Rave Posted May 31, 2012 Report Share Posted May 31, 2012 A friend of mine recently found out her dog has heartworms. She was very consistent in giving Interceptor on the first of every month. From what my friend learned from her vet and from my own internet research, I was shocked to learn the effectiveness of most heartworm preventative is not 100% and has been on the decline in recent years, now with the possibility of a resistant strain of heartworm. I wanted to bring this to everyone's attention so that you could do your own research and talk to your vet about what's best for your dog in terms of testing and preventative. Below are a few articles I've run across in my research. http://veterinarynews.dvm360.com/dvm/Veterinary+news/Leading-parasitologist-reveals-heartworm-preventiv/ArticleStandard/Article/detail/703785 http://www.bayeranimalhealthsymposium.com/blagburn_heartworm.shtml http://www.sundogcatmoon.com/2/post/2011/03/new-information-regarding-the-effectiveness-of-heartworm-prevention.html http://www.mdvma.org/newsletter/2011/spring/heartworm.pdf Quote Link to comment Share on other sites More sharing options...
Liz P Posted May 31, 2012 Report Share Posted May 31, 2012 No medication is 100% effective or predictable. That is why vets do annual HW tests, even if a dog takes a tablet every single month. I've seen a few HW positive dogs who took medication faithfully. Quote Link to comment Share on other sites More sharing options...
Rave Posted May 31, 2012 Author Report Share Posted May 31, 2012 The recent study those articles cite shows Advantage was 100% effective in that particular study. What I'm concerned with is the effectiveness appears to be decreasing in recent years and also the possibility of a resistant strain. Quote Link to comment Share on other sites More sharing options...
Liz P Posted May 31, 2012 Report Share Posted May 31, 2012 The recent study those articles cite shows Advantage was 100% effective in that particular study. What I'm concerned with is the effectiveness appears to be decreasing in recent years. Yes, but studies and real life aren't the same thing. You are limited in studies to a very small number of individuals. When a drug actually hits the market you find out the true effectiveness and unexpected side effects. You also have variables that are not present in a lab setting, like how people apply the product. The biggest problem I hear about with topical heartworm prevention is incorrect application leading to a pet not getting a full dose. Sure, worms develop resistance to dewormers. It is possible that is happening or will happen with heartworm. Quote Link to comment Share on other sites More sharing options...
Mark Billadeau Posted May 31, 2012 Report Share Posted May 31, 2012 The last time I looked at the published studies there were 3 likely causes for heartworm preventative failures: 1. preventative dose was incorrect, not given, or not retained by dog (i.e. spit out) 2. resistance was being developed in heartworms 3. the minimum effective dose developed 10+ years ago for the estimated level of microfilaria load is no longer valid for the current microfilaria load The data the last time I reviewed the literature was inconclusive about #2. There was sufficient data to indicate the number of infected mosquitoes has increased over the years; this means the likely microfilaria load dogs are getting each month is higher than when the minimum effective dose was determined for the preventatives. There was a study where one specific strain of microfilaria maintained in a laboratory obtained from one dog which showed some resistance to all heartworm preventatives (see Rave's first link for a summary). In this study "All dogs were infected with 100 MP3 infective larvae and treated once as follows 30 day later...". I've highlighted the 3 key points in this study which may or may not apply in the real world. The number of larvae may not be representative of what dogs get each month, the abundance of this strain in the field is not known, and dogs get treated each month which has an additive effect on preventing the development of larvae into adult heartworms. Quote Link to comment Share on other sites More sharing options...
Rave Posted May 31, 2012 Author Report Share Posted May 31, 2012 Yes Liz I realize there is often a gap between lab testing and real world, which is why I was very clear about that in the section you highlighted. I have never felt comfortable using a topical for hw preventative for the reasons you mentioned. However I find it interesting that delivery system, when used properly, appears to be more effective and less prone to create a potentially-resistant heartworm. I also read and believe that improved testing may be another reason we're learning of more positives. The purpose of this post was to make people aware of the issues. Several people I talked to after learning about my friend's dog had no idea Interceptor may only be 92% effective (that was the number the vet told my friend). Not all vets require testing to get HW preventative. The American Heartworm Society discusses the lack of efficacy and summarizes the studies done: http://www.heartwormsociety.org/veterinary-resources/canine-guidelines.html Quote Link to comment Share on other sites More sharing options...
Mark Billadeau Posted May 31, 2012 Report Share Posted May 31, 2012 I've done another review of the literature on this subject and found this study which has been published and is presented here in slide form: Macrocyclic lactone resistance in Dirofilaria immitis The study shows a correlation between macrocyclic lactone (all heartworm meds) effectiveness and the frequency of a genotype in the surviving microfilaria after treatment (lower effectiveness = higher frequency of genotype). The study indicated that while the adult heartworms could be cleared, resistant microfilaria remained in the blood. They are working on developing a test for resistant microfilaria. On a side note the study indicated that the MP3 strain discussed above was not found to be resistant. Quote Link to comment Share on other sites More sharing options...
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