它的起源、发病机理、传播机制和途径尚不清楚。
It has been not known the disease origins, pathogenesis, transmission mechanisms and mode.
但到目前为止，其发病的原因和机制尚不清楚。
But so far, the reason and mechanism of this disease still are unclear.
但到目前为止，其发病的原因和机制尚不清楚。
But so far, the reason and mechanism of this disease still are unclear.
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