Heartworm Treatment Dogs

Pretreatment Work-up for Heartworm Infections

Staging of the heartworm infection means determining how severe the heartworm disease is.  Heartworm disease is the pathology or damage caused by the presence of the heartworms.  In some cases there is heartworm infection with little or no heartworm disease.  In some cases there is severe heartworm disease with very few heartworms.   Some pets with heartworms have severe allergic reaction in the lungs that needs to be treated before the heartworms are killed.  Some pets with caval syndrome need surgical intervention before the heartworms are killed.


We recommend a complete blood screen, radiographs and ultrasound to determine the Stage of Heartworm Disease.

  • Stage I disease – Pets show no outward signs of disease and the blood tests and radiographs and ultrasound are essentially normal with little or no pathology.
  • Stage II disease – Pets show little or no outward signs at rest.  There may be mild evidence of liver, or kidney damage or may be mild anemia. The radiographs will show right heart enlargement and enlargement of the pulmonary arteries.  The ultrasound may show dilatation of the right atrium and ventricle.
  • Stage III disease  - The pet is showing outward signs of disease. There may be weight loss, ill thrift, poor hair coat and skin problems.  There may be coughing, pale mucous membranes and a heart murmur.  The pet may have fluid accumulation in the abdomen called ascites.  Radiographs will show heart enlargement, enlarged pulmonary arteries, with increased twisting and curving of the pulmonary arteries as well as pruning of these arteries.  All of these signs indicate increased pulmonary blood pressure, increased blood clots or thrombi in the vessels feeding the lungs and are indicators of severe heartworm disease. The ultrasound shows marked enlargement of the right side of the heart, as well as moderate enlargement of the left side of the heart.
  • Stage IV disease – Caval Syndrome – The pet is showing all the signs of Stage III disease plus on physical examination there may be bloody urine, yellow gum tissue and a severe murmur with a gallop or third heart sound and a jugular pulse.  The pet may be weak and unable to stand with rapid respirations and a panic look on the face.  Blood tests will confirm hemolytic anemia, liver enzyme elevation and high bilirubin.   The radiographs will show severe damage to the lungs.  The ultrasound will show heartworms trapped in the tricuspid valve on the right side of the heart.

Once the degree or severity of heartworm infection is determined, a careful matching of medications and timing of treatment and surgery is instituted to maximize the chances of a successful outcome.

Heartworm Treatment Protocol

The following is the treatment protocol for heartworm therapy in a pet that is stable and not needing emergency care now used at Safari Animal Care Centers by Dr. Garner.

1)      Blood tests, urine tests, radiographs and ultrasound is used to stage the severity of the heartworms. Depending on the stage additional medications or therapy may be necessary.

2)      As soon as heartworm disease is confirmed start the pet on Advantage Multi.  Because there may be young adult heartworms that will not be killed by the medications used in the heartworm treatment we may keep a pet on Advantage Multi for up to 3 or 4 months prior to the treatment.

3)      Start the pet on Doxocycline at 10mg/kg twice a day for thirty days prior to killing the adult heartworms.  This medication has been shown to improve the outcome and lessen the tissue damage done during a heartworm treatment.  These effects are achieved because of the effect doxocycline has on Wolbachia.

4)      30 Days from the diagnosis one dose of Melarsomine is given as a deep intramuscular injection into the back muscles of the pet.  This medication starts to kill the adult heartworms one by one over a period of 5 to 6 weeks.

5)      Reducing dose of prednisone 0.5mg/kg BID for the first week, followed by 0.5mg/kg SID for the second week, followed by 0.5mg/kg every other day for the next two weeks.

6)      The pet is kept with severe exercise restriction and confinement.  The client is instructed to bring the pet in for a weekly examination for five visits.  Exercise restriction is the most important part of the therapy.  Pets with few worms (15) that are allowed to exercise have much more severe damage to their lungs than pets with many worms (50) that are kept quiet.

7)      Then the pet is hospitalized over night where two injections of the Melarsomine are given 24 hours apart deep intramuscular as before.

8)      Reducing dose of prednisone 0.5mg/kg BID for the first week, followed by 0.5mg/kg SID for the second week, followed by 0.5mg/kg every other day for the next two weeks.

9)      The pet is again restricted and visits the hospital every two weeks for three visits for a physical examination and consultation.

10)  6 months after the second set of adulticide injections a heartworm test is done to confirm clearing of all adult heartworms.

11)  Aspirin therapy is no longer recommended as a part of the therapy.

12)  Microfilaria treatment is no longer necessary as they are made incapable of causing infection by the doxocycline and are eventually killed by the monthly administration of the Advantage Multi.

13)  Stay on Advantage Multi and repeat testing every 6 months.

What Are Wolbachia?

Wolbachia are bacteria that live within the heartworm that participate in the reproduction of the female heartworm.  When the wolbachia are killed with doxocycline it has beneficial effects on the heartworm treatment.  Adult female heartworms are rendered sterile and they expel their baby heartworms.  It appears these baby heartworms cause significant inflammation in the lungs when the females are killed as a part of the heartworm treatment.  By removing them first it reduces the damage to the lungs during the therapy.  Also death of the wolbachia retards the development of the larval stages of the heartworms making them no longer infectious to other animals.

What is Pulmonary Thromboembolism

When the adult heartworms die, the dead worm bodies float inside the blood vessels until the vessel size matches the heartworm size.  These worms then plug the vessel causing the lung tissue that is fed by this vessel to suffer.  If too many adult heartworms die during a short period of time, the pet may die of this pulmonary thromboembolism.  The most common time during a heartworm treatment for this to happen is 8 to 10 days after the injections of melarsomine.  Hospital therapy with cortisone, bronchiodialators, diuretics and oxygen therapy may be necessary.  The pet may show signs of fever, cough, coughing up blood, or vomiting up blood.