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 via mosquitoes and can affect dogs, cats, and other mammals within your neighborhood. Learn how ivermectin is used for dogs to treat or prevent heartworm disease.

What is Ivermectin?

Ivermectin is a broad-spectrum antiparasitic drug that can kill several different parasites found in dogs. It's in a category of drugs called macrocyclic lactones, which cause neurological damage to parasites, leading to paralysis and death of the parasite.

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 spaghetti-like worms that live in the heart and blood vessels of dogs and other mammals. As the worms reproduce and grow in number, they damage the heart, lungs, and surrounding blood vessels.

Transmission

When an uninfected mosquito takes a blood meal from a dog infected with heartworms, that mosquito will take up heartworm larvae. When that same mosquito then takes a blood meal from an uninfected dog, some of the larvae are transmitted to the dog. When you give your dog their monthly heartworm prevention, you aren't preventing future mosquitoes from transmitting, but rather, the prevention is killing the larvae that your dog has been exposed to over the course of the past month. If you want to help prevent mosquito bites altogether, try using a dog mosquito repellant.

There are different larval stages of heartworms and not all stages are susceptible to the preventatives. Missed doses can let larvae mature past the stages where the preventatives are effective, so consistent use is important in preventing disease.

Diagnosis

Heartworm disease is initially diagnosed with a blood test at your veterinarian's office. Since treatment can be costly and risky, most vets will also send a blood sample to a laboratory for confirmation. Once a heartworm infection is confirmed with additional blood testing, the vet will want to stage the heartworm disease. They'll take a thorough history to discuss the signs your dog has shown at home and perform chest radiographs (X-rays).

Stages

There are four distinct classes of heartworm disease in dogs. The veterinarian needs to determine the dog's stage of heartworm disease so they can assess the risk of the disease as well as the 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.

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 and the veterinarian may recommend ivermectin to kill the worms gradually over time. This is often called the 'slow-kill method.'

Slow-Kill Method

This involves giving a dog with heartworm disease a preventative that contains ivermectin or moxidectin, with an antibiotic called doxycycline. This treatment can be less expensive than melarsomine treatment, but it's called the slow-kill method because it can take much longer to eradicate heartworm disease. It may take up to a year or more for the adult heartworms to die. Dogs being treated with this method must have restricted exercise for the duration of the treatment.

Unfortunately, the problem with the slow-kill method is that the heartworms can continue to damage a dog's heart, lungs, and blood vessels throughout treatment. This method also requires absolute compliance with medication administration and exercise restriction; faltering at any point can put the dog at further risk.

There are concerns that this method leads to the development of heartworm larvae resistant to preventative medications, which is another reason why it is not recommended.

Adulticide Therapy

Immiticide treatment of heartworm disease starts quite similarly to the slow-kill method. The first steps involve putting the dog on a heartworm preventative and doxycycline. However, as opposed to continuing these medications for months and months and months, melarsomine is added a month after finishing the ivermectin and doxycycline.

Melarsomine is an injection that goes into the back muscle of a dog. The American Heartworm Society recommends three individual injections. The first and second are given 30 days apart and the third is given one day after the second.

Dogs receiving adulticide therapy will still need strict exercise restriction. Although the duration is usually not nearly as long as it is for the slow-kill method, it's much more important to keep the dog as calm and inactive as possible until cleared by the vet. As the heartworms die, the dog's body works to break them down. Activity or exercise can cause a dead worm to break apart, creating a risk factor for an embolus, similar to a blood clot in the bloodstream, that can lodge in a small vessel and prevent blood flow.

If your dog has difficulty remaining calm, your veterinarian may prescribe a medication to promote relaxation, such as trazodone. However, it's still difficult for owners to follow through on this vital part of the treatment plan, especially if they have a high-energy dog.

How to Prevent Heartworm Disease

Heartworm disease can be treatable, but that does not mean it's easy or safe to treat. Preventing heartworm disease is still the best therapy out there. Today, many heartworm prevention products are on the market, some of which contain ivermectin. There are once-monthly oral chews, once-monthly topical products, and even injectables that your veterinarian can administer every six or 12 months (depending on the product the veterinarian carries). If you have concerns about your dog's risk for heartworm disease or are confused about which prevention product is right for your dog, speak to your veterinarian.