Almost all breeds and types of horses, ponies and donkeys can be affected, from tiny Shetland ponies to heavyweight draught horses, although the condition is rare in English Thoroughbreds. In South Australia reports say that as many as 60% of horses and ponies are affected. About 5% of the UK horse population is thought to suffer. In France, more and more horses who never suffered from sweet Itch start now to swish.  

Although known by different names (e.g. Sommer Ekzem in Germany, Kasen in Japan, Queensland Itch in Australia and Sand Fly bite in the Middle East), the symptoms are the same. These include severe pruritus [itching], hair loss, skin thickening and flaky dandruff. Exudative dermatitis [weeping sores, sometimes with a yellow crust of dried serum] may occur. Without attention sores can suffer secondary infection.

The top of the tail and the mane are most commonly affected. The neck, withers, hips, ears, forehead and around and below the eyes can suffer. In more severe cases, the mid-line of the belly, the saddle area, the sheath or udder and the legs may also show symptoms.

The animal may swish its tail vigorously, roll frequently and attempt to scratch on anything within reach. It may pace endlessly and seek excessive mutual grooming from field companions. When kept behind electric fencing with nothing on which to rub, sufferers may scratch out their mane with their hind feet and bite vigorously at their own tail, flanks and heels. They may drag themselves along the ground to scratch their belly or sit like a dog and propel themselves round to scratch the top of their tail on the ground.

Diagnosis is not usually difficult - the symptoms and its seasonal nature (spring, summer and autumn) are strong indicators. However symptoms can persist well into the winter months, with severely affected cases barely having cleared up before the onslaught starts again the following spring.

Horses that develop Sweet Itch usually show some signs of the disease between the ages of 1 and 5 years although there is anecdotal evidence that stress (e.g. moving to a new home, sickness, castration or severe injury) can be a factor when more mature animals develop the condition. It is common for the symptoms to first appear in the autumn, although they may be fairly mild at this time and could go unrecognised.