Heartworm disease is a serious and potentially life-threatening condition affecting dogs worldwide. This disease, caused by the parasitic worm Dirofilaria immitis, can severely impact a dog’s health by lodging in the heart, lungs, and associated blood vessels. This presence often leads to respiratory complications, heart failure, and other serious conditions, like seizures, if left untreated. Despite risks posed by heartworm disease, many pet owners are uncertain about how to best protect their pets and the methods of treatment available. Cynthia Maro, DVM, CVA, CAC, VMRT, VNAET, emphasizes that regular screening tests are important, in addition to the use of heartworm preventives in at-risk dogs and cats. Pets that travel to high-risk regions, such as snow-bird owners, need to have year-round prevention. In addition, owners with pets who have displayed sensitivity to heartworm preventives should be aware of the need for increased screening frequency – 2-4 times yearly. Their pets MAY also benefit from use of natural repellants and oral herbal preventives.
Transmission and Lifecycle of Heartworm Disease
The spread of heartworm disease relies on the mosquito as an intermediary host, allowing the disease to become widespread, especially in regions with high mosquito populations. The process begins when a mosquito carrying infective heartworm larvae bites an infected dog, releasing the larvae into the dog’s bloodstream. Over the course of six to seven months, these larvae mature into adult worms within the circulatory system, specifically in the pulmonary arteries and the right side of the heart. These adult worms are capable of reproducing, releasing microfilariae into the dog’s bloodstream, which can then be picked up by other mosquitoes and transmitted to new hosts, including other dogs, cats, ferrets and even immunocompromised humans.
This cycle of transmission means that a single infected dog has the potential to spread heartworm disease to others in its community, up to 3 miles away from the original infected dog. The presence of adult worms in a dog’s body places strain on the heart and lungs, obstructing blood flow and affecting respiratory function. At the onset of infection, symptoms are often mild and may go unnoticed, including a light cough or decreased energy levels. However, as the disease progresses, dogs can exhibit more severe signs, including difficulty breathing, fainting, and in advanced cases, heart failure. Early detection is critical in managing heartworm disease, making regular veterinary check-ups essential for catching any signs of infection.
Prevention Strategies for Heartworm Disease
Prevention remains the cornerstone of managing heartworm disease, particularly because treatment can be costly, lengthy, and challenging for both dogs and owners. Year-round prevention is typically achieved through monthly medications, which eliminate any larvae before they can mature. These medications come in various forms, such as oral tablets, topical applications, and injectable options, each with unique benefits and potential side effects. Veterinarians often recommend monthly preventive measures even in cooler climates or during seasons when mosquitoes seem less active, as mosquitoes can thrive in various environments, including indoors.
For added protection, pet owners can consider mosquito control methods around the home to reduce exposure. Although these methods do not replace preventives, they can lower the risk of mosquito bites, particularly during peak mosquito seasons. Many people may mistakenly believe that indoor pets are safe from heartworm, but mosquitoes can enter homes, putting all dogs and indoor cats at risk. Preventive measures should be consistent and regular, forming a core part of every pet care routine.
A more resistant form of heartworm disease has been found in the southern states, so year-round use of prevention is recommended in northern pets who visit the south. And, with milder winters, monthly preventives are even recommended in the northern states.
Treatment Options for Heartworm Disease
If a dog tests positive for heartworm disease, treatment becomes essential to prevent further complications. The treatment process is rigorous and requires strict adherence to protocols to ensure the dog’s safety. The most common approach is a multi-step process involving a pre-treatment evaluation of stage of disease and extent of organ damage. This is followed by injections of an adulticide medication to kill the adult worms, typically administered by a veterinarian over the course of a few days. These injections are often followed by rest and limited activity for the dog, as physical exertion can increase the risk of complications during treatment.
During treatment, dogs may experience side effects, including fever, soreness, and other symptoms as the worms begin to die and break down. Following adulticide injections, additional medications will be prescribed to help eliminate the microfilariae (the tiny baby heartworms), ensuring that any remaining larvae in the bloodstream are neutralized to prevent further infection. Recovery can be challenging for both the dog and the owner, with strict limitations on exercise and careful monitoring required throughout the process. Many veterinarians will also perform follow-up blood tests and imaging to confirm that the treatment has successfully eradicated the heartworms. Monitoring also provides insight into the internal organ function and management of secondary health problems, such as kidney, liver or blood disorders from the parasites and the medications necessary to eradicate them.
Heartworm Disease in Cats
Indoor cats are at a higher risk for the development of heartworm disease than outdoor cats. This is related to fact that mosquitoes are more likely to bite an indoor pet, once they become trapped in the home. Outdoors, mosquitoes can find blood meals through abundant numbers of wild animals and rodents.
In cats, pets who display coughing, signs of allergies and or asthma, and vomiting should all be screened for feline heartworm disease.
Once diagnosed, the disease is treated differently than in dogs. Management of symptoms has been most successful in my practice when an integrative approach to treating the cardiac and respiratory symptoms is followed. Decreasing inflammation through nutraceuticals and dietary management has produced a high rate of recovery.
Adulticide treatment is NOT recommended, because the disease is self-limiting. A monthly preventive program is advantageous in cats.
Importance of Regular Heartworm Testing and Monitoring
For both prevention and post-treatment care, regular heartworm testing is essential. Veterinarians usually recommend an annual heartworm test for dogs on preventive medication, as no preventive measure is 100% effective. These tests provide early detection, allowing for timely intervention should an infection occur. For dogs who have undergone treatment, follow-up testing and monitoring are equally important to confirm the absence of heartworms and to address any potential lingering health issues caused by the disease. Regular check-ups provide a safety net, ensuring that any signs of infection are identified and addressed quickly, maximizing the dog’s chances for a full recovery.
Heartworm disease remains a significant threat to dogs, and to a lesser extent to cats, but with the right preventive and treatment measures, pet owners can protect their companions from this potentially devastating condition. By maintaining preventive care, staying vigilant with annual testing, and consulting with veterinarians about the best care plan, pet owners can help ensure that their pets lead long, healthy lives free from the dangers of heartworm disease.
Author: Dr. Cynthia Maro – Integrative Veterinarian, Cynthia Maro, DVM, CVA, CAC, VMRT, VNAET