Spotthedog you've done an awesome job with Gus.
And both of you are right, things like this shouldn't happen. Every dog from a shelter should've been there long enough to have gone through a quarantine period after arrival, and had it's basic medical and physical needs attended to. Which is at least two weeks. If a dog from a shelter still has parasite problems after two weeks then the whole shelter has a problem and it should be addressed as an animal welfare issue.
There's no excuse for not doing these things, it doesn't matter how busy they are.