The latest revisions are based on findings from the 14th Triennial Heartworm Symposium held in September 2013. I’ve outlined a summary of the guideline changes here, and urge you to visit our website at heartwormsociety.org to download the complete documents.
- Acknowledgement 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. In the new guidelines, these same factors are implicated, but the documented presence of resistant sub-populations of heartworms is noted.
The revised guidelines also note that, while resistance apparently affects all macrocyclic lactones, differences among active ingredients, doses and product formulations can result in varying rates of failures.
- Increased emphasis on heartworm prevention. As noted above, heartworm resistance to preventives is real. This is not, however, a cue to abandon prevention; in fact, the opposite is true. More than ever, we as practitioners need to emphasize proper use of preventives, which means ensuring that all at-risk animals—both dogs and cats—are on prevention 12 months a year.
- Antigen and microfilaria testing recommended. Antigen testing is considered the most sensitive diagnostic method when screening asymptomatic dogs or seeking verification of a suspected heartworm infection. A study conducted on shelter dogs found a 7.1% 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 administration of doxycycline and macrocyclic lactones prior to adulticide therapy. The AHS heartworm treatment protocol for dogs includes administration of a macrocyclic lactone (ML) 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 MLs 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; however, infections with Toxocara cati and Aelurostrongylus spp. can produce thoracic radiographic patterns similar to those caused by D. immitis, necessitating their consideration in a differential diagnosis.
While largely preventable, heartworm disease today remains one of the most serious diseases affecting our patients. By following the American Heartworm Society’s guidelines, you can help reverse this trend.
C. THOMAS NELSON, DVM
ANIMAL MEDICAL CENTER
AMERICAN HEARTWORM SOCIETY