Did the vet say what kind of parasite? Can you ask them what parasite and then why Revolution wouldn't work on them? Maybe ask if there's any other medication at all that would help. I'd be very leery of going back for the second injection of Ivermectin, personally, but I'm not sure what else could be done as an alternative.
~*~*~ Kelsey ~*~*~
RIP my sweet Lily ~ 6/12/08 - 1/20/12