Blood is drawn from the dog & tested for a protein shed by the female worm as she passes microfilaria (babies). This test is very reliable.
Then X-rays, blood labs, and urinalysis are evaluated to determine the impact of the heartworm infection on the dog’s health.
Your dog will then be staged for heartworm disease as part of the evaluation.
It assists your veterinarian in choosing the best method of treatment for eliminating the heartworms:
Lowest risk... young, healthy, minimal heartworm disease evident on X-rays, all other tests are normal.
Moderately affected... some coughing, some difficulty breathing, changes on X-rays, and blood work may have kidney and/or liver damage.
Severely affected... weight loss, coughing, and difficulty breathing, more damage visible on x-rays, and blood tests show kidney and/or liver damage.
Caval Syndrome... the dog is collapsing in shock, all of the above abnormalities are more intense and the dog is dying.
Another factor is when some animals develop a severe allergic reaction to the heartworms, or microfilaria. This more elusive kind of reaction occurs most often in cats. It is not uncommon for infected cats to suddenly die from the effects of just a few worms.