2 dogs in home one becoming aggressive towards other

Question
Last September, my husband my husband and i added a new family member to our family, an 8 week old female boxer. We brought her home and joined her with our older dog, Monkie, a 7 year old lab mix. The 2 dogs got along great, both curious of each other, but have gotten along great. We had only 2 spats through the past year, one in which the boxer was being a playful puppy and bouncing around and the older dog(whom happens to be very laid back and lazy) wasnt in the mood they growled and had one snap at each other and it was done, maybe lasted 10 seconds.  The other incident was about 2 months ago when they got into an argument over a t-bone bone. They batted, growled, and bit at each other for about 15 seconds when we got them seperated, and spat was over. Never having to seperate them for longer than maybe 2 minutes and it was over and they would be running around playing again.
Just last thursday, 3 days before Bella turned 1, they were playing in the basement, laying by each other, and getting along great. About 2 hours later we gave them their heartworm medicine (which is disguised at a chewy treat), and Bella went bizzerk, she ate hers and decided she wanted the older dogs treat also, she went straight for her nick and was biting and batting at the older dog and would not stop. When we get them seperated, we yelled bad dog at her, and put ehr outside for a few minutes. As soon as we brought her back in the house she went after Monkie again, only this time monkie defended herself and bit down on bellas neck, causing a large laceration. In the process of trying to seperate them my husband got bit through the thumb. That night we made them sleep in seperate kennels right next to each other (both are used to sleeping with us and NEVER spend overnight in a kennel)
The next day Bella had to go to the vet as the bite in her neck became very swollen overnight.They cleaned her all up, put her on antibiotics and sent her home.
These 2 dogs whom have never spent a day apart since last September have now been completely seperated for 4 days. We have tried each day while having them on leashes to put them close together, and as soon as they are in reach the boxer jumps and starts fighting. Throughout the day we keep one in the living room and the other in the kitched seperated by a dog gate. At night they sleep in their kennels now, however even when a foot apart behind bars they show no aggressive interest in each other. However as soon as they are in the same room and nothing is in between them Bella tries attacking.
Like ive said we have had them seperated since last thursday, but both my husband and i are so confused by the recent fighting and are at a loss for what to do. They were perfect together for the first year, just last week they were at the lake together, swimming, digging, and they were never more than 4 feet apart as always, and now they cant be in the same room together?? Any thoughts or suggestions would be greatly appreciated.

Answer
Hi Karie. I've experienced a few of my own intra-household dog fights over the past 20 years.  I can tell you that they are mostly over resources (in your case the heartworm pill), but they can ultimately be about establishing a position in the pack.

Bella isn't a puppy anymore.  She's reaching social maturity (and possibly sexual maturity if she's not spayed) and the license to be a silly puppy is long gone.  Monkie will no longer tolerate her antics and has probably been "telling" her that for some months now and Bella has been accepting that message.  However, it sounds to me like Bella has decided that she wants to take over the role of alpha dog in the household and is just at that age to do it.  Once a dog has been established in that position, fighting generally ceases, as long as the other dog is OK with giving up that top slot.  

Keep in mind that Bella is not feeling good right now.  She's sore, she's on meds and she's not trusting her old friend anymore.  Monkie has lost trust, too, so you have 2 dogs who are anxious and wary of each other.  And sometimes the best way to protect yourself is to lash out first.

You're doing the right things keeping them separated, but don't rush to put them together too soon.  During a fight, cortisol is released into the dog's bloodstream and remain there for quite some time. I can't find the citation right now, but I believe this can last for up to a week.  Increased cortisol in the dog's system causes increases in heart rate and BP, muscles are ready for fight or flight, vigilance is heightened.  All of these things make the dog more reactive and less able to "think".  Each time they have a stressful encounter, this situation continues to cascade and will take even longer to resolve.

I would give it at least 2 weeks continuing your management of keeping the dogs separated.  Use crates and baby gates so they can see each other as normally as possible.  After that, go ahead and try to allow them in the same area.  Have a leash attached, but don't hold the other end.  That causes more tension.  Do it casually as if by accident, but remain calm yourself.  Breathe normally, because when you don't, the dogs pick up on it.  Be sure all potential resources (toys, food) are put away so there's not a chance of a fight developing.  Don't pet either dog, ignore them both as if everything is fine.  Watch for potential trouble signs (head lowering, staring for more then 2 seconds, sniffing each other for more than a few seconds, high tail position on either dog). Keep this first re-introduction short, then return them to their own areas.  Each day, gradually allow them more time together.

There's a techique that I recently used successfully in a client's home with 2 dogs who were having serious fights.  It's called BAT - Behavior Adjustment Technique and was developed by Grisha Stewart.  Basically, BAT allows the dog to make good choices  - before she makes bad ones like aggressing - and then creating a distance between the dog and the "trigger" - Monkie.  All work is done below trigger threshold, so you would begin at a distance where she can see Monkie, but not react, then retreat.  The website is at:  http://ahimsadogtraining.com/blog/bat/  Purchase the 1.5 hour DVD to get more details than what you'll find on the website.

I recommend you consult with a local trainer who uses positive reinforcement methods.  No type of physical punishment or shock collars will help this situation.  If you'd like I can refer you to someone in your area.  Going for the neck is a serious type of fighting - Bella meant to kill Monkie, so this isn't something that's going to magically go away without some type of professional help.  Obedience training and learning patient and polite behaviors is key to maintaining a harmonious multi-dog household.  

Good luck and I hope your husband heals up as well.  Let me know if you have further comments or questions about anything I've suggested.