American Heartworm Society issues revised guidelines


Resistance acknowledged; new veterinary testing methods recommended.

ORLANDO—An increased emphasis on the importance of heartworm prevention, as well as new scientific information about heartworm resistance, testing and treatment, have prompted the American Heartworm Society (AHS) to revise its guidelines for dogs and cats. The updates to the guidelines were announced during the AHS’s half-day symposium at the North American Veterinary Conference earlier this week.

“The body of knowledge we have on heartworm is constantly expanding,” says Stephen Jones, DVM, who began his three-year term as AHS president last fall. “This was especially apparent at the 2013 Heartworm Symposium in September, where more than 40 presentations were delivered on a range of heartworm issues, including heartworm resistance.”

The AHS board conducted a comprehensive review of the scientific papers submitted for the symposium, resulting in a series of updates to the guidelines. Here’s an overview:

> Acknowledgment of heartworm resistance. Previously, resistance was cited as one possible factor among others that can lead to lack of efficacy, including inadequate compliance, failed administration and failed absorption. The new guidelines still implicate these factors but also note the documented presence of resistant subpopulations of heartworms. The new information about resistance has also prompted the AHS to place additional emphasis on the importance of year-round administration of heartworm preventives.

The revised guidelines acknowledge that, while resistance apparently affects all macrocyclic lactones, differences among active ingredients, doses and product formulations can result in varying rates of failure.

> Increased emphasis on heartworm prevention. Previous AHS guidelines were organized under the headings of heartworm diagnosis, prevention and treatment. The current version leads with heartworm prevention, highlighting the need for veterinarians and their clients to ensure that all at-risk animals are on prevention 12 months a year.

> Antigen plus microfilaria testing recommended. Antigen testing is considered the most sensitive diagnostic method when screening an asymptomatic dog or seeking verification of a suspected heartworm infection. But a study conducted on shelter dogs found a 7.1 percent false negative rate due to the formation of antigen-antibody complexes. The AHS now recommends that microfilaria testing be done in tandem with antigen testing to determine if this life stage of heartworms is present; previously the guidelines suggested that microfilaria testing “may be done.” This practice will help in the identification of these heartworm-infected dogs that have negative antigen test results yet may have circulating microfilariae.

> Support for doxycycline and macrocyclic lactone administration prior to adulticide therapy. The AHS heartworm treatment protocol for dogs includes administration of a macrocyclic lactone preventive coupled with doxycycline to suppress embryogenesis of heartworms, weaken adult heartworms and decrease post-treatment complication rates. This regime begins 60 days before the first of three administrations of melarsomine. Any treatment method utilizing only marcrocyclic lactones as a slow-kill adulticide is not recommended.

> Additional diagnostic differentials for heartworm in cats. Diagnosis of heartworm infection in cats is challenging, in part because of limitations in accuracy of interpreting antigen and antibody tests. Thoracic radiography and echocardiography are useful methods of confirming infections. According to information presented at the 2013 Triennial Symposium, however, infections with Toxocara cati and Aelurostrongylus species can produce thoracic radiographic patterns similar to those caused by Dirofilaria immitis, necessitating their consideration in a differential diagnosis.

The revised AHS guidelines can be found at In addition, the AHS also has a newly revised AHS resistance statement.

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