Heartworm in Dogs

Heartworm disease is caused by the parasite Dirofilaria immitis. Dogs become infected when a mosquito carrying infective larvae bites them. The larvae migrate through the body, mature over about 6 months, and adult worms settle mainly in the pulmonary arteries and sometimes the heart. Adult worms can live for 5 to 7 years in dogs and may produce microfilariae that continue the cycle.

Risk factors include: • Living in or traveling to areas with mosquitoes; heartworm is found in all 50 U.S. states. • Missing or delaying year-round heartworm prevention. • Outdoor exposure, though indoor dogs are still at risk because mosquitoes enter homes. • Lack of routine annual testing, which can delay diagnosis. • Ongoing exposure in high-incidence regions such as the southeastern U.S., the Mississippi River Valley, and Texas.

There is no known breed predisposition. Any dog can be infected, though highly active dogs may show exercise intolerance sooner once damage develops.

Diagnosis usually starts with one or more blood tests. The most common test detects antigens from adult female heartworms. Many veterinarians also check for microfilariae in the bloodstream. Because no single test explains the full picture, positive dogs often need staging tests before treatment.

At the appointment, your veterinarian may recommend: • A heartworm antigen test. • A microfilaria test. • Chest X-rays to look for lung and blood vessel changes. • Bloodwork such as a CBC and chemistry panel to assess organ health. • Echocardiography in some dogs to evaluate heart changes or visualize worms. • Additional assessment if severe disease is suspected.

Testing matters even in dogs on preventives because missed doses, vomiting after a dose, or product failure can happen. Annual testing is still recommended for dogs 7 months and older.

Treatment is carefully managed and should always be directed by a veterinarian. The standard adulticide treatment for dogs uses melarsomine, and many veterinarians follow a 3-injection protocol because it is considered safer and more effective at clearing infection. Dogs are often started on heartworm preventive and doxycycline as part of the treatment plan, and some may also need anti-inflammatory or supportive medications depending on disease severity.

Treatment commonly includes: • Strict exercise restriction, often beginning at diagnosis and continuing through treatment and recovery. • Heartworm preventive to address susceptible larval stages and help prevent new infections. • Doxycycline to target Wolbachia bacteria associated with heartworms. • Melarsomine injections to kill adult worms. • Monitoring for complications such as pulmonary thromboembolism when worms die. • Hospitalization or stabilization in more severe cases. • Emergency referral for suspected caval syndrome, which may require surgical removal of worms before adulticide treatment.

'Slow-kill' approaches are generally reserved for situations where standard treatment is not possible, because infection can persist for months to years while ongoing damage continues. Prognosis is often good when heartworm is found earlier and treatment restrictions are followed closely, but some heart, lung, and vessel damage may be permanent.