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Resistance to Monthly Heartworm Preventatives: A Wake Up Call?

Recently there has been some concern by researchers and scientists that monthly heartworm preventatives are becoming less effective. Scientists are studying the fact that there are some strains of the parasite that causes heartworm disease(Dirofilaria) that are becoming resistant to the effects of this class of drugs. These drugs, also called macrolide antibiotics (including Heartgard, Sentinel, Revolution, Trifexis and ProHeart) have revolutionized heartworm preventative because of their effectiveness (almost 100%), safety and because they interrupt the development of 2 large stages after infection (hence the reason for once per month dosing rather than daily dosing). Unfortunately many pet owners and even some practicing new veterinarians do not realize that these products also provide control against several intestinal worm diseases, including those that can also be transmitted to people, particularly young children.

First a little biology, then some history, followed by new developments. There are 5 larval stages to this worm disease, two of which (the first two) occur in the mosquito gut and the last three which occur in the tissues and heart of the dog. The adult worms produce an egg form called a microfilaria, which circulates in the blood of the infected dog. Microfilaria look like small worms – small enough to fit through the capillary beds of a dog. Studies on dog owner compliance reveal that of all the dogs in the US, only 55% are treated with heartworm preventative. Of that group of dogs, only 56% receive adequate heartworm prevention defined as year round preventative use. Thus the actual number of treated dogs in the US is approximately one third of the total US dog population. To highlight this vulnerability, in the aftermath of Hurricane Floyd in 1999, 67% of dogs rescued from the floodwaters in a portion of North Carolina were infected with the heartworm parasite.

The new developments? A recent heartworm parasite strain from dogs in northeastern Georgia showed resistance to the effects of a monthly administered heartworm preventative. More recently (2013 data) demonstrated 4 resistant strains of the heartworm parasite from the Mississippi River delta area that showed diminished effectiveness to all of the heartworm preventatives now on the market. The cause for the development of resistance is currently being debated. Until new recommendations are developed or new medications developed, the best method of preventing heartworm disease remains year round heartworm prevention. Much like problems created by over usage of antibiotics, under usage of heartworm preventative creates the potential for genetic resistance to heartworm preventative. The American Heartworm Society, an organization that sponsors and furthers research into heartworm disease study, recommends proper and complete usage of heartworm preventative on a year round basis.