A 1.5 year-old male castrated mixed-breed dog presented to the Cornell University Hospital for Animals in May 2016 for evaluation following a positive heartworm antigen test performed the month before. The dog had tested negative for heartworm infection in October 2015 when he was rescued as a stray in North Carolina; the client started him on monthly heartworm preventatives when they received custody of the dog at the beginning of November 2015. On presentation the dog was bright, alert, and responsive. Cardiovascular evaluation was unremarkable as was the remainder of the physical exam. Confirmatory tests for heartworm infection included a SNAP 4Dx, a DiroCHEK® Heartworm Antigen Test at the NYS Animal Health Diagnostic Laboratory, a direct blood sample examination, and a Knoll's test. Both antigen tests were positive, but there was no evidence of microfilariae. The patient was treated and cleared of infection. This paper will discuss the transmission, diagnosis, and treatment of heartworm infections.