The one thing I can think of is possibly destroying the "good" bacteria in their gut...you may want to let them have some ManaPro OptiZyme or a similar product with live and active bacteria...plain yogurt also works.
My Gunner took the Panacur Power Pac (five day double dose), and then Quest for bots a few weeks later with no side affects.
"Sit tall in the saddle, hold your head up high. Keep your eyes fixed where the trail meets the sky. And live like you ain't afraid to die...don't be scared, jut enjoy the ride." - Chris LeDoux