与光滑双脐螺相似的蜗牛可能是非洲双脐螺物种的起源。此后，曼氏血吸虫已传播到其他新热带双脐螺物种和哺乳动物宿主中。曼氏血吸虫的分布处于不断变化的状态。在埃及，曼氏血吸虫在尼罗河三角洲几乎完全取代了埃及血吸虫，并已传播到该国其他地区。一种易感宿主螺类——稻草双脐螺已被引入亚洲，并且有证据表明尼泊尔存在曼氏血吸虫传播。水坝和拦河坝的建设导致塞内加尔曼氏血吸虫疫情爆发，该寄生虫在巴西仍在继续传播。由于与外来水生物种的竞争和环境变化，光滑双脐螺以及曼氏血吸虫在加勒比群岛的数量已减少。使用吡喹酮和奥沙尼喹控制曼氏血吸虫已降低了全球患病率，但控制难以持续，而且曼氏血吸虫可能会对吡喹酮产生耐受性/抗性，这引发了对其未来疗效的担忧。由于合理的环境问题，在未来的控制措施中，控制螺类不太可能成为一种选择。全球变暖将影响双脐螺和曼氏血吸虫的分布，但影响的程度和性质尚不清楚。
