Cellulitis is typically a more rapidly spreading infection with more systemic signs (fever, malaise), whereas erysipeloid is a localized, slowly advancing, violaceous plaque.
Erysipelas is a more superficial infection (upper dermis) with a more prominent, raised, advancing border and greater systemic toxicity than erysipeloid.
This is a recurrent lesion in the same spot triggered by a drug, not an occupational infection from handling animals or fish.
A spider bite reaction is often more painful and may become necrotic, and is a single event, unlike the slowly migrating plaque of erysipeloid.