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. 

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. 

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. 

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. 

How can heartworm infections be prevented? 

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. 

Clinical signs  

Clinical signs only become apparent after the heartworms start living in the dog’s lungs and heart. 

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 

How is a heartworm infection diagnosed? 

• 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.  

When should dogs be tested? 

• Dogs over 7 months of age 

• Dogs taking monthly preventives  

• 7 months after possible infection (due to missed doses) 

• Annually — or biannually if at a higher risk  

What happens if your dog tests positive? 

Your veterinarian will discuss treatment options and may recommend additional testing, such as: 

• Complete blood count (to assess blood cells)  

• Chemistry panel (to assess organ function) 

• Urinalysis (to assess kidney function and other aspects of urinary tract health) 

How are heartworm infections treated? 

Treatments are tailored to each individual dog based on the severity of their symptoms and the results of diagnostic testing. 

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. 

What happens if I don’t treat my dog for heartworm disease? 

Without treatment, damage to your dog’s heart and lungs will worsen, and other symptoms will also become more severe. These conditions can progress to life-threatening stages. 

How successful is heartworm treatment? 

The 3-part injection of melarsomine is 98% effective at killing the adult worms. However, dogs who do not receive adequate rest after the injections are at risk for pulmonary embolisms, which can be fatal, due to dying worms blocking blood flow in the lungs. 

For dogs with heart failure or caval syndrome, the overall prognosis is less favorable, but treatment can still be successful. 

The 3-injection protocol of melarsomine is 98% effective at killing the adult worms.  

Can humans be infected with heartworms?  

It is possible, but highly uncommon, for people to be infected with heartworms. The first reported case in the United States was diagnosed in 1941, and only 81 cases have been reported since then. 

In these cases, infection still occurs through mosquito bites. The most common symptoms are nodules under the skin and around the eye, and in humans, the infection rarely affects the lungs or brain. 

Prevention involves avoiding being bitten by mosquitoes, especially in parts of the country that are at higher risk.