If they test positive (or even not, since he said the poo samples aren't 100% accurate, as in there might not be any eggs etc in the particular poop you brought, diarrhea is just a pain to test). The roundworm stuff (I know for sure and I assume it's similar for most other common worms) doesn't exactly kill the worm. It paralyzes them and then they are passed. It doesn't do much to eggs or larvae, and the life cycle is about 2 weeks, so if you pass a bunch from the first worming, but they laid eggs, then those eggs will have hatched and grown enough in 2 weeks to get the second batch cleared. Blake had pretty bad worms when we got him and about a week or so after the 2nd dose, he started up with loose stinky poop again, so we wormed a 3rd time and that seemed to do the trick--perfect little sausages in the box from here on out.

It is entirely possible, however, that Britain being more advanced than the US that you actually have wormers that do a clean sweep of all lifestages and so you don't have to do the multiple treatments, but I do believe I've read some posts from people on here who were recommended to do just what we did.

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My babies
Aber, Ragnar, Kelso (the corgi trio), Daisy (our rescue mystery-mix puppy), Ellie, Henry, Gwennie, Jezebel (the kitty quartet), Blake, Chaucer, Molly and Milton (the kittens), and Bingo the African Grey parrot

Over the Bridge:
Pippin: 1/6/08-17/5/11
Freja: 11/8/10-30/10/10
Kirby: 11/8/10-1/11/10

"Praise liberty/The freedom to obey/It's a song that strangles me/Well, don't cross the line" Billie Joe Armstrong