I don't think it matters, ultimately, whether we say God 'created', or God 'designed', parasites.
In MOST cases it is the same deal either way. Until recently, anyone who accepted creation, accepted that God 'designed' the creation. A good portion of those, however, believed in something like the Fall. I remember growing up and hearing that 'evil' did not exist in the world before the Fall, and this may be true, but (sorry) how far do we take that?
I don't think you are asking whether an intelligent being could create parasites. You are asking about a benevolent being creating parasites.
Basically, if I look around, I see groundhogs eating my veggies, I see beetles destroying my cleomes, my cat is eating birds and moles, I am eating someone else's veggies, I am eating animals, insects are eating me...that's life. That is the same life which has always been.
So, what does intelligence do? It protests. It creates a God who also understands that 'life' as we know it, is hard and sad. It creates a God Who ultimately wants the 'intelligent' part of us to be free from sadness and pain. If humans evolved the intelligence to see a better way, what does that mean? Nothing, perhaps. It may just be our crutch to bear, to seek an ideal in a world where we all have to consume or be consumed, and where we will all end up being devoured by other life forms no matter what we believe. Does our intelligence ever seperate us from our own animal nature? Could we survive in a world where there were no controls of populations? Could we survive without animal consumers of life? If not, then it is very 'intelligent' to allow nature to be nature.