Heartworm Resistance

Heartworm resistance: CAPC, AHS issue new guidance for veterinarians


National Report -- Lack of efficacy to heartworm preventives (macrocyclic lactones) remain geographically limited, according to a new joint statement issued by two prominent veterinary groups -- the Companion Animal Parasite Council (CAPC) and American Heartworm Society (AHS).

The consensus statement that has been in the works since its August meeting, as first reported in DVM Newsmagazine, was released to offer veterinarians guidance, explains Michael Paul, CAPC's executive director.

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.

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.
* CAPC and AHS testing guidelines should be followed even in the "face of reports of lack of efficacy."

Here are the specific recommendations from the panel to address each of the four areas:

Resistance of canine heartworm populations
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 of dogs.

Testing
The panel agreed that current CAPC and AHS guidelines continue to be relevant even in the face of reports of LOE, and that emphasis continue to be placed on annual heartworm testing.

Prevention
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.

Treatment
Existing CAPC and AHS treatment guidelines remain intact and 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 heartworm disease.
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."

recent research indicates that some field isolates and laboratory strains of Dirofilaria immitis may not be fully susceptible to preventive medications.