Can you leave food out for the cat, with worm treatment in it?
As it's effecting your dogs health I'd do something to discourage the cats visits, ie leaving citrus peel where it messes. Spraying it with a hose or water gun.
It must be frustrating for you. We have a cat highway in our yard, we watch cats (two adults and a kitten) walk across our yard- the same path, several times a day. They don't stay to mess, thank goodness (think they've been chased in the past by our Border Collie).
__________________ Caleb   Pippi |