Heartworm Disease in Dogs: Treatment

What is heartworm disease?

Heartworm disease is caused by a parasite called Dirofilaria immitis, better known as heartworm. Heartworms live in the right side of the heart and the pulmonary artery (the large blood vessel that carries blood from the heart to the lungs). 

Dogs become infected when they are bitten by an infected mosquito carrying infective heartworm larvae, and the larvae pass from the mosquito into the dog. It is a serious and potentially fatal disease. Treatment is highly successful in most dogs but requires careful monitoring and strict exercise restriction.

How is heartworm disease treated?

There is some risk involved in treating dogs with heartworms, although fatalities are rare. Many dogs have advanced heartworm disease when they are diagnosed, meaning there may already be substantial damage in the heart, lungs, blood vessels, kidneys, and liver. In rare cases, the disease may be so advanced that it is safer to treat organ damage and keep the dog comfortable than it is to risk negative effects associated with killing the heartworms. Dogs in this condition are not likely to live more than a few weeks or months.

Treatment usually includes two types of medication: one to kill the heartworm larvae (microfilariae) and one to kill adult heartworms, along with strict exercise restriction during recovery. Your veterinarian will recommend the most appropriate treatment approach based on your dog’s condition and the latest recommendations from the American Heartworm Society.

Treatment to kill microfilariae (heartworm larvae)

Before treatment to kill adult heartworms, your dog will receive a drug to kill microfilariae. Most common heartworm preventives will kill microfilariae. Your dog may need to stay in the hospital for observation on the day this medication is administered. After microfilariae treatment, your dog will continue on a heartworm preventative.

Prednisone (Deltasone®, Meticorten®) may be prescribed to minimize inflammation from dying microfilariae or heartworms. In addition, many dogs will be treated with the antibiotic doxycycline (Vibramycin®, Vibra-Tabs®) to combat potential infection with a bacteria (Wolbachia) that inhabits the heartworm. These bacteria are symbiotic, meaning they help the worm to survive and reproduce.

Treatment to kill adult heartworms

Treatment to kill adult heartworms requires an injectable medication called melarsomine (brand name Immiticide® or Diroban®). Melarsomine kills adult heartworms in the heart and adjacent vessels and is given as a series of injections. Most dogs receive an initial injection followed by a 30-day rest period, and then two additional injections given 24 hours apart. Your veterinarian will determine the specific injection schedule according to your dog's condition. 

Your dog may need to stay in hospital for observation on the day of each injection. Pain medication may be given as well, because melarsomine can cause muscle pain. Your dog will be tested for microfilariae approximately one month after the final injection of melarsomine.  At nine months after the last injection, they will be tested for the presence of adult heartworms with an antigen test. If any of these results are positive, treatment will be modified.

Exercise restriction and monitoring

Complete rest is essential after treatment. Adult heartworms die within a few days of treatment and start to break down. As they decompose, fragments are carried to the lungs, where they become lodged in small blood vessels before eventually being reabsorbed by the body.

This resorption can take several weeks to months, and most post-treatment complications are caused by these fragments of dead heartworms. For this reason, your dog must be kept as quiet as possible and should not exercise for one month following the final melarsomine injection. 

The first week after each injection is particularly important because this is when the worms are dying. A cough may persist for seven to eight weeks after treatment in heavily infected dogs. If the cough becomes severe, contact your veterinarian. 

If your dog has a significant reaction in the weeks after the initial treatment, prompt treatment is essential. Treatment with anti-inflammatories, antibiotics, cage rest, supportive care, and intravenous fluids are usually effective if complications occur. 

Notify your veterinarian if your dog shows:

• Loss of appetite
• Shortness of breath
• Severe coughing 
• Coughing up blood
• Fever
• Depression

The “slow-kill” treatment

An alternative to melarsomine, the “slow-kill” heartworm treatment protocol uses a continuous monthly heartworm preventive (typically containing moxidectin or ivermectin), along with an oral antibiotic (doxycycline), to gradually eliminate heartworms.

This approach is generally not recommended because it may take a year or longer to eliminate all heartworms, allowing additional damage to occur. It also requires strict exercise restriction for a much longer period. However, this method may be considered in certain situations when the standard treatment is not appropriate (e.g., if the risk of rapid-kill methods outweigh the benefits, if the dog is likely to die from an unrelated disease, or if melarsomine is not available). Your veterinarian will select the correct drug and administration time based on your dog's condition.

Are any other treatments necessary?

Dogs with severe heartworm disease may require antibiotics, pain relief medications, special diets, diuretics to remove fluid accumulation in the lungs, and medication to improve heart function before treatment for heartworms. Even after heartworms have been eliminated, some dogs may require lifelong treatment for heart failure, which may include diuretics, heart medications such as ACE inhibitors, beta-blockers or cardiac glycosides, and special low-sodium diets.

What is the prognosis after treatment?

Many owners are surprised at the improvement they see after treatment. Dogs that previously showed clinical signs often experience increased energy and vitality, improved appetite, and weight gain.

How can I prevent my dog from getting heartworms?

The best way to avoid a heartworm infection is by giving your dog a heartworm preventive and reducing their exposure to mosquitos. Measures to reduce mosquitos include:

• Using FDA-approved or EPA-approved mosquito repellents or insecticides designed for dogs
• Eliminating standing water where mosquitoes breed
• Limiting outdoor activity during peak mosquito activity at dawn and dusk

Dogs that have been successfully treated for heartworms can become infected again if they are not protected. With the safe and affordable heartworm preventives available today, no pet should ever have to endure this disease.  Talk with your veterinarian about the best heartworm prevention program for your dog.