Revised heartworm guides add resistance information

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The American Heartworm Society’s guidance on heartworm infection in dogs and cats now includes information on resistance to heartworm preventives.
The organization also increased the prominence of prevention instructions in its 2014 guides on prevention, diagnosis, and management of heartworm infection. The AHS published in January updated editions of the two sets of guidelines—one each for dogs and cats. They are available here.

Through the guides, the AHS also now recommends microfilaria testing along with antigen testing to reduce the likelihood of false-negative tests from antigen testing alone. The guides include updated recommendations on adulticide treatment in dogs as well as added diagnostic differentials for infections in cats.

AHS officials published a separate statement that describes resistance to macrocyclic lactones as an issue of great importance in veterinary medicine.

“Every compound currently marketed in every form of administration (oral, topical, and parenteral) has been shown to be less than perfect in at least one study,” the publication states. “However, while the evidence indicates that resistance affects all macrocyclic lactones, differences in active ingredients, doses, and product formulation among the available preventives can result in varying rates of failures.”

The statement notes that heartworm preventives remain effective in the “vast majority” of uses, and appropriate use according to the label is of paramount importance. Inappropriate uses include administering macrocyclic lactones alone to treat heartworm-infected dogs and administering products that are intended for use in large animals as heartworm preventives in dogs or cats.