Heartworm is a parasitic worm that lives inside the blood vessels of infected animals and can cause heart failure over time. When established inside the blood vessels, a reproducing worm releases large numbers of very small worms (microfilariae) that circulate around the bloodstream. Mosquitos pick up the microfilariae when they take a blood meal and can deposit them in the bloodstream of another dog when they take their next blood meal.
This cycle can be interrupted with the simple application of a medication once monthly to the skin at the back of the neck. It is important to have a blood test to check to see if heartworm parasites are already there, and this can be performed in your own home. Prevention is usually applied just once monthly from June 1 to November 1, or during winter months if travelling to warmer climates.