And now back to Denver. You don't say how old Denver is or whether the behavior has gotten worse over time. If it is getting worse as Bulldogs become more popular, perhaps there could be a cause-and-effect relation at work. Suppose Denver always felt uneasy about Bulldogs, but didn't get pushed over the edge when he saw one only occasionally. As Bulldogs have increased in popularity in your area and Denver sees them more and more often, he may not have time to relax completely between sightings, so each one is a bit more stressful than the one before.

So, what to do to address Denver’s behavior? A good idea is to engage in a program of counterconditioning and desensitization. This is a behavioral modification technique used by trainers in which the trigger of your dog’s negative response, in this case Bulldogs, is presented in a controlled way and your dog is taught to associate Bulldogs’ presence with something he just adores and doesn't get at any other time. Before undertaking counterconditioning and desensitization, please get professional advice from someone who can spend some time with Denver and get to know him and his specific responses to Bulldogs. Counterconditioning and desensitization must be carefully planned by a professional well-versed in this technique and implemented slowly and patiently in order to see good results. An alternative method to counterconditioning and desensitization is called Constructional Aggression Treatment (CAT).  CAT is relatively new behavioral modification technique in which your dog is taught alternative, nonaggressive responses. However, because CAT is relatively new on the scene, it may be difficult to find someone experienced.