All About Heartworm – Part 2

A close-up look at prevention and treatment of heartworm, and why both conventional and alternative therapies play important roles.

If you have a dog, you already know something about heartworm. But there are several misunderstandings surrounding this disease that need clarification. In the second of this two-part article, we’ll take a detailed look at treatment and prevention options, and why an integrative approach is the best defense.


If you live in a region where heartworm is prevalent, you need to be proactive about prevention. Start by taking steps to minimize your dog’s risk of being bitten by mosquitoes.

Your dog may also need to receive a conventional heartworm preventive. Many people shy away from these medications, but it’s important to realize that depending on where you live, they are an effective way to protect your dog, and are part of an integrative prevention approach. There are several choices – oral, topical, or injectable.

  1. Oral monthly medications use medicine such as ivermectin or milbemycin. In general, these drugs are nearly 100% effective when given following label instructions, and are actually among the safest medications used in clinical practice. In general, most holistic veterinarians prefer oral medications for conventional heartworm prevention because they don’t last as long in the body as topical and injectable medications (only 24 to 48 hours as opposed to months). While some people have expressed concerns about ivermectin, because certain breeds are more sensitive to these chemicals, the sometimes fatal reactions found in some lines of collies and collie crosses involve much higher levels of this chemical than what is found in heartworm preventives.
  2. Topical medicines are applied monthly to the dog’s skin. After application, they enter the bloodstream through the skin; concentrations of the medicine in the blood and tissues prevent heartworm disease. Because they persist in the dog’s body for several months, however, they aren’t the first choice with most holistic veterinarians.
  3. There is currently one injectable medication for heartworm prevention. It was recently returned to the market after being removed several years ago due to severe side effects in a number of dogs. There are still significant warnings and restrictions on the label, and veterinarians must be certified in order to administer this product. Because the medication lasts in the dog’s body for at least six months, most holistic doctors don’t prefer it.


It’s important that oral preventive medication be given per the manufacturer’s recommendations. I’m often asked if it’s okay to give the medication every 45 to 60 days rather than every 30 days. The people who ask this tend to be those who really resist giving their animals any medication. While I would like to find (or maybe even invent!) a natural heartworm preventive that’s as effective as the monthly oral medications currently prescribed, the low dose of medication contained in these products is way below the toxic or lethal dose.

In general, most holistic veterinarians prefer oral medications for conventional heartworm prevention because they don’t last as long in the body as topical and injectable medications.

When you administer the medication, it is quickly absorbed into the body. If an infected mosquito bites your dog that day, the medication will kill any immature heartworm larvae injected into him by the mosquito. If the mosquito bites the dog a few days later, the medication has already been expelled by the dog’s body and the injected heartworm larvae will begin developing. However, if you give the next dose of medication 30 days later, it will kill the developing larvae (despite what some people think, the medication does not work “retroactively” but actually kills developing larvae at the time of administration). If you wait longer than 30 days to give the next dose, then the drug is less effective due to maturation of the heartworm larvae. For this reason, it’s really important to give the drug as close to every 30 days as possible.


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