Q: Someone told me that there may be some resistance to Heartgard developing. Is this true? My dog uses Heartgard. Should I consider changing brands?
For several years now veterinarians, mostly from states bordering the Gulf of Mexico, have been saying they think there could be some resistance developing to the current heartworm preventives. Last September the Companion Animal Parasite Council and the American Heartworm Society (AHS) gathered leading parasitologists, veterinary cardiologists, and scientific researchers to look into this very topic.
After reviewing scientific and clinical field observations, the groups “acknowledge the possibility of resistance and encourage further resistance-related research.” It also expressed confidence in the effectiveness of current prevention protocols. Dr. Jay Stewart, president of CAPC, noted “we are confident that existing products are extremely effective when administered as directed.” Dr. Wallace Graham, president of AHS, concluded, “we want to stress that compliance and year-round administration of preventives for non-infected pets remain the most important methods in preventing heartworm disease.”
Even to the experts, it remains unclear whether or not there may be some isolated cases of resistance. However, here are some things I can confidently tell you:
(1) Most cases of “failure” come from owners not using the product every four weeks throughout the year. Late administration and skipped months are found to be the cause of infection in almost every case.
(2) In recent years, some pet owners have chosen to purchase preventives through online pharmacies and catalogs instead of their veterinarian. In almost every case, the drug manufacturers do not sell through these sources and they have been warning that unknown storage and shipping conditions could lead to product failure. It is possible that the chickens are coming home to roost on this one.
(3) One would expect that if there is Heartgard resistance that there should also be resistance to Interceptor and Revolution. All three are macrolide antibiotics and work by the same mechanism. As such, it would be surprising to see one brand have resistance problems and have the other ones be unaffected.
(4) The link with Heartgard comes from a recent lawsuit filed by a terminated employee. He is suing the company alleging they are covering up data indicating resistance. One of two things will come from this suit. Either data will become public answering the question once and for all or the ex-employee will be shown to be trying to harm the company out of bitterness. Unfortunately, sometimes just the accusation is enough to cause distrust in the public’s eye.
(5) If there is resistance developing, right now it seems geographically limited. My colleagues and I have not seen the issue in Virginia. In the past five years, I have only personally seen two cases of heartworm disease in dogs on prevention products. Both were situations where the product was ordered online. One was using Interceptor and one was using Revolution. Neither product was backed by the manufacturer due to the source of purchase. Both companies claimed potential product damage due to excessive heat exposure during diversion from veterinary channels.
(6) All three major drug companies continue to back their heartworm products when purchased through a veterinarian. That means if a dog does come up positive, the company will pay all of the costs associated with treatment. This is a good reason to stay away from generics, off-label use, and non-veterinarian sources. If there is resistance and if one of your dogs is affected, the treatment will be taken care of.
(7) Most infections that are caught early are treated without long-term effects. If there is limited resistance, it should not be enough to cause a significant worm burden in a dog that gets monthly prevention all year and screened for infection annually. We should still be able to catch a resistant infection early and treat with minimal side effects to the pet and no cost to the owner.
(8) Hopefully drug companies will work on developing a preventive solution that involves a different mechanism of action than macrolides. If we are not seeing resistance now, it is almost certain that we will at some point in the future.
Dr. Watts is a companion animal general practitioner and owner of Clevengers Corner Veterinary Care. He can be reached through ClevengersCorner.com or by calling 428-1000.