The American Heartworm Society estimates that about 27 million dogs are not protected from heartworms. Heartworm cases are found in all 50 states but are most prevalent within 150 miles of the Gulf and Atlantic coasts as far north as New Jersey and along the Mississippi River region. In these areas, almost half the dogs not on heartworm prevention are infected. And that's scary, because heartworms kill.It's also scary because the more infected dogs there are in an area, the more infected mosquitoes there are as well. Dogs become infected through the bite of a mosquito that has bitten another infected animal. When the mosquito bites an infected dog, it ingests young pre-larval heartworms called microfilariae that are circulating in the infected dog's blood.The microfilariae mature into larvae within the mosquito in 10 to 14 days. When the mosquito feeds on another dog, it injects the larvae, which then travel to the new dog's heart, lungs and associated blood vessels,and eventually mature into adults that may live for five to seven years.One dog may harbor as many as 250 heartworms, each up to a foot long. The worms initially cause inflammation of the surrounding arteries, and later, enlargement of the heart, congestive heart failure and death.Yet prevention is simple with either daily or monthly drugs. Most owners use a monthly preventive because it is easier to remember. Puppies can start a preventive as early as eight weeks of age. Adult dogs should be checked to see that they are free of heartworms before beginning prevention. Your veterinarian can check for heartworms by looking at a blood sample for microfilariae, the presence of which indicates that there are adult heartworms in the dog. However, because not all infestations produce detectable microfilariae, a more sensitive antigen test (sometimes called an occult heartworm test) can detect the presence of adult female heartworms that are at least eight months old. X-rays and ultrasounds can help determine the extent of the infection.Dogs with heartworms can be treated, but there can be some risks involved--fortunately, not as many as there were several years ago. The major complication comes from lung embolisms caused by inflammatory reactions and pieces of dead heartworms in the lung's blood vessels. This complication can be reduced by enforcing strict rest, giving anti-inflammatory drugs and even by surgically removing as many worms as possible before beginning treatment. Obviously, it's far safer, cheaper and easier to simply prevent them in the first place.In fact, heartworms are now perhaps the most preventable potentially fatal disease of dogs. Yet even dogs on preventive medication can get heartworms, mostly because the owner forgets to the give the preventive on time. That's why dogs on preventives should still be checked every other year or following any lapse in dosing. It's also why the American Heartworm Society supports year-round administration of heartworm preventives. When preventives are given year-round, a missed dose is less likely to lead to an infection.Since their discovery 100 years ago, heartworms have taken far too many lives.With the means to prevent them from taking more, there's no excuse to ignore them.To find out more, visit Pet Vet's Canine Heartworm Pet Vet page.