Symptoms of flea allergies in pets include intense itching, which causes chewing, licking, and excessive scratching. Hair loss is a secondary symptom. Bacterial skin infections are not uncommon. The animal's rump is the most usually affected area.
Ironically, most animals who are allergic to fleas groom excessively and therefore have only a few fleas – but only a couple bites every couple of weeks are enough to cause persistent itching.
Interestingly, fleas love humidity. So if you have a pet allergic to fleas, a dehumidifier may help give them some relief.