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Heartworm prevention for dogs is, in my opinion, one of the most important things you can do for your dog to keep him healthy if you live in an area where there are mosquitoes and heartworms. I listened today to a webinar that presented the latest in emerging concerns about heartworm prevention in dogs.
I’ve talked about potential heartworm medicine resistance before. However, there have been a few new findings since that article was published.
Are Heartworms Becoming Resistant to Currently Used Heartworm Prevention Medicines?
According to Dr. Byron Blagburn, an expert in parasitology who has studied heartworms extensively, there are some strains of heartworms that are indeed resistant to all of the currently available heartworm prevention medicines.
As drastic as that sounds, that does not mean we should stop giving our dogs heartworm prevention medicines. However, it does mean that we need to be aware of the potential for failure. We can no longer assume that our heartworm prevention is going to be 100% effective in preventing heartworms in all dogs.
The good news is that the highest concentration of dogs that have become infected with heartworms despite being on heartworm prevention are in the areas where the heartworm burden is the heaviest. By far, the most numerous cases have been found in the Mississippi Valley and Mississippi Delta regions in the southern part of the United States.
What Does This Mean for Dog Owners? How Should Heartworm Prevention Be Approached?
As noted previously, dog owners should continue using heartworm prevention medicines. However, it is more important that ever to follow label directions carefully.
- Heartworm prevention medicines labeled for monthly use should be administered monthly. Dog owners should not try to stretch out the dose to every six weeks or every other month.
- Heartworm prevention medicines should be given year round and should not be discontinued in the winter, even in climates with severe winter weather.
- Testing for heartworms should be performed at least once yearly. Though the chance of infection is low, failure is possible. If your dog does become infected, early detection will allow early intervention.
Interestingly, Dr. Blagburn’s work with one particular strain of resistant heartworms (known as the MP3 strain) seems to indicate that medications containing moxidectin may be more effective in killing these worms than other heartworm prevention medicines. (Moxidectin is the active ingredient in Advantage Multi™.) Blagburn postulates that this may be because moxidectin remains in the system of the dog at higher dosages for a longer period of time than other heartworm prevention medicines, such as ivermectin, selamectin or milbemycin. However, it is not clear at this time how prevalent the MP3 strain is in naturally infected dogs and whether other strains of heartworms react in the same fashion. So, the verdict is still out on the validity of these findings.
Potential Causes for the Appearance of Resistant Strains of Heartworms
Blagburn has some theories about why these resistant strains of heartworms have surfaced. These are some of the factors that he believes contributed to their appearance.
- Misuse of heartworm prevention medications containing moxidectin. Proheart 6™ is an injectable heartworm prevention medicine containing moxidectin. The injections need to be given every 6 months to prevent heartworms. In particular, Blagburn believes that dogs that received only one dose of Proheart 6™ and never returned for subsequent treatments may have contributed to the development of resistant strains. He theorizes that the increased blood levels of moxidectin that result from the injections select for the resistant strains which are able to survive once the dog is no longer receiving the medication. This differs, according to Blagburn, from the monthly doses of moxidectin present in Advantage Multi™.
- The second factor that Blagburn credits as problematic is the administration of heartworm prevention medications to heartworm positive dogs in an attempt to elicit a slow-kill or soft-kill of heartworms. Blagburn believes, along with many of his colleagues, that this slow-kill method should not be used for most cases of heartworm disease and that melarsomide (Immiticide™) is the proper medication medication for treating heartworm disease.
Research is still ongoing in this area by Dr. Blagburn as well as many other scientists and veterinarians. We will continue to keep you posted of the latest developments as we learn them.
Source: Emerging Issues in Heartworm Prevention, webinar presented by DVM360 on April 20, 2011 featuring Dr. Byron Blagburn, MS, PhD
Photo Courtesy of kyolsen/Flickr.com
About Lorie Huston, DVM
Lorie Huston is a veterinarian, pet health and pet care expert, professional writer, blogger, social media and blogging consultant, and SEO strategist.
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{ 10 comments… read them below or add one }
Great information. Last I heard we also had a national shortage of Immiticide. Is that still the case? Are other drug companies stepping up to make their own version of heartworm treatment?
Hi, Kyla. There are no other companies that make Immiticide, only Merial. I’m not sure if Merial still has the patent on it or not. As of now, I’m hearing that we can get as much Immiticide as we need. I have three heartworm-positive dogs that we’ll be treating within the next few weeks, so I’ll let you know if we have any difficulty getting the Immiticide for them. They’re all relatively large dogs so we’ll need quite a bit of it. (In case any of you are wondering, none of these dogs have been on preventive medication previously. They are all rescues, two of them from the southeastern US…I’m not sure what state exactly they came from.)
Thanks for the info, Lorie. Heartworm is such a scary thing – we do monthly preventative treatments and the yearly tests for Bella, and I hope we never have to deal with a case of it!
I hope you never have to deal with it either, Pup Fan. It is scary. But it sounds like you’re doing everything you can to keep your four-legged friend safe from it. Keep up the good work
As a show person and breeder of multipurpose Golden Retrievers I am concerned about the issue of heart worm and have a few questions:
1) how early should a litter of pups start receiving interceptor, I usually start at 8 weeks when 1st series of vaccines and they are evaluated.
2) I live in Kalamazoo MI, and the vets here are reporting 11 new cases as well as my vets office, these dogs contacting heart worm are they mostly rescue or has it increased with responsible pet owners as well.
3) My dogs go in the water and work in the field is there any preventive measures I can take.
Thanks so much!
Mara
MaraKoyne Goldens
You’ve asked some useful questions, Mara. Thank you. You can find the answers here: Heartworm Prevention Questions and Answers
My new vet uses the injectable Proheart for heartworm prevention, which was just administered to my six-month-old goldendoodle. Subsequently, I was informed that Proheart had been recalled by the manufacturer in 2004 because of reports of severe illnesses and deaths. A 2005 study I found (Glickman et al., Intern J Appl Res Vet Med • Vol. 3, No. 2, 2005, 49-61) concludes that Proheart is no more risky, and in some respects less risky, than other medications. The study was funded by CDC but also by Fort Dodge, the manufacturer of Proheart.
Proheart is obviously available again, but I can’t find any information on what other studies have been done, whether the origins of the apparently high rate of reporting of adverse effects were clarified, or whether, for example, the formulation or dosage has changed. Clearly, it’s more convenient to administer than monthly doses of, say, Heartgard, which I’ve been using for earlier dogs for some years.
Can you clarify this for me?
Thanks.
Bob Lichter
Hi Bob.
Proheart Six has been used overseas for quite a while, apparently without the incidence of adverse effects experienced here in the US. I’m not quite sure why that is. It was recalled for a time in the US, as you mentioned. However, it has since been released back into the market with stricter usage requirements implemented by the FDA. That’s why it cannot be administered at the same as vaccines or other medications. The formulation and the dosage has not changed though. I hope that helps to clarify the situation for you a little bit.
My 2 year old border collie tested positive for heartworms today. The vet suggested I give him a 30 day dose of doxycycline along with he advantage multi topical prevention medicine for 6 months, and then re test and see if they have cleared up. I really dont want to take a chance and I feel that there is probably a better treatment method out there. Has anyone ever heard of this working???
Hi RileysMom.
What you are describing is what many refer to as the “slow kill” method of treating heartworms. There is some controversy about whether this is the best way to treat but, in some cases, when the Immiticide treatment is not possible for whatever reason, it may be the only other option. The concern with this method is that the adult worms remain in the heart and vessels of the infected dog until they die of “natural” causes and this can take months or sometimes even years. During that time, they can still cause damage. It will work eventually, it’s just a matter of how long it takes to work.
Did you discuss other options with your veterinarian? If not, you may want to touch with him or her. There may be health reasons for choosing this form of treatment over the Immiticide treatment but you could discuss it and find out if your dog is a candidate.
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