A problem does exist with lack of efficacy of heartworm preventives, according to a panel of representatives from the American Heartworm Society and Companion Animal Parasite Council, but the scope and severity of the problem are unclear.
"Most credible reports of lack of efficacy that are not attributable to compliance failure are geographically limited at this time," according to the panel report. "The extent of this problem is obscured by demonstrated lack of owner and veterinary compliance, possible changes in environmental/vector factors, and newer, more effective antigen testing for heartworms, all of which contribute to what is interpreted to be a lack of efficacy."
The panel of representatives from the AHS and the CAPC met in Atlanta this past August to discuss reports of a lack of efficacy of macrocyclic lactones (avermectins and milbemycins) against canine heartworms. In November, the organizations released a report on the roundtable discussion.
The panel concluded that the potential for resistance is not a reason to abandon macrocyclic lactones. The panel identified issues for further study and recommended that practitioners follow current AHS and CAPC guidelines on heartworm testing, prevention, and treatment.
Dr. Charles Thomas Nelson, a panel member and a past president of the AHS, re-emphasized the importance of dog owners giving heartworm preventives to their dogs every 30 days.
"Until we get more data, until we get more answers, we really can't say a lot more than to be sure to insist that people give the product every single month," Dr. Nelson said. "And a dog with heartworms needs to be treated appropriately. The practice of using preventives long term to treat heartworms may lead to selection of a resistant strain."
The panel report as well as guidelines on heartworm control are available on the AHS website at www.heartwormsociety.org and CAPC website at www.capcvet.org.