Summer mastitis, often known as “August Bag” is a common problem in the UK in grazing animals. As the name suggests, it is usually in the summer months from June to September. As the numbers of flies increases, the risk also increases as the disease is spread between animals by biting flies. Unsurprisingly cases will often be found in fields with ponds or trees, which provide a good environment for the flies. It is an acute disease of the non-lactating udder and is common in both dairy and beef herds. Dry cows and heifers are particularly susceptible, but calves can also be affected. The primary causative agent is Trueperella pyogenes, but mixed infections can also occur.