Biting

Question
My Jack Russell just turned 1 year old & he has become more aggressive over the past month.  Now over the holiday's he has bit my daughter (20 yrs old# & myself.  Both times were following an event at our house of which we had a house full of people for.  My daughter was 1st on Xmas night, he threw up a little bit so my daughter went to pick him up so my husband could clean it up & he bit her.  The 2nd time was New Years day I found him licking a toothpick that had fallen on the floor.  When I went to pick it up he snarled at me so I stood up & told him no & pointed at him & he jump up & latched onto & bit my thumb.  Now, he has a liver shunt which we were told this could make him a little edgy & he has not been fixed & we were told that may help too.  I am looking to get as many informed opinions as I can on this.  Please advise how you think is best to deal with this.  THANK YOU

Answer
Hi Rebecca,
I answered this a week ago but it never registered, sorry not sure why it bounced back.
I believe you've already answered a part of your own question - yes absolutely the crowd and loud hectic environment can cause your dog to act out. Jacks are pretty sensitive to all their surroundings and get very (overly) excited and stressed when their "space" is occupied or taken over by things or people they can't control.  It's just the way they "protect" their environment. Having said that, it's NOT OK that they bite... anyone/ever.
I know it's not often that you have that many people in your house but next time you do, it's probably better if you remove  him from the stress and stimulation - just for his sake and peace of mind.
I don't know when he got the liver shunt, you didn't mention if it was a recent change, but if it was, he most definitely could be feeling uncomfortable and protective - you know how WE feel when we don't feel good (last thing you want to deal with is a bunch of people).
Lastly, ABSOLUTELY neutering him will bring him down quite a few notches on the aggression; intact males are very protective and can be aggressive in times of stress.  
I'm glad you picked up the toothpick, that could have killed him!
Now that it's calm and quieter I bet his disposition has turned back to normal, but neutering should be your number one priority with him as well as some more behavior training.
You were correct when you told him NO, but saying it in a LOW, firm, loud voice works better than a panicky shrill (not saying this is what you did), and the MOST IMPORTANT part (which takes time) is to learn his "signs" of turning from calm to aggressive. The correction has to be absolutely immediate after the offense. The SECOND you see him even THINKING of getting that way, its up to you to stop it. Use your voice and your body to move forward toward him and use the corrective word... ("no" is so overused that it's a good idea to use a different corrective word only when dealing with aggression). Don't use his name or "no".  Act very very fast with the correction so he understands what you're correcting. The longer the time span between offence and correction is critical. Anything after a few seconds will be futile and confusing to the dog.
It takes time in cases like these because your dog is not generally aggressive.
Please get him neutered and have patience, he's still young and trainable.
I wish you both nothing but the best!!! Good luck!