All of the following are monthly heartworm preventives except:

The correct answer is diethylcarbamazine, which is not classified as a monthly heartworm preventive. Diethylcarbamazine (DEC) is typically administered on a daily basis rather than monthly, which distinguishes it from the other options listed.

In contrast, milbemycin oxime, selamectin, and Heartgard Plus are all formulations that are designed for monthly administration to effectively prevent heartworm disease in pets. Milbemycin oxime is known for its effectiveness against several parasitic infections and is commonly used in a monthly prevention regimen. Selamectin is a topical solution that not only prevents heartworms but also treats and controls other parasites, administered once a month. Heartgard Plus, which contains ivermectin and pyrantel, is another popular monthly chewable tablet specifically aimed at preventing heartworm infection.

The distinction of diethylcarbamazine as a daily preventive highlights different approaches to heartworm prevention and underlines the importance of adhering to proper dosing schedules for effective parasite control.