OG 12, SC 37，关于it使用的问题

37, Although schistosomiasis is not ofter fatal, it is so debilitating that it has become an economic drain on many developing countries.
(A) it is so debilitating that it has become an economic
(B) it is of such debilitation, it has become an economical
(C) so debilitating is it as to become an economic
(D) such is its debilitating, it becomes an economical
(E) there is so much debilitation that it has become an economical

搜索了一下，大家关于这条题目的问题集中在so X that Y，和so X as to Y。但我的问题在于正确答案A中两个it的使用。

OG上笼统地说"it" clearly refers to schistosomiasis（名词，血吸虫病），并被debilitating修饰。那么放回句子里：“schistosomiasis is so debilitating that schistosomiasis has become an economic drain on many developing countries” ？怎么理解？