If they need to be muzzled to protect sheep and other dogs, they shouldn't be worked in hot weather, or open wire muzzles that allow them to pant might be an option. The signs of heat stress include very rapid shallow breathing, weakness because of muscle damage then collapse because of kidney failure, and the next stage is death. In less severe cases the only signs are when the dog gets home and it is lethargic and reluctant to eat. Mr Burnell says that if the dog is to be left on the back of the truck in the heat of the day, some thought must be given to the siting of the truck so that it remains in shade as the sun moves round. He says that to prevent heat stress on hot days, working dogs must have ready access to drinking water at frequent intervals and be offered shade and rest from time to time to prevent the distressing effects of heat stress.
Heat stress can be fatal but it can readily be avoided.