Michael Paul, DVM
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The Companion Animal Parasite Council (CAPC) and the American Heartworm Society (AHS) have released a joint statement prompted
by concerns of veterinary parasitologists and clinicians over regional reports of lack of efficacy of macrocyclic lactones
used to prevent heartworm infection in animals. Representatives of both organizations—along with university clinicians with
particular interest in heartworm disease and experts in pharmacology, parasitology, and resistance research—met to address
the issues surrounding this lack of efficacy.
The goal was to take steps to clarify current understanding about resistant populations of heartworms that at this time appear
to be limited geographically to the upper and lower Mississippi River region. The consensus of the group was that resistance
has indeed been demonstrated to occur. While such populations have been identified, it was stressed that a lack of dispensing
consistency and client compliance are major factors that contribute to lack of efficacy.
The report stresses that macrocyclic lactones are the most important weapon in preventing heartworm infection currently available
and that they should continue to be prescribed year-round as the need for improved client compliance is emphasized.
The report also outlines recommendations for testing protocols that take into account some life cycle concerns that are frequently
misunderstood or neglected. Annual testing is still recommended, with additional testing recommended in some situations such
as delayed initiation of preventive, which may allow for latent infections, or after a lapse in administration.
Although the emphasis of the discussions was on canine heartworm disease, effective prevention relies on monthly administration
of a macrocyclic lactone to all dogs and cats year-round. Additional measures, including screening in kennels when possible,
confining pets indoors during the heaviest hours of mosquito exposure, and the use of mosquito repellents, can significantly
reduce exposure to infected mosquitoes.
Treatment protocols were discussed and agreed to by consensus. One of the most important outcomes was the admonishment not
to use so-called "slow kill" or "soft kill" treatments. The treatment protocol outlined is the same as that described on the
CAPC and AHS websites and emphasizes the administration of macrocyclic lactones, doxycycline, and melarsomine injected three
times in the regimen.
Several key points from this meeting:
1. The extent of the lack of efficacy problem is not fully known. As more studies are done, amendments to this statement
will be necessary.
2. The single greatest tool we have to prevent heartworm infection remains the macrocyclic lactone group of drugs.
3. The most basic step we can take to reduce lack of efficacy is to improve compliance, which involves veterinarians recommending
and clients administering preventives.
4. The timing and frequency of testing must be linked to an understanding of the life cycle of heartworms.
5. Treatment is not complete until the patient is free of microfilaria.
6. Infected dogs should not be transported until proven clear of microfilaria.
The AHS and CAPC are extremely proud of the fact that this was a joint effort, and we look forward to future collaborations.
Dr. Paul is the executive director of the Companion Animal Parasite Council and a former president of the American Animal
Hospital Association. He is retired from practice and lives in Anguilla, British West Indies.
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