Fly strike
The thought of flystrike can send a chill through any shepherd; though no illness or disease in sheep is pleasant, this is one of the worst. After a mild, very wet winter, the numbers of fly larvae around will be greater than usual, so it will be doubly important to watch out for strike and deal with it without risking compromise to the health of the sheep or the potential to use the wool.
The economic cost of flies is estimated to reduce milk yields by 0.5 litres per cow per day and growth rates by 0.3kg per head per day in cattle, which can easily be extrapolated to sheep - it's not just the damage to an animal which has been struck, but the irritation which distracts the whole flock from getting on with eating and growing.