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Are Heartworms Developing Resistance to Preventatives?

More dogs on year-round preventatives are testing positive for heartworms.

News item written by Mary Straus, published in the Whole Dog Journal, March 2011

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In August, 2010, representatives of the American Heartworm Society (AHS), the Companion Animal Parasite Council (CAPC), and experts in the field of nematode resistance met in Atlanta. Their goal was to discuss the possibility of heartworms becoming resistant to “macrocyclic lactones,” the scientific name for the heartworm preventatives we know as Heartgard (ivermectin), Interceptor (milbemycin oxime), Revolution (selamectin), and ProHeart (moxidectin).

Dr. Everett Mobley, a veterinarian who practices in Kennett, Missouri, wrote about this issue in his “Your Pet’s Best Friend” blog in May 2009. In his six-part post, “Are Heartworms Getting Worse?,” he says that he first began noticing an increase in the number of dogs in his clinic who tested positive for heartworms despite being on year-round heartworm preventatives in 2006. He learned that other veterinarians were reporting similar experiences, and that “These reports come from the Mississippi valley, starting about 100 miles south of St. Louis, and getting worse as one goes south.”

Experts dismissed these reports for a long time as being due to “client noncompliance,” that is, owners failing to give the preventatives to their dogs twelve months a year. It was not until April 2009 that they began to say, “We know that something has changed, but we don’t know what it is.  There is a problem, but the underlying cause has not been determined.”

The issue was a primary topic of discussion at the American Heartworm Society’s 2010 Triennial Symposium held in April. A landmark initial study was presented that evaluated heartworm microfilariae in different regions of the Mississippi Delta. The study revealed differences in sensitivity of the samples to macrocyclic lactones. Separate experiments revealed genetic variability of heartworms in different geographic locations, which could potentially be associated with varying responses to the drugs.


The AHS and CAPC issued a consensus statement in November regarding the findings of the meeting in Atlanta, acknowledging the problem and calling for further study. They believe that any heartworm resistance is geographically limited (presumably to the Mississippi valley) at this time based on credible reports of lack of efficacy. They recommend that pet owners continue to give preventatives year-round, following label directions, as they continue to be effective for the vast majority of dogs. There is no evidence that higher doses or more frequent dosing would increase protection. They also recommend yearly heartworm testing for all dogs, even if they have been kept on preventatives.

In the past, I have recommended that people might safely extend the time between doses of heartworm preventatives to six weeks, and decrease the dosage when using Interceptor, based on the efficacy studies that were done when the FDA approved these drugs. It is safer to administer preventatives monthly and to give the full label dosage. Following these steps will also ensure that, should your dog become infected, the product manufacturer’s guarantee will be honored and treatment costs will be covered. (Manufacturers will guarantee products only when purchased from a vet).

I still question the need to give preventatives year-round in cold climates, where mosquitoes cannot survive during the winter. The heartworm life cycle requires the larvae to spend some time inside a mosquito in order to develop into adults; without mosquitoes, there is no risk of infection. In warm climates such as are found in the southern half of the U.S. (below the 37th parallel), give heartworm preventatives year-round. This is also necessary to avoid voiding the manufacturers’ guarantees.

If you choose not to give heartworm preventatives year-round, keep in mind that all of these drugs work “backwards,” killing larvae that may have infected your dog in the previous month. Give the last dose after temperatures have dropped, and start them up again a month after your area warms up. If temperatures remain above about 45 to 50 degrees, day and night, you should give your dog monthly heartworm preventatives.

Keep in mind when testing for heartworms that it takes at least six months following exposure before a dog will test positive, using either antigen or microfilariae testing. This interval may be increased if the dog is being treated with heartworm preventatives during this time. The AHS now recommends that three consecutive negative tests, each six months apart, may be needed before we can feel confident that a dog is not infected with heartworms.

Research into possible resistance of heartworms to current medications is ongoing in a number of universities and other centers in the United States, Canada, and Italy. We’ll keep you posted.

Update: Protecting your dog from mosquitoes may help to reduce the incidence and transmission of heartworm disease when used along with heartworm (macrolytic lactone) preventatives, especially in areas where heartworm is endemic and resistance has been found. See Research targets mosquito's role in heartworm disease and New research cited in support of change to heartworm protocol.

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