In dogs, the most important cause of pustular lesions is infection
with Staphylococcus intermedius (ie pyoderma). It could
be suggested that pustules on canine skin should be assumed to be
staphylococcal in origin until proven otherwise! However,
pustules may be a feature of sterile, immune-mediated diseases (all
of which are rare in all veterinary species). The most important
example is pemphigus foliaceus in dogs, cats and horses.