Abstract
BACKGROUND: There are concerns that the chronic use of macrocyclic lactone preventives to kill adult heartworms ("soft-" or "slow-kill") may have contributed to the development of macrocyclic lactone resistance. This prospective analysis was designed to expand our understanding of currently employed treatment decisions, protocols utilized in a "slow-kill" methodology, and trends in heartworm prevention in a region with concerns about macrocyclic lactone resistance. We tested the hypothesis that practitioners underestimate the actual percentage of heartworm-positive dogs treated with "slow-kill" therapy. Owners' financial concerns and veterinarians' efforts at meeting client preferences are the primary reasons for employment of "soft-kill" treatment. METHODS: A prospective analysis of dogs determined to be heartworm-positive when presented to a mixed-animal practice in northern Mississippi was conducted for the second quarter of 2016. Client records were scrutinized for heartworm preventive purchase history. Veterinarians in the four-doctor practice completed a questionnaire regarding their beliefs and practices of heartworm treatment. RESULTS: Forty of 321 canine patients tested heartworm-positive with a commercial antigen test kit. Of these, two were considered to be due to possible product failure. The majority (75.0%) of patients received a "slow-kill" method, a percentage greater than that estimated by the practitioners. Patients were equally likely to have received adulticidal treatment as they were to receive no treatment (12.5%). Injectable moxidectin was the most common preventive used in "slow-kill" treatment (80.65%). Client financial concerns were cited as the primary reason for choosing "slow-kill" treatment (79.0%). CONCLUSIONS: Despite American Heartworm Society recommendations, clients and veterinarians prefer the "slow-kill" method of heartworm treatment. Trends in patient heartworm preventive history show that poor client compliance remains the predominant explanation for heartworm infection. Thus, consistent use of existing, effective heartworm preventives should be the primary goal in reducing prevalence of heartworm infection, regardless of the recognized threat of resistance. It is also noteworthy that practitioner estimates may be suspect in their accuracy. Further study is needed on the risks and efficacy of "slow-kill" treatment and the effects of different ML preventives for this off-label use.