That depends on multiple factors including the age of the dog, how many worms were involved, how much the worms or treatment may have affected the animal's organs, and any problems that existed prior to the dog contracting heartworm disease. The work-up and tests done before treatment should tell you the dog's status at that time and it can be repeated after treatment to determine the animal's status now.
Although some dogs with heartworm may show obvious signs like coughing or exercise intolerance, many appear normal when diagnosed, experience minimal side-effects from the treatment, and function normally afterwards.
And although shi-tzus might be small, they're tough little dogs. Where they get into trouble is when their owners forget that and instead relate to them (babytalk, lots of reactive responses) in ways that actually communicate that person's inability to take care of themselves, let alone the dog. Being responsible for meeting the emotional needs of the owner and even protecting that person is the last thing any dog needs, but especially a little one with heartworm disease. Far better to relate to the animal in a way that communicates full confidence in yourself and your dog to make it through this just fine.
If you're not sure what you communicate to your dog, stand in front of a mirror and talk to it the same way you would to your dog. Pay attention to your body language and expressions as well as tone of voice. (Dogs respond to our tone of voice much more than our words with a higher sing-songy tone communicating submission rather than confidence.) Ask yourself whether what you see and hear would inspire confidence in another, or something lesser, such as pity, anxiety, or an inability to cope. If the latter, for your dog's sake, put your own fears on hold and get a grip for your dog, even if you have to fake it at first.
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