If if bio-engineered a virus...?

Assuming you had a strong background in Biochemistry, Biology and Molecular Biology, plus a lot of high-level experience in bio engineering, you MIGHT be able to eventually come up with a virus specific to the Cane Toad. But it would probably take you 20 or 30 years to come up with an effective virus that didn't affect other desirable varieties of toad. There are Bio-Ethics involved here.