I've never seen fly strike in cattle before, but yesterday we brought a Highland heifer calf in for tagging and to clean its dirty bum. It had been observed flicking its tail a lot, and was found to have newly-hatched maggots in the milky dung which had stuck to its legs and tail.
A timely thorough wash down to remove the faeces, and treatment with spot-on, prevented any further problem, but another 24 hours would have seen the maggots into the flesh.