Heartworm Prevention Part 1

I’m going to spend a little time covering the two oral heartworm preventive products that we carry at Pet Authority. I’ll explain how they work in general and then give the spectrum of parasites that they cover.

Oral heartworm prevention has been around for a long time. Initially, a tablet had to be given every single day to prevent heartworms. We’ve come a long way from the Filaribits (diethylcarbamazine)! Currently, the most common type of heartworm prevention is a tablet that is given orally once a month.

As we discussed briefly in the last post, heartworms are transmitted by mosquitos. The mosquito’s bite allows heartworm larva to enter the dog’s body. They migrate to the heart and major vessels that go to the lungs. There are a few stages that the larva goes through as it makes its way from the skin bite to the heart/lungs. Heartworm prevention medications have to try to kill those larva at some point between the mosquito bite and adulthood.

The most common class of medication used to kill heartworm larva are the macrocyclic lactones. Drugs in this family came originally from a bacteria that lives in the soil. They have an extremely broad efficacy when used to treat parasites — internal and external. They are effective at relatively low doses and with a relatively high safety margin. What this means is that we’ve got a readily available, safe, effective way to prevent and treat many parasitic diseases. Ivermectin and Milbemycin are the two specific macrocyclic lactones that I will mention here.

Nearly all heartworm products out there on the market also kill a number of intestinal worms (which ones will vary by product). A second drug in the tablet may be present to accomplish this task. (Milbemycin kills intestinal worms on its own. Ivermectin -can- but not at the dosage in the heartworm preventives.) Pyrantel pamoate is the dewormer found in the most common monthly preventive tablets. The major sorts of worms that we target are hookworm, roundworm, and whipworm.

How the Drugs Work:

Basically, each dose of a macrocyclic lactone clears out a number of the heartworm larva from the dog’s system that have been ‘collected’ over the previous 30 days. We once believed that a single dose of any macrocyclic lactone was enough to clear out ALL of the larva a dog contracted in the prior 30 days. Some recent data shows that it may take as many as 3 consecutive doses to clear out the larva from an infected mosquito bite. This simply proves that what we’ve been recommending all along is the best policy: keep dogs on heartworm prevention year round. The manufacturers will guarantee that dogs are protected from heartworm if the prevention is given year-round. They help pay for treatment if there is a product failure.

The dewormers also attack and paralyze/kill the intestinal worms when they are given monthly. If the products are given year-round, the manufacturers will guarantee that pets will remain free of the intestinal worms covered by that product. It’s a great guarantee, especially since hooks and rounds are contagious to people in various ways.

Products We Carry:

Top bannerTri-Heart Plus (ivermectin/pyrantel): This is a tried-and-true formula that most people are familiar with. Given once a month, it will protect dogs from heartworm, hookworm, and roundworm. It’s a beef-flavored chewable tablet. This is essentially the same as Heartgard Plus, which is the meaty chew biscuit. We carry this because the manufacturer offers a better guarantee than the competitors and it’s priced a bit lower than Heartgard Plus. About 98% of the dogs that would willingly eat the Heartgard Plus will also eat Tri-Heart Plus. We haven’t had any problems with this product at all. More information on Tri-Heart Plus can be found here.

Index 01Interceptor (milbemycin): The milbemycin in this product takes care of heartworm, hookworm, roundworm, and whipworm. The Whipworm protection is what sets this oral chewable tablet apart. We typically dispense this product for dogs that are at risk for, or have had, whipworm. Whipworm is not contagious to people, but it’s a nuisance for dogs. It’s also exceedingly hard to clear the eggs out of the outdoor environment, so dogs that have a positive fecal test for whipworm have to be on Interceptor monthly forever to protect them from reinfection. (One of our topical products also covers for Whips, but I’ll deal with that one later on. It’s a more complicated product.) More information on Interceptor can be found here.

Choosing the Right Prevention

It’s not that difficult, really. We base our decisions first and foremost on what a patient is at risk for contracting. All dogs are at risk for contracting heartworm, whether they go outside a lot or a tiny bit. Ivermectin or Milbemycin are great options for taking care of heartworm prevention. I don’t have a preference as far as heartworms themselves are concerned.

“Stay at home” suburban dogs are at a lower risk of whipworms than a dog that goes hunting, goes to dog parks, or has a lot of wildlife (foxes, mostly) in its area. Most dogs fall into this category. Those dogs will do just fine with Tri-Heart Plus. If a patient is at risk for, or has, whipworms, we need to use Interceptor. I send Tri-Heart Plus home with most of my patients. Those at risk for whips will get Interceptor.

A note on Interceptor: If a dog is POSITIVE for heartworm, a dose of milbemycin could cause a very severe allergic/anaphylactic reaction. This can and often is life-threatening for the dog. It’s absolutely necessary for a dog to have a current negative heartworm test before starting Interceptor!

The tl;dr version is: Keep your dog on prevention year-round and pick a product based on which parasites your dog will be at risk of contracting. “Heartworm” prevention also kills intestinal parasites.


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