What is the survival rate for heartworms in dogs?

Goals of heartworm treatment

Pet owners and veterinarians should have a clear understanding of the goals of heartworm treatment. While many dog owners administer heartworm prevention on a regular basis, too many dogs do not receive these treatments and eventually develop the disease.

When adult heartworms become lodged within the pulmonary vessels, they cause significant inflammation within the vessel walls and surrounding tissues. This leads to the clinical signs associated with heartworm disease: coughing, exercise intolerance, syncope, abnormal heart and lung sounds, ascites, and a host of other symptoms.

The primary goals of heartworm treatment are to minimize the clinical signs associated with heartworm disease and to eliminate all heartworms (adults, juveniles, larvae, and microfilariae) from the body.

While live worms cause significant inflammation in the pulmonary vessels, dying and dead worms can result in even more damage. As such, treating heartworms does not result in an immediate risk reduction or the immediate resolution of inflammation. In fact, dogs are often at a higher risk of clinical signs of heartworm disease as heartworms die, regardless of whether the parasite’s death is caused by adulticide treatment or the natural end of the life cycle.

Treating heartworms early, using an adulticide, prevents worsening damage and leads to faster resolution of clinical signs.

Avoiding complications: pre-treatment heartworm diagnostics

Prior to beginning treatment for heartworm disease, diagnostics should be performed to assess the patient’s health status. These diagnostics can better prepare the veterinarian and pet owner for complications that may arise during treatment and suggest approaches to minimize these risks.

Pre-treatment thoracic radiographs are often recommended for patients with heartworm disease. A technical proficiency in diagnostic imaging and proper positioning on the vet table are paramount in assessing the degree of pulmonary damage and predicting the likelihood of treatment-related complications.

Bloodwork, including a complete blood cell count and serum biochemistry, is also often recommended prior to treatment. This can help in assessing for uncommon complications of heartworm disease, such as liver and kidney damage, while also assessing the dog’s overall health status.

Adulticide treatment side effects

Adult heartworms are treated with melarsomine dihydrochloride. This medication is administered by intramuscular injection, deep into the epaxial muscles.

Melarsomine injections are often associated with localized, injection-site swelling and soreness. These local effects may vary from mild to severe, depending on the patient. Many patients benefit from the administration of pain medication in conjunction with their melarsomine injections.

A more significant side effect associated with melarsomine treatment is pulmonary thromboembolism: dead or dying worms dislodge and travel into distal pulmonary arteries, where they are broken down by the immune system. Some degree of pulmonary thromboembolism is virtually guaranteed with adulticide therapy, but not all dogs will show clinical signs.

Clinical signs can be minimized by ensuring that owners strictly restrict their pet’s activity during and after treatment, which reduces the degree of pulmonary thromboembolism that occurs.

Heartworm treatment protocol: melarsomine dosage and adjunct medications

Melarsomine label recommendations discuss two treatment options: a two-dose protocol and a three-dose protocol.

The manufacturer recommends the two-dose protocol for dogs with Class 1 (asymptomatic or mild) or Class 2 (moderate) heartworm disease. The three-dose protocol is recommended for use in dogs with Class 3 (severe) heartworm disease, including dogs with cough, dyspnea, muscle wasting, and/or fatigue.

In contrast to the manufacturer recommendations, however, the American Heartworm Society recommends that all infected dogs be treated with the three-dose protocol. The two-dose protocol kills only 90% of adult worms (according to label claims), while the three-dose protocol kills 98% of adult worms. Additionally, the three-dose protocol is associated with a lower risk of effects related to pulmonary thromboembolism than the two-dose protocol.

In addition to melarsomine, a number of adjunct medications are recommended in dogs undergoing heartworm treatment. These medications, which help improve treatment efficacy and minimize side effects, include:

Anti-inflammatory doses of steroids minimize clinical signs associated with pulmonary thromboembolism.

This antibiotic is effective against Wolbachia, a symbiotic bacterium found within heartworms. Doxycycline is thought to decrease pulmonary inflammation while making adult heartworms more susceptible to adulticide therapy.

These are used to kill microfilariae and 3rd and 4th stage larvae. Because melarsomine is ineffective against heartworm larvae and young adult worms, macrocyclic lactones should be administered for two months prior to adulticide therapy, thus ensuring that all heartworms in the dog are mature enough to be susceptible to melarsomine.

Macrocyclic lactones should be used with caution in dogs with high microfilarial counts, as rapid microfilarial die-off may lead to significant clinical signs.

Heartworm disease is a serious condition that can lead to irreparable organ damage in dogs.  In this article, staff at Carolina Veterinary Specialists explain what pet owners should be prepared for during their dog's heartworm treatment.

What is heartworm disease?

Heartworm is spread through the bite of an infected mosquito, which can pass a parasitic worm called dirofilaria immitis into a dog's bloodstream. Heartworm is not contagious, and cannot be passed from one host dog to another, it is only through Mosquito bites that heartworm passes. Do not make the mistake of thinking that means the risk of heartworm is low; heartworm has been reported in all 50 states while being especially common between New Jersey and the Gulf of Mexico, as well as along the Mississippi River and its major tributaries.

If your pet is bitten by an infected mosquito, the worms will mature into adults, mate, and produce offspring while living inside your pet's heart, lungs, and blood vessels.

What is heartworm treatment in dogs?

The team at Carolina Veterinary Specialists cannot stress enough that, when it comes to heartworm, prevention is vastly superior to treatment.  We recommend contacting your vet post-haste to formulate a preventative treatment plan for your canine companion if you have not already.  Most commonly Heartworm prevention is administered through a monthly medication that can be prescribed by your vet.   

In the cases Where preventative measures fail to prevent infection, there are treatment options available for your pet, though all come with the possibility for serious side effects and health complications, though fatalities are rare.

Because Heartworm is undetectable until at least 5 months after infection, many dogs are suffering from advanced Heartworm Disease by the time they are diagnosed, requiring swift and intense treatment.  In rare cases, the damage to the dog's internal organs may be so severe by the time the condition is detected that it is better to treat the damage and keep the patient comfortable rather than assume the additional risks associated with attempting to kill the heartworms.  Dogs in this advanced condition have a life expectancy of only a few weeks or months. 

Thankfully, a new medication has been developed for killing adult heartworms while having fewer dangerous side effects.  Melarsomine is an injectable drug that kills adult heartworms that is administered over the course of multiple injections.  Typically your dog will be given a 30 day rest period after their first injection, after which they will receive two more injections 24 hours apart.  Antibiotics will also be prescribed to combat any infectious bacteria the heartworms may be carrying.  With this new medication, 95% of dogs with heartworms are now able to be successfully treated.

Your dog will also receive treatment to kill juvenile heartworms (microfilaria) either before or after their Melarsomine treatment.  Your dog may need to spend the night in the hospital for observation on the day this treatment is administered.

What should I do after my dog's heartworm treatment?

It is critical that your dog be allowed to rest following their injection.  Heartworm treatment in dogs kills the Adult heartworms within a few days, but further complications can occur while their corpses are decomposing.  It can take several months for the heartworms to be reabsorbed into the patient's bloodstream.  Most post-treatment complications arise from these fragments of decomposing heartworms, so to minimize this risk your dog must not be allowed to exercise and should be kept as quiet as possible for the first month following treatment.  For seven to eight weeks following injection, a cough will be noticeable.  If this cough persists beyond this or is especially severe, as well as if your dog is demonstrating shortness of breath or fever, contact your veterinarian right away.

What are the side effects of heartworm treatment in dogs?

Treatment for heartworm can cause serious complications for your pet's health and can be potentially toxic to the dog’s body.  Many dogs experience soreness and swelling at the site of their injections.  The most severe side effects are related to a large number of worms suddenly dying.  You must contact your vet immediately if your dog is panting excessively, has difficulty breathing, is suddenly lethargic or collapses, begins to reject food, begins to vomit, or develops diarrhea.

Our Hospital offers emergency care. If you suspect your dog is suffering from heartworms, or side effects from heartworm treatments, contact us right away. We will work with your primary Veterinarian to ensure the best treatment for your valued pet.