Pasko, Swan and Dan Adkins share the third chapter of the plague story, in Superman 313 (July 1977).
It’s not a huge surprise when Amalak fails to kill Supergirl at the top of the issue. Superman had used his heat vision to short out Amalak’s gun.
Perhaps Superman could have found another way, because the short-circuiting star-cannon explodes while Supergirl and Amalak are in battle, destroying his asteroid home. Superman has brought Nam-Ek to the resort, where his horn proves effective at curing people of the plague. But to Superman’s dismay, the outbreaks continue.
Steve Lombard’s nephew Jamie seems to have some resistance to it, but Clark has other issues to deal with. He ought to have paid more attention, as he winds up in a battle with a mysterious alien, who just happens to also be Jamie’s dog.
Only then does Superman put the pieces together, realizing that the dog was at the scene of every plague outbreak.
But the creature is not exactly a shape changer. Rather, it has the ability to appear to people in any form it wishes, hypnotically. So while we and Superman see only an alien, all Jamie can see is his dog, which Superman is ready to kill.
The story concludes in the next issue.