In the mouth early lesions generally appear as whitish ('blanched'), roughly circular areas on the dorsum of the tongue. As fluid collects in these they become more prominent and stand out from the normal surrounding epithelium. When viewed along its surface the tongue may appear corrugated. Vesicles on the tongue frequently reach several centimetres in diameter and are situated characteristically on the dorsum, but may involve the tip of the tongue or occur on the sides close to the teeth.

When vesicles are developing an animal is likely to be hyporexic and show signs of depression. Excess salivation usually occurs later, especially when vesicles in the mouth have ruptured and fresh erosions are present.

A febrile response, range 39.5ºC to 41.0ºC, is usually detectable from just before vesicles first become evident or coincident with their first detection, and for 3-4 days afterwards. A nasal discharge, serous at first, and mucopurulent 1-2 days later, are other early clinical signs.

The epithelia of vesicles on the tongue rapidly become necrotic and rupture, creating deep, red erosions, initially with a ragged edge. Handling the tongue is likely to result in pieces of epithelium becoming detached.

Vesicles followed by erosions also appear in other parts of the mouth, particularly on the dental pad, hard palate, gums and inside the lips. In addition, vesicles on the muzzle and in the nares are quite common.

Erosions on the tongue and other parts of the mouth usually heal rapidly. Within 3-4 days the base of the erosions becomes covered with a sero-fibrinous exudate and regeneration of the epithelium is complete within about 10 days, unless secondary bacterial infection supervenes.