Heartworm Part II: Medication - Benefits and Issues

Patty Khuly, DVM
Updated: June 02, 2016
Published: October 03, 2006
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Do you ever wonder why it is you administer heartworm medication to your dogs (and some of you, to your cats)? If you’re reading this I know you’re not illiterate. You can surely tackle the print on the side of a box with ease. However, did you ever wonder how common these wormy creatures are and whether you actually need to give a drug each month to kill it?

An increasing number of my clients do. Surprisingly similar (albeit less widespread) to the raging debate on vaccines, heartworm resistance is on the rise—and I don’t mean drug resistance. The one I refer to is a purely human phenomenon.

Ever since Filaribits Plus (back in the seventies and eighties) was found to cause devastating liver damage to some dogs and then  Ivermectin (the active ingredient in Heartgard) was determined to be hazardous to the health of select breeds (collies and Aussies) a movement of anti-heartworm proponents has been slowly gaining ground.

No doubt, drugs are an imperfect way to deal with disease. Natural immunity is always preferred. However, dogs don’t have natural immunity against heartworms. The slithery things are much too smart for that. They have been living successfully inside dogs for centuries, perhaps even for millennia.

Heartworms don’t really aim to hurt dogs, they just want to have a place to live and breed. Killing off their host is an unwanted by-product of their lifestyle. Because dogs have only recently achieved a greater lifespan (due to their overwhelmingly successful interaction with humans in recent decades and the medical attention that comes with it) heartworms have become a more serious health risk, particularly to the geriatric population. But all dogs, even young dogs, are at risk for disease, even for serious reactions to the worms in some cases (such as liver and or kidney failure and, more commonly, respiratory disease).

For this reason preventative heartworm medication is a must for dogs living in climates that support mosquitoes. Year-round, monthly administration is by far the safest means of control. Eliminating mosquitoes entirely is impossible.

Some of my clients contend that heartworms are preventable via natural supplements that boost the immune system. No study bears this out. Some believe that controlling the mosquitoes (eliminating standing water in the environment and using citronella collars) is the way to go. But there’s no way to ensure that NO mosquito gets to your dog.

I even have some clients that boast that their dogs have never received a heartworm pill and yet have never tested positive. This is fairly common. It’s the rest of us that keep their dog free of heartworms by eliminating our dogs as hosts, thereby reducing the overall population of larvae in the environment. If that`s you, I hope I can help you  reconsider.

Much like the issue of vaccination, it’s always safest to be the one unvaccinated animal in a population of vaccinates. Of course it’s best not to assume the risk of a reaction if you know that everyone else is protected and can’t transmit any disease to you. So it’s perfectly safe to not administer heartworm medication as long as the rest of the dogs receive their heartworm pills.

My contention, then, is that it’s our civic duty to keep as many dogs free of heartworm disease as possible by taking our dogs out of the host population with monthly medication.  Sure, all drugs have their risks (I repeat that constantly in this blog), but the risk of heartworm medication reactions is miniscule by drug standards. The dose needed to kill larvae is teeny tiny; only one dog in about 5,000 reacts adversely (usually minimally) to these drugs.

If you don’t give your dog any other drug and you live in a mosquito prone environment, please consider this one—if not for the sake of your own dog, for that of all the strays that have no access to healthcare and might be helped by your generous contribution to the reduction of heartworm in the overall population. Thanks!