As a new grad, you may occasionally run across reports of heartworm infections that are resistant (or thought to be resistant) to heartworm prevention. Heartworm resistance became a hot topic in the early 2000s, when several cases were discovered.
The buzz has died down a bit since that time, but the topic still periodically surfaces in the veterinary field and the media. Clients may ask questions, the topic may come up at CE, and you will likely hear about it in journals. Having some basic background information on heartworm resistance will help you educate your clients and make the best possible recommendations for your patients.
If a dog tests heartworm positive, despite a previous negative test and a history of heartworm prevention, there are three possible causes:
- The dog missed one or more doses of heartworm prevention. Only about half of dogs on heartworm prevention receive it every month, year-round. Most cases of heartworm prevention “failures” are caused by missed doses of prevention.
- The dog wasn’t truly heartworm negative when prevention was started. Heartworms have a prepatent period of six months or more. If a dog tests negative before starting prevention, then positive one year later, the dog probably had a prepatent infection when the test was performed. The American Heartworm Society recommends that dogs starting heartworm prevention be tested before starting prevention, then every six months for the first year. This ensures that the dog is truly heartworm negative. At this time, testing frequency can be decreased to once yearly.
- The dog is infected with a resistant strain of heartworms. This is probably the least likely explanation, but certainly the most concerning.
The first documented case of heartworm resistance occurred in a dog found after Hurricane Katrina. This dog couldn’t be cleared of microfilariae, despite repeated dosing with heartworm preventatives. In-vitro testing confirmed that the dog was infected with a heartworm strain that was resistant to macrocyclic lactones. Additional testing found resistant heartworm strains in a number of dogs from the Mississippi Delta region.
Resistant heartworm strains may be resistant to any or all of the four macrocyclic lactone molecules (ivermectin, moxidectin, selamectin and milbemycin). Strains vary in their degree of resistance; some are susceptible to macrocyclic lactones only with repeated dosing, while other strains show no susceptibility to heartworm preventatives.
Macrocyclic lactone resistance is a hereditary trait. This raises concerns that resistance could spread, making our current heartworm preventatives less effective. Fortunately, this does not appear to be happening at this time. In fact, reports of resistance have actually decreased in recent years. Resistance does occur, and resistant strains have been isolated from a range of geographic regions, but this does not appear to be a growing problem at this time.
What can be done?
Noncompliance with heartworm prevention recommendations is a far greater contributor to heartworm disease than macrocyclic lactone resistance. Therefore, the most important thing you can do to prevent heartworms is to recommend effective, year-round heartworm prevention for every single patient.
If resistant heartworm strains are a concern, mosquito repellant can add another layer of protection. Studies have shown that transmission of heartworms (including macrocyclic lactone-resistant strains) is markedly decreased by the use of mosquito repellant. Some flea and tick preventatives (K9 Advantix®, Vectra 3D®, and others) contain a mosquito repellant; these products can play an adjunct role in heartworm prevention.
While all veterinarians should remain aware of the potential for macrocyclic lactone resistance, it does not appear to be a widespread problem at this time. Stay informed, though, because changes may occur! The American Heartworm Society’s website is the best place to receive accurate, up-to-date information on all heartworm-related issues, including heartworm resistance.
- American Animal Hospital Association. Heartworm Update.
- American Heartworm Society. Heartworm Preventive Resistance: Is it Possible?
- Atkins, C. Heartworm Resistance. Colorado VMA.
- Cima G. 2013. Heartworm preventive resistance is real. AVMA.
- DVM360. 2018. Heartworm resistance: What we know (and what we don’t).