Sudden aggression

Question
I have a 4 yo old fixed male boxer named tank, and a 8 mo to 1 yo unfixed mixed female puppy.  I believe she has just started her first heat.  These 2 have been the best of friends since day one, always running and playing with each other.  a disagreement every once and awhile, nothing major.  until the other night.  she went after him, full force.  took 5-7 min to separate the two.  there was no food around, which has caused the minor altercations before.  ever since then the boxer is scared of the puppy, he cringes and cowers when she is near. and the puppy has become overly aggressive towards him. so much so we have kept her on a leash when they are out of their kennels. i don't know what to do.  has her 1st heat caused this?  or could it be something more?  any help is appreciated.

Answer
Hi Mike, Neutered or not, an intact female in heat will protect herself until she is ready to be bred. When she went after him, was there bloodshed or just a bunch of threatening? Being her first heat, she may not know how to respond to a male. If there are lost of threats, but no bloodshed or fierce biting, I would let them settle it. Since she is older (you probably had her longer) and he is young, she may be either being protective of herself or her territory. Usually the females are in charge over a neutered male. You must establish yourself as the pack leader and firmly tell them both "NO BITING" is allowed. I use the command "GENTLE" with my pack. It is important that you show no fear and that you stand between them and even growl at the one who starts the altercation. Sometimes you must make them both get in the submissive down position to establish your authority. Give this a try, leave me some feedback, and if you have further questions along the line, do not hesitate to write me back. Let me know how it goes. Thank you for writing me.  Warm Regards, Susan