Looks like the heartworm protection in this lot isn't working:
Iverhart Max Lot Recalled on Efficacy Concern
Virbac Animal Health voluntarily recalled one lot of its canine heartworm and internal parasite preventive Iverhart Max Chewable Tablets (ivermectin, pyrantel pamoate, praziquantel) because the ivermectin failed to meet the company’s stability specifications.
Some dogs dosed with tablets from the affected lot (#110482 for large dogs, 50.1 to 100 lbs.) may not be protected against heartworm disease, the company said.