Heartworm is serious and infections with the parasite can lead to death in affected dogs and cats. Although prevention is nearly 100% successful, more than 1 million pets in the United States have heartworm infections.

Medications to prevent heartworm target the larval stage of the parasite – that stage of the parasite that is transmitted to an animal by an infected mosquito. If there is a lapse in prevention, those larvae go on to develop into adult worms that live in the animal’s heart and arteries of the lungs.

While we do have preventives for cats, unfortunately, there is no approved drug available for treating adult heartworm infections in cats – treatment focuses on monitoring and managing signs and symptoms associated with the infection.¹ Long-term care is individualized but may include:

• Prednisolone to reduce inflammation that leads to coughing or other respiratory distress
• Antibiotics (doxycycline) to kill bacteria that live in heartworms (Wolbachia) which can help adult worms to survive and reproduce
• Other supportive measures as needed – hospitalization for IV fluid support, oxygen, and other respiratory therapies

Conversely, we do have approved treatment options for dogs, but it’s not a “one and done” therapy – it takes months to kill all stages of heartworm living in the bloodstream, heart and lungs. Treatment requires a combination of medications – given at different time points – to kill the adults, immature worms that could lead to new infections, as well as bacteria (Wolbachia) that live within heartworms and that help them to survive.^2,3

Medications used in dogs may include:

• Antibiotics (doxycycline) to kill the bacteria that live in heartworms (Wolbachia) and that help adult worms to survive and reproduce
• Melarsomine to kill adult heartworms
• Preventives to eliminate the immature larvae and prevent new infections
• Other adjunctive therapies (as needed) including anti-inflammatories (e.g. steroids) and anti-clotting medications (e.g. aspirin)

Surgical removal of heartworms as a treatment can be done in both dogs and cats. Although it is not a common approach to treating animals with heartworm infection, its advantage is that it lessens the possibility of inflammatory reactions that can develop as adult worms die with medical therapy.

Dogs undergoing treatment for heartworm require strict activity restriction throughout treatment and for 6 to 8 weeks afterward, as physical exertion can be dangerous while the worms are dying off.

To summarize, we have no approved treatment for cats that develop heartworm infection; prevention is the key to protecting cats from disease. While we do have approved treatment for dogs, “treatable” doesn’t mean it’s easy, cheap, or risk-free. Additionally, a 2023 Banfield Pet Hospital review of medical record data found that dogs who tested positive and underwent treatment for heartworm had a higher risk of complications over time, including heart failure and cardiomyopathy.⁴ 

Year-round prevention for dogs and cats – as recommended by the American Heartworm Society⁵ – is nearly 100% successful in preventing heartworm infection. Owners should talk to their veterinarian about annual testing and preventive options that best suit them and their pet.

Resources:

1) The American Heartworm Society - Prevention, Diagnosis, and Management of Heartworm Infection (Dirofilaria immitis) in Cats (2024)

2) The American Heartworm Society - Heartworm Treatment Guidelines for the Pet Owner (2024)

3) The American Heartworm Society - Prevention, Diagnosis, and Management of Heartworm Infection (Dirofilaria immitis) in Dogs (2024)

4) New pet owner survey suggests common myths fueling significant gaps in prevention of deadly parasite for U.S. dogs and cats. PR Newswire. April 2024

5) The American Heartworm Society - Heartworm Prevention for Dogs