Does stranger anxiety occurs only in breastfed babies?

Not at all: This is a normal stage of infant development that begins around 6 mo & can last well into the 2nd yr or longer. I find it a good marker for mental awareness/intellectual ability long before they learn to speak. They realize their primary caretaker is more dependable to provide for their needs & prefer them. It may be lost in kids who never have a consistent caretaker (multiple & unpredictable).