What is the primary action of currently-available monthly heartworm preventatives in dogs?

The primary action of currently available monthly heartworm preventatives in dogs is to kill any larvae (specifically, the juvenile stages called L3 and L4 larvae) acquired by the dog in the preceding 30-day period. These preventatives work by targeting the larval stage as it develops in the dog's body, thus stopping the progression to adult heartworms, which can lead to serious health issues.

By effectively eliminating these immature larvae, the preventatives significantly reduce the risk of developing heartworm disease, which can cause severe cardiovascular damage and other health complications. It is important to note that while the preventatives are effective against larvae, they do not kill adult heartworms that may already be present, nor do they remove eggs from the bloodstream. They are also not designed to prevent all types of worm infections, as their efficacy is specific to heartworm larvae.