Ivermectin for Heartworms in Dogs

Key Takeaways

• Ivermectin is widely used to prevent heartworm disease in dogs through products like Heartgard, which kills immature heartworms before they mature.

• The American Heartworm Society recommends adulticide therapy using melarsomine injections as the primary treatment for heartworm-positive dogs.

• The slow-kill method, using ivermectin and doxycycline, is an alternative but comes with risks, including potential continued damage to the dog's organs and development of drug resistance.

Ivermectin is an antiparasitic drug commonly used to prevent heartworm disease in dogs. Heartworm disease is a serious parasitic infection that any dog can contract. It's transmitted through mosquito bites and can affect dogs, cats, and other mammals. Learn how ivermectin is used for dogs to treat or prevent heartworm disease.

How to Treat Heartworm Disease in Dogs

What is Ivermectin?

Ivermectin is a broad-spectrum antiparasitic drug that can kill various parasites found in dogs. It's classified as a macrocyclic lactone, a type of drug that causes neurological damage to parasites, resulting in their paralysis and death.1

Ivermectin is a common ingredient in heartworm prevention products, such as Heartgard and Iverhart. These products contain low doses of ivermectin that are safe for dogs, but capable of killing immature heartworms (microfilaria) before they mature into adult heartworms and infiltrate the heart.

Heartworm Disease in Dogs

Heartworms are thin, thread-like worms that reside in the heart and blood vessels of dogs and other mammals. As these worms reproduce and increase in number, they cause damage to the heart, lungs, and nearby blood vessels.

Transmission

When an uninfected mosquito feeds on a dog infected with heartworms, it ingests heartworm larvae. If the mosquito then feeds on an uninfected dog, the larvae are transmitted. Monthly heartworm prevention does not stop future mosquito transmission but instead kills larvae that your dog has encountered over the past month. To further protect your dog from mosquito bites, consider using a dog mosquito repellant.

Heartworms have various larval stages, and not all are susceptible to preventatives. Missing a dose can allow larvae to mature beyond the stages where preventatives are effective, making consistent use crucial for disease prevention.

Diagnosis

Heartworm disease is first diagnosed with a blood test at your veterinarian's office. Due to the potential cost and risks of treatment, veterinarians often send a blood sample to a laboratory for confirmation. After confirming the infection, the vet will stage the heartworm disease, take a detailed history of your dog's signs at home, and perform chest radiographs (X-rays).

Stages

Heartworm disease in dogs is classified into four distinct stages. The veterinarian must determine the stage to assess the risk and plan the appropriate treatment.

• Mild: The mildest form of heartworm disease, dogs with this stage may have almost no symptoms. If a dog is symptomatic, they will have only mild signs such as an occasional cough.

• Moderate: Dogs may have a moderate cough and become tired after normal activity. The vet may also start to see radiographic changes in the dog's lungs. This can be accompanied by a change in lung sounds when the vet listens to the dog’s breathing.

• Severe: Severely affected dogs will have a persistent cough and notable exercise intolerance, meaning they get winded after minimal play or activity. They may also have episodes of difficulty breathing and other signs of heart failure, including fluid accumulation in the abdomen or fainting episodes. Chest radiographs of these dogs will also show definitive changes due to the heartworm disease. Dogs in this category can die suddenly.

• Caval Syndrome: This is when a dog's heartworm burden is so high that the adult worms block blood flow back to the dog's heart. These dogs are weak, have difficulty breathing, and may have destruction of their red blood cells, which can result in pale gums, a reddish color to the urine, or a yellowish color to the skin or whites of the eyes. This requires a dangerous but necessary procedure to remove the worms from the dog.

Can I Catch Heartworms From My Dog?

Ivermectin and Treating Heartworm Disease in Dogs

The American Heartworm Society recommends treating heartworm disease in dogs using melarsomine (Immiticide or Diroban) injections to kill the adult worms. This is known as adulticide therapy.

In rare cases, a dog may have health conditions that make adulticide therapy too risky, leading the veterinarian to recommend ivermectin. This approach gradually kills the worms and is often referred to as the 'slow-kill method.'

Slow-Kill Method

This method involves administering a preventative containing ivermectin or moxidectin and an antibiotic called doxycycline to a dog with heartworm disease. It is less expensive than melarsomine treatment but is termed the slow-kill method because it takes much longer to eradicate the infection, potentially over a year. During this treatment, dogs must have restricted exercise.

The major issue with the slow-kill method is that the heartworms can continue damaging a dog's heart, lungs, and blood vessels during treatment. This approach requires strict adherence to medication schedules and exercise restrictions; any lapse can further endanger the dog.2

Concerns exist that this method may cause heartworm larvae to become resistant to preventative medications, making it a less recommended option.

Adulticide Therapy

Immiticide treatment for heartworm disease begins similarly to the slow-kill method, with initial steps involving a heartworm preventative and doxycycline. However, rather than continuing these medications indefinitely, melarsomine is introduced a month after the ivermectin and doxycycline course.

Melarsomine is injected into the dog's back muscle. The American Heartworm Society advises three injections: the first and second are administered 30 days apart, with the third given one day after the second.

Dogs undergoing adulticide therapy also require strict exercise restriction. While typically shorter than the slow-kill method, maintaining the dog's calmness and inactivity is crucial until the veterinarian provides clearance. When heartworms die, the dog's body breaks them down, but exercise can cause dead worms to fragment, posing a risk of an embolus blocking small blood vessels.

If your dog struggles to remain calm, your veterinarian might prescribe a sedative like trazodone. Despite this, keeping a high-energy dog inactive is challenging, but it remains a crucial part of the treatment plan.

How to Prevent Heartworm Disease

While heartworm disease is treatable, it isn't easy or risk-free. Prevention remains the most effective strategy. Numerous heartworm prevention products are available, some containing ivermectin. Options include once-monthly oral chews, topical products, and injectables your veterinarian can administer every six or 12 months, based on availability. For guidance on heartworm disease risk or selecting the right prevention product, consult your veterinarian.