Giardiasis prevention

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Would love to know if anyone has any tips or advice.
Following on from my other thread about dogs with diarrhoea. Am now fairly convinced they've got giardiasis, lab probably had it for ages with symptoms coming and going.
Hoping the antibiotics will clear it up. I didn't realise it was so common and easy to catch,have let dogs drink from ditches and dirty puddles will have to start training them not to and carrying a large bottle of water and bowl with me on walks. Problem is they're big dogs and need a big bowl.
Also lab loves swimming, will I just have to watch him carefully and make sure he doesn't drink the water?

I don't think there is a magic answer - I think dpup had this a few months back - he's not a scavenger of food but was an enthusiastic drinker of muddy puddles

I do take water out with me - I've got one of those drinking bottles for dogs so that cuts down on the amount of stuff to cart around - and I try as much as possible to watch him like a hawk and I take him to places with clean, running water rather than standing water to help him itch his retriever's water scratch

Currently there is no way to prevent giardia.

At what point should it be treated?
My lab might have had giardia for ages, he'd get loose mucousy stools/diarhoea for a few days, get better and I'd assume it was something he'd eaten on a walk. Then maybe a month or so later same thing again, never went to vets as he always seemed to completely recover.
I wouldn't want him to keep having antibiotics so is it ok to only treat if diarrhoea lasts longer than a week? Assuming diarrhoea is the only symptom and dog fine otherwise.