This severe and life-threatening condition is preventable. The parasite is transmitted by mosquitoes and can affect both dogs and cats alike. The heartworm parasite is transmitted by mosquitoes and can affect both and cats. There are several different products available for the prevention of heartworm, including convenient once a year injection. We are happy to discuss the options with you to find which one works best for you and your pet. Please remember that ” all-wormer” products do not protect against heartworm.