This may not be the source of the giardia, but grocery store meat is BAD to feed raw because it sits out for extended periods of time. Sealed does not mean sterile, there's no way of knowing how long it sat out and any bacteria it picked up will be sealed in there with it to grow and spread. It's not an issue for us because we cook it before eating, but feeding it raw to a pet is a risk.We get it sealed in plastic, but not frozen. So it should not contain any parasites I don't think?
If you want to prepare your own raw food you'd be better off going to an actual meat market or butcher where it is processed on site and source your meat fresh even if it costs more. When you get it home wash it off, make a batch and separate it into daily portions, then put it in the freezer and only thaw out enough for a day at a time. Surface cooking wouldn't be a bad idea either.