1. The most frequent reasons are that doses of heartworm preventive were not given.
   • Insufficient amount of product was purchased for each pet in the household.¹
   • A monthly dose of preventive was available but not given.
2. The dog may have been infected with heartworms, including immature adult worms, when started on preventive.
3. The heartworm preventive may have failed to eliminate all migrating tissue larvae at the time of administration.

¹Atkins CE, et al. Heartworm ‘lack of effectiveness’ claims in the Mississippi delta: Computerized analysis of owner compliance - 2004-2011. Veterinary Parasitology 2014;206;106-113