Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I'm sorry to hear that Missy has a cough and I understand that you are concerned about her.
A cough simply means she is experiencing airway irritation.
A cough can be related to heart disease, including heartworm, lung disease secondary to a viral, bacterial, fungal or parasitic infection or tracheal irritation due to a bacterial, viral or fungal infection.
Functional diseases of the larynx and trachea due to weakening cartilage, or poor nerve function, are also possible causes of a more chronic cough in an older dog. These are both worsened by secondary bacterial infections.
All of the possibilities I mentioned above could cause a cough.
In a young dog infectious disease is by far the most common cause. Of those kennel cough or infectious tracheobronchitis is one of the most common causes. Even dogs that have been vaccinated can get sick because we cannot vaccinate against all of the agents that cause kennel cough, so some dogs will still get sick even with the vaccine. The good news is that dogs that are vaccinated usually don't get as sick as dogs that haven't been vaccinated.
But in an older, over weight, small breed dog that has a chronic cough, without a fever, laryngeal disease and/or a collapsing trachea are more common. Heart disease, especially in a dog with a previously diagnosed heart murmur and an enlarged heart on radiographs is another common cause.
A less common disease, but one seen more in older, small breed terrier dogs, is Idiopathic pulmonary fibrosis (IPF). This is a chronic and progressive disease characterized by development of scarring in the lungs. It is very difficult to diagnose so we will often rule out other diseases first and then consider that as a possible cause. A definitive diagnosis is achieved only with a CT scan, which many owners understandably cannot afford.
It would be reasonable to try antibiotic therapy, especially if she has some tartar and gum disease.
Did she seem to improve at all with the medications given?
I understand that she has had some blood tests. Have any radiographs been taken?
Has she had an echocardiogram (ultrasound of the heart) to evaluate her for primary heart disease?
If she is not coming along as hoped with treatment then it is time for further diagnostic testing. That will be the only way to pin down her disease and direct treatment, which is more likely to be beneficial for her.
Please let me know if you have any further questions.