NATIONAL REPORT — Studies are in the works to determine the scope of heartworm resistance and are expected to be unveiled to the veterinary
market in the next six months.
Until then, a joint statement from the Companion Animal Parasite Council (CAPC) and American Heartworm Society (AHS) says
the lack of efficacy to heartworm preventives (macrocyclic lactones) remain geographically limited.
The consensus statement was crafted from a roundtable discussion in early August, explains Michael Paul, CAPC's executive
director. The roundtable was attended by veterinary experts Drs. Stephen Ettinger (chair), Byron Blagburn, Dwight Bowman,
Sharon Patton, Matt Miller, John McCall and Tom Nelson.
The panel concludes, "Most credible reports of lack of efficacy (LOE) that are not attributable to compliance failure are
geographically limited at this time. The extent of this problem is obscured by demonstrated lack of owner and veterinary compliance,
possible changes in environmental/vector factors..."
The statement was created to offer veterinarians further guidance in four areas—resistance, testing, prevention and treatment,
the groups report.
When it comes to resistance, both groups agree there is evidence of genetic variation within some heartworm populations that
may be associated with decreased susceptibility to macrocyclic lactones. "The potential for lack of efficacy of traditional
control products is not a reason to abandon their use."
Paul adds that veterinarians need to remain vigilant and aggressive in recommending heartworm preventives, step up testing
and treat if necessary.
"This statement recognizes that something is going on," Paul adds. "To me, that is the number one issue. Something is happening
in this area that is resulting in the lack of efficacy. In spite of those questions, these products are still the best thing
we got, and in the vast, vast majority of dogs, they do protect. We want to make sure that dogs stay on these products," he
"We want to stress to clients that we want them [to use] these products year-round and in all dogs. And we have to stress
the necessity of compliance. It's about compliance, compliance, compliance. It is something that we can do something about."
Dr. Wallace Graham, president of the American Heartworm Society (AHS), adds that the current recommendations will be updated
as new information arises. "My confidence level is good in what we have. But it is not adequate to the problem. There are
things we don't yet know," Graham says. "I am confident the information will come. We have really good people investigating
all facets of this thing."
Key recommendations for veterinarians include:
- When used properly, macrocyclic lactones remain effective as a prophylactic agent in the vast majority of dogs.
- Don't abandon prevention strategies.
- Don't "vary from label directions on the dose and frequency of administered products or concurrently administer multiple products."
- Stage-specific medical management should be used based on the classification of heartworm disease.
- Both organizations strongly opposed "soft kill" strategies (using preventives to try to "treat" heartworm positive dogs).
- CAPC and AHS testing guidelines should be followed even in the "face of reports of lack of efficacy."
The following are specific recommendations from the panel to address each of the four areas:
Resistant heartworm populations
The panel agreed that there is evidence of genetic variation within some heartworm populations that may be associated with
decreased susceptibility to macrocyclic lactones. The potential for lack of efficacy of traditional control products is not
a reason to abandon their use. When properly used, macrocyclic lactones remain effective prophylactic agents in the vast majority
The panel agreed that current CAPC and AHS guidelines continue to be relevant even in the face of reports of LOE. Veterinarians
should re-emphasize the need for annual heartworm testing.
The panel recommended that existing guidelines for the use of macrocyclic lactones as a preventive be followed and that accurate
medical records be kept regarding pre-prevention and post-prevention testing. Both CAPC and AHS emphasized the need for client
education about heartworm disease and to take the steps necessary to reduce exposure.
Further, the panel recommended that veterinarians not vary from label directions on the dose and frequency of administered
products, or concurrently administer multiple products.
Existing CAPC and AHS treatment guidelines should continue to be followed at this time. Because the severity of heartworm
disease varies, veterinarians are encouraged to follow the stage-specific medical management based on the classification of
Both organizations are strongly opposed to "soft kill" and the panel stated: "Based upon recent findings, veterinary medical
knowledge and acceptable stewardship of our available medications and patients, slow kill or soft kill is not recommended and should not be used."
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