Heartworm disease, scientifically known as dirofilariasis, is a silent yet deadly condition caused by the parasitic nematode Dirofilaria immitis. This insidious parasite primarily targets the pulmonary arteries, leading to inflammation, vascular dysfunction, and pulmonary hypertension. As the disease progresses, it increases right-side cardiac pressure and causes gradual pulmonary insufficiency. In severe cases, complications such as aberrant heartworm migration, caval syndrome, disseminated intravascular coagulation (DIC), and heart failure can occur. While many infected dogs remain asymptomatic, common clinical signs include weight loss, exercise intolerance, coughing, and difficulty breathing.

Prevention is paramount in managing heartworm disease. Understanding the life cycle and transmission dynamics of heartworm disease is crucial for effective prevention and treatment.

Commonly Affected Species

Heartworm disease affects various species, with dogs and coyotes being the most commonly impacted. The transmission of Dirofilaria immitis relies on over 70 mosquito species, with Aedes, Anopheles, and Culex being the primary vectors. Wild animals like wolves, foxes, California sea lions, gray seals, and raccoons also serve as reservoirs, while among companion animals, dogs are the primary hosts, followed by cats and ferrets. The presence of these wildlife reservoirs contributes to the persistence and spread of heartworm disease in different regions.

Geographic Distribution and Risk Factors

Dirofilariasis is prevalent in regions with temperate, semitropical, or tropical climates, including the US, Canada, Australia, Latin America, and southern Europe. Risk factors for companion animals include outdoor housing, with medium to large-sized dogs aged 3 to 8 years in endemic areas being most susceptible. Although indoor animals are not immune, the majority of infections occur in outdoor canines. The changing climate and urbanization patterns are also influencing the distribution and incidence of heartworm disease, making it a growing concern for veterinarians and pet owners alike.

Human Health Implications

While infected mosquitoes can transmit heartworm larvae to humans, there are no documented cases of patent infections. Larval maturation in humans can lead to pulmonary encapsulation and granulomatous reactions, often resembling lung cancer on thoracic radiographs. These "coin lesions" can be mistaken for more serious conditions, leading to unnecessary anxiety and medical procedures. Therefore, awareness and preventive measures are essential to mitigate the risk of heartworm transmission to humans.

April 2024: Updated AHS Canine Heartworm Guidelines

On April 9, 2024, the American Heartworm Society (AHS) released an updated version of its Canine Heartworm Guidelines, incorporating the latest research on heartworm prevention, diagnosis, and treatment. This update aims to provide veterinarians with the most current and practical recommendations to combat this disease. The guidelines were developed through a comprehensive scientific review by experts in parasitology, cardiology, and clinical practice, ensuring they reflect the most recent advancements and insights in the field.

Key Updates

Prevention: The AHS continues to advocate for the year-round use of macrocyclic lactone heartworm preventives. Additionally, the updated guidelines now recommend both FDA-approved isoxazolines and EPA-approved mosquito repellent products to kill mosquitoes and help prevent heartworm transmission. Previously, only EPA-approved products were endorsed. This expanded recommendation aims to provide veterinarians with more options to tailor preventive measures based on individual patient needs and regional considerations.

Diagnosis: The guidelines recommend annual antigen and microfilaria testing for canine patients. Microfilaria testing is especially crucial if the dog’s prevention history is unknown or if infection is suspected. Dogs over seven months old should be tested for heartworms before starting preventive treatment to avoid inadvertently administering "slow kill" treatment to untested, heartworm-positive dogs. Testing should also occur seven months after any lapse in preventive medication. The emphasis on accurate and timely testing helps ensure that heartworm infections are detected and treated early, preventing severe complications.

Treatment: The AHS treatment protocol involves administering a macrocyclic lactone on Day 0, followed by four weeks of doxycycline to eliminate Wolbachia, additional macrocyclic lactone on Days 30, 60, and 90, and a series of three melarsomine injections on Days 60, 90, and 91. Activity restriction is recommended throughout the treatment period and for at least 6-8 weeks after completion. The guidelines also stress the importance of repeating heartworm tests if results are unexpected, such as when a dog is microfilaria-positive but antigen-negative. In such cases, blocked antigen caused by an antigen-antibody complex may be the issue. Repeating the antigen test with a heat-treated new blood sample can provide a more accurate result. This comprehensive treatment approach aims to eradicate the infection while minimizing the risk of complications.

Spectrum-of-Care Guidance

The updated guidelines include a spectrum-of-care section for cases where the optimal treatment protocol is not feasible. This section covers alternative treatment protocols, including those with fewer melarsomine injections and guidance for interrupted treatment. This flexibility ensures that veterinarians can provide effective care even in challenging circumstances, such as when resources are limited or patient-specific factors prevent the use of standard protocols.

Rising Heartworm Incidence

The AHS conducts a heartworm incidence survey every three years, gathering data from veterinary practices and shelters. The latest survey, conducted in early 2023, reflects data from 2022, showing an upward trend in heartworm rates, particularly in regions like Arkansas, the Carolinas, Georgia, Mississippi, and Texas. Unexpected increases were also noted in states with historically low heartworm rates, such as Connecticut, Kansas, Massachusetts, North Dakota, Oregon, and Washington. These findings underscore the need for continued vigilance and adaptation of preventive strategies to address emerging trends.

A study published in January in Frontiers in Veterinary Science by Banfield Pet Hospitals confirmed these trends, showing increases in the Northwest and New England. The study's findings align with the AHS survey data, highlighting the growing challenge of heartworm disease in diverse geographic areas.

“Heartworm disease in dogs continues to be far too common, despite the availability of highly effective medications to prevent and treat it,” said Dr. Rizzo. “Our hope is that the updated AHS guidelines provide the practical and proven support veterinarians need to significantly reduce the rate of heartworm disease.” The updated guidelines are designed to equip veterinarians with the knowledge and tools necessary to combat heartworm disease effectively, ultimately improving the health and well-being of dogs everywhere.

The updated Canine Heartworm Guidelines are the most comprehensive and rigorously researched resource on heartworm epidemiology, biology, life cycle, prevention, and treatment. By staying informed and adopting the latest guidelines, veterinarians can play a crucial role in mitigating the impact of this pervasive disease on the canine population.