Written by: Kristyn Carr, DVM of Kindness Pet Hospital

Cause

Canine heartworm disease is caused by a parasite called Dirofilaria immitis. Dirofilaria immitis has a somewhat complicated lifecycle involving several larval stages and multiple hosts. Immature heartworms, referred to as microfilariae, circulate in the bloodstream of infected dogs. When an infected dog is bitten by a mosquito, the microfilariae enter the mosquito’s salivary glands and mature through several stages. Once the microfilariae reach a certain stage (known as the L3 stage), they become able to infect other dogs through the bite of the mosquito.

When a dog is bitten by a mosquito containing the infective microfilariae, the microfilariae mature further and migrate through the dog’s skin and muscle tissue to enter the dog’s circulatory system. Full maturation of microfilariae into adult heartworms takes 6-9 months. Adult heartworms live in a dog’s pulmonary artery (a major blood vessel between the heart and lungs). Adult heartworms reproduce in the dog’s pulmonary artery, creating more microfilariae that circulate in the dog’s bloodstream. These microfilariae are ingested by mosquitoes and result in infection of other dogs.

Clinical Signs and Diagnosis

While most cases of canine heartworm disease are diagnosed with routine testing of asymptomatic dogs, common clinical signs of heartworm disease include coughing, increased resting respiratory rate, and exercise intolerance or shortness of breath. Signs of advanced heartworm disease include a distended abdomen, lethargy, severe exercise intolerance or shortness of breath, and collapse.

Several tests exist for detecting canine heartworm disease. Most tests detect a protein (known as heartworm antigen) that is produced by female heartworms. These antigen tests are typically performed as part of a dog’s annual veterinary visit. Other tests detect circulating microfilariae in a dog’s blood. The recommended tests will vary depending on a particular dog’s situation, i.e., whether the dog has been on heartworm prevention, the presence or absence of clinical signs, etc.

Treatment

Once a diagnosis of heartworm disease is made, your veterinarian may recommend further testing to “stage” the disease or determine its severity. Staging tests commonly include bloodwork and chest X-rays, but further testing, such as an abdominal ultrasound, may be recommended if the disease is severe.

The “gold standard” treatment for heartworm disease is a series of three injections of a medication called melarsomine. Melarsomine is an arsenic-based compound that kills adult heartworms. Steroids and an antibiotic called doxycycline are used in conjunction with melarsomine to maximize treatment efficacy and minimize treatment side effects. The exact timing of these adjunct medications and injections is beyond the scope of this article, but can be discussed with your veterinarian. Also, additional resources are provided at the end of this article. After treatment, it takes 6-9 months for a dog to test negative for heartworms. Of course, not all infected dogs can tolerate treatment this aggressive, and in those cases, veterinarians or shelter staff may reach for the “slow kill” method.

The “slow kill” method is a bit of a misnomer, as it does not kill adult heartworms. Rather, the slow kill method involves administering heartworm prevention to infected dogs to prevent the circulation of microfilariae and keeping infected dogs from serving as reservoirs of infection for other dogs. Thus, the slow-kill method acts as heartworm “birth control.” With this method, the adult heartworms present in the infected dog’s pulmonary artery live out their normal lifetime (1-3 years). During these 1-3 years, the adult heartworms cause continued inflammation and can lead to permanent changes in the dog’s heart and lungs. It is for this reason that melarsomine therapy is considered the “gold standard,” as it minimizes the amount of time that adult heartworms are present in the pulmonary artery and limits the amount of inflammation and damage that can be caused by the heartworms.

Prevention

The phrase “an ounce of prevention is worth a pound of cure” could not be truer than when discussing canine heartworm disease. Heartworm disease is far easier, safer, and less expensive to prevent than to treat. Heartworm preventives work by eliminating any microfilariae deposited by mosquitoes before the microfilariae can mature into adult heartworms.

There are numerous different heartworm preventives available, with most preventives being oral tablets or chews, but some topical products are also available. Heartworm preventives are prescription products, and your veterinarian can recommend the best product for your dog. Heartworm prevention should be used year-round on the Gulf Coast, due to a heavy mosquito burden and persistence of warm, wet weather even in winter months.

Most veterinarians also recommend annual heartworm testing, even when your dog is consistently using prevention, to ensure the prevention is effective and to monitor for resistance among the heartworm population. Recent studies have also shown that using products that repel mosquitoes in dogs along with traditional heartworm preventives is more effective than using traditional preventives alone.

A Quick Note on Cats

While dogs are the definitive host for Dirofilaria immitis, cats can also be infected with heartworms. Heartworm disease can be harder to diagnose in cats, as they typically are infected with fewer worms, making antigen tests less reliable, and feline heartworm disease can closely resemble feline asthma. However, feline heartworm disease is like canine heartworm disease in that it is easier to prevent than treat. Numerous heartworm preventives are available for cats, with most being topical products that combine heartworm with “ea and/or tick prevention.

Additional Resources

For more information on canine and feline heartworm disease, please visit the American Heartworm Society. You can also listen to Dog-Harmony’s Pupcast episode co-hosted by founder, Nancy Bown, CPDT-KA, and Sydney Bassett on canine heartworm disease with a guest appearance from the South Walton Mosquito Control District.