Heartworm Disease

Overview 

• Heartworms are spread by mosquitoes, but infections can be prevented by using routine medications. While heartworm disease can be life-threatening, there are effective treatment options for established heartworm infections, and strict rest during the treatment period is critical to recovery.  

• Domestic dogs (and some wild canids) are the definitive host for heartworms, but other animals, including cats and ferrets, can also be infected. 

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Transmission and life cycle 

• Heartworms are caused by the parasite Dirofilaria immitis. Immature heartworms, called microfilaria, are ingested by female mosquitoes when feeding on infected dogs. While the microfilaria are living inside the mosquitoes, they mature into larva. As the mosquitoes continue to feed on more dogs, they then inject the mature larva into dogs’ tissues, starting a new infection. 

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• After the larva enter a dog’s body, they migrate through tissues and eventually enter the bloodstream, which allows them to travel to the heart and lungs. The larva settle in those organs and grow into worms. Males can reach 5-6 inches in length, and females can reach 10-12 inches in length. 

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• The worms start producing new microfilaria around 6-9 months after infection, and the adult heartworms can survive in dogs for 5-7 years. 

• Heartworms can be found in countries all around the world. In the United States, they are most commonly diagnosed in the Southeast and along the Mississippi River Valley. However, heartworms can be found in all 50 states and in Canada.

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Clinical Signs  

• Stage 1: Mild disease 

  • Low numbers of worms  

  • Some dogs will not show any clinical signs 

• Stages 1-2: Mild to moderate disease 

  • Coughing 

  • Weakness  

  • Exercise intolerance 

• Stage 3: Severe disease  

  • Heart failure (coughing, labored breathing, abdominal enlargement) 

  • Dark urine  

  • Caval syndrome   

  • Large numbers of worms occupy the right side of the heart and blood vessels 

  • Red blood cell destruction causes anemia 

  • Life-threatening 

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Diagnosis 

• Blood tests can detect both adult worms and microfilariae (immature worms).  

• Echocardiology is used to visualize worms in the heart and blood vessels. 

• X-rays (radiographs) reveal of the chest.

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Treatment

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• Stabilization : 

  • Expect strict exercise restriction to be needed after adulticide therapy but moderately affected dogs will need restriction from the first. If the dog has heart failure, this will need to be controlled as well. Some dogs will need anti-inflammatory doses of steroids to control the inflammation that stems from the presence of worms. This is often done concurrently with the second step as described below.

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• Treatment for stages 1-2: 

  • Exercise restriction and strict rest must be continued throughout treatment. 

  • First, a heartworm preventative medication will be given to kill the microfilaria, and the dog needs to be closely monitored for any rare, but adverse reactions.  

  • Then a 4-week course of both prednisone and doxycycline will be prescribed. Prednisone is a corticosteroid that will help reduce blood clots in the lungs, and doxycycline is an antibiotic that will kill the bacteria living inside the heartworms. 

  • One or two months after this course is started, your veterinarian will administer a drug called melarsomine. This injection, given in the hospital, kills the adult worms. The most common side effect of this drug is pain at the injection site. 

  • Following the melarsomine, your dog will be prescribed another course of prednisone. One month later, two more melarsomine injections will be given 24 hours apart. 

  • It is essential that you keep your dog on strict rest for an additional 6-8 weeks while the worms are dying. Your veterinarian will recommend additional visits over the next nine months to evaluate treatment success, and some steps may be repeated as necessary. 

• Treatment for stage 3: 

  • Dogs showing signs of heart failure need to be stabilized prior to beginning heartworm treatment. 

  • If dogs already have caval syndrome, then they require emergency surgical removal of the adult worms. 

  • There is a protocol involving only two injections of melarsomine instead of three, but it is not recommended as the first choice of treatment. 

  • The use of heartworm preventives to slowly kill the adult worms is an option when there is another medical reason that melarsomine cannot be used. 

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Prevention 

• Heartworm preventative medications are safe and highly effective. They should be given once a month, all year-round. Examples include: 

  • Oral medications:  

    • Ivermectin (e.g. Heartgard Plus, Tri-Heart)  

    • Milbemycin (e.g. Interceptor, Sentinel)  

  • Topical medications:  

    • Selemectin (e.g. Revolution) 

    • Moxidectin (e.g. Advantage Multi, Simparica Trio)  

  • Injectable medications:  

    • Moxidectin (ProHeart 6, ProHeart 12) 

• A topical product (Advantix) to repel and kill mosquitoes can be used in addition to heartworm preventives.