That is an interesting concept. I attempted to look some of this up and what it looks like is that if someone is afflicted with radiation poisoning, one of four things happen: Rupture of the strand, alteration to bases, destruction of sugars, crosslinks and formation of dimers. Basically what this sounds like is the breakdown of whatever DNA exists in your blood. Somehow I would think this would just kill an Animorph like every other person. I believe it would not be different under the assumption that nothing crazy would happen to a person who accepted a blood transfusion if they suffered from radiation poisoning. I suppose this would really depend on how stored DNA in an Animorph interacts with their human DNA though.....entirely speculation.
As far as howlers go. I don't think howler's sleep or drop guard, so I don't know how a Yeerk would make it's way inside one, presuming they have ears. Obviously a Yeerk would do this to exploit their howling ability, but I don't think that being inside it's head would make it any less susceptible to the howl.
My first post, not big on introductions, but my name is Chris, and I just discovered this place and I like it a lot. So hopefully I'll be around more often.