Pet parents know the importance of maintaining a monthly regimen of heartworm preventative for their four-legged family members but one popular brand may no longer be working. Heartworms are nasty parasites. Treatments for infected pets are often lengthy and expensive and left untreated, can be deadly. That's why preventive drugs are so important. Heartworms are transmitted through the bite of an infected mosquito and since we live in an area that has a large mosquito population, especially in the summer months, the risk of heartworms is that much higher. Personally, my cat, Jupiter Jack, doesn't even venture outside, but because mosquitos do sometimes find their way inside the house from time to time, we keep him on preventative as well out of an abundance of caution.