What to Know About Treating Heartworm Disease with Ivermectin

Heartworm disease is a very real, very scary parasitic infection that any dog can contract. It is transmitted via mosquitoes and can effect dogs, cats, and mammals within your neighborhood, like squirrels, raccoon, skunks, foxes, coyotes, etc.

Heartworm Basics: What It Is and How Dogs Get It

Heartworms are exactly what they sound like, worms that live in the heart and blood vessels of dogs and other mammals. 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.

The problem arises in the fact that 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.

Diagnosing Heartworm Disease in Dogs

Heartworm disease is initially diagnosed with a blood test. Since treatment can be costly and risky, most vets will send out a blood sample for confirmation tests. Once a heartworm infection is confirmed with additional blood testing, the vet will want to stage the heartworm disease. This can involve taking a thorough history to assess for any symptoms the dog is showing at home as well as chest radiographs. There are four distinct classes of heartworm disease in dogs. It's important for a vet to know which class a dog with heartworm disease is in to know the risk the dog faces not only from the heartworms but from the treatment as well. Each class is characterized by a progression of symptoms and diagnostic changes.

Mild: The mildest form of heartworm disease, dogs with this stage will oftentimes be asymptomatic at home, meaning they have almost no symptoms at all. If a dog is symptomatic, they will have only mild signs such as an occasional cough.

Moderate: Symptoms can include a moderate cough as well as acting tired after normal activity. The vet may also start see radiographic changes in the lungs of a dog moderately affected by heartworm disease. 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. They will also have 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 as a result of the heartworm disease. Dogs in this category can die suddenly.

Caval Syndrome: This when a dog's heartworm burden is so high that the adult worms quite literally 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 physically remove the worms from the dog's heart.

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. For rare pets with health conditions that make adulticide therapy risky, the veterinarian may recommend ivermectin to kill the worms gradually over time. This is known as the slow-kill method.

Slow-Kill Method

Sometimes called the slow-kill method, this involves giving a dog with heartworm disease an ivermectin-based preventative, such as Heartgard Plus, Iverhart Plus, or Tri-Heart Plus, with or without an antibiotic such as doxycycline. This treatment can be less expensive than the immiticide treatment but it's called the slow-kill method because it can take much longer to fully treat a dog with heartworm disease. Some dogs may take up to a year or more to be fully rid of adult heartworms! Dogs being treated with this method also have restricted exercise for the duration of the treatment. The problem with the slow-kill method, though, is that throughout treatment the heartworms can continue to damage a dog's heart, lungs, and blood vessels.

The slow-kill method also requires absolute compliance on the part of the owner with the medication administration as well as 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 out quite similarly to the slow kill method. In fact, the first steps involve putting the dog on an ivermectin-based preventative and doxycycline just like the slow kill method. However, as opposed to continuing these medications for months and months and months, melarsomine is added in a month after finishing the ivermectin and doxycycline. Melarsomine is an injection the goes into the back muscle of a dog. The American Heartworm Society recommends three individual injections, the first and second one being 30 days apart and the second and third one being 1 day apart. Most veterinarians will want to keep a dog for observation for the first few hours after an injection. This is because the medication can cause discomfort for the dog and the vet will be watching for signs of pain. Dogs receiving adulticide therapy will still need strict exercise restriction, but the duration is usually not nearly as long as it is for the slow-kill method.

This method isn't without its own drawbacks, though. For starters, a dog receiving this injection has to stand completely still for the veterinarian to administer the drug into the dog's back muscle. This may be more difficult for some dogs than others, whether they have a lot of fear anxiety at the vet or are over-excited and over-stimulated at the vet and can't sit still because they just want to play. The other drawback with this method is that, although the exercise restriction duration is shorter, it is much more important to keep a dog as calm and as quiet and as sedentary as possible before they are cleared by their veterinarian. This is because, as the heartworms are dying off, the dog's body will work to break them down. In the face of activity or exercise, though, a dead worm could possibly break apart. This would create a risk factor for an embolus, similar to a blood clot in the bloodstream, that can then lodge in a small vessel and prevent blood flow.

There are medications, such as trazodone, that a veterinarian can prescribe to keep dogs quiet during this period of exercise restriction, but for some higher energy dogs, it's still difficult for owners to follow through on this vital part of the treatment plan.

Prevention Truly is the Best Medicine

Heartworm disease can be treatable, but that does not mean it's easy and safe to treat. Preventing heartworm disease is still the best therapy out there. Today, there are a variety of heartworm prevention products on the market. There are once monthly oral prevention, once monthly topical prevention, 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.