Heartworm Disease in Dogs

12 min read


Diagnosis of Heartworm in Dogs


Any patient presented because of the suspicion of heartworm disease will have a thorough medical history taken and undergo a complete physical exam, chest X-rays and routine blood chemistry tests.


The definitive diagnosis (proof of diagnosis) is usually made through the use of a heartworm antigen test. Whole blood is drawn from the dog, stabilized so it will not clot, and tested for the presence of a protein shed by the female worm as she passes microfilaria. This test is very reliable and will detect heartworm burdens of 2-3 worms or higher.


Another common test that can be used in conjunction with the antigen test is the Knotts test or Modified Knotts test. This is where whole blood is drawn and treated to cause the blood cells to break open. Then the sample is spun in a centrifuge, the top portion is poured off, and the bottom sediment is looked at under the microscope for the presence of microfilaria.


Usually once a diagnosis is made via a blood test, then X-rays, CBC (complete blood count), chemistry profile (evaluates the function of the body’s organs), and urinalysis are evaluated to determine the impact of the heartworm infection on the dog’s health. Pets displaying signs of heart disease side effects may have a complete cardiac evaluation, or evaluation of any other area of the body that is indicated by the initial test results.


Your dog will 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:


Stage I

Lowest risk... young healthy dogs with minimal heartworm disease evident on X-rays and all other tests are normal.


Stage II

Moderately affected dogs... some coughing is noticed, some difficulty breathing, changes are seen on X-rays, and blood work may reveal some kidney and/or liver damage.


Stage III

Severely affected dogs...  the patient has weight loss, coughing, difficulty breathing, more damage visible on x-rays, and blood tests shows kidney and/or liver damage.


Stage IV

Vena Cava Syndrome or Caval Syndrome...  the dog is collapsing in shock, all of the above abnormalities are more intense and the dog is dying. They are initially treated with surgical jugular removal of some worms if possible. There is no guarantee this treatment will be successful and many patients with Caval Syndrome die in spite of treatment.


Treating Heartworms in Dogs


By now, it is clear that the treatment varies from dog to dog. Each animal’s personal condition is evaluated and the treatment protocol tailored to best effect a full recovery with the least side effects. Therefore, the following will be very general regarding the medications used for treating heartworm disease and the more common side effects. In the end, you and your veterinarian will choose the best treatment protocol for your pet.


Treatment involves two basic areas:


1.  Patient evaluation and stabilizing for treatment procedure.
2.  Elimination of all forms (adult, larvae, and microfilaria) of the heartworm parasite.


Patient Evaluation and Stabilization


The veterinarian evaluates the overall health of the animal by conducting X-rays and blood and heart tests, then determines how to best proceed with treatment. Part of this evaluation is staging the severity of the heartworm disease in the dog (see above).


Some animals need to have certain conditions stabilized before heartworm treatment can proceed. Those in third stage heartworm disease, for example, may require deliberation to decide if it is best to try surgical removal of some worms through the jugular vein before any other steps of parasite elimination are considered.


Elimination of the Heartworm Parasite


This is a two-step process. The adult worms and the microfilaria are eliminated separately. No one medication kills both. The adults are treated first, then a different treatment is used to kill the microfilaria and migrating larvae.


The most serious side effects usually occur with the treatment of the adult heartworms. As the worms die they lodge in the lung arteries and block even more blood vessels than before treatment.  Besides the usual inflammation caused by the presence of the worms, the inflammation is amplified due to the decomposing worms within the blood vessels.


The worm destruction also releases foreign substances in to the dog’s circulation as the worms break down and are eliminated from the dog by the immune systems. A large amount of inflammation and swelling generally occurs during this period.


Before treatment begins, it is very important to ask your veterinarian any questions you may have about the treatment and what to expect. Some veterinarians will keep the dogs in the hospital during treatments for observation. Your doctor will make the decisions on an individual basis regarding what would be best for your dog.


The prescription medicine used to treat the adult heartworms are called adulticides. The two adulticides used most commonly are derivatives of arsenic. It is not known exactly how these medications work to kill the heartworms. We just know they do work.


(NOTE: New medications may be available at any time; this listing of treatments may not be complete!)


The first one is thiacetarsamide. It has been used for at least half a century and is effective but can be toxic to the liver, kidneys, or cause severe irritation if the solution gets outside of the vein. The second medication is called Melarsomine dihydrochloride. With fewer side effects than thiacetarsamide, it is also an arsenic derivative and is administered by a careful intramuscular injection. It appears to be as effective and possibly more so in dogs than thiacetarsamide. It also has potential for significant side effects, so close veterinary monitoring is paramount.  


Side effects from Melarsomine dihydrochloride can be immediate or take up to 2 weeks to appear. One aspect of the side effects are due to the destruction of the adult worms and the resulting blood vessel blockage and inflammation.  


No matter what adulticide is used, it is very important to keep your dog very quiet and follow all of your doctor’s instructions. If you have any doubt about what to do or what is going on, do not hesitate to call your veterinarian ASAP.


As the inflammation peaks after adulticide treatment at 5-10 days, sometimes anti-inflammatory medications are used. However, some anti-inflammatory medications can reduce the effectiveness of the adulticide, so a veterinarian will recommend when it is best to use the anti-inflammatory medication, if at all. Some patients even require a second set of adulticide treatments since the very immature L5 heartworms and young female adults are more resistant to the treatment.


After the adulticide treatment and its side effects are resolved (usually at about 1 month post treatment), the microfilaria are then eliminated with one or another of two common heartworm preventives, Ivermectin or Mibemycin oxime. This will be done approximately one month after the adulticide treatment, depending on your veterinarian’s final decision regarding when it can be done. 


Approximately four months after adulticide therapy, the dogs are retested for the presence of heartworm(s). This will determine if a second treatment is needed.


Heartworm Prevention in Dogs


It is much simpler to prevent heartworm disease from occuring than treating it afterward. The most common preventatives on the market kill the immature heartworm larvae before they molt to the L5 stage. As long as they are given to the dog every month (or according to the medication instructions), they are very effective in preventing heartworm infection and subsequent development of heartworm disease.


The choice of which type of prescription heartworm prevention to use will be determined by a discussion with your veterinarian and what is best for your pet. Ideally puppies are started on monthly heartworm preventatives by 8 weeks of age. They should have a heartworm blood test at around 7 months of age and then be retested on an annual basis or according to the veterinarian's recommendations. 


The latest recommendation by the American Heartworm Society is once every 2-3 years in dogs that NEVER miss a dose of preventative. Any missed preventative doses should be communicated to your veterinarian and re-testing should be scheduled accordingly.