The Life cycle of Dirofilaria immitis:
No matter which prevention method you choose for your pet, they both work by killing the L3 and L4 larvae found in the body. Preventions do not kill mosquitoes nor do they prevent the larvae from entering the body. When the prevention is given monthly, the medication kills the larvae that have been in the body for the past 30 days (Petmd.com), which means the medication does not stay in your dog’s system for the whole 30 days. This is why it is so important to give your dog medication every month. If more than 30 days lapses between doses, the prevention cannot kill the immature or mature adult heartworms and your dog is at a high risk of getting heartworm disease. For this reason, the National Heartworm Society recommends Heartworm preventions should be given every month, year around no matter where you live.
Prohart-6 is an injection that your dog gets every six months under the skin to prevent heartworms. It is very convenient; you do not have to worry that you may skip a dose or your pet spitting out the pill.