The Heartworm Cycle in Dogs
There's a lot of information and mis-information floating around about Heartworm Prevention Effectiveness. We wanted to let you know where we stand on this.
Our recommendations for monthly heartworm prevention have changed recently due to some new and alarming information collected in the Gulf States. We are now recommending Advantage Multi to prevent heartworms, intestinal parasites (hookworms, roundworms, and whipworms), as well as adult fleas. It does not control ticks, so if that is an issue for your dog, we currently recommend Scalibor collars, as they are highly effective, keep ticks from attaching, are water proof, and are good for 6 months. They release the smallest amount of chemical compared to the topical Permethrin products (Vectra 3D and Advantix). Frontline does not prevent all ticks from attaching, and only seems to last for 2 to 3 weeks. Another emerging problems is tick borne disease, like Lyme disease, so if we can prevent the ticks from attaching, then we will have prevented the transmission of Lyme disease, Rocky Mountain Spotted Fever, Erlichiosis, and Bartonelosis.
There was a recent study by Dr. Byron Blagburn at the University of Alabama regarding the efficacy of heartworm preventatives. Before we get into the results of the study, we'd like to give you a bit of history. In the years following Hurricaine Katrina, veterinarians in the Gulf States began seeing heartworm prevention failures. The drug companies were insisting the failures were because the pet owners were not giving prevention consistently, but it quickly became clear that was not the case. Next it was theorized that dogs were being exposed to too many larvae and that was why the heartworm prevention was not working. While the numbers of larvae had increased, that was also not the case. Finally, Dr. John McCall was able to isolate the reason for the heartworm prevention failures- a resistant strain of heartworm. This strain has been tracked all over the United States, but is most prevalent coming up the Mississippi River and wherever large groups of dogs were relocated from the Katrina-devastated states. While the strain has not reached the South Carolina coast as yet, it is in the Atlanta area and will be here soon.
Once the strain was isolated, the FDA became concerned that the preventives were not performing according to their label directions, and so they designed the study that Dr. Blagburn has just completed. The study involved 5 groups of 8 dogs each that had been born in a lab facility, therefore, they had never been exposed to mosquitoes outside. The five groups were a control, Heartgard, Interceptor, Revolution, and Advantage-Multi. The dogs were given an intravenous dose of 100 heartworm larvae of the resistant strain, and then given the assigned heartworm prevention for 5 months, sacrificed, and necropsied. The control group dogs all had somewhere between 34 and 70 worms removed from the heart and lungs. This is consistent with naturally acquired heartworm disease. The dogs on Advantage Multi were all 100% protected. The dogs on Revolution, Interceptor and Heartguard all had one dog that would have tested positive for heartworms. This means that there were at least 6 to 9 worms present. For a dog to test positive for heartworms, the dog must have at least 3 adult female heartworms present to produce enough antigen for the test to detect disease. More disturbing is that of the remaining 7 dogs in the above groups, 6 of the dogs had 1 to 4 worms present. The low worm count in the 6 dogs means that they likely would not test positive, but actually have heartworm disease.
It has been theorized that the resistance problem developed for 2 reasons. The first reason is based on the way the most heartworm preventives work. When you give a heartworm pill, it kills everything it can in 24 hours. Then the drug level drops but is never completely gone from the blood stream, so the worms learn how to survive around the drug. Another theory for resistance derives directly from how we've treated heartworm disease in the past. About 15 years ago, veterinarians discovered that if you treated a heartworm-positive dog with prevention, within 12 months, the dog would test negative. We called this the look-back effect. Over time, the adult worms are killed off, and this treatment is easier on dogs than injectable therapies like Immiticide. Unfortunately the adult worms that are present develop larvae called microfilaria, and because they have been able to survive monthly applications, they can build resistance to the drugs we were using.
Advantage Multi was 100% protective because the active ingredient, Moxidectin, is absorbed from topical application within 30 minutes into the fat below the skin, and from there picked up by the blood stream. After 3 to 4 months of use, Moxidectin achieves a steady state in the blood stream so that it kills every stage of heartworm and intestinal parasite daily. Proheart 6 also contains Moxidectin, but because of the way the drug is encapsulated for the injection, it has had failures, as the drug is insufficient to achieve the steady state.
There are other strains of heartworms as well as the resistant one, but the current heartworm failure rates in the endemic areas are around 2%. If you consider that 1 dog out the 8 in a test group will test positive, but an additional 6 have heartworms that cannot be detected, then this may mean that 14% of the population is actually infected with heartworms. Because we are just beginning to understand the problem at hand, we truly do not know how big or small the problem is.
I know many of you had been trying to get away from topically applied products, and so there are a few other options. Advantage Multi is topically applied, but as mentioned previously, the heartworm prevention portion, Moxidectin, is 100 % absorbed within 30 minutes of application. If you are not having flea issues, you can theoretically bathe your pet in 2 hours after application, and wash off the alcohol base and the Imidacloprid portion of the Advantage Multi. Imidacloprid is the ingredient that kills adult fleas. This means you will need to make sure that you have done a good job controlling fleas in the environment, and have a back-up plan like Capstar or Program to help keep them controlled.
Please contact our office with any questions or concerns. Thank you for allowing us to take care of your pets.
Ruth M. Roberts, DVM CVA CVH
Sun Dog Cat Moon, LLC
Our recommendations for monthly heartworm prevention have changed recently due to some new and alarming information collected in the Gulf States. We are now recommending Advantage Multi to prevent heartworms, intestinal parasites (hookworms, roundworms, and whipworms), as well as adult fleas. It does not control ticks, so if that is an issue for your dog, we currently recommend Scalibor collars, as they are highly effective, keep ticks from attaching, are water proof, and are good for 6 months. They release the smallest amount of chemical compared to the topical Permethrin products (Vectra 3D and Advantix). Frontline does not prevent all ticks from attaching, and only seems to last for 2 to 3 weeks. Another emerging problems is tick borne disease, like Lyme disease, so if we can prevent the ticks from attaching, then we will have prevented the transmission of Lyme disease, Rocky Mountain Spotted Fever, Erlichiosis, and Bartonelosis.
There was a recent study by Dr. Byron Blagburn at the University of Alabama regarding the efficacy of heartworm preventatives. Before we get into the results of the study, we'd like to give you a bit of history. In the years following Hurricaine Katrina, veterinarians in the Gulf States began seeing heartworm prevention failures. The drug companies were insisting the failures were because the pet owners were not giving prevention consistently, but it quickly became clear that was not the case. Next it was theorized that dogs were being exposed to too many larvae and that was why the heartworm prevention was not working. While the numbers of larvae had increased, that was also not the case. Finally, Dr. John McCall was able to isolate the reason for the heartworm prevention failures- a resistant strain of heartworm. This strain has been tracked all over the United States, but is most prevalent coming up the Mississippi River and wherever large groups of dogs were relocated from the Katrina-devastated states. While the strain has not reached the South Carolina coast as yet, it is in the Atlanta area and will be here soon.
Once the strain was isolated, the FDA became concerned that the preventives were not performing according to their label directions, and so they designed the study that Dr. Blagburn has just completed. The study involved 5 groups of 8 dogs each that had been born in a lab facility, therefore, they had never been exposed to mosquitoes outside. The five groups were a control, Heartgard, Interceptor, Revolution, and Advantage-Multi. The dogs were given an intravenous dose of 100 heartworm larvae of the resistant strain, and then given the assigned heartworm prevention for 5 months, sacrificed, and necropsied. The control group dogs all had somewhere between 34 and 70 worms removed from the heart and lungs. This is consistent with naturally acquired heartworm disease. The dogs on Advantage Multi were all 100% protected. The dogs on Revolution, Interceptor and Heartguard all had one dog that would have tested positive for heartworms. This means that there were at least 6 to 9 worms present. For a dog to test positive for heartworms, the dog must have at least 3 adult female heartworms present to produce enough antigen for the test to detect disease. More disturbing is that of the remaining 7 dogs in the above groups, 6 of the dogs had 1 to 4 worms present. The low worm count in the 6 dogs means that they likely would not test positive, but actually have heartworm disease.
It has been theorized that the resistance problem developed for 2 reasons. The first reason is based on the way the most heartworm preventives work. When you give a heartworm pill, it kills everything it can in 24 hours. Then the drug level drops but is never completely gone from the blood stream, so the worms learn how to survive around the drug. Another theory for resistance derives directly from how we've treated heartworm disease in the past. About 15 years ago, veterinarians discovered that if you treated a heartworm-positive dog with prevention, within 12 months, the dog would test negative. We called this the look-back effect. Over time, the adult worms are killed off, and this treatment is easier on dogs than injectable therapies like Immiticide. Unfortunately the adult worms that are present develop larvae called microfilaria, and because they have been able to survive monthly applications, they can build resistance to the drugs we were using.
Advantage Multi was 100% protective because the active ingredient, Moxidectin, is absorbed from topical application within 30 minutes into the fat below the skin, and from there picked up by the blood stream. After 3 to 4 months of use, Moxidectin achieves a steady state in the blood stream so that it kills every stage of heartworm and intestinal parasite daily. Proheart 6 also contains Moxidectin, but because of the way the drug is encapsulated for the injection, it has had failures, as the drug is insufficient to achieve the steady state.
There are other strains of heartworms as well as the resistant one, but the current heartworm failure rates in the endemic areas are around 2%. If you consider that 1 dog out the 8 in a test group will test positive, but an additional 6 have heartworms that cannot be detected, then this may mean that 14% of the population is actually infected with heartworms. Because we are just beginning to understand the problem at hand, we truly do not know how big or small the problem is.
I know many of you had been trying to get away from topically applied products, and so there are a few other options. Advantage Multi is topically applied, but as mentioned previously, the heartworm prevention portion, Moxidectin, is 100 % absorbed within 30 minutes of application. If you are not having flea issues, you can theoretically bathe your pet in 2 hours after application, and wash off the alcohol base and the Imidacloprid portion of the Advantage Multi. Imidacloprid is the ingredient that kills adult fleas. This means you will need to make sure that you have done a good job controlling fleas in the environment, and have a back-up plan like Capstar or Program to help keep them controlled.
Please contact our office with any questions or concerns. Thank you for allowing us to take care of your pets.
Ruth M. Roberts, DVM CVA CVH
Sun Dog Cat Moon, LLC
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