Which heartworm preventative is considered a microfilaricide?

Ivermectin is recognized as a microfilaricide, meaning it is capable of killing the juvenile forms of heartworms known as microfilariae. This property makes it particularly effective not only as a heartworm preventative but also in treating existing infections, as it can reduce the microfilarial load in the bloodstream of infected animals.

While other medications like milbemycin oxime and selamectin are effective heartworm preventatives, they do not have the ability to eliminate microfilariae. Diethylcarbamazine (DEC), while effective in certain contexts, is less commonly used today due to its side effects and is not as effective in a preventative role as ivermectin. Therefore, ivermectin's dual role as both a preventative and a microfilaricide distinguishes it as the correct answer in this context.