Retired cattle rancher here, who went through screwworm inspections/quarantine in the '70s & I think into the '80s. (Ticks were a BIG problem back them, too.) Cattle w/ screwworms don't get past the auction barn and usually don't get into the auction barn as their employees are pretty eagle eyed. 1/
2/ Screwworms are devastating. New born calves have to have their ambilocal cords doctored within hours of birth or be eaten alive by them. Any small cut is open season from screwworms that will make a major mess of an animal within a day or so of infestation. The load on ranchers will be enormous.